- Haskell-Cafe - Haskell.org

Galois is hiring again!
by Iavor Diatchki 31 Jan '17

31 Jan '17

Hello, We have more openings at Galois, for a variety of positions, including researchers, principle investigators, software engineers, project leads, and some others. The full list is available at: http://galois.com/careers/#careers-list We are looking for people with strong background in one or more of the following areas: functional programming, formal methods, machine learning, embedded systems, computer security, and networking. Galois has two offices, one on the west coast of the US, in Portland, Oregon, and one on the east coast, in Arlington, Virginia. There are positions available at both offices. Generally, we are looking for candidates who are allowed to work in the US, and are interested in working locally at one of the two offices. If this sounds exciting, please consider sending us your resume. The descriptions of the positions each contains an application link. Cheers, -Iavor

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Lazy ST test code
by David Feuer 31 Jan '17

31 Jan '17

I've come up with what might be a fix for a major thread safety bug in lazy ST. Fixing it involved making multiple non-trivial changes to a number of basic functions. Unfortunately, the GHC test suite barely touches lazy ST. Does anyone out there have some code that uses lazy ST in interesting ways but that only relies on the libraries that ship with GHC?

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Haskell IDEs
by Gregory Guthrie 31 Jan '17

31 Jan '17

The Wiki on Haskell IDEs seems to be out of date. It mentions KDevelop, which no longer has any references to Haskell on its site, and to FPeclipse which was orphaned in 2015. I find Leksah rather cumbersome to use/learn (for students), and it is still a bit hard to convince them after Netbeans & Eclipse to fall back to Vi and DOS command windows. :)

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Re: [Haskell-cafe] Haskell IDEs
by Marian Jancar 31 Jan '17

31 Jan '17

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Sequence of lifting transformation operators
by Tyson Whitehead 31 Jan '17

31 Jan '17

A couple of times I find myself wishing for an easy way to transform a function by picking and choosing which of its operators are lifted. For example, consider f <$> x1 <*> pure x2 <*> x3 <*> pure x4 we can put all the details of this into f itself f' x1 x2 x3 x4 = f <$> x1 <*> pure x2 <*> x3 <*> pure x4 and then just write f' x1 x2 x3 x4 I think it might be nice to have a series of transformation functions that made this easy to write. Something like a series of operator like so f' = f <$_$_> where it would be read that f' is a lifted f such that the first argument is lifted, the second is not, the third is, and the fourth is not (this is consistent with f <$> returning a being a lifted f with first argument being lifted). This would make code such as the following flip (go finalX finalY) y `liftM` mx = go <__$_> finalX finalY mx y https://github.com/snoyberg/conduit/blob/be803218b5b2acaad2eb720ca3a27a4d07… much more robust to write (how it gets written now depends very much on the number of arguments, their order, which ones are lifted, etc.), a whole lot easier to read, and don't even get me started on the points free use (go <__$_>)! A library could easily provide the first six or so variants (2^6 = 64 functions). If they were really useful the compiler could provide the rest. <$> :: (a -> b) -> f a -> f b <$$> :: (a -> b -> c) -> f a -> f b -> f c <$_> :: (a -> b -> c) -> f a -> b -> f c <_$> :: (a -> b -> c) -> a -> f b -> f c ... Cheers! -Tyson PS: It would be nice to also have ones that work with f being lifted. That is like how we have <$> for an unlifted f and <*> for a lifted f. <*> :: f (a -> b) -> f a -> f b <**> :: f (a -> b -> c) -> f a -> f b -> f c <*_> :: f (a -> b -> c) -> f a -> b -> f c <_*> :: f (a -> b -> c) -> a -> f b -> f c ... A bit of a pain here is that <**> is actually already taken as <*> with reversed arguments. Possibly this would call for something like this instead <$^> :: (a -> b) -> f a -> f b <$^^> :: (a -> b -> c) -> f a -> f b -> f c <$^_> :: (a -> b -> c) -> f a -> b -> f c <$_^> :: (a -> b -> c) -> a -> f b -> f c ... and <*^> :: f (a -> b) -> f a -> f b <*^^> :: f (a -> b -> c) -> f a -> f b -> f c <*^_> :: f (a -> b -> c) -> f a -> b -> f c <*_^> :: f (a -> b -> c) -> a -> f b -> f c ... where they are prefixed by $ or * to indicate what type f is and then _ encodes an unlifted argument position and ^ a lifted argument position.

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Re: [Haskell-cafe] Monad transformer performance - Request to review benchmarking code + results
by Saurabh Nanda 29 Jan '17

29 Jan '17

Please tell me what to INLINE. I'll update the benchmarks. Also, shouldn't this be treated as a GHC bug then? Using monad transformers as intended should not result in a severe performance penalty! Either monad transformers themselves are a problem or GHC is not doing the right thing. -- Saurabh. On 29 Jan 2017 7:50 pm, "Oliver Charles" <ollie(a)ocharles.org.uk> wrote: I would wager a guess that this can be solved with INLINE pragmas. We recently added INLINE to just about everything in transformers and got a significant speed up. On Sun, 29 Jan 2017, 11:18 am David Turner, <dct25-561bs(a)mythic-beasts.com> wrote: > I would guess that the issue lies within HtmlT, which looks vaguely > similar to a WriterT transformer but without much in the way of > optimisation (e.g. INLINE pragmas). But that's just a guess after about 30 > sec of glancing at https://hackage.haskell.org/package/lucid-2.9.7/docs/ > src/Lucid-Base.html so don't take it as gospel. > > My machine is apparently an i7-4770 of a similar vintage to yours, running > Ubuntu in a VirtualBox VM hosted on Windows. 4GB of RAM in the VM, 16 in > the host FWIW. > > > On 29 Jan 2017 10:26, "Saurabh Nanda" <saurabhnanda(a)gmail.com> wrote: > > Thank you for the PR. Does your research suggest something is wrong with > HtmlT when combined with any MonadIO, not necessarily ActionT? Is this an > mtl issue or a lucid issue in that case? > > Curiously, what's your machine config? I'm on a late 2011 macbook pro with > 10G ram and some old i5. > > -- Saurabh. > > On 29 Jan 2017 3:05 pm, "David Turner" <dct25-561bs(a)mythic-beasts.com> > wrote: > > The methodology does look reasonable, although I think you should wait for > all the scotty threads to start before starting the benchmarks, as I see > this interleaved output: > > Setting phasers to stun... (port 3002) (ctrl-c to quit) > Setting phasers to stun... (port 3003) (ctrl-c to quit) > Setting phasers to stun... (port 3001) (ctrl-c to quit) > benchmarking bareScotty > Setting phasers to stun... (port 3000) (ctrl-c to quit) > > Your numbers are wayyy slower than the ones I see on my dev machine: > > benchmarking bareScotty > Setting phasers to stun... (port 3000) (ctrl-c to quit) > time 10.94 ms (10.36 ms .. 11.52 ms) > 0.979 R² (0.961 R² .. 0.989 R²) > mean 12.53 ms (11.98 ms .. 13.28 ms) > std dev 1.702 ms (1.187 ms .. 2.589 ms) > variance introduced by outliers: 66% (severely inflated) > > benchmarking bareScottyBareLucid > time 12.95 ms (12.28 ms .. 13.95 ms) > 0.972 R² (0.951 R² .. 0.989 R²) > mean 12.20 ms (11.75 ms .. 12.69 ms) > std dev 1.236 ms (991.3 μs .. 1.601 ms) > variance introduced by outliers: 50% (severely inflated) > > benchmarking transScottyBareLucid > time 12.05 ms (11.70 ms .. 12.39 ms) > 0.992 R² (0.982 R² .. 0.996 R²) > mean 12.43 ms (12.06 ms .. 13.01 ms) > std dev 1.320 ms (880.5 μs .. 2.071 ms) > variance introduced by outliers: 54% (severely inflated) > > benchmarking transScottyTransLucid > time 39.73 ms (32.16 ms .. 49.45 ms) > 0.668 R² (0.303 R² .. 0.969 R²) > mean 42.59 ms (36.69 ms .. 54.38 ms) > std dev 16.52 ms (8.456 ms .. 25.96 ms) > variance introduced by outliers: 92% (severely inflated) > > benchmarking bareScotty > time 11.46 ms (10.89 ms .. 12.07 ms) > 0.986 R² (0.975 R² .. 0.994 R²) > mean 11.73 ms (11.45 ms .. 12.07 ms) > std dev 800.6 μs (636.8 μs .. 975.3 μs) > variance introduced by outliers: 34% (moderately inflated) > > but nonetheless I do also see the one using renderTextT to be > substantially slower than the one without. > > I've sent you a PR [1] that isolates Lucid from Scotty and shows that > renderTextT is twice as slow over IO than it is over Identity, and it's > ~10% slower over Reader too: > > benchmarking renderText > time 5.529 ms (5.328 ms .. 5.709 ms) > 0.990 R² (0.983 R² .. 0.995 R²) > mean 5.645 ms (5.472 ms .. 5.888 ms) > std dev 593.0 μs (352.5 μs .. 908.2 μs) > variance introduced by outliers: 63% (severely inflated) > > benchmarking renderTextT Id > time 5.439 ms (5.243 ms .. 5.640 ms) > 0.991 R² (0.985 R² .. 0.996 R²) > mean 5.498 ms (5.367 ms .. 5.631 ms) > std dev 408.8 μs (323.8 μs .. 552.9 μs) > variance introduced by outliers: 45% (moderately inflated) > > benchmarking renderTextT Rd > time 6.173 ms (5.983 ms .. 6.396 ms) > 0.990 R² (0.983 R² .. 0.995 R²) > mean 6.284 ms (6.127 ms .. 6.527 ms) > std dev 581.6 μs (422.9 μs .. 773.0 μs) > variance introduced by outliers: 55% (severely inflated) > > benchmarking renderTextT IO > time 12.35 ms (11.84 ms .. 12.84 ms) > 0.989 R² (0.982 R² .. 0.995 R²) > mean 12.22 ms (11.85 ms .. 12.76 ms) > std dev 1.159 ms (729.5 μs .. 1.683 ms) > variance introduced by outliers: 50% (severely inflated) > > I tried replacing > > forM [1..10000] (\_ -> div_ "hello world!") > > with > > replicateM_ 10000 (div_ "hello world!") > > which discards the list of 10,000 () values that the forM thing generates, > but this made very little difference. > > Hope this helps, > > David > > > [1] https://github.com/vacationlabs/monad-transformer-benchmark/pull/2 > > > > On 29 January 2017 at 07:26, Saurabh Nanda <saurabhnanda(a)gmail.com> wrote: > > Hi, > > I was noticing severe drop in performance when Lucid's HtmlT was being > combined with Scotty's ActionT. I've tried putting together a minimal repro > at https://github.com/vacationlabs/monad-transformer-benchmark Request > someone with better knowledge of benchmarking to check if the benchmarking > methodology is correct. > > Is my reading of 200ms performance penalty correct? > > -- Saurabh. > > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. > > > > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post.

