May 2010 - Haskell-Cafe

Help debugging code broken after upgrading debian to GHC 6.12: "invalid argument"
by Brandon Simmons 15 May '10

15 May '10

> > GHC 6.12's runtime handles input and output encoding, instead of > > simply truncating Chars; my guess is it's locale-related. And sure > > enough, I see several non-ASCII characters in mycology.b98 which are > > likely to do the wrong thing if the runtime doesn't know which > > character set to use. > > Yup. Converting mycology.b98 to utf-8 makes it run. > However, why does it run with --quiet on the original file?? That actually makes sense. That mycology Befunge source is actually a combination of Befunge-98 (which can use non-ascii characters. I'd forgotten that) and befunge-93 which uses only printable ascii characters. Without the --quiet flag, we end up inspecting a Bool value returned after taking the `length` of a row and a column, which looks to see if the grid is bigger than 80x25 characters. I guess with the --quiet flag, we ignore that Bool value and so the program never has to take the length of a line, and consequently never sees any of the non-ascii characters (because the source file is written in such a way that a befunge-93 interpreter will naturally stop execution after the '93 section of code. That also explains why simply printing the file string causes the interpreter to fail, even with --quiet. Thanks for the help,

1 0

Help debugging code broken after upgrading debian to GHC 6.12: "invalid argument"
by Brandon Simmons 15 May '10

15 May '10

>On May 14, 2010, at 20:24 , Brandon Simmons wrote: >> The other baffling thing is this: if the debugging line 426 is >> uncommented, then even running: >> >> $> runghc Befunge.hs --quiet mycology.b98 >> >> ...will fail. But all we're doing is a call to `putStr`! Why would >> that trigger an error?! Maybe there was a bug in my code that was > > > GHC 6.12's runtime handles input and output encoding, instead of > simply truncating Chars; my guess is it's locale-related. And sure > enough, I see several non-ASCII characters in mycology.b98 which are > likely to do the wrong thing if the runtime doesn't know which > character set to use. Thanks, that makes a lot of sense. Is there something I can read to enlighten myself as to how I would go about getting my program to be liberal about non-ascii characters in a file? Also, I noticed yesterday that $> cabal list --installed shows the 'old-locale' package: * old-locale Synopsis: locale library Latest version available: 1.0.0.2 Latest version installed: 1.0.0.2 License: BSD3 I'm not sure if that could be part of the issue, or whether that is normal. Thanks again, Brandon Simmons http://coder.bsimmons.name/ > > -- > brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com > system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu > electrical and computer engineering, carnegie mellon university KF8NH

2 1

Help debugging code broken after upgrading debian to GHC 6.12: "invalid argument"
by Brandon Simmons 14 May '10

14 May '10

I developed my Befunge interpreter that I posted here: http://coder.bsimmons.name/blog/2010/01/a-befunge-93-interpreter/ ...under GHC 6.8 Debian packages, and after revisiting the code, my interpreter fails on a particular file in a way that is truly baffling me. I don't even know where to begin. I am certain this code was working before, because the Befunge source it fails on is in my darcs test suite: The error I get is an InvalidArgument IOError. I'm not even sure what that means. Here are the imports I'm using in the module in case that makes the problem with my upgrade obvious: import System.Environment import Data.Char import Control.Arrow (first,second,(&&&),(|||)) import Data.Array.Unboxed import Control.Monad.State.Strict import System.Random import System.IO import System.IO.Error If anyone wants to look at my code and help me with this, they can download the three attached files and run the following: $> runghc Befunge.hs aturley.bf $> runghc Befunge.hs --quiet aturley.bf ...which should both print a pretty picture. Now try the file giving me problems: $> runghc Befunge.hs --quiet mycology.b98 ... which should give correct output (a bunch of line of "GOOD: blah...."). And finally: $> runghc Befunge.hs mycology.b98 ...which will (for me) simply print: "error processing options: invalid argument" The problem seems to have to do with line 427 and the call to `buildGrid` and the Bool value `wasBig`: while debugging I can print the array returned on that line, but trying to print the Bool simply fails with the error. The other baffling thing is this: if the debugging line 426 is uncommented, then even running: $> runghc Befunge.hs --quiet mycology.b98 ...will fail. But all we're doing is a call to `putStr`! Why would that trigger an error?! Maybe there was a bug in my code that was there before, but wasn't showing its head because something was being lazier than it is now? If anyone wants to take the time to help me with this, I would be most grateful. SIncerely, Brandon Simmons http://coder.bsimmons.name/

3 2

MultiParamTypeClasses, FunctionalDependencies and FlexibleInstances using GHCi
by Julian Fleischer 14 May '10

14 May '10

Hello, i'm playin' around with GHCs Haskell and some extensions. I'm already aware of that functional dependencies are "very very tricky", but there is something I don't understand about there implementation in GHC. I've constructed my own TypeClass "Num" providing a signature for (+), having multiple params a, b and c. I'm than declaring a (flexible) Instance for Prelude.Num, simply using (Prelude.+) for the definition of my (+) - and it does not work as I expect it to. First, this is the code: > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, TypeSynonymInstances, FlexibleInstances #-} > import qualified Prelude > > class Num a b c | a b -> c where > (+) :: a -> b -> c > > instance (Prelude.Num x) => Num x x x where > (+) = (Prelude.+) now if I load it into GHCi and type "3 + 4" i get a whole bunch of error-messages. I do understand that > (3::Prelude.Int) + (4::Prelude.Int) works, since I've explicitly declared 3 and 4 to be Prelude.Int and there is a functional dependency stating that (+) :: a b determines the results type c, by the Instance declaration cleary c will be the same as a and b. Now, if I type > 3 + 4 it does not work, and i really don't understand why. If i ask GHCi for 3's type ($ :t 3) it will answer "3 :: (Prelude.Num t) => t". But, if 3 and 4 are Prelude.Nums and there is an instanfe Num x x x for x of Prelude.Num - than why can't GHC deduce from the definitions that 3 and 4, both Prelude.Nums, can be used with (+) since there is an instance for Prelude.Num and my class Num - and the result will of course be something of Prelude.Num? best regards, Julian

