January 2012 - Haskell-Cafe

PatternSignatures (rant)
by Johannes Waldmann 07 Apr '18

07 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 10 Jul '16

10 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 14 Sep '15

14 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 04 Mar '13

04 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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XCode Dependency for HP on Mac
by Tom Murphy 22 Jan '13

22 Jan '13

This may sound ignorant because, well, it is ignorant: I know very little about the underlying mechanics here. Installing the Haskell Platform currently requires XCode developer tools. To get XCode on my 10.6 machine, I... [*** begin ranty details (skippable) ... was told I could get a free version by registering as an Apple Developer. So I lie on the forms (phone number and address, for example, are _required_ fields!), and lie on the _required_ 2-3 page survey. I put in a code that they sent to my email (couldn't lie on that!), and log in. The page tells me, in the exact box that told me if I registered I could get XCode for free, that I... *** end ranty details] ...have to either pay to upgrade to their newer OS (10.7: Lion), or pay $99/year for a Mac OS Developer Membership. Is there a way to install HP without XCode? Could there be in the future? I'm tired of dealing with Apple's constant upgrade requirements, registration requirements, etc., and it seems like a small function that XCode actually performs in the Haskell development "toolchain." Again, I'm ignorant of the details and I'm sorry if this is ranty, but I'd love to hear your reactions. Thanks! Tom

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arr considered harmful
by Ryan Ingram 22 Dec '12

22 Dec '12

I know it's a bit of an 'intentionally provocative' title, but with the recent discussions on Arrows I thought it timely to bring this up. Most of the conversion from arrow syntax into arrows uses 'arr' to move components around. However, arr is totally opaque to the arrow itself, and prevents describing some very useful objects as arrows. For example, I would love to be able to use the arrow syntax to define objects of this type: data Circuit a b where Const :: Bool -> Circuit () Bool Wire :: Circuit a a Delay :: Circuit a a And :: Circuit (Bool,Bool) Bool Or :: Circuit (Bool,Bool) Bool Not :: Circuit Bool Bool Then :: Circuit a b -> Circuit b c -> Circuit a c Pair :: Circuit a c -> Circuit b d -> Circuit (a,b) (c,d) First :: Circuit a b -> Circuit (a,c) (b,c) Swap :: Circuit (a,b) (b,a) AssocL :: Circuit ((a,b),c) (a,(b,c)) AssocR :: Circuit (a,(b,c)) ((a,b),c) Loop :: Circuit (a,b) (a,c) -> Circuit b c etc. Then we can have code that examines this concrete data representation, converts it to VHDL, optimizes it, etc. However, due to the presence of the opaque 'arr', there's no way to make this type an arrow without adding an 'escape hatch' Arr :: (a -> b) -> Circuit a b which breaks the abstraction: circuit is supposed to represent an actual boolean circuit; (Arr not) is not a valid circuit because we've lost the information about the existence of a 'Not' gate. The arrow syntax translation uses arr to do plumbing of variables. I think a promising project would be to figure out exactly what plumbing is needed, and add those functions to a sort of 'PrimitiveArrow' class. All of these plumbing functions are trivially implemented in terms of 'arr', when it exists, but if it doesn't, it should be possible to use the arrow syntax regardless. -- ryan

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Partial statical linking
by Jason Dusek 27 Sep '12

27 Sep '12

Some time ago, I wrote to this list about making shared libraries with GHC, in such a way that the RTS was linked and ready to go. Recently, I've been looking a similar but, in a sense, opposite problem: linking Haskell executables with some of their non-Haskell dependencies, for distribution. I tried passing a few different sets of options to the linker through GHC, with -optl: -optl'-Wl,-r' -optl'-Wl,-r,-dy' -optl'-Wl,-static,-lffi,-lgmp,-dy' None of these had the desired effect. In the end, running GHC with -v and carefully editing the linker line produced the desired change (I have linked to and provided the diff below). The effect -optl seems to be to introduce options in the linker line just before -lHSrtsmain, which would seem to prevent one from linking libffi and libgmp differently. Is editing and storing away the linker script the best option at present for partially static linking? -- Jason Dusek () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments https://github.com/solidsnack/arx/commit/90ec5efdb0e991344aa9a4ad29456d466e… #@@ -122,10 +122,8 @@ # -lHSarray-0.3.0.2 \ # -lHSbase-4.3.1.0 \ # -lHSinteger-gmp-0.2.0.3 \ #- -lgmp \ # -lHSghc-prim-0.2.0.0 \ # -lHSrts \ #- -lffi \ # -lm \ # -lrt \ # -ldl \ #@@ -136,4 +134,7 @@ # -lgcc_s --no-as-needed \ # /usr/lib/gcc/x86_64-linux-gnu/4.6.1/crtend.o \ # /usr/lib/gcc/x86_64-linux-gnu/4.6.1/../../../x86_64-linux-gnu/crtn.o \ #+ -static \ #+ -lgmp \ #+ -lffi \

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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 10 Jun '12

10 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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Offer to mirror Hackage
by Dan Knapp 25 Apr '12

25 Apr '12

With Hackage down, now seemed like a good time to push this issue again. It's such an important site to us that it's really rather a shame there are no mirrors of it. I have a personal-and-business server in a data center in Newark, with a fair chunk of bandwidth, which I'd like to offer for a permanent mirror. Is there interest in this? Who do I need to talk to for it to happen? Strategy-wise, I think the best approach is round-robin DNS, since that's transparent to the end user - everything would still appear at the URL it's at now, but behind-the-scenes magic would let things keep working when one or the other site is down. I haven't personally set up such a system before but I'm willing to take on the burden of figuring it out. So I have a better idea of what I'm signing up for, can anyone tell me how much disk space and how much bandwidth per month Hackage uses? I have a fair chunk of both, as I say, but I'd like to know in advance to ensure that things go smoothly. As for what I'd want in return for this, really nothing. I wouldn't say no to an unobtrusive mention somewhere on the site, but I'd be happy just knowing I'd given something back to the Haskell community, which has given a lot to me. -- Dan Knapp "An infallible method of conciliating a tiger is to allow oneself to be devoured." (Konrad Adenauer)

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