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Re: [Haskell-cafe] Monad transformer performance - Request to review benchmarking code + results
by Saurabh Nanda 29 Jan '17

29 Jan '17

Thank you for the PR. Does your research suggest something is wrong with HtmlT when combined with any MonadIO, not necessarily ActionT? Is this an mtl issue or a lucid issue in that case? Curiously, what's your machine config? I'm on a late 2011 macbook pro with 10G ram and some old i5. -- Saurabh. On 29 Jan 2017 3:05 pm, "David Turner" <dct25-561bs(a)mythic-beasts.com> wrote: The methodology does look reasonable, although I think you should wait for all the scotty threads to start before starting the benchmarks, as I see this interleaved output: Setting phasers to stun... (port 3002) (ctrl-c to quit) Setting phasers to stun... (port 3003) (ctrl-c to quit) Setting phasers to stun... (port 3001) (ctrl-c to quit) benchmarking bareScotty Setting phasers to stun... (port 3000) (ctrl-c to quit) Your numbers are wayyy slower than the ones I see on my dev machine: benchmarking bareScotty Setting phasers to stun... (port 3000) (ctrl-c to quit) time 10.94 ms (10.36 ms .. 11.52 ms) 0.979 R² (0.961 R² .. 0.989 R²) mean 12.53 ms (11.98 ms .. 13.28 ms) std dev 1.702 ms (1.187 ms .. 2.589 ms) variance introduced by outliers: 66% (severely inflated) benchmarking bareScottyBareLucid time 12.95 ms (12.28 ms .. 13.95 ms) 0.972 R² (0.951 R² .. 0.989 R²) mean 12.20 ms (11.75 ms .. 12.69 ms) std dev 1.236 ms (991.3 μs .. 1.601 ms) variance introduced by outliers: 50% (severely inflated) benchmarking transScottyBareLucid time 12.05 ms (11.70 ms .. 12.39 ms) 0.992 R² (0.982 R² .. 0.996 R²) mean 12.43 ms (12.06 ms .. 13.01 ms) std dev 1.320 ms (880.5 μs .. 2.071 ms) variance introduced by outliers: 54% (severely inflated) benchmarking transScottyTransLucid time 39.73 ms (32.16 ms .. 49.45 ms) 0.668 R² (0.303 R² .. 0.969 R²) mean 42.59 ms (36.69 ms .. 54.38 ms) std dev 16.52 ms (8.456 ms .. 25.96 ms) variance introduced by outliers: 92% (severely inflated) benchmarking bareScotty time 11.46 ms (10.89 ms .. 12.07 ms) 0.986 R² (0.975 R² .. 0.994 R²) mean 11.73 ms (11.45 ms .. 12.07 ms) std dev 800.6 μs (636.8 μs .. 975.3 μs) variance introduced by outliers: 34% (moderately inflated) but nonetheless I do also see the one using renderTextT to be substantially slower than the one without. I've sent you a PR [1] that isolates Lucid from Scotty and shows that renderTextT is twice as slow over IO than it is over Identity, and it's ~10% slower over Reader too: benchmarking renderText time 5.529 ms (5.328 ms .. 5.709 ms) 0.990 R² (0.983 R² .. 0.995 R²) mean 5.645 ms (5.472 ms .. 5.888 ms) std dev 593.0 μs (352.5 μs .. 908.2 μs) variance introduced by outliers: 63% (severely inflated) benchmarking renderTextT Id time 5.439 ms (5.243 ms .. 5.640 ms) 0.991 R² (0.985 R² .. 0.996 R²) mean 5.498 ms (5.367 ms .. 5.631 ms) std dev 408.8 μs (323.8 μs .. 552.9 μs) variance introduced by outliers: 45% (moderately inflated) benchmarking renderTextT Rd time 6.173 ms (5.983 ms .. 6.396 ms) 0.990 R² (0.983 R² .. 0.995 R²) mean 6.284 ms (6.127 ms .. 6.527 ms) std dev 581.6 μs (422.9 μs .. 773.0 μs) variance introduced by outliers: 55% (severely inflated) benchmarking renderTextT IO time 12.35 ms (11.84 ms .. 12.84 ms) 0.989 R² (0.982 R² .. 0.995 R²) mean 12.22 ms (11.85 ms .. 12.76 ms) std dev 1.159 ms (729.5 μs .. 1.683 ms) variance introduced by outliers: 50% (severely inflated) I tried replacing forM [1..10000] (\_ -> div_ "hello world!") with replicateM_ 10000 (div_ "hello world!") which discards the list of 10,000 () values that the forM thing generates, but this made very little difference. Hope this helps, David [1] https://github.com/vacationlabs/monad-transformer-benchmark/pull/2 On 29 January 2017 at 07:26, Saurabh Nanda <saurabhnanda(a)gmail.com> wrote: > Hi, > > I was noticing severe drop in performance when Lucid's HtmlT was being > combined with Scotty's ActionT. I've tried putting together a minimal repro > at https://github.com/vacationlabs/monad-transformer-benchmark Request > someone with better knowledge of benchmarking to check if the benchmarking > methodology is correct. > > Is my reading of 200ms performance penalty correct? > > -- Saurabh. > > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. >

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Monad transformer performance - Request to review benchmarking code + results
by Saurabh Nanda 29 Jan '17

29 Jan '17

Hi, I was noticing severe drop in performance when Lucid's HtmlT was being combined with Scotty's ActionT. I've tried putting together a minimal repro at https://github.com/vacationlabs/monad-transformer-benchmark Request someone with better knowledge of benchmarking to check if the benchmarking methodology is correct. Is my reading of 200ms performance penalty correct? -- Saurabh.

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Munich Haskell Meeting, 2017-01-30 @ 19:30
by Christian Neukirchen 28 Jan '17

28 Jan '17

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, January 28 at Augustiner-Gaststätte Rumpler at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-jan-2017/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Desugaring ArrowLoop notation
by Francesco Ariis 28 Jan '17

28 Jan '17

Hello cafe, I was reading Stephen's Arrow Tutorial [1], where (among other things), a way to compute the running mean of a stream of integers using circular programming (ArrowLoop) is shown: mean5 :: Fractional a => Circuit a a mean5 = proc value -> do rec (lastTot, lastN) <- delay (0,0) -< (tot, n) let (tot, n) = (lastTot + value, lastN + 1) let mean = tot / n returnA -< mean To understand the example better I tried to convert the `rec do` notation to plain loop/(>>>) one (i.e. without syntactic sugar), but failed. So I ask: what's the sour version of `mean5`? -F [1] https://en.wikibooks.org/wiki/Haskell/Arrow_tutorial

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Re: [Haskell-cafe] Instrumentation without requiring code-level changes?
by Saurabh Nanda 28 Jan '17

28 Jan '17

> > Yes, I think so, with the PackageImports extension. > > https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_ > guide/syntax-extns.html#package-imports > Thank you for this. If this works, it makes using instrumentedscotty & instrumentedopaleye easier to use.. Btw, there's a note in the docs: [...] Note: you probably don't need to use this feature, it was added mainly so that we can build backwards-compatible versions of packages when APIs change. It can lead to fragile dependencies in the common case: modules occasionally move from one package to another, rendering any package-qualified imports broken. [...] I hope GHC doesn't plan to remove this extension any time soon. -- Saurabh.