6 5

Modular type inference
by Simon Peyton-Jones 14 May '10

14 May '10

Friends Many of you will know that I've been muttering about re-engineering GHC's type inference engine for some time now. Dimitrios, Tom, Martin and I have just completed an epic paper describing the Glorious New Framework that forms the substance of the above mutterings: http://haskell.org/haskellwiki/Simonpj/Talk:OutsideIn We'd love comments and feedback. Dimitrios and I plan to roll up our sleeves and implement it in June. Simon Modular type inference with local assumptions: OutsideIn(X) Abstract. Advanced type system features, such as GADTs, type classes, and type families have have proven to be invaluable language extensions for ensuring data invariants and program correctness among others. Unfortunately, they pose a tough problem for type inference, because they introduce local type assumptions. In this article we present a novel constraint-based type inference approach for local type assumptions. Our system, called OutsideIn(X), is parameterised over the particular underlying constraint domain X, in the same way as HM(X). This stratification allows us to use a common metatheory and inference algorithm. Going beyond the general framework, we also give a particular constraint solver for X = type classes + GADTs + type families, a non-trivial challenge in its own right.

2 1

Re: [Haskell-cafe] Why is TChan GHC specific?
by Edward Amsden 14 May '10

14 May '10

On Wed, May 12, 2010 at 3:29 PM, Peter Robinson <thaldyron(a)gmail.com> wrote: > As far as I know, TChan needs the 'retry' combinator which requires GHC's RTS. > Same is true for TMVar, I think. (sorry for the doubling peter, I forgot reply-all) OK. I'm new to this and probably didn't know where to look, but I didn't know that 'retry' was GHC specific.

3 3

tweak vacuum-* output
by Ozgur Akgun 14 May '10

14 May '10

Hi all, I am using vacuum-opengl and vacuum-ubigraph to visualise and analyse some of my data structures. They are quite helpful most of the time, however sometimes I feel the need to tweak the generated output -- such as removing the auto-generted identifiers from constrcutor names, pack some things together, or similar. Is there a way to configure their output? And for the vacuum-ubigraph option, I like it's output generally, however while creating the expression tree, is doesn't respect my structures. If there is a flag or so to fix this issue, I'd appreciate it. The Problem is like the following: data Expr = Sum Expr Expr | Mult Expr Expr | Single Int e = Sum (Single 2) (Mult (Single 3) (Single 4)) And it orients the tree in such a way that Mult looks like the root node, instead of Sum, as I would expect. Thanks, Ozgur Akgun

2 9

Concurrent Rings in Erlang and Haskell: A Haskell answer to Joe Armstrong's Erlang Challenge
by Aran Donohue 13 May '10

13 May '10

I wrote this a couple of weeks ago and forgot about it. I thought it might be of interest to some on this list: http://workshop.arandonohue.com/ring/

2 1

Ideas of a polymorphic Tree in Haskell
by Edgar Z. Alvarenga 13 May '10

13 May '10

Hi, I created a small Genetic Algorithm program, replicating this work ("Statistical mechanics of program systems" - JP Neirotti, N. Caticha, Journal of Physics A Math and Gen) made in Lisp. When a restricted the problem just for one type, the Haskell program worked perfectly with much less lines and a lot faster than the Lisp program. The problem was when I tried to generalize to polymorphic types. I'm using this datatype to represent a program: data Tree a = Bin (String, (a -> a -> a)) (Tree a) (Tree a) | Un (String, (a -> a)) (Tree a) | V And can convert a Tree to a function with: treeToFunc :: Tree a -> a -> a treeToFunc (Bin f l r) = (snd f) <$> treeToFunc l <*> treeToFunc r treeToFunc (Un f u) = (snd f).(treeToFunc u) treeToFunc V = id I already create another datatype to represent a polymorphic program (using GADT or existentials), but doesn't see a way to convert this kind of tree to a function. Anyone has any idea? Thanks, Edgar

3 3

Re: Functions of type foo :: f a -> g a
by Gordon J. Uszkay 13 May '10

13 May '10

Thank you for all these answers - I learned a lot (especially that the responses come in REALLY quickly, so the digest option is not a good choice). I think I need to rephrase the problem, taking into account what I have learned. class (Functor f, Functor g) => Foo f g where foo :: f a -> g a as per John Meacham's suggestion to isolate the transform. Really, f and g are 'containers' which I want to reshape (and there can be many different re-shapings of g for each f, and different f's can map to the same g, so no fundeps relation holds in general). I am trying to use classes as an interface description, with this transformation being part of that interface, and also to allow me to experiment with different constraints (e.g. Monad f, Applicative f, etc). Edward Kmett points out (thank you for that detailed reply!) that there is no canonical foo -- which is a good point, and is indeed the right model -- "natural transformation" was likely a red herring. What I am trying to do is take a container of data f, and partition it into a structure g as a 'chunking' of that data. The f container is a potentially infinite stream of data obtained from a generator, and I want to be able to control how much data is extracted, so an eager 'fmap' won't be sufficient (an eager process will be applied to the 'chunks' defined by g). I am going to purse the hylomorphism model, but am interested if anyone else has any similar situations, and if using classes to manage the interface is the right strategy. Thanks! Gordon J. Uszkay uszkaygj(a)mcmaster.ca

4 4