March 2009 - Haskell-Cafe

[Probably a dumb question] Quasiquoting
by Andrew Coppin 31 Mar '09

31 Mar '09

Can somebody explain to me how [$expr| 1 + 2 |] is different from ($expr "1 + 2") Other than a superficially different type signature, I'm not seeing what the fundamental difference is...

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Haddock: inserting interactive sessions in the documentation
by Manlio Perillo 31 Mar '09

31 Mar '09

Hi. What is the suggested (if any) convention for inserting an interactive session in the documentation? Right know I'm doing (in random-shuffle package): -- |Convert a sequence @(e1...en)@ to a complete binary tree. -- -- @ -- System.Random.Shuffle> buildTree ['a','b','c','d','e'] -- Node 5 (Node 4 (Node 2 (Leaf 'a') (Leaf 'b')) -- (Node 2 (Leaf 'c') (Leaf 'd'))) -- (Leaf 'e') -- @ buildTree :: [a] -> Tree a As an example, in Python we use """ >>> 3 + 2 5 """ One nice feature of this special syntax is that it can be parsed, using doctest. Thanks Manlio

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type-level programming support library
by spoon 31 Mar '09

31 Mar '09

I've been doing some basic work on a support library for type level programming ( see http://hackage.haskell.org/trac/summer-of-code/ticket/1541 ). I know there have been similar attempts using fundeps ( Edward Kmett showed me some of his work, but I've lost the address... ) but this approach uses type families. Anyway, I would like to hear your critique! Currently I have higher order type functions and ad-hoc parametrized functions. Here's what foldl looks like: type family Foldl ( func :: * -> * -> * ) val list type instance Foldl func val ( Cons first rest ) = Foldl func ( Eval ( func val first ) ) rest type instance Foldl func val Nill = val Notice the use of Eval - this is a trick to enable us to pass around data with kind * -> *, or whatever, and then trip this into becoming a value. Here's an example, using this trick to define factorial: -- multiplication type family Times x y type instance Times x Zero = Zero type instance Times x ( Succ y ) = Sum x ( Times x y ) -- The "first order" function version of Times data TimesL x y -- Where what Eval forced TimesL to become. type instance Eval ( TimesL x y ) = Times x y -- multiplies all the elements of list of Nat together type Product l = Foldl TimesL ( Succ Zero ) l -- here list to creates a list from ( Succ Zero ) to the given number type Factorial x = Product ( ListTo x ) We can now use the function like this: *TPrelude> result ( ) :: Factorial ( Succ ( Succ ( Succ ( Zero ) ) ) ) Succ (Succ (Succ (Succ (Succ (Succ Zero))))) Using the parametrized types kinda reminds me of programming in Erlang: -- What would conventionally be the monad type class, parametized over m type family Bind m ma ( f :: * -> * ) type family Return m a type family Sequence m ma mb Here's the maybe monad: -- Monad instance type instance Bind ( Maybe t ) Nothing f = Nothing type instance Bind ( Maybe t ) ( Just a ) f = Eval ( f a ) type instance Sequence ( Maybe t ) Nothing a = Nothing type instance Sequence ( Maybe t ) ( Just a ) b = b type instance Return ( Maybe t ) a = Just a type instance Eval ( Just x ) = Just x Here's an example: *TPrelude> result ( ) :: Bind ( Maybe Nat ) ( Just Zero ) Just Just Zero For more information and to download the loose collection of module implementing this please see: http://www.killersmurf.com/projects/typelib John Morrice

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GHC on AIX
by nitralime 31 Mar '09