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hash tables
by Dennis Raddle 27 Jan '17

27 Jan '17

I want to build a hash table and then freeze it and use it for fast lookup later. Is there a way to build it in IO or ST but freeze it and use it in transparent code later? D

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Re: [Haskell-cafe] Using stringize and string concatenation in ghc preprocessing
by Ben Franksen 27 Jan '17

27 Jan '17

Am 20.08.2016 um 20:33 schrieb Brandon Allbery: > You might want to look at cpphs as an alternative preprocessor. There are > some ancient K&R-era hacks that could be used if absolutely necessary, but > cpphs should be a much simpler and cleaner solution. Is there any reason /not/ to prefer cpphs over cpp? If not, why does anyone still use cpp for Haskell? Cheers Ben -- "Make it so they have to reboot after every typo." ― Scott Adams

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Problem using external package from git with stack
by Juan Luis Martty 27 Jan '17

27 Jan '17

Hello, I am trying to use binary-serialise-cbor in my project. Since it's not on hackage I followed the docs and added the following to stack.yaml: packages: - '.' - location: git: https://github.com/well-typed/binary-serialise-cbor.git commit: 1e11ea5cc7810d241b2fe1bca0ed435baa4d4371 extra-dep: true flags: binary-serialise-cbor: newtime15: true And in my .cabal file I have: build-depends: base >= 4.7 && < 5, network-simple, time, binary-serialize-cbor, bytestring When I do stack build the package is downloaded and built but then I get the following error: Error: While constructing the build plan, the following exceptions were encountered: In the dependencies for hcopia-0.1.0.0: binary-serialize-cbor must match -any, but the stack configuration has no specified version I attached both files just in case. Regards, Juan

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Re: [Haskell-cafe] Instrumentation without requiring code-level changes?
by David Turner 27 Jan '17

27 Jan '17

On 27 Jan 2017 07:16, "Saurabh Nanda" <saurabhnanda(a)gmail.com> wrote: (copying the mailing list -- I don't think your intention was to keep this off the mailing list) Apologies, yes, I failed to CC the mailing list. Thanks. Double checked that this one is addressed properly. I've looked at middleware for Scotty (or any WAI application), but it has limitations. I want deeper insights about how much time is being spent where, when serving a web request -- which is why the need to instrument SQL, Redis, and HTML rendering. One example that doesn't necessarily need to be in the IO monad is HTML rendering using Lucid, where the data is being passed separately to it (standard Controller <> View separation). Ok, this is definitely something that's hard to define, let alone measure: Lucid's rendering uses Bytestring.Builder to interleave the rendering and the transmission of traffic over the network, so a simple endTime-minus-startTime calculation doesn't work. You'd have to do something within ByteString.Builder itself to count the time spent filling each buffer vs the time spent sending it. I don't know of an existing implementation, but I can see how you'd do it (and it's worth noting that the relevant code is in IO, hidden behind a call to `unsafePerformIO` or similar): https://hackage.haskell.org/package/bytestring-0.10.8.1/docs/Data-ByteStrin… Also, if everything needs to be run in MonadIO, what does that do to fine-grained effect tracking? I wanted to have HasDb, HasRedis, HasEmail kind of monads (or type-classes) that give me some guarantees about the **type** of IO interactions a particular function call can do. If I instrument them and add an additional MonadIO constraint, does it mean any IO can be lifted into such monads? Your type classes can avoid mentioning IO, it's the instance definitions that would want to contain the instrumentation code. But they need IO anyway so that's no big deal. It's unfortunate to know that the RTS doesn't have these kind of instrumentation capabilities built-in. Is it absolutely not possible to do this at an RTS level? Ah, actually, there's the RTS event log: https://ghc.haskell.org/trac/ghc/wiki/EventLog and https://hackage.haskell.org/package/base-4.9.1.0/docs/Debug-Trace.html#g:2 But it emits a _lot_ of events from the RTS if you switch it on. Cheers, David -- Saurabh. On Fri, Jan 27, 2017 at 12:32 PM, David Turner < dct25-561bs(a)mythic-beasts.com> wrote: > Hi Saurabh, > > There's no direct equivalent to monkey-patching in Haskell (AFAIK (*)). > Why not instrument things always? The performance impact is usually quite > negligible, and the ability to switch instrumentation on at runtime in > production is sometimes invaluable. > > For instrumenting Scotty (in particular) or WAI-based apps (in general) > you can instrument things simply by inserting an appropriate `Middleware`, > a decision that can be deferred until service-startup time, and which > involves no change to the Scotty-side code. > > There's no equivalent to WAI for database access so I don't have a general > answer in that case, but typically database APIs have only a few core > `execute` or `query` functions that are quite straightforward to wrap with > instrumentation if needs be. > > This all kinda _has_ to be in IO anyway, as you're trying to measure a > real-world activity. Certainly there's no problem with either Scotty or > databases in this regard, as they're running in IO. Do you have an example > where you need instrumentation of a pure (i.e. non-IO) computation? In such > a case you could use the `unsafePerformIO` escape hatch (like `Debug.Trace` > does) but you'll have to be very careful to measure what you think you're > measuring: the language is quite free to reorder computations and avoid > repeating duplicated computations via referential transparency etc. > > (*) I mean, you could do something with dynamic libs and LD_PRELOAD (the > original monkey-patching) but please don't! > > Hope that helps, > > David > > On 27 Jan 2017 05:21, "Saurabh Nanda" <saurabhnanda(a)gmail.com> wrote: > >> (another cross post from Reddit - https://www.reddit.com/r/haske >> ll/comments/5qfd36/instrumentation_without_required_application/) >> >> Is it possible to collect instrumentation data from a Hakell application >> without requiring the core application to change code? Here are some >> examples of instrumentation data: >> >> * Time taken to serve an HTTP request (not average time, but individual >> times, for individual requests) along with the incoming request >> params+headers and outgoing response length. >> * Time taken to make an SQL query along with the actual query (again, not >> average times, but individual times, for individual queries) >> * Time taken to fetch data from Redis along with the Redis command >> * Time taken to render an HTML page along with path to the HTML file >> >> Most dynamic languages (like Ruby) allow monkey-patching OR "decorating" >> existing functions during runtime. Most instrumentation agents (like >> Skylight, Newrelic), simply monkey-patch the API surfaces of known >> libraries to collect this data. Therefore, using these instrumentation >> agents is a one line code change (just include the gem, or drop the JAR, or >> whatever). >> >> What is the equivalent of this in Haskell? >> >> Here's what I've tried so far [1] , but it has the following >> disadvantages: >> >> * Requires changes to the application's cabal file to replace the >> non-instrumented versions of the libraries with the instrumented version >> (replace scotty with instrumentedscotty, in my example) >> * Requires the application to explicitly import the instrumented versions >> of the libraries (replace import Scotty with import InstrumentedScotty, in >> my example) >> * Forces all the code interacting with instrumented APIs to be run in a >> MonadIO environment - because of the underlying calls to getCurrentTime >> >> I'm sure there's a better way, but I couldn't get my hands on it! >> >> [1] https://gist.github.com/saurabhnanda/27592da0269bc35569ec6239e1a91b75 >> >> -- Saurabh. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> > -- http://www.saurabhnanda.com

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Re: [Haskell-cafe] Instrumentation without requiring code-level changes?
by Saurabh Nanda 27 Jan '17

27 Jan '17

(copying the mailing list -- I don't think your intention was to keep this off the mailing list) I've looked at middleware for Scotty (or any WAI application), but it has limitations. I want deeper insights about how much time is being spent where, when serving a web request -- which is why the need to instrument SQL, Redis, and HTML rendering. One example that doesn't necessarily need to be in the IO monad is HTML rendering using Lucid, where the data is being passed separately to it (standard Controller <> View separation). Also, if everything needs to be run in MonadIO, what does that do to fine-grained effect tracking? I wanted to have HasDb, HasRedis, HasEmail kind of monads (or type-classes) that give me some guarantees about the **type** of IO interactions a particular function call can do. If I instrument them and add an additional MonadIO constraint, does it mean any IO can be lifted into such monads? It's unfortunate to know that the RTS doesn't have these kind of instrumentation capabilities built-in. Is it absolutely not possible to do this at an RTS level? -- Saurabh. On Fri, Jan 27, 2017 at 12:32 PM, David Turner < dct25-561bs(a)mythic-beasts.com> wrote: > Hi Saurabh, > > There's no direct equivalent to monkey-patching in Haskell (AFAIK (*)). > Why not instrument things always? The performance impact is usually quite > negligible, and the ability to switch instrumentation on at runtime in > production is sometimes invaluable. > > For instrumenting Scotty (in particular) or WAI-based apps (in general) > you can instrument things simply by inserting an appropriate `Middleware`, > a decision that can be deferred until service-startup time, and which > involves no change to the Scotty-side code. > > There's no equivalent to WAI for database access so I don't have a general > answer in that case, but typically database APIs have only a few core > `execute` or `query` functions that are quite straightforward to wrap with > instrumentation if needs be. > > This all kinda _has_ to be in IO anyway, as you're trying to measure a > real-world activity. Certainly there's no problem with either Scotty or > databases in this regard, as they're running in IO. Do you have an example > where you need instrumentation of a pure (i.e. non-IO) computation? In such > a case you could use the `unsafePerformIO` escape hatch (like `Debug.Trace` > does) but you'll have to be very careful to measure what you think you're > measuring: the language is quite free to reorder computations and avoid > repeating duplicated computations via referential transparency etc. > > (*) I mean, you could do something with dynamic libs and LD_PRELOAD (the > original monkey-patching) but please don't! > > Hope that helps, > > David > > On 27 Jan 2017 05:21, "Saurabh Nanda" <saurabhnanda(a)gmail.com> wrote: > >> (another cross post from Reddit - https://www.reddit.com/r/haske >> ll/comments/5qfd36/instrumentation_without_required_application/) >> >> Is it possible to collect instrumentation data from a Hakell application >> without requiring the core application to change code? Here are some >> examples of instrumentation data: >> >> * Time taken to serve an HTTP request (not average time, but individual >> times, for individual requests) along with the incoming request >> params+headers and outgoing response length. >> * Time taken to make an SQL query along with the actual query (again, not >> average times, but individual times, for individual queries) >> * Time taken to fetch data from Redis along with the Redis command >> * Time taken to render an HTML page along with path to the HTML file >> >> Most dynamic languages (like Ruby) allow monkey-patching OR "decorating" >> existing functions during runtime. Most instrumentation agents (like >> Skylight, Newrelic), simply monkey-patch the API surfaces of known >> libraries to collect this data. Therefore, using these instrumentation >> agents is a one line code change (just include the gem, or drop the JAR, or >> whatever). >> >> What is the equivalent of this in Haskell? >> >> Here's what I've tried so far [1] , but it has the following >> disadvantages: >> >> * Requires changes to the application's cabal file to replace the >> non-instrumented versions of the libraries with the instrumented version >> (replace scotty with instrumentedscotty, in my example) >> * Requires the application to explicitly import the instrumented versions >> of the libraries (replace import Scotty with import InstrumentedScotty, in >> my example) >> * Forces all the code interacting with instrumented APIs to be run in a >> MonadIO environment - because of the underlying calls to getCurrentTime >> >> I'm sure there's a better way, but I couldn't get my hands on it! >> >> [1] https://gist.github.com/saurabhnanda/27592da0269bc35569ec6239e1a91b75 >> >> -- Saurabh. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> > -- http://www.saurabhnanda.com

