March 2009 - Haskell-Cafe

alex documentation
by j.romildo＠gmail.com 19 Mar '09

19 Mar '09

Hello. I need the documentation of alex (a lexical analyser generator in Haskell) in PDF format, so I used the Makefile in the doc directory of the distribution of alex to get it. But unfortunatly, the documentation includes some code listings that are too wide and does not fit in the default page width used by the tools generating the PDF file. So, it would be nice if someone could rearrange the listings int the documentation source, so that it fits better in a printed page. José Romildo PS: I am going to use alex in the Compiler Construction classes I am teaching.

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Re[2]: [Haskell] ANNOUNCE: jhc 0.6.0 Haskell Compiler
by Bulat Ziganshin 19 Mar '09

19 Mar '09

Hello sylvain, Thursday, March 19, 2009, 2:16:06 AM, you wrote: > Something is wrong: how can Haskell be faster than C? 1. you measure efficiency of libs, not compilers 2. don't forget about -O2/-O3 -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Announce: language-python
by Bernie Pope 19 Mar '09

19 Mar '09

Language-python provides a parser (and lexer) for Python written in Haskell. Currently it only supports version 3 of Python (the most recent version), but it will support version 2 in the future. The parser is implemented using Happy, and the lexer is implemented using Alex. Web page: http://projects.haskell.org/language-python/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/language-python Darcs: darcs get http://code.haskell.org/language-python/ Example: $ ghci Prelude> :m Language.Python.Version3.Parser Prelude Language.Python.Version3.Parser> parseModule "def inc(x): return x + 1\n" [] Right (Module [Fun {fun_name = Ident "inc", fun_args = [Param {param_name = Ident "x", param_annotation = Nothing, param_default = Nothing}], fun_result_annotation = Nothing, fun_body = [Return {return_expr = Just (BinaryOp {operator = Plus, left_op_arg = Var (Ident "x"), right_op_arg = Int 1})}]}]) Prelude Language.Python.Version3.Parser> :m Language.Python.Version3.Lexer Prelude Language.Python.Version3.Lexer> lexOneToken "def inc(x): return x + 1\n" [] Right (Def (Sloc {sloc_filename = "", sloc_row = 1, sloc_column = 1})," inc(x): return x + 1\n") This is the first release of the package (version 0.1.1) and there is still much to be improved, in particular: - Unicode support is incomplete. - Source locations are sub-optimal, and will eventually become source spans. - The pretty printer needs polish. - The parser only supports version 3 of Python. Support for Version 2 is a major goal, and should be straightforward. - Only minimal performance tuning and correctness testing has been performed. Version 0.1.1 is not intended for production use. Cheers, Bernie.

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What *not* to use Haskell for
by Dave Tapley 19 Mar '09

19 Mar '09

Hi everyone So I should clarify I'm not a troll and do "see the Haskell light". But one thing I can never answer when preaching to others is "what does Haskell not do well?" Usually I'll avoid then question and explain that it is a 'complete' language and we do have more than enough libraries to make it useful and productive. But I'd be keen to know if people have any anecdotes, ideally ones which can subsequently be twisted into an argument for Haskell ;) Cheers, Dave

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Tweaking the garbage collector for realtime usage
by Peter Verswyvelen 19 Mar '09

19 Mar '09

The GHC documentation lists a lot of tweaks that can be done to the garbage collector. However, Haskell spin-offs like Timber <http://www.timber-lang.org/> implement their own incremental garbage collector that is better suitable for real-time usage. Did someone already fiddle with GHC's gc flags so it works better for real-time applications, like games and simulations?

