- Haskell-Cafe - Haskell.org

GHC AT inference bug?
by Kevin Quick 04 Jul '10

04 Jul '10

I started with the following: {-# LANGUAGE TypeFamilies #-} class DoC a where type A2 a op :: a -> A2 a data Con x = InCon (x (Con x)) type FCon x = x (Con x) foldDoC :: Functor f => (f a -> a) -> Con f -> a foldDoC f (InCon t) = f (fmap (foldDoC f) t) doCon :: (DoC (FCon x)) => Con x -> A2 (FCon x) doCon (InCon x) = op x fCon :: (Functor x, DoC (FCon x)) => Con x -> A2 (FCon x) fCon = foldDoC op I then changed the rank of op, but forgot to update the foldDoC accordingly---see below. Attempting to compile this causes GHC to run forever using 100% cpu. The corrected definition of foldDoC works fine. Should the GHC (6.12.1) behavior in the face of my foolishness be reported as a bug or is this a legitimate infinite recursion of type deduction? {-# LANGUAGE TypeFamilies #-} class DoC a where type A2 a type A3 a op :: a -> A2 a -> A3 a data Con x = InCon (x (Con x)) type FCon x = x (Con x) -- should have been changed to this, which works -- foldDoC :: Functor f => (f a -> a) -> A2 (FCon f) -> Con f -> a -- foldDoC f i (InCon t) = f (fmap (foldDoC f i) t) -- this original version causes GHC to hang foldDoC :: Functor f => (f a -> a) -> Con f -> a foldDoC f (InCon t) = f (fmap (foldDoC f) t) doCon :: (DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x) doCon (InCon x) = op x -- note that if this is commented out then there's no hang: presumably because GHC doesn't have to perform type deduction for foldDoC. fCon :: (Functor x, DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x) fCon = foldDoC op -- -KQ

1 0

Type classes
by Andrew Coppin 04 Jul '10

04 Jul '10

Yesterday I read the Typeclassopedia [again]. In the course of doing this, I discovered that several types have class instances that do something useful. (Indeed, some of these instances do things that I've written custom functions to do.) The trouble is, when you read the documentation, it might say something like data Foo instance Monoid Foo This tells me that Foo is an instance of Monoid. It utterly fails to tell me what the heck the Monoid instance actually *does*. Now, sometimes it's quite obvious. (E.g., just about the only binary operation on lists is (++).) But sometimes it's not less obvious, and sometimes it's so utterly mystifying that just about the only way to find the answer is to either read the source code or run example code and try to deduce some kind of pattern in the answers. In summary, I think we need to devise a way of better-documenting class instances. Sometimes important functionallity can only be accessed using a class, and if it isn't too obvious what a particular class instance does, you can miss that functionallity and end up implementing it again by hand. (Or worse, thinking it's just impossible.) And there's also little quirks like the way Eq usually means value-equality, but for mutable structures it's usually defined as reference-equality. And then of course there are non-standard user-defined classes and their instances...