1 1

Suggestions for "Advanced" Topics for Haskell-based Class
by Conrad Cunningham 27 Jan '17

27 Jan '17

I am teaching a class in Haskell-based functional programming for advanced undergraduates and beginning graduate students at my institution. None of the students have previously used Haskell and for most of my students functional programming is new. Because I am teaching this in a "multiparadigm programming" course, I want to expand beyond what I have usually covered in the Haskell-based "functional programming" course and cover a few topics in areas such as parallel, concurrent, distributed, reactive, or metaprogramming (domain-specific languages, Template Haskell, etc.). Assuming my course has more or less covered the topics in *Learn You a Haskell for Great Good *(with likely shallow coverage of monads) at that point, what would be good additional topics to cover, libraries to use, and tutorial or teaching resources to use? Although I have taught fundamental Haskell FP topics for many years, I have not delved into any of these "advanced" topics. Thanks, Conrad -- H. Conrad Cunningham, Professor Computer and Information Science University of Mississippi (USA)

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Instrumentation without requiring code-level changes?
by Saurabh Nanda 27 Jan '17

27 Jan '17

(another cross post from Reddit - https://www.reddit.com/r/haskell/comments/5qfd36/instrumentation_without_re… ) Is it possible to collect instrumentation data from a Hakell application without requiring the core application to change code? Here are some examples of instrumentation data: * Time taken to serve an HTTP request (not average time, but individual times, for individual requests) along with the incoming request params+headers and outgoing response length. * Time taken to make an SQL query along with the actual query (again, not average times, but individual times, for individual queries) * Time taken to fetch data from Redis along with the Redis command * Time taken to render an HTML page along with path to the HTML file Most dynamic languages (like Ruby) allow monkey-patching OR "decorating" existing functions during runtime. Most instrumentation agents (like Skylight, Newrelic), simply monkey-patch the API surfaces of known libraries to collect this data. Therefore, using these instrumentation agents is a one line code change (just include the gem, or drop the JAR, or whatever). What is the equivalent of this in Haskell? Here's what I've tried so far [1] , but it has the following disadvantages: * Requires changes to the application's cabal file to replace the non-instrumented versions of the libraries with the instrumented version (replace scotty with instrumentedscotty, in my example) * Requires the application to explicitly import the instrumented versions of the libraries (replace import Scotty with import InstrumentedScotty, in my example) * Forces all the code interacting with instrumented APIs to be run in a MonadIO environment - because of the underlying calls to getCurrentTime I'm sure there's a better way, but I couldn't get my hands on it! [1] https://gist.github.com/saurabhnanda/27592da0269bc35569ec6239e1a91b75 -- Saurabh.

1 0

Question about STRef
by Antoine Rimlet 26 Jan '17

26 Jan '17

Hi list, I try to get the following little program (a slightly modified "Man or boy", it prints -14254067) work "as expected", that is, without consuming lots of memory: import Control.Monad.ST import Data.STRef a k x1 x2 x3 x4 x5 = do kk <- newSTRef k let b = do k <- modifySTRef kk pred >> readSTRef kk; a k b x1 x2 x3 x4 if k <= 0 then do x3' <- x3; x4' <- x4; return (x3' + x4') else do x5' <- x5; b' <- b; return (x5' + b') main = print (runST (a 22 (return 1) (return (-1)) (return (-1)) (return 1) (return 0))) I use GHC 7.8.4, and executing this program uses about 2.5 GB of memory (or about 3.5 GB with runghc). However, the "equivalent" program in OCaml only needs 4 MB (or 9 MB with the interpreter): let rec a k x1 x2 x3 x4 x5 = let kk = ref k in let rec b () = begin decr kk; a !kk b x1 x2 x3 x4 end in if k <= 0 then let v = x3 () in v + x4 () else let v = x5 () in v + b ();; print_int (a 22 (fun () -> 1) (fun () -> -1) (fun () -> -1) (fun () -> 1) (fun () -> 0));; print_newline ();; Therefore I suspect I'm doing something wrong, but I can't see what. I did try to use the strict version modifySTRef' as indicated in the manual, but with no visible improvement. Thanks, Antoine

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Request for feedback on two GHC proposals
by Mario Blažević 26 Jan '17

26 Jan '17

The time is nigh for closing the discussion on the Eval class proposal for GHC, so please pipe in if you have any interest in it, either on the practical (what's the point of this terrible idea!?) or the theoretical side (I can finally get those free theorems and I'd love unlifted products!). https://github.com/ghc-proposals/ghc-proposals/pull/27 I would also appreciate feedback on a tangentially related and wholly practical INCOMPLETE_CONTEXTS pragma proposal at https://github.com/ghc-proposals/ghc-proposals/pull/34 Thank you.

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Re: [Haskell-cafe] Question about STRef
by Johannes Waldmann 26 Jan '17

26 Jan '17

What Michael did not mention in the mail, but it's in his code: numerals defaulted to Integer, and the story changes (somewhat) if we force Int. I updated my measurements: http://www.imn.htwk-leipzig.de/~waldmann/etc/mob/ and ghc-8 looks better now - but not quite wins. > there's still some more low hanging fruit here ... for whom? What could we reasonably expect of the compiler here? It could do the strictification? the unboxing? It certainly could not replace Integer by Int, since the range of numerical values has no obvious bound. But then, all these Integers are small, but we pay quite an overhead (for represeting Int as Integer) which has increased for current ghcs? - J.

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Re: [Haskell-cafe] Question about STRef
by Johannes Waldmann 26 Jan '17

26 Jan '17

Thanks for pointing out this (Knuth's) nice test case. I should use this as an exam question ... Allocation and run-time can be reduced by replacing return (x3' + x4') with let x = (x3' + x4') in x `seq` return x And, modifySTRef' *does* help. I *do* notice a regression (for the seq-ed and primed version) ghc-8.0.2 : 2.6 GB alloc, 2.3 sec ghc-6.10.4: 2.0 GB alloc, 1.7 sec (measured on X5365 @ 3.00GHz ) Numbers are for total allocation, residency is small (it runs with +RTS -H10k -A10k but then the number of GCs goes up) - J.

2 2

Bug-free, leak-free, battle-tested library for broadcast channels?
by Saurabh Nanda 25 Jan '17

25 Jan '17

(cross-posted from Reddit because I'm not sure of the audience overlap between haskell-cafe & reddit) I want to broadcast some instrumentation data from deep within my Haskell app. This data will have listeners in some environments (say, debug), but not others (say, production). Which library should I be using? A little searching threw two possible candidates: * http://hackage.haskell.org/package/split-channel * https://hackage.haskell.org/package/broadcast-chan But, I'm not sure if these are battle-tested. Any help would be appreciated. -- Saurabh.

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Early feedback on a GHC proposal
by Chris Smith 25 Jan '17

25 Jan '17

Anyone want to offer some early feedback on a GHC proposal I'm working on? It's called OverloadedApplication. Current version at https://github.com/cdsmith/ghc-proposals/blob/overloadedapplication/proposa… I am proposing something a bit more ambitious than what I need, because the proposal as written seems more likely to be broadly useful. The bit that I actually need is about desugaring of infix operators in the presence of RebindableSyntax. I'd love to get early thoughts from anyone who has a solid use case for the rest of the proposal. It feels like a thing that *ought* to be useful, but I'm drawing blanks at the moment.

