July 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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ANNOUNCE: ioctl 0.0.1
by Maciej Piechotka 13 Sep '15

13 Sep '15

A package for type-safe I/O control. Currently only ioctl is supported. Currently simply a extract from my tuntap fork TODO: - Return the integer as well as structure (will break the API) - Port for Windows Network.Socket.IOCtl (as soon as I manage to setup some sane environment on this platform) - Wrapping around DeviceIoControl Example (in hsc): data NotRead = NotRead instance NotRead Int where ioctlReq _ = #const FIONREAD notRead s = ioctlsocket' s NotRead Regards

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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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Fundeps and overlapping instances
by Simon Peyton-Jones 09 Jun '12

09 Jun '12

Oleg points out, and Martin also mentions, that functional dependencies appear to interact OK with overlapping instances, but type families do not. I this impression is mistaken, and I'll try to explain why in this message, in the hope of exposing any flaws in my reasoning. We can't permit overlap for type families because it is unsound to do so (ie you can break "well typed programs don't go wrong"). But if it's unsound for type families, it would not be surprising if it was unsound for fundeps too. (I don't think anyone has done a soundness proof for fundeps + local constraints + overlapping instances, have they?) And indeed I think it is. So the short summary of this message is: if it works for fundeps it works for type families, and vice versa. (NB this equivalence is not true about GHC's current implementation, however. GHC doesn't support the combination of fundeps and local constraints at all.) Such an equivalence doesn't argue against fundeps; I'm only suggesting the that the two really are very closely equivalent. I much prefer type families from a programming-style point of view, but that's a subjective opinion. Simon Imagine a system "FDL" that has functional dependencies and local type constraints. The big deal about this is that you get to exploit type equalities in *given* constraints. Consider Oleg's example, cut down a bit: class C a b | a -> b instance C Int Bool newtype N2 a = N2 (forall b. C a b => b) t2 :: N2 Int t2 = N2 True We end up type-checking (True :: forall b. C Int b => b). From the functional dependency we know that (b~Bool), so the function should typecheck. GHC rejects this program; FDL would not. But making use of these extra equalities in "given" constraints is quite tricky. To see why look first at Example 1: module X where class C a b | a -> b data T a where MkT :: C a b => b -> T a module M1 where import X instance C Int Char where ... f :: Char -> T Int f c = MkT c module M2 where import X instance C Int Bool g :: T Int -> Bool g (MkT x) = x module Bad where import M1 import M2 bad :: Char -> Bool bad = g . f This program is unsound: it lets you cast an Int to a Bool; result is a seg-fault. You may say that the problem is the inconsistent functional dependencies in M1 and M2. But GHC won't spot that. For type families, to avoid this we "eagerly" check for conflicts in type-family instances. In this case the conflict would be reported when compiling module Bad, because that is the first time when both instances are visible together. So any FDL system should also make this eager check for conflicts. What about overlap? Here's Example 2: {-# LANGUAGE IncoherentInstances #-} module Bad where import X -- Overlapping instances instance C Int Bool -- Instance 1 instance C a [a] -- Instance 2 f :: Char -> T Int f c = MkT c -- Uses Instance 1 g :: T a -> a g (MkT x) = x -- Uses Instance 2 bad :: Char -> Int bad = g . f Again, a seg fault if it typechecks. But will it? When typechecking 'g', we get a constraint (C a ?), where 'a' is a skolem constant. Without IncoherentInstances GHC would reject the program on the grounds that it does not know what instance to choose. But *with* IncoherentInstances it would probably go through, which is unsound. So IncoherentInstances has moved from causing varying dynamic behaviour to causing seg faults. Very well, so FDL must get rid of IncoherentInstances altogether, at least for classes that have functional dependencies (or that have superclasses that do). But at the moment GHC makes an exception for *existentials*. Consider Example 3: class C a b | a -> b -- Overlapping instances instance C Int Bool -- Instance 1 instance C a [a] -- Instance 2 data T where MkT :: C a b => a -> b -> T f :: Bool -> T f x = MkT (3::Int) x -- Uses Instance 1 g :: T -> T g (MkT n x) = MkT n (reverse x) -- Uses Instance 2 bad :: Bool -> T bad = g . f In the pattern match for MkT in g we have the constraint (C a b), where 'a' is existentially bound. So under GHC's current rules it'll choose the (C a [a]) instance, and conclude that (b ~ [a]). So it's ok to reverse x. But it isn't; see function bad! So to avoid unsoundness we must not choose a particular instance from an overlapping set unless we know, absolutely positively, that the other cases cannot match. (GHC's exception for existentials was introduced in response to user demand. Usually, overlapping instances are somehow semantically coherent, and with an existential we are *never* going to learn more about the instantiating type, so choosing the best available seems like a good thing to do.) But even nuking IncoherentInstances altogether is not enough. Consider this variant of Example 3, call it Example 4: module M where class C a b | a -> b instance C a [a] -- Instance 2 data T where MkT :: C a b => a -> b -> T g :: T -> T g (MkT n x) = MkT n (reverse x) -- Uses Instance 2 module Bad where import M instance C Int Bool -- Instance 1 f :: Bool -> T f x = MkT (3::Int) x -- Uses Instance 1 bad :: Bool -> T bad = g . f This is nasty. In module M we have only one instance declaration for C, so there's no overlap, and function g typechecks fine. Now module Bad adds an overlapping instance declaration and 'f' uses it; moreover, it's clear that the new instance declaration is the right one to use. It's not clear how to fix this. The only way I can think of is to insist that all the instances appear in a single module, so that you cannot add more "later". But that loses part of the point of overlap in the first place, namely to provide a generic instance and then later override it. So FDL must * embody eager checking for inconsistent instances, across modules * never resolve overlap until only a unique instance can possibly match * put all overlapping instances in a single module Type families already implement the first of these. I believe that if we added the second and third, then overlap of type families would be fine. (I may live to eat my words here.) I'd be interested to know what Chameleon does on these examples.