5 13

One problem with Cabal-install and dependencies
by Michael Lazarev 04 Jul '10

04 Jul '10

Hi, everybody! I think there's either an error in cabal-install, or I don't have an idea on haskell packages. I tried to install one particular library that depends on containers-0.2.0.1 and got the following error: Data/IntMap.hs:182:7: Could not find module `Data.Data': It is a member of the hidden package `base'. Perhaps you need to add `base' to the build-depends in your .cabal file. Use -v to see a list of the files searched for. Then I tried to install containers-0.2.0.1 separately, andin the process of the struggle I found that issuing the following command in containers-0.2.0.1 directory: ~/.cabal/bin/cabal install --constraint="base >= 4" -v ... prints this: Resolving dependencies... selecting containers-0.2.0.1 (hackage) selecting array-0.3.0.1 (installed or hackage) and discarding array-0.1.0.0, 0.2.0.0 and 0.3.0.0 selecting base-3.0.3.2 (installed) and 4.2.0.1 (installed) and discarding syb-0.1.0.0, 0.1.0.1, 0.1.0.2, 0.1.0.3, 0.2 and 0.2.1 selecting ffi-1.0 (installed) selecting ghc-prim-0.2.0.0 (installed) selecting integer-gmp-0.2.0.1 (installed) selecting rts-1.0 (installed) selecting syb-0.1.0.2 (installed) In order, the following would be installed: containers-0.2.0.1 (new version) Configuring containers-0.2.0.1... Dependency array ==0.3.0.1: using array-0.3.0.1 Dependency base ==3.0.3.2: using base-3.0.3.2 And proceeds to building the library: /usr/local/bin/ghc --make -package-name containers-0.2.0.1 -hide-all-packages -fbuilding-cabal-package -i -idist/build -i. -idist/build/autogen -Idist/build/autogen -Idist/build -Iinclude -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package-id array-0.3.0.1-fb28b07fd38a6011f4f73844e0d8281f -package-id base-3.0.3.2-260693a92016991a03206b5a55f6a411 -O -XDeriveDataTypeable -XMagicHash -XRank2Types -XCPP Data.Graph Data.IntMap Data.IntSet Data.Map Data.Sequence Data.Set Data.Tree where, as you see, it really uses base-3.0.3.2, and thus fails. So, my qusetion is, whether I'm doing something wrong, or there are really two errors: 1) cabal-install makes a wrong choice for dependency version 2) cabal-install does not respect the constraint, and proceeds to building the library without any warning

3 4

The Haskell module landscape
by Christopher Done 04 Jul '10

04 Jul '10

I was pondering how naming Haskell packages with a proper hierarchy is quite important, and wondered what the general hierarchy of the Hackage packages was like. So I grabbed the tarball of pakage descriptions[1] from Hackage and ran a little messy Haskell script[2] and got this list: http://gist.github.com/raw/463423/f8458d83b1a7cc26cdbf812747188993e50cd8a2/… Here's a snippet: hsdns: ADNS hsdns: ADNS.Base hsdns: ADNS.Endian hsdns: ADNS.Resolver cv-combinators: AI.CV.ImageProcessors HOpenCV: AI.CV.OpenCV.CV HOpenCV: AI.CV.OpenCV.CxCore HOpenCV: AI.CV.OpenCV.HighGui It's interesting to see how there are quite a few packages that could be easily put into the proper namesace, e.g. ADNS below should be in the Network hierarchy. It's also cool to see clusters of separate packages providing functionality for the same module, e.g. check out Control.Applicative! special-functors: Control.Applicative base: Control.Applicative applicative-extras: Control.Applicative.Backwards applicative-extras: Control.Applicative.Compose applicative-extras: Control.Applicative.Error InfixApplicative: Control.Applicative.Infix category-extras: Control.Applicative.Parameterized action-permutations: Control.Applicative.Permutation applicative-extras: Control.Applicative.State yjtools: Control.Applicative.Tools unicode-symbols: Control.Applicative.Unicode base-unicode-symbols: Control.Applicative.Unicode Control.Concurrent also has a lot going on inside it. There are quite a lot of packages under Network, too. One idea: Hackage could have an additional page which displays this, fully linked up to the packages in question, with an MSDN-style complete collapsible/expandable module hierarchy menu on the left and documentation on the right. I'm not saying that's the right way to do it, but it certainly makes for fun browsing for Haskell's entire open source codebase. (It would also encourage library writers to put their libraries in a useful namespace.) [1]: http://hackage.haskell.org/packages/archive/00-index.tar.gz [2]: http://gist.github.com/463472

1 0

Haskell and Standard ML modules
by José Romildo Malaquias 04 Jul '10

04 Jul '10

Hello. I am writing a Haskell program based on some guidelines written for Standard ML (The Tiger compiler from the "Modern Compiler Implementation in ML" book). The author uses modules for abstracting things like specifities of the taraget machine. For intance, there is a general Frame signature that abstracts the frame representation which depends on the target machine. For each target machine there is a structure compliant to that signature. For instante MipsFrame, PentiumFrame and SparcFrames would be structures implementing the Frame signature. Many parts of the program just see the Frame signature. Is there any guidelines on how to achieve somethying similar to this in Haskell? Regards, Romildo