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Re: [Haskell-cafe] A proof that the list monad is not a free monad
by David Feuer 25 Jan '17

25 Jan '17

The ConstraintKinds extension just lets you deal with kinds involving constraints. Turn it on, import the Constraint kind from somewhere (I know it's exported from GHC.Exts and also from Data.Constraint in the constraints package; I don't know if it has a nice home in base). Then you can write things like class C f a where p :: f a => proxy f -> a -> Int etc. On Jan 23, 2017 4:19 PM, "Olaf Klinke" <olf(a)aatal-apotheke.de> wrote: On a mostly unrelated note, Dan Doel elaborated [1] that the list monad is not the monad of free monoids either, even though mathematically the free monoid over x is the set of words of finite length with alphabet x. Briefly, the reason is that Haskell types are domains, not sets. Below is a proof that the free monad is not the free algebra. One approximant to free algebraic structures is to look at the free F-algebra for a functor F. This approach does not take into account any equations between the algebraic operations. {-# LANGUAGE Rank2Types,MultiParamTypeClasses,FlexibleInstances,FlexibleContexts #-} module FreeFAlg where import Control.Applicative import Control.Monad -- F-algebras (sans laws). class Functor f => Alg f a where alg :: f a -> a -- The free F-algebra over x, following Dan Doel's ideas, -- with the universal property baked in. newtype FreeAlg f x = FreeAlg (forall a. Alg f a => (x -> a) -> a) runFree :: Alg f a => FreeAlg f x -> (x -> a) -> a runFree (FreeAlg f) = f universal :: Alg f a => (x -> a) -> FreeAlg f x -> a universal = flip runFree instance Functor f => Alg f (FreeAlg f x) where alg ff = FreeAlg (\f -> alg (fmap (($f).runFree) ff)) instance Functor (FreeAlg f) where fmap h (FreeAlg fx) = FreeAlg (\f -> fx (f.h)) instance Monad (FreeAlg f) where return x = FreeAlg ($x) (FreeAlg fx) >>= k = FreeAlg (\f -> fx (\x -> runFree (k x) f)) instance Applicative (FreeAlg f) where pure = return (<*>) = ap data FreeMonad f x = Pure x | Roll (f (FreeMonad f x)) instance Functor f => Alg f (FreeMonad f x) where alg = Roll -- This looks like the same universal property... universal' :: Alg f a => (x -> a) -> FreeMonad f x -> a universal' f (Pure x) = f x universal' f (Roll ffree) = alg (fmap (universal' f) ffree) freeAlg2freeMonad :: Functor f => FreeAlg f x -> FreeMonad f x freeAlg2freeMonad = universal Pure freeMonad2freeAlg :: Functor f => FreeMonad f x -> FreeAlg f x freeMonad2freeAlg = universal' return The free monad is not the free F-algebra, because uniqueness fails. Consider the constant functor data Point a = Point instance Functor Point where fmap f Point = Point -- universal' f (Pure x) = f x -- universal' f (Roll Point) = alg Point -- but we have another function with the same type as universal': notUnique :: Alg Point a => (x -> a) -> FreeMonad Point x -> a notUnique f = const (alg Point) For classes like Monoid, which have equations between the algebraic operations (such as mappend mempty = id), we'd need the language extension that allows constraints as type parameters. I'd like to write Free Monoid x = Free (forall a. Monid a => (x -> a) -> a) but I am not experienced with the ConstraintKinds extension. Anyone help? Olaf [1] http://comonad.com/reader/2015/free-monoids-in-haskell/ _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

1 0

GHC 8.0.2 Stackage LTS8 plan
by Luke Murphy 25 Jan '17

25 Jan '17

Hi folks, Please see the following brief write up regarding what the Stackage curator team have planned for a smooth transition to GHC 8.0.2 on LTS builds. http://lwm.github.io/stackage-8.0.2/ Best, Luke

1 0

Re: [Haskell-cafe] Datomic-like db for Haskell?
by Ovidiu Deac 24 Jan '17

24 Jan '17

I'll do some research to evaluate my options. So far I have: Homegrown implementation on top of some DB (Riak/Postgres/Mongo...) https://github.com/agentm/project-m36 https://hackage.haskell.org/package/filestore Thanks for the answers! On Fri, Dec 23, 2016 at 8:07 PM, Ertugrul Söylemez <esz(a)posteo.de> wrote: > >> * Another option is to run an insertion-only relational model on > >> really any database system you want like PostgreSQL. There is > >> nothing about preserving history that needs special handling to be > >> efficient. > > > > I assume that some sort of structural sharing is needed once the > > collections grow above a certain size. Also some support for caching > > would be nice. > > Yeah, sharing could improve disk usage and cache utilisation. I'm not > aware of any DBMS-side sharing features in systems I have used > (PostgreSQL and SQLite mainly). The usual way to get sharing in > relational systems is to normalise the schema. That also allows you to > use domain-specific knowledge to get even better sharing. > > Caching is of course supported, at least by PostgreSQL. It aims to keep > as much of the database in memory as possible (or as you tell it to). >

2 1

PLDI workshop and proposal deadline - Jan 30
by Tobias Grosser 24 Jan '17

24 Jan '17

Reminder! The final deadline for PLDI W&T proposals is coming up on January 30. Please see the PLDI website for more information. http://conf.researchr.org/track/pldi-2017/pldi-2017-workshops-and-tutorials. Tobias Grosser PLDI Publicity Chair

1 0

is it normal for docs to be pending for 24 hours?
by Patrick Pelletier 24 Jan '17

24 Jan '17

This is only my second time uploading a package to Hackage, so I don't yet have a feel for how it's supposed to go. I uploaded normalization-insensitive-2.0.0.1 about 24 hours ago: https://hackage.haskell.org/package/normalization-insensitive-2.0.0.1 Under "Status", it says "Docs pending". (And the module names are all non-clickable.) Is it normal to take this long to build the docs? Is there some way to find out where in the queue my job is? Is this an indication that something has gone wrong? How do I fix it? Thanks, --Patrick

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[ANN] FastPush 0.1.0.0
by William Yager 24 Jan '17

24 Jan '17

In response to a bit of an obscure use case I had, I created this very simple library which gives you a Monad with a single operation, `push :: a -> m ()`. This operation pushes an item onto a stack (represented under the hood by a mutable ST vector). When you run the monad, you get the stack out as a vector. The library is quite fast. It's about 2x faster than C++'s std::vec::push_back compiled without optimizations, and about 40% as fast as std::vec::push_back when compiled with G++ -O2. There's almost certainly some low-hanging optimization fruit for anyone skilled at Haskell optimizing. The library is parametric over choice of mutable vector. You can use Unboxed, Storable, or normal vectors. I've even included a Push Monad Transformer based on Control.Monad.ST.Trans, but that's almost certainly unsafe, per the caveats listed in the Control.Monad.ST.Trans docs. https://hackage.haskell.org/package/FastPush-0.1.0.0 https://github.com/wyager/FastPush/ Cheers, Will

2 3

[ANN] wiringPi-1.0
by Patrick Pelletier 24 Jan '17

24 Jan '17

https://hackage.haskell.org/package/wiringPi-1.0 https://github.com/ppelleti/hs-wiringPi I released wiringPi-0.1.0.0 less than two weeks ago, but already I have a new major version, with a breaking change. wiringPi-0.1.0.0 kept close to the C wiringPi library, which required calling a setup function. There were four different setup functions, and the choice of setup function determined the pin numbering scheme, which would affect all subsequent functions which used pin numbers. This sort of far-reaching global state felt very un-Haskell-like, and antithetical to modularity. So, in wiringPi-1.0, the setup function is called automatically the first time a wiringPi function is used. The wiringPi binding always configures the underlying C library to use the "Broadcom GPIO" pin numbering scheme. However, the Pin type now has three different constructors, to allow a pin to use any of the three pin-numbering schemes. These pin numbers are automatically converted to the "Broadcom GPIO" scheme internally. So now you can use any pin numbering scheme on a case-by-case basis, without having to make a global choice. About wiringPi: This is a Haskell binding to the wiringPi library, which allows you to interface with the GPIO pins on the Raspberry Pi. Unlike some other solutions for using the Raspberry Pi's GPIO pins, wiringPi provides access to more advanced features, such as enabling the internal pull-up or pull-down resistors. --Patrick

1 0

Re: [Haskell-cafe] A proof that the list monad is not a free monad
by Olaf Klinke 23 Jan '17