31 Mar '09

Hi, I have tried to install the newest available binary distribution of GHC for AIX (6.8.2). Unfortunately, it didn't work properly. Following the trivial installation steps as described in INSTALL file I executed the following commands: 1) ./configure --prefix=/home/nik/local 2) gmake install But I got the following error message: -------------------------------------------------------------------------------------------------------------------------------------- .... Reading package info from stdin ... done. warning: can't find GHCi lib HSrts.o Saving old package config file... done. Writing new package config file... done. /tmp/ghc-6.8.2/utils/mkdirhier/mkdirhier /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2 for i in libHSrts*.a ; do \ case $i in \ *.a) \ /tmp/ghc-6.8.2/install-sh -c -m 644 $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2; \ : /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/`basename $i` ;; \ *.dll) \ /tmp/ghc-6.8.2/install-sh -c -m 644 -s $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2 ;; \ *.so) \ /tmp/ghc-6.8.2/install-sh -c -m 755 $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2 ;; \ *.dylib) \ /tmp/ghc-6.8.2/install-sh -c -m 755 $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2; \ install_name_tool -id /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/`basename $i` /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/`basename $i` ;; \ *) \ /tmp/ghc-6.8.2/install-sh -c -m 644 $i /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2; \ esac; \ done gmake[1]: Leaving directory `/tmp/ghc-6.8.2/rts' gmake -C libraries install DOING_BIN_DIST=YES gmake[1]: Entering directory `/tmp/ghc-6.8.2/libraries' if ifBuildable/ifBuildable base; then \ cd base && \ ../installPackage/installPackage '/tmp/ghc-6.8.2/utils/ghc-pkg/ghc-pkg.bin' '/home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/package.conf' '/home/nik/local/ghc-6.8.2' '/home/nik/local/ghc-6.8.2' '/home/nik/local/ghc-6.8.2/bin' '/home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/lib' '/home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/lib' '/home/nik/local/ghc-6.8.2/share/ghc-6.8.2' '/home/nik/local/ghc-6.8.2/share/doc/ghc/libraries/$pkg' '/home/nik/local/ghc-6.8.2/share/doc/ghc/libraries/$pkg' '/home/nik/local/ghc-6.8.2/share/doc/ghc/libraries/$pkg' ; \ fi Installing: /home/nik/local/ghc-6.8.2/lib/ghc-6.8.2/lib/base-3.0.1.0 gmake[1]: *** [install.library.base] Error 127 gmake[1]: Leaving directory `/tmp/ghc-6.8.2/libraries' gmake: *** [install] Error 2 -------------------------------------------------------------------------------------------------------------------------------------- Am I missing something? Any help is very much appreciated! Regards Nik

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Converting -fglasgow-exts into LANGUAGE pragmas
by Martijn van Steenbergen 31 Mar '09

31 Mar '09

Hello café, I want to replace the -fglasgow-exts in the snippet below by LANGUAGE pragmas. Rank2Types alone doesn't suffice. Which do I need to get the snippet to compile? > {-# OPTIONS -fglasgow-exts #-} > > module RunMud where > > import Control.Monad.State > > type Mud = StateT MudState IO > data MudState = MudState { mRunMud :: RunMud } > type RunMud = forall a. Mud a -> IO a > > getRunMud :: Mud RunMud > getRunMud = do > s <- get > return (mRunMud s) Thanks in advance, Martijn.

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Re: Exception handling in numeric computations
by John Lato 31 Mar '09

31 Mar '09

> From: Donn Cave <donn(a)avvanta.com> > Quoth John Lato <jwlato(a)gmail.com>, > >> An exception is caused by some sort of interaction with the run-time >> system (frequently a hardware issue). The programmer typically can't >> check for these in advance, but can only attempt to recover after >> they've happened. >> >> An error is some sort of bug that should be fixed by the programmer. > > I have never felt that I really understood that one. Honestly, me neither, until recently. I'm only barely starting to understand it, and I do think there's a great deal of overlap. Even if an error is a bug that can be fixed by the programmer, certain exceptional situations can also be fixed by the programmer by handling the exception, even if they can't be detected in advance. In general I would say that I agree with JCC's responses. > > What about invalid inputs? Say someone encounters a disk full error, > and the resulting partly written file is now unreadable data for its > intended application because of an invalid file encoding? Is that > an exception, or a bug that should be fixed? > > My guess is that you'll say it's a bug, i.e., that application's > file decoding result should be an Either type that anticipates that > the file encoding may be invalid. Not necessarily. I would be satisfied with a Maybe :) I've encountered malformed data frequently enough that any decoders I write (and I have done so recently) would anticipate invalid data. > > I will also guess if the file is unreadable because of an external > I/O problem like no read access to file or filesystem, you would > similarly expect this to be treated like that - I mean, ideally, e.g., > hGetLine :: Handle -> IO (Either IOError String) Not necessarily, but possibly. The big difference, of course, is that decoding can be a pure operation, while reading never is. I personally wouldn't mind if hGetLine had the type you give. The way I see it, there are two advantages to exceptions in this case. The first is that it's very easy for exceptions to trickle up and be handled at a higher level. The second 'advantage' is that the programmer doesn't need to explicitly handle exceptions, whereas an Either would require at least a pattern match to use the resulting value. John

