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Shared modification inside lambdas
by Andrew Hunter 28 Nov '08

28 Nov '08

I'm developing an embedded DSL in Haskell (for exact real arithmetic work, but that's irrelevant right now.) Among other things, the language contains functions and application, as well as various terms. In one iteration, the abstract syntax looked kinda like this (in spirit, don't worry about the actually available terms...) data Expr = Var String | Const Int | Plus Expr Expr | Quantified Range String Expr | Lambda String Expr | App Expr Expr Here, lambda-terms just have the name of their bound variable and the RHS. Quantified terms are much like specialized lambdas; one note is that you *will* have to evaluate the RHS of the quantified term with many values for the argument. It's obvious how to write an evaluator on these--just keep an environment, etc. All well and good, but it seems to me that if I'm embedding the DSl, shouldn't I be able to use the host language's facilities--for example, function abstractions and applications--directly? Well, I tried this, and it seems it works OK, like so: data Expr = Var String | Const Int | Plus Expr Expr | Quantified Range (Int -> Expr) I replaced Lambda terms and applications with Haskell-level functions. Note that my quantifiers still need internal functions, but I replaced them in the same fashion. However, I have a problem; one thing I often have to do with expressions is refine them, where refine has type: refine :: Expr -> Expr and transforms expressions into more interesting ones via certain, mostly simple rules. The problem comes with functions, either just plain lambdas or quantified ones. The rule for refining functions boils down to just refining (recursively) the RHS; in the old version, that was easy! refine (Lambda var rhs) = Lambda var (refine rhs) I only refine while evaluating terms of type Expr, so top level functions aren't important--by the time I'm evaluating them, I've applied them to arguments--but quantified terms are a problem. I could write something like: refine (Quantified range pred) = Quantified range pred' where pred' = \c -> refine (pred c) But the problem is that this refines the term, again, every time I apply an argument to pred': I know I'm going to apply arguments many times to that new pred, and I want to refine it /once/: refinement is argument-agnostic, it only rearranges the structure of the AST, so in theory it'd be nice if I could refine that structure, just duplicating references to the appropriate free variable where I would have duplicated refences to the appropriate (Var String) before. Am I sore out of luck here? Is there a reasonable way I can implement my internal functions via Haskell functions, but apply argument-agnostic transformations to the RHS in a shared fashion? Or, is there some optimization in GHC that means I don't need to worry about this? Thanks, AHH

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Re: Suggestion: Syntactic sugar for Maps!
by Don Stewart 28 Nov '08

28 Nov '08

bulat.ziganshin: > Hello circ, > > Thursday, November 27, 2008, 9:59:08 PM, you wrote: > > So why not {"hello": 1, "there": 2} ? > > mymap "hello:1 there:2" > > where mymap implementation is left to the reader :) Hey, well, even easier: {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE FlexibleInstances #-} import Data.Map import Data.String import Text.JSON instance IsString (Map Int Bool) where fromString = fromList . read -- or, say, JSON syntax for assoc lists. {- fromString s = case resultToEither (decode s) of Right a -> a Left s -> error s -} test :: Map Int Bool test = "[(7, True), (1, False)]" main = print test -- Don

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I have forgotten .. ghci question
by Galchin, Vasili 28 Nov '08

28 Nov '08

Hello, I have an experimental version of a package ~/FTP/Haskell/blah. Who I "point" ghci ath this experimental package version so I can poke around? Kind regards, Vasili

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Need machine for DPH benchmarking
by Roman Leshchinskiy 28 Nov '08

28 Nov '08

Hi all, we, the DPH team, are at the moment in the very unfortunate situation of not having a proper machine for running our benchmarks on. Could a kind soul maybe give us (i.e., me) access to a quadcore or 2xquadcore x86 Linux or OS X machine? I only need to build ghc on it and run small benchmarks which never take more than a couple of minutes, maybe once every couple of days or so. We do need to use all cores, though, so no other CPU-intensive processes can be running during benchmarking. This is only for a week or two, until we get our own machine. We would be eternally grateful and won't forget you when DPH takes over the world. Roman

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RE: [Haskell-cafe] followedBy parser in Parsec
by Paul Keir 27 Nov '08

27 Nov '08

'lookAhead' is exactly what I needed: try $ array_element >>= \ae -> lookAhead (reservedOp "(") >> return ae Many thanks, Paul Maybe you're looking for 'lookAhead'? > > [...] //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

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Building QuickCheck 2 Under GHC 6.10.1
by Dominic Steinitz 27 Nov '08

27 Nov '08

Having been a happy user of QuickCheck 2 for many years, I now find it won't build under ghc 6.10.1. Before I investigate further, has anyone encountered this problem and has a fix? Thanks, Dominic. > C:\Users\Dom\QuickCheck>Setup build > Preprocessing library QuickCheck-2.0... > Building QuickCheck-2.0... > [2 of 9] Compiling Test.QuickCheck.Exception ( Test\QuickCheck\Exception.hs, dis > t\build\Test\QuickCheck\Exception.o ) > > Test\QuickCheck\Exception.hs:12:31: > Class `Exception' used as a type > In the type `Exception' > In the type `Either Exception a' > In the type `IO (Either Exception a)' > > Test\QuickCheck\Exception.hs:15:36: > Class `Exception' used as a type > In the type `Exception' > In the type `Either Exception a' > In the type `IO (Either Exception a)'

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followedBy parser in Parsec
by Paul Keir 27 Nov '08

27 Nov '08

Hi, Is there a way in Parsec to check what the next token is, and if it is what you're hoping for, leave it there. This is an example of something which doesn't work at all: testpar = try $ do ae <- array_element option [] $ try $ satisfy (\c -> c /= '(') >> unexpected "" return ae I'm finding this totally confusing by now %0 Can I invert "notFollowedBy" somehow, or maybe there's a "peek" function I don't know about? Help! Thanks, Paul

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HaskellWiki image upload fails because of - probably - the filesize.
by And1 27 Nov '08

27 Nov '08

Hey dear Haskell-Cafe-Readers, I tried several times to upload several images - screenshots that is - to my HaskellWiki/Xmonad site: http://haskell.org/haskellwiki/Image:And1_xmonad.png I always get the error, that the connection to the site/server was reset. I mailed both administrators of the HaskellWiki but I got, for about 2-4 weeks now, no reply. I tried to upload the following images, maybe that's important: http://and1.donnergurgler.net/misc/screen_2008-09-26.png http://and1.donnergurgler.net/misc/screen_2008-11-02.png http://and1.donnergurgler.net/misc/screen_2008-11-10.png I was told, that the problem is maybe the filesize of the new images, because it is bigger, then the filesize of the image already uploaded. Thanks for reading this and hopefully finding the problem, -- Regards, Andi R.

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Possible issue with Hoogle and Haddock?
by Stephen Hicks 27 Nov '08

27 Nov '08

Hi, I was noticing recently that there seems to be a problem with Hoogle and Haddock. In particular, I just hoogled "bracket" and got the following result: bracket :: IO a -> a -> IO b -> a -> IO c -> IO c Clearly this is the wrong type, as it should be bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c The Haddock-generated documentation at least has newlines to group the terms, but this is still far from correct. I noticed the same thing a while back in Parsec's documentation for token, token :: Stream s Identity t => t -> String -> t -> SourcePos -> t -> Maybe a -> Parsec s u a which again should have parens. Is this a bug? Is is something that's well-known? Cheers, steve

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haddock question
by allan 27 Nov '08

27 Nov '08

Dear all I'm trying to locally build the documentation for the haskell-src-exts package and running into a bit of bother. If