23 Jan '17

On a mostly unrelated note, Dan Doel elaborated [1] that the list monad is not the monad of free monoids either, even though mathematically the free monoid over x is the set of words of finite length with alphabet x. Briefly, the reason is that Haskell types are domains, not sets. Below is a proof that the free monad is not the free algebra. One approximant to free algebraic structures is to look at the free F-algebra for a functor F. This approach does not take into account any equations between the algebraic operations. {-# LANGUAGE Rank2Types,MultiParamTypeClasses,FlexibleInstances,FlexibleContexts #-} module FreeFAlg where import Control.Applicative import Control.Monad -- F-algebras (sans laws). class Functor f => Alg f a where alg :: f a -> a -- The free F-algebra over x, following Dan Doel's ideas, -- with the universal property baked in. newtype FreeAlg f x = FreeAlg (forall a. Alg f a => (x -> a) -> a) runFree :: Alg f a => FreeAlg f x -> (x -> a) -> a runFree (FreeAlg f) = f universal :: Alg f a => (x -> a) -> FreeAlg f x -> a universal = flip runFree instance Functor f => Alg f (FreeAlg f x) where alg ff = FreeAlg (\f -> alg (fmap (($f).runFree) ff)) instance Functor (FreeAlg f) where fmap h (FreeAlg fx) = FreeAlg (\f -> fx (f.h)) instance Monad (FreeAlg f) where return x = FreeAlg ($x) (FreeAlg fx) >>= k = FreeAlg (\f -> fx (\x -> runFree (k x) f)) instance Applicative (FreeAlg f) where pure = return (<*>) = ap data FreeMonad f x = Pure x | Roll (f (FreeMonad f x)) instance Functor f => Alg f (FreeMonad f x) where alg = Roll -- This looks like the same universal property... universal' :: Alg f a => (x -> a) -> FreeMonad f x -> a universal' f (Pure x) = f x universal' f (Roll ffree) = alg (fmap (universal' f) ffree) freeAlg2freeMonad :: Functor f => FreeAlg f x -> FreeMonad f x freeAlg2freeMonad = universal Pure freeMonad2freeAlg :: Functor f => FreeMonad f x -> FreeAlg f x freeMonad2freeAlg = universal' return The free monad is not the free F-algebra, because uniqueness fails. Consider the constant functor data Point a = Point instance Functor Point where fmap f Point = Point -- universal' f (Pure x) = f x -- universal' f (Roll Point) = alg Point -- but we have another function with the same type as universal': notUnique :: Alg Point a => (x -> a) -> FreeMonad Point x -> a notUnique f = const (alg Point) For classes like Monoid, which have equations between the algebraic operations (such as mappend mempty = id), we'd need the language extension that allows constraints as type parameters. I'd like to write Free Monoid x = Free (forall a. Monid a => (x -> a) -> a) but I am not experienced with the ConstraintKinds extension. Anyone help? Olaf [1] http://comonad.com/reader/2015/free-monoids-in-haskell/

1 0

Dropbox sdk
by Pieter Laeremans 23 Jan '17

23 Jan '17

Hi all, I've tried using the dropbox sdk package from package today. But it is a bit outdated I can't get it to compile. I 've tried using stack init, with various resolvers. One of the problems seems to be that the dependencies for the certificate package used to by dropbox-sdk, enforces base < 4.8. How should I best tackle this problem ? Should I assume that the certificate package should be replaced by something else? And just try to modify the code ? I know it's open source, so I would prefer trying out a fix, and submitting a patch request, rather than bothering the maintainers. Thanks for your help, Pieter -- Pieter Laeremans pieter(a)laeremans.org

2 2

gloss from ghci
by Johannes Waldmann 23 Jan '17

23 Jan '17

Dear Cafe - I am trying to use gloss https://hackage.haskell.org/package/gloss from ghci. The "display" command will open a window and draw the picture on it just fine. But when I close this window by hitting ESC, ghci prints <interactive>: interrupted and exits, so I'm back at the shell from where I started ghci. This is somewhat annoying - especially since ghci's command line history does not get written, so when I re-start it, I have to re-type (work-around: before "display", close ghci, this will write history, then restart and draw. Ugly.) The gloss FAQ suggests ghci -fno-ghci-sandbox but this does not help (ghci still exits, this time, without printing the message). - J.

2 1

A proof that the list monad is not a free monad
by Ingo Blechschmidt 23 Jan '17

23 Jan '17

Dear lovers of generalized abstract nonsense, recall that we can construct, for any functor f, the free monad over f: data Free f a = Pure a | Roll (f (Free f a)) Intuitively, Free f a is the type of f-shaped trees with leaves of type a. The join operation merely grafts trees together and doesn't perform any further computations. Values of the form (Roll _) shall be called "nontrivial" in this posting. Some of the usual monads are in fact free monads over some functor: * The Tree monad is the free monad over the functor Pair, where data Pair a = MkPair a a. The intuition as Pair-shaped trees is most strong in this example. Every parent node has a pair of children. * The Maybe monad is the free monad over the functor Unit, where data Unit a = MkUnit. In the graphical picture parent nodes have a Unit of children, so no children at all. So there are exactly two Unit-shaped trees: the tree consisting only of a leaf (corresponding to Just _) and the three consisting of a childless parent node (corresponding to Nothing). * The Identity monad is the free monad over the functor Void, where Void a is a type without constructors. * The Partiality monad is the free monad over Maybe. * The Writer [r] monad is the free monad over the functor (r,). In an (r,)-shaped tree, a parent node is decorated with a value of type r and has exactly one child node. Traversing such a path yields a list of r's and a leaf value of type a, so in total a value of type ([r],a) = Writer r a. (In particular, Free (r,) () is isomorphic to [r]. This observation led to the claim that the list monad is free. But this claim is false and indeed doesn't follow from the observation. To show that a monad m is free, one has to exhibit a monad isomorphism forall a. m a -> Free f a for some functor f. In contrast, exhibiting an isomorphism of m a with Free (f a) () is neither sufficient nor required.) Even more monads are quotients of free monads. For instance, State s is a quotient of Free (StateF s), where data StateF s a = Put s a | Get (s -> a). -- NB: This functor is itself a free functor over a type constructor -- which is sometimes called StateI [1], rendering Free (StateF s) what -- Oleg and Hiromi Ishii call a "freer monad" [2]. The quotient is exhibited by the following monad morphism, which gives semantics to the purely syntactical values of the free monad. run :: Free (StateF s) a -> State s a run (Pure x) = return x run (Roll (Put s m)) = put s >> run m run (Roll (Get k)) = get >>= run . k This point of view is the basis of the operational package by apfelmus [1] and is also talked about in an StackOverflow thread [3]. It's the main reason why free monads are useful in a programming context. So, is the list monad a free monad? Intuitively, it's not, because the join operation of the list monad (concatenation) doesn't merely graft expressions together, but flattens them [4]. Here is a proof that the list monad is not free. I'm recording it since I've been interested in a proof for quite some time, but searching for it didn't yield any results. The threads [3] and [5] came close, though. In the free monad over any functor, the result of binding a nontrivial action with any function is always nontrivial, i.e. (Roll _ >>= _) = Roll _ This can be checked directly from the definition of (>>=) for the free monad or alternatively with Reid Barton's trick of exploiting a monad morphism to Maybe, see [3]. If the list monad was isomorphic-as-a-monad to the free monad over some functor, the isomorphism would map only singleton lists [x] to values of the form (Pure _) and all other lists to nontrivial values. This is because monad isomorphisms have to commute with "return" and return x is [x] in the list monad and Pure x in the free monad. These two facts contradict each other, as can be seen with the following example: do b <- [False,True] -- not of the form (return _) if b then return 47 else [] -- The result is the singleton list [47], so of the form (return _). After applying a hypothetical isomorphism to the free monad over some functor, we'd have that the binding of a nontrivial value (the image of [False,True] under the isomorphism) with some function results in a trivial value (the image of [47], i.e. return 47). Cheers, Ingo [1] http://projects.haskell.org/operational/Documentation.html [2] http://okmij.org/ftp/Haskell/extensible/more.pdf [3] http://stackoverflow.com/questions/14641864/what-monads-can-be-expressed-as… [4] https://hackage.haskell.org/package/free-4.12.4/docs/Control-Monad-Free.html [5] https://www.reddit.com/r/haskell/comments/50zvyb/why_is_liststate_not_a_fre…

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interfacing to C library - c2hs/hsc2hs/GreenCard?
by Brian Sammon 23 Jan '17

23 Jan '17

I'd like to write a haskell app that calls a C library that I wrote. This is my first time trying to do something like this. What would people here recommend I start with? c2hs? hsc2hs? GreenCard? One of the newer tools like bindings-dsl or inline-c? It's a relatively small library -- a handful of structs and a bunch of functions that take them as input, often returning another as output, and ultimately doing some libcurl to exchange data with websites. Also, can anyone recommend some documentation to read, that's applicable to GHC 8? A lot of the stuff I found on google predates GHC 8 (and a fair amount predates GHC 7)--not sure if that's likely to be a problem.

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free monad versus operational for blackjack game
by Michael Litchard 23 Jan '17

23 Jan '17

I'm studying the code for a blackjack game: See marked answer. http://codereview.stackexchange.com/questions/46809/blackjack-in-haskell It uses the operational package, and mentions one could write equivalent code using a free monad. But my question is, for this use case is there any benefit for using a free monad?