2 2

Rank2Types and pattern matching
by Yves Parès 04 Jul '10

04 Jul '10

Hello everybody, I'm trying to implement the type protection used by ST to prevent a monad from returning a certain type. There's my code: import Control.Monad.Identity newtype SomeMonad s a = SomeMonad { unSome :: Identity a } deriving (Monad) newtype SomeType s = SomeType Int runSomeMonad :: (forall s. SomeMonad s a) -> a runSomeMonad (SomeMonad x) = runIdentity x And when compiled, I got this error: phantom.hs:10:14: Couldn't match expected type `forall s. PwetMonad s a' against inferred type `PwetMonad s a1' In the pattern: PwetMonad x In the definition of `runPwetMonad': runPwetMonad (PwetMonad x) = runIdentity x But when I change line 10 to: runSomeMonad x = runIdentity . unSome $ x then it works, so obviously, the trouble is about pattern matching. What was I doing wrong?

5 8

understanding Peano
by Patrick Browne 04 Jul '10

04 Jul '10

Hi, I would like to understand the Peano module below. I do wish to implement or improve the code. I am unsure of the semantics of (suc = Suc) and then the subsequent use of (Suc n) Is this an example of type-level programming? module Peano where data Nat = Zero | Suc Nat deriving Show class Peano n where suc :: n -> n eq :: n -> n -> Bool plus :: n -> n -> n instance Peano Nat where suc = Suc eq Zero Zero = True eq (Suc m) (Suc n) = eq m n eq _ _ = False plus m Zero = m plus m (Suc n) = Suc (plus m n) -- Some evaluations let t1 = plus (Suc (Zero)) (Suc ( Suc (Zero))) let t2 = plus (Suc (Suc(Zero))) ( Suc (Zero)) is 1+2=2+1? eq t1 t2 -- true *Peano> :t plus (Suc (Suc(Zero))) ( Suc (Zero)) regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

4 3

What is the point of many and some functions in Control.Applicative (for Alternative?)
by Thomas Hartman 03 Jul '10

03 Jul '10

When I load up Control.Applicative in ghci and try, eg many [1,2] or many (Just 1) or some [1,2] or some (Just 1) this never returns. What are the practical uses of these combinators, or for using the Alternative class in general?

4 3

Read Instance code.
by Andy Stewart 03 Jul '10

03 Jul '10

Hi all, I have some incorrect "Read instance" make i got error "Prelude.read: no parse", and i don't know how to fix it. ------------------------------> code start <------------------------------ newtype SerializedWindow = SerializedWindow (Maybe DrawWindow) instance Show SerializedWindow where show _ = "SerializedWindow Nothing" instance Read SerializedWindow where readsPrec _ str = [(SerializedWindow Nothing, idStr) | (val :: String, idStr) <- reads str] ------------------------------> code end <------------------------------ Any help? Thanks! -- Andy

2 3

Spiking neural networks in Haskell
by Dmitry V'yal 03 Jul '10

03 Jul '10

Greetings anyone, Some time ago I with a friend of mine implemented a toy interactive simulator of neural network activity. It has a totally imperative design and uses Gtk2hs for user interface parts. The darcs repository is available here: http://vyal.ru/code/spiking_neuro Now we decided to develop it further to try out some nice biologically inspired ideas. At first I wanted to rewrite the simulator in C++, since that language seems to match the current design more closely. But recently I've read some FRP related papers and have decided to rethink the overall approach. I'd like to hear some comments from the community on the current code and advises on possible improvements and other designs and their pros and conses. The performance is not critical for me, I'm looking for maximal flexibility and clearness so the model can easily be modified and extended. Best wishes, Dmitry

1 0