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ghc: Var/Type length mismatch message - what does it mean?
by John Lask 31 Mar '09

31 Mar '09

consider the following types (from the paper: "Stream Fusion From Lists to Streams to Nothing at All") > data Stream a = forall s. Stream ( s ->(Step a s)) s > data Step a s = > Done > | Yield a s > | Skip s an instance of this data type is: > stream0 :: Stream () > stream0 = Stream (\ s -> Yield () s) () now consider: > data StreamM m a = forall s. StreamM ( s ->m (Step a s)) s and the following instance: > stream0IO :: StreamM IO () > stream0IO = StreamM (\ s -> return (Yield () s)) () loads ok, but as soon as we introduce the following constraint: > data Monad m => StreamM' m a = forall s. StreamM' ( s ->m (Step a s)) s > stream0IO' :: StreamM' IO () > stream0IO' = StreamM' (\ s -> return (Yield () s)) () we get the following message when we load into ghci (6.8.2) Var/Type length mismatch: [] [base:GHC.Base.(){(w) tc 40}] is this a bug? if not, what is this message telling us ?

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Unique monad?
by Andrew Coppin 31 Mar '09

31 Mar '09

Maybe I'm just being blind here, but I don't see a monad transformer (or even a monad) in the standard libraries for producing "unique" values. Have I missed something? Related, I don't see a monad for gradually consuming input. We've got the Reader monad, but that appears to just give you global access to a single monolithic value. That value could be some kind of collection, but there's no way to track where you've read up to (or efficiently access that point). We've got Parsec, which is a pretty efficient and powerful text parser. But what if you don't need full parsing capabilities? You just need to consume input, but you're not exactly sure where you're going to consume it? The Writer monad seems to let you incrimentally write values out, but there's no matching monad for incrimentally reading stuff back in. (I guess ultimately you can build all these specialised monads out of the general State monad if you want. I'm just surprised they're not already defined somewhere...)

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ANN: new release of HTTP, version 4000.0.5
by Sigbjorn Finne 30 Mar '09

30 Mar '09

Hi, a new version of the HTTP package - 4000.0.5 has been uploaded, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP Includes a bunch of fixes and cleanups (thanks to all that have reported&contributed), along with some (shock, horror) API documentation. Let me know if it appears to be well behaved and dependable. thanks --sigbjorn

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abou the Godel Numbering for untyped lambda calculus
by Algebras Math 30 Mar '09

30 Mar '09

Hi all, I am reading the book "The lambda calculus: Its syntax and Semantics" in the chapter about Godel Numbering but I am confused in some points. We know for Church Numerals, we have Cn = \fx.f^n(x) for some n>=0, i.e. C0= \fx.x and C 1 = \fx.fx. >From the above definition, I could guess the purpose of this kind of encoding is trying to encode numeral via terms. How about the Godel Numbering? From definition we know people say #M is the godel number of M and we also have [M] = C#M to enjoy the second fixed point theorem : for all F there exists X s.t. F[X] = X. What the mapping function # is standing for? How could I use it? What the #M will be? How to make use of the Godel Numbering? Thank you very much! alg

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