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Possible to use Cabal-1.18 with ghc-6.12.3?
by Simon Peter Nicholls 23 Jan '17

23 Jan '17

Can I improve my setup, so that Cabal-1.18 will work better with ghc-6.12.3? I have an old project that I’m updating, which is currently compilable with 6.12.3. I’ve managed to get Cabal-1.18.1.7 and cabal-install-1.18.0.1 to build it, which gives me some handy extra tooling, but only if I pre-install all the project dependencies before updating Cabal. If I try to use a sandbox instead, the build will fail at the point where template-haskell is used: > Loading package utf8-string-0.3.7 ... <command line>: can't load .so/.DLL for: HSutf8-string-0.3.7 (libHSutf8-string-0.3.7.so: cannot open shared object file: No such file or directory) That’s a dependency in my sandbox, but this also fails if it’s installed to the default `~/.cabal`. I see a similar error if I try `runhaskell Setup.hs` instead of cabal: > Setup.hs: <command line>: can't load .so/.DLL for: HSCabal-1.18.1.7 (libHSCabal-1.18.1.7.so: cannot open shared object file: No such file or directory) So I figure Cabal-1.18 and my default 6.12.3 are not co-operating as well as they could. If I use —enable-shared in my dependency builds, I still get this error, and I also see it if I try to build statically. The template-haskell “Loading” steps always seems to fail by not finding a shared library, as if it’s stepping outside of Cabal (and my!) control. I note that installation locations are different in Cabal-1.18 (vs 1.16), but figure the GHC registration process takes care of something like `.cabal/lib/x86_64-linux-ghc-6.12.3/utf8-string-0.3.7/libHSutf8-string-0.3.7-ghc6.12.3.so`. A pre-installed parsec dependency looks like `ghc-6.12.3/lib/parsec-3.1.1/ghc-6.12.3/libHSparsec-3.1.1-ghc6.12.3.so`. Neither has the name format reported by the error. Is there something in my Cabal or my GHC setup that I can change, to get these working together? A clean fix would be great, but a bodge is fine, since this transition work is isolated in a docker container. Thanks.

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Soliciting feedback on my lens json parsing tutorial (please don't share yet!)
by Cody Goodman 22 Jan '17

22 Jan '17

Hi All, I wrote a lens json parsing tutorial here: https://github.com/codygman/concise-json-parsing-in-haskell I'd appreciate any feedback. My approach tries to use a prior understanding of Python into understanding lens enough to do something useful. Perhaps I can include links to tutorials that cover more of the theoretical foundations as well. Any criticism and/or ideas is welcome! Thanks, Cody Goodman

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Announce: Haskell Platform 8.0.2
by Gershom B 21 Jan '17

21 Jan '17

On behalf of the Haskell Platform team, I'm happy to announce the release of Haskell Platform 8.0.2 Now available at https://www.haskell.org/platform/ This includes GHC 8.0.2, cabal-install 1.24.0.2, and stack 1.3.2, all with many bugfixes and improvements since the last platform release. This platform release we've cleaned up the webpage a bit, and renamed the "minimal" distribution to the "core" distribution to highlight that it is the recommended approach (and simplified the accompanying text). A number of improvements have been made to the windows installer -- notably the /S option for silent install is now in fully working order, and we have the flags /STACK and /D (for stack and platform-ghc install paths). The installer expects no quoting of paths, even with spaces within the paths, like so: HaskellPlatform-8.0.2-full-x86_64-setup.exe /S /STACK=c:\My install path for stack\local\bin /D=c:\program files\Haskell\Platform\8.0.2 There is also an updated version of msys2 included, which includes pacman, et al. to ease the installation of libraries with more complex foreign dependencies. Changes to Contents: * As a result of transitive dependencies of platform packages, the integer-logarithms and call-stack packages have been added to the full platform. A full list of contents is available at https://www.haskell.org/platform/contents.html Thanks to all the contributors to this release, thanks to all the package and tool maintainers and authors, and a big thanks to the GHC team for all their hard work. A list of new GHC changes is available at: https://ghc.haskell.org/trac/ghc/blog/ghc-8.0.2-released A list of cabal changes is available at: https://hackage.haskell.org/package/cabal-install-1.24.0.2/changelog The new cabal documentation page is at: https://cabal.readthedocs.io/en/latest/ A list of stack changes is at: https://docs.haskellstack.org/en/stable/ChangeLog/ Happy Haskell Hacking all, Gershom (Note -- one feature we have not implemented yet for the platform but would like to is a generic linux installer that allows one to set a custom location and does not require root. Anyone who would like to volunteer to help with this, please get in touch!)

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Call for Participation: BOB 2017 (February 24, Berlin)
by Stefan Wehr 20 Jan '17

20 Jan '17

Reminder: early registration ends this Monday! ================================================================ BOB 2017 Conference "What happens if we simply use what's best?" February 24, 2017 Berlin http://bobkonf.de/2017/ Program: http://bobkonf.de/2017/program.html Registration: http://bobkonf.de/2017/registration.html ================================================================ BOB is the conference for developers, architects and decision-makers to explore technologies beyond the mainstream in software development, and to find the best tools available to software developers today. Our goal is for all participants of BOB to return home with new insights that enable them to improve their own software development experiences. The program features 14 talks and 8 tutorials on current topics: http://bobkonf.de/2017/program.html The subject range of talks includes functional programming, advanced front-end development, and sophisticated uses of types. The tutorials feature introductions to Haskell, Swift, PureScript, React, QuickCheck, Agda, CRDTs and Servant. John Hughes will give the keynote talk. Registration is open online: http://bobkonf.de/2017/registration.html NOTE: The early-bird rates expire on January 23, 2017! BOB cooperates with the :clojured conference on the following day. There is a registration discount available for participants of both events. http://www.clojured.de/

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Re: [Haskell-cafe] Haskell with C++: removing runtime dependencies on Windows 10.
by David Banas 20 Jan '17

20 Jan '17

Hi Deech, Not sure this is helpful, as I haven’t tested it on Windows 10, but here are the gcc/ghc compile/link options I use for a project that has a similar need: CC = gcc CFLAGS += -I/usr/lib/ghc-7.4.2/include/ -g -fPIC HC = ghc HC_OPTS = -cpp -O3 $(EXTRA_HC_OPTS) EXTRA_HC_OPTS = -package parsec -dynamic -fPIC -rtsopts HC_LOPTS = -shared -dynamic -package parsec -lHSrts -lm -lffi -lrt -db

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Strange encoding issue with Text and Servant
by Jan von Löwenstein 20 Jan '17

20 Jan '17

Hi, I have got a two places with a `QueryParam "q" Text` and call it with a Text that contains a `=` literal. In one place the `=` is correctly encoded as %3D, in the other I see a `!D(MISSING)`. This has to happen somewhere in Servant or the lower layers. Both Texts print out nicely with an `=` sign if I just print them to stdout. Google does not find `!D(MISSING)` anywhere. Any idea what could possibly be the problem here? Best Jan

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monad prompt example missing
by Michael Litchard 20 Jan '17

20 Jan '17

There's an example from the Monad.Prompt documentation https://hackage.haskell.org/package/MonadPrompt-1.0.0.5/docs/Control-Monad-… That is a dead link. http://paste.lisp.org/display/53766 Does anyone have a copy of this example, and of course any other ones you may know of.

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Haskell with C++: removing runtime dependencies on Windows 10.
by aditya siram 19 Jan '17

19 Jan '17

Hi, I am trying to create an executable on Windows 10 with some Haskell->C++ bindings I wrote but having trouble getting rid of some runtime DLL dependencies. I'm currently compiling and linking using the MSYS2 and GHC environment that ships with Windows Stack installer but no matter what I do I end up with the following dependencies: $ ldd <executable> ... libwinpthread-1.dll => /c/Users/user/AppData/Local/Programs/stack/x86_64-windows/ghc-8.0.1/mingw/bin/libwinpthread-1.dll (0x64940000) libstdc++-6.dll => /c/Users/user/AppData/Local/Programs/stack/x86_64-windows/ghc-8.0.1/mingw/bin/libstdc++-6.dll (0x6fc40000) libgcc_s_seh-1.dll => /c/Users/user/AppData/Local/Programs/stack/x86_64-windows/ghc-8.0.1/mingw/bin/libgcc_s_seh-1.dll (0x61440000) I have verified that the standalone versions of those libs are in available in stack install: libstdc++: ../stack/x86_64-windows/ghc-8.0.1/mingw/lib/gcc/x86_64-w64-mingw32/5.2.0/libstdc++.a ../stack/x86_64-windows/ghc-8.0.1/mingw/lib/gcc/x86_64-w64-mingw32/5.2.0/libstdc++.dll.a ../stack/x86_64-windows/ghc-8.0.1/mingw/lib/gcc/x86_64-w64-mingw32/5.2.0/libgcc.a libgcc: ../stack/x86_64-windows/ghc-8.0.1/mingw/lib/gcc/x86_64-w64-mingw32/5.2.0/libgcc_eh.a ../stack/x86_64-windows/ghc-8.0.1/mingw/lib/gcc/x86_64-w64-mingw32/5.2.0/libgcc_s.a libpthread: ../stack/x86_64-windows/ghc-8.0.1/mingw/x86_64-w64-mingw32/lib/libpthread.a ../stack/x86_64-windows/ghc-8.0.1/mingw/x86_64-w64-mingw32/lib/libpthread.dll.a I've unsuccessfully tried to link them into the executable using many combinations of `-optl-static`, `-optl-static-gcc`, and `-optl-static-stdc++` and also passing along these libraries " raw" to the linker like: `ghc-Options: -pgml g++ "-optl-Wl,--whole-archive" "-optl-Wl,-Bstatic" "-optl-Wl,-lstdc++" "-optl-Wl,-Bdynamic" "-optl-Wl,--no-whole-archive" Short of shipping these DLL with the executable (which I'd really rather not do) I'm running out of ideas so any suggestions would be appreciated. Thanks! -deech

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Haskell Weekly - 01/18/2017
by Haskell Weekly 19 Jan '17