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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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Re: [Haskell-cafe] Re: Functional dependencies and Peano numbers (and hoogle-bug?)
by Oscar Finnsson 03 May '11

03 May '11

Thanks for the great feedback. The bijective example was especially interesting. While reading "Fun with Type Functions" I notices GNum as an interesting alternative to the Num type class but I couldn't find any such package on hackagedb. Do anyone know if there is anything like GNum on hackagedb? On an unrelated note: I hoogled "to" (i.e. http://haskell.org/hoogle/?hoogle=to) and just got a blank page. Nothing. Nil (not even <html>...</html>). Is this a bug or a "feature"? :) The reason I hoogled it was because I'm searching for something like explicit casting found in many other languages, something similar to > class To f t where > to :: f -> t > > instance To a a where > to x = x > > instance (Real a, Fractional b) => To a b where > to = realToFrac > > instance (Read a) => To String (Maybe a) where > to = maybeRead -- not from cgi :) so I can write something like > ("23.2" `to`) :: Maybe Double or > ((42 :: Integer) `to`) :: Float Anyone made a module/package that solves this problem already? I cannot be the first that needs generic type safe conversion... . -- Oscar

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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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Downloading Haskell repos from GitHub
by Gwern Branwen 20 Mar '11

20 Mar '11

Along the lines of http://blog.patch-tag.com/2010/03/13/mirroring-patch-tag/ for downloading all patch-tag.com repositories, I've begun to wonder how to download all Github repositories since more and more people seem to be using it. Nothing in http://develop.github.com/ seems especially useful for grabbing the git:// URLs of all repos by language - just by user. The only real list of repos by language seems to be gotten at via http://github.com/languages/Haskell/updated or http://github.com/languages/Haskell/created . (You might think http://github.com/languages/Haskell would be good, but no, it's just a few random repos by interest and not a full listing.) I looked at the HTML, and it looks possible to use tagsoup to get all 98 pages and then parse the entries to get the HTTP URLs of the repos, and then turn *that* into git:// URLs suitable for shelling out to 'git clone', but I can't help but wonder if maybe there's a better approach someone more familiar with Github would know. -- gwern

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