January 2010 - Haskell-Cafe

PatternSignatures (rant)
by Johannes Waldmann 06 Apr '18

06 Apr '18

Dear all, (this is a rant - please ignore) why has "PatternSignatures" been renamed to "ScopedTypeVariables" (in ghc-6.10)? show me the alleged "type variable" in this code: do x :: Int <- [ 1 .. 10 ] ; return $ x^2 and why on earth do I need to use a language extension at all to get to the most basic thing of declarative programming: to declare the type of an identifier? This seriously hurts (me, at least) when teaching Haskell. Thanks - J.W.

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Commutative monads vs Applicative functors
by Ronald Guida 09 Jul '16

09 Jul '16

I have a few questions about commutative monads and applicative functors. >From what I have read about applicative functors, they are weaker than monads because with a monad, I can use the results of a computation to select between alternative future computations and their side effects, whereas with an applicative functor, I can only select between the results of computations, while the structure of those computations and their side effects are fixed in advance. But then there are commutative monads. I'm not exactly sure what a commutative monad is, but my understanding is that in a commutative monad the order of side effects does not matter. This leads me to wonder, are commutative monads still stronger than applicative functors, or are they equivalent? And by the way, what exactly is a commutative monad?

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Proposal: TypeDirectedNameResolution
by Johannes Waldmann 03 Mar '13

03 Mar '13

While browsing Haskell-Prime I found this: http://hackage.haskell.org/trac/haskell-prime/wiki/TypeDirectedNameResoluti… This is not some April Fool's day hoax? Because, it might actually turn Haskell into a somewhat usable (and marketable) language ... well, you know what I mean. Is there 'ghc -XTypeDirectedNameResolution' yet? - J.W.

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Could not find module `Text.Regex'
by zaxis 16 Sep '12

16 Sep '12

import Text.Regex date_by_ntday dateStr ntday = do let [y,m,d] = map (\x -> read x::Int) $ splitRegex (mkRegex "-") dateStr .... %ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 Which package(s) do i need to use Text.Regex ? Sincerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/Could-not-find-module-%60Text.Regex%27-tp27366745p273… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Cannot understand liftM2
by Nicola Paolucci 13 Feb '12

13 Feb '12

Hi All, I'm loving learning Haskell quite a bit. It is stretching my brain but in a delightfull way. I've googled, I've hoogled but I haven't found a clear explanation for what exactly liftM2 does in the context below. Using the cool lambdabot "pointless" utility I found out that: > \x -> snd(x) - fst(x) is the same as: > liftM2 (-) snd fst I like the elegance of this but I cannot reconcile it with its type. I can't understand it. I check the signature of liftM2 and I get: Prelude> :t liftM2 Prelude> liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r Can someone help me understand what's happening here ? What does a Monad have to do with a simple subtraction ? What is actually the "m" of my example ? I am sure if I get this I'll be another step closer to illumination ... Thanks, Nick

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Anyone recommend a VPS?
by Michael Snoyman 21 Mar '11

21 Mar '11

Hi all, I'm working with a client right now on deploying an app. I was wondering if anyone had some recommendations for a Haskell-friendly host. I'm inclined to go with a VPS for this setup, but if there's reliable shared hosting, that would do as well. This client is fairly price-sensitive, so I don't think anything past the $20/month range to start off with. The only other requirement for this project is PostgreSQL. As far as wants: * Linux is preferable * A server that isn't on everyone's spam list would be better. Of course, I can route through an outside e-mail provider, but local would be nicer. * Have room to grow. For now, this is a fairly small app, but the hope of course is for it to expand. Thanks in advance. Michael

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Infinite types should be optionally allowed
by Job Vranish 22 Feb '11

22 Feb '11

There are good reasons against allowing infinite types by default (mostly, that a lot of things type check that are normally not what we want). An old haskell cafe conversation on the topic is here: http://www.nabble.com/There%27s-nothing-wrong-with-infinite-types!-td771373… However, I think infinite types should be allowed, but only with an explicit type signature. In other words, don't allow infinite types to be inferred, but if they are specified, let them pass. I think it would be very hard to shoot yourself in the foot this way. Newtype is the standard solution to situations where you really need an infinite type, but in some cases this can be a big annoyance. Using newtype sacrifices data type abstraction and very useful type classes like Functor. You can use multiparameter type classes and functional dependencies to recover some of the lost abstraction, but then type checking becomes harder to reason about and the code gets way more ugly (If you doubt, let me know, I have some examples). Allowing infinite types would fix this. I'm imagining a syntax something like this: someFunctionThatCreatesInfiniteType :: a -> b | b = [(a, b)] Thoughts? Opinions? Am I missing anything obvious? - Job

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IORef memory leak
by Jim Snow 15 Oct '10

15 Oct '10

I'm having some trouble with excessive memory use in a program that uses a lot of IORefs. I was able to write a much simpler program which exhibits the same sort of behavior. It appears that "modifyIORef" and "writeIORef" leak memory; perhaps they keep a reference to the old value. I tried both ghc-6.8.3 and ghc-6.10.1. Is this a known limitation, or is this a ghc bug, or am I using IORefs in the wrong way? -jim module Main where import Data.IORef import Control.Monad -- Leaks memory leakcheck1 ior = do go 1000000000 where go 0 = return () go n = do modifyIORef ior (+1) go (n-1) -- Leaks memory leakcheck2 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior writeIORef ior (x+1) go (n-1) -- Runs in constant memory leakcheck3 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior go (n-1) main :: IO () main = do ior <- newIORef 0 leakcheck2 ior compiled with: ghc -O2 --make Leak.hs -o Leak

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ANN: ForSyDe DSL v3.1
by Seyed Hosein Attarzadeh Niaki 25 Jun '10

25 Jun '10

Dear All, A new version of the ForSyDe DSL is uploaded to the Hackage database. "The ForSyDe (Formal System Design) methodology has been developed with the objective to move system design to a higher level of abstraction and to bridge the abstraction gap by transformational design refinement. This library provides ForSyDe's implementation as a Haskell-embedded Domain Specific Language (DSL). For more information, please see ForSyDe's website: http://www.ict.kth.se/forsyde/." In addition to compatibility with base 4 and GHC 6.10.x, the new version provides more freedom for declaration of process functions (e.g., support for when and let blocks). BR/ Hosein Attarzadeh

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ANN: parameterized-data library v0.1.4
by Seyed Hosein Attarzadeh Niaki 25 Jun '10

25 Jun '10

Dear Haskell developers, A new version of the parameterized-data library providing fixed-sized vectors (based on the type-level library) is pushed into the repository and uploaded to the Hackage database. This is only a minor update to fix the compatibility issues with base 4, GHC>=6.10 and cabal-install (not backward compatible). Bests, Hosein Attarzadeh

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