19 Jan '17

View this email in your browser (http://us10.campaign-archive2.com/?u=49a6a2e17b12be2c5c4dcb232&id=d1f16b3bf…) ** Haskell Weekly (http://news.us10.list-manage1.com/track/click?u=49a6a2e17b12be2c5c4dcb232&i…) ------------------------------------------------------------ ** Issue 38 (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) ------------------------------------------------------------ Welcome to Haskell Weekly! Haskell (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) is an advanced, purely functional programming language. This is a weekly summary of what’s going on in its community. You can subscribe to the email newsletter (http://news.us10.list-manage1.com/track/click?u=49a6a2e17b12be2c5c4dcb232&i…) or the Atom feed (/haskell-weekly.atom) . Want to contribute to Haskell Weekly? Send us a message on Twitter (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) or open an issue on GitHub (http://news.us10.list-manage1.com/track/click?u=49a6a2e17b12be2c5c4dcb232&i…) . ** News from the Haskell community ------------------------------------------------------------ * Brent Yorgey’s new programming languages course (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) My new PL course is now finished, and all the course materials are freely available. Working through all the exercises should be a great option for anyone wishing to learn some basics of programming language design and implementation. * CLaSH 0.7 released: GHC 8 support and more (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) Our company has been hired to do FPGA development work, and best of all, we get to use CLaSH. As a consequence, I cannot put as much time into CLaSH development as during my days as a PhD/PostDoc. However, we do now get to use CLaSH in industry! And as part of the contract there is time to find and fix bugs in CLaSH! * safe-prelude: a thought experiment (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) This is a thought experiment in a different point in the alternative prelude design space. After my blog post on readFile, I realized I was unhappy with the polymorphic nature of readFile in classy-prelude. Adding that with Haskell Pitfalls I’ve been itching to try something else. * Selecting a platform: JavaScript vs Elm vs PureScript vs GHCJS vs Scala.js (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) There are not many people who have worked on production applications in multiple frontend FP systems, and lived to tell the tale. So here goes, my guide to selecting a functional programming platform for the browser. * The design and use of QuickCheck (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) The typical examples you’ll find online have deceptively simple code. The examples make it hard to learn how their succinct tests are possible, and how to customize them when things go wrong. This guide puts it all together piece by piece so you can use QuickCheck with confidence. * Monads made difficult (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) This is a short, fast and analogy-free introduction to Haskell monads derived from a categorical perspective. This assumes you are familiar with Haskell and basic category theory. * Constraint kinds (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) In this blog post, I will show some examples of using the ConstraintKinds GHC extension. Classes and contexts were not first-class citizens in Haskell, but the introduction of the Constraint kind has changed this and allows them to be used as parameters of types. * Expressive types, not oppressive types (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) Uncle Bob looks at advanced type systems and sees them as more oppressive rather than more expressive. Being able to describe whether or not a function can return null is an opportunity, not a constraint. * LiquidHaskell (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) LiquidHaskell (LH) refines Haskell’s types with logical predicates that let you enforce critical properties at compile time. ** Package of the week ------------------------------------------------------------ This week’s package of the week is dump-core (http://news.us10.list-manage.com/track/click?u=49a6a2e17b12be2c5c4dcb232&id…) , a GHC plugin for rendering Core. Send us a message on Twitter (http://news.us10.list-manage2.com/track/click?u=49a6a2e17b12be2c5c4dcb232&i…) to nominate next week’s package! ============================================================ Copyright © 2017 Haskell Weekly, All rights reserved. You are receiving this email because you opted in at our website haskellweekly.news. Our mailing address is: Haskell Weekly 13140 Coit Rd Ste 405 Dallas, TX 75240 USA Want to change how you receive these emails? You can ** update your preferences (http://news.us10.list-manage.com/profile?u=49a6a2e17b12be2c5c4dcb232&id=ffb…) or ** unsubscribe from this list (http://news.us10.list-manage.com/unsubscribe?u=49a6a2e17b12be2c5c4dcb232&id…) Email Marketing Powered by MailChimp http://www.mailchimp.com/monkey-rewards/?utm_source=freemium_newsletter&utm…

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Approaches for a DSL for API-level testing
by Damian Nadales 19 Jan '17

19 Jan '17

Hello, I'm writing an embedded DSL for API level testing. The idea in a nutshell is that I can write tests like the following one: createProjectCreates = do p <- createProject "foobar" ps <- getProjects ps `shouldContain` p Here http://stackoverflow.com/questions/41740658/separating-concerns-in-a-dsl-us… I show a sketch of the solution I'm working on, and the issues I'm running into. I wonder whether there are other approaches to embedded DSL writing in Haskell that allows for a better compositionality and separation of concerns. Thanks! Damian.

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[ANNOUNCE] Glasgow Haskell Compiler 8.0.2 is available!
by Ben Gamari 19 Jan '17

19 Jan '17

=============================================== The Glasgow Haskell Compiler -- version 8.0.2 =============================================== The GHC team is happy to at last announce the 8.0.2 release of the Glasgow Haskell Compiler. Source and binary distributions are available at http://downloads.haskell.org/~ghc/8.0.2/ This is the second release of the 8.0 series and fixes nearly two-hundred bugs. These include, * Interface file build determinism (#4012). * Compatibility with macOS Sierra and GCC compilers which compile position-independent executables by default * Compatibility with systems which use the gold linker * Runtime linker fixes on Windows (see #12797) * A compiler bug which resulted in undefined reference errors while compiling some packages (see #12076) * A number of memory consistency bugs in the runtime system * A number of efficiency issues in the threaded runtime which manifest on larger core counts and large numbers of bound threads. * A typechecker bug which caused some programs using -XDefaultSignatures to be incorrectly accepted. * More than two-hundred other bugs. See Trac [1] for a complete listing. * #12757, which lead to broken runtime behavior and even crashes in the presence of primitive strings. * #12844, a type inference issue affecting partial type signatures. * A bump of the `directory` library, fixing buggy path canonicalization behavior (#12894). Unfortunately this required a major version bump in `directory` and minor bumps in several other libraries. * #12912, where use of the `select` system call would lead to runtime system failures with large numbers of open file handles. * #10635, wherein -Wredundant-constraints was included in the -Wall warning set A more detailed list of the changes included in this release can be found in the release notes, https://downloads.haskell.org/~ghc/8.0.2/docs/html/users_guide/8.0.2-notes.… Please note that this release breaks with our usual tendency to avoid major version bumps of core libraries in minor GHC releases by including an upgrade of the `directory` library to 1.3.0.0. Also note that, due to a rather serious bug (#13100) affecting Windows noticed late in the release cycle, the Windows binary distributions were produced using a slightly patched [2] source tree. Users compiling from source for Windows should be certain to include this patch in their build. This release is the result of six months of effort by the GHC development community. We'd like to thank everyone who has contributed code, bug reports, and feedback to this release. It's only due to their efforts that GHC remains a vibrant and exciting project. [1] https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.0.2&col=id… [2] http://downloads.haskell.org/~ghc/8.0.2/0001-SysTools-Revert-linker-flags-c… How to get it ~~~~~~~~~~~~~ Both the source tarball and binary distributions for a wide variety of platforms are available at, http://www.haskell.org/ghc/ Background ~~~~~~~~~~ Haskell is a standardized lazy functional programming language. The Glasgow Haskell Compiler (GHC) is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating efficient code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces. GHC is distributed under a BSD-style open source license. Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, can be found on the GHC wiki http://ghc.haskell.org/trac/ghc/wiki/Platforms Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: http://ghc.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on getting started with hacking on GHC are available from GHC's developer site, http://ghc.haskell.org/trac/ghc/ Community Resources ~~~~~~~~~~~~~~~~~~~ There are mailing lists for GHC users, develpoers, and monitoring bug tracker activity; to subscribe, use the web interfaces at http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-tickets There are several other Haskell and GHC-related mailing lists on www.haskell.org; for the full list, see https://mail.haskell.org/cgi-bin/mailman/listinfo Some GHC developers hang out on the #ghc and #haskell of the Freenode IRC network, too: http://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: http://www.haskell.org/ghc/reportabug

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Re: [Haskell-cafe] Control.Parallel seemingly randomly behaviour
by Johannes Waldmann 18 Jan '17

18 Jan '17

Hi Juan, I am interested in this because I use Control.Parallel in teaching currently. But - * what is your actual Main program / main function (it was not included in your message?) * what is your actual cabal / stack config file? * on what machine (OS, CPU) are you running this (it seems pretty slow)? * have you looked at eventlogs with threadscope? - J.

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Yesod devel issue: cannot satisfy -package-key main
by Geraldus 18 Jan '17

18 Jan '17

Hi folks! I haven't used Yesod quite a while; now when I'm trying to run development server of freshly scaffolded PostgreSQL stack's template I see this error message: Rebuilding application... (using cabal) Starting development server... *<command line>: cannot satisfy -package-key main* (use -v for more information) Exit code: ExitFailure 1 I've found several issues on the web, however I wasn't able to resolve this. I've tried to upgrade my Yesod binary first via `stack install yesod-bin`. Now I have: stack exec yesod version *yesod-bin version: 1.4.18.7* stack --version *Version 1.1.2 x86_64 hpack-0.14.0* I also tried to comment out the test suite section in cabal file, but devel server still fails to run. Can someone clarify how to deal with this?

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