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Crash in GHCI - what is the correct behavior here?
by Zachary Turner 19 Mar '09

19 Mar '09

Basically just learning haskell, I would have posted this in the beginners list but since it involves a segfault of GHCI, I figured it might be worth posting here. I was trying to get a good understanding of local variable scoping issues, so I tried the following: f :: (Num a) => a -> a f x = let p = x*x in let p = x*p in p I have some background in ML, which led me to believe that what should happen here is that the function would return x^3. Instead, GHCI just completely terminates, I guess with a segfault. What's the "correct" behavior here? Should it even compile? I understand that you can't redefine the same symbol twice in the same scope, so I tried this specifically to see what would happen if you defined the same variable again in a nested scope. I thought it would just shadow the original declaration, while still using the original p to calculate the value of the new p. I don't think the problem is the re-declaration of the same symbol in a nested scope (although if someone could clarify that would be nice), but rather the fact that I've attempted to use the previous declaration of p in defining the new declaration of p. Would it be a safe assumption that a bug report should be submitted over this?

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Type equality proof
by Martijn van Steenbergen 19 Mar '09

19 Mar '09

Olá café, With the recent generic programming work and influences from type-dependent languages such as Agda, the following data type seems to come up often: > data (a :=: a') where > Refl :: a :=: a Everyone who needs it writes their own version; I'd like to compile a package with this data type and related utilities, if such a package doesn't exist already (I couldn't find one). Below is what I have so far; I'd much appreciate it if you added your ideas of what else the package should contain. > {-# LANGUAGE GADTs #-} > {-# LANGUAGE TypeOperators #-} > > module Eq where > > data (a :=: a') where > Refl :: a :=: a > > class Eq1 f where > eq1 :: f a -> f a' -> Maybe (a :=: a') > > class Eq2 f where > eq2 :: f a b -> f a' b' -> Maybe (a :=: a', b :=: b') > > class Eq3 f where > eq3 :: f a b c -> f a' b' c' -> Maybe (a :=: a', b :=: b', c :=: c') Thank you, Martijn.

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Fw: hw do i solve this problems:
by THANDO NTUANE 19 Mar '09

19 Mar '09

----- Forwarded Message ---- From: THANDO NTUANE <thandodarl85(a)yahoo.com> To: Haskell-Cafe(a)haskell.org Sent: Thursday, March 19, 2009 10:22:40 AM Subject: hw do i solve this problems: kindly help me.. im stack but managed to solve exercise 1 and 2.

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Memoizing partially evaluated computations.
by Sebastiaan Visser 19 Mar '09

19 Mar '09

Suppose I have a list of IO computations that depends on a few very time consuming pure operations. The pure operations are not dependent on the real world: > computation :: [IO Int] > computation = [ > smallIOfunc timeConsumingPureOperation0 > , smallIOfunc timeConsumingPureOperation1 > , smallIOfunc timeConsumingPureOperation2 > , smallIOfunc timeConsumingPureOperation3 > ] > where smallIOfunc a = print a >> return a In my main function I would like to repeatedly print the values > main = forever $ > sequence_ (map (>>=print) computation) When I do this, all the time consuming operations will be reevaluated every run of the main loop. Is there a any (simple or smart) way to prevent the garbage collector from cleaning up the fully evaluated thunks inside my computation? As if it were something like this: > computation :: [IO Int] > computation = [smallIOfunc 42, smallIOfunc 34385, smallIOfunc 3, smallIOfunc 55] Of course I could plugin some kind of Int memoizer inside my computation, but I do not really have the control to change things `deep' inside the code. I want to have some form of snapshot of a list of partially evaluated IO computations... Any suggestions? Tanks, Sebastiaan

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Haskell Style - Pattern Matching with case vs. function declarations
by Tom.Amundsen 18 Mar '09

18 Mar '09

Hi All, I am new to Haskell. I just started reading "Real World Haskell" a few days ago, so I apologize for being such a noob. But, I am curious why I see a lot of code where people do pattern matching via multiple function declarations instead of using the case ... of ... construct? For example: [code] foldl' _ zero [] = zero foldl' step zero (x:xs) = let new = step zero x in new `seq` foldl' step new xs [/code] instead of this, which I prefer: [code] foldl' f acc xs = case xs of [] -> acc (x:xs) -> let new = f acc x in new `seq` foldl' f new xs [/code] -- View this message in context: http://www.nabble.com/Haskell-Style---Pattern-Matching-with-case-vs.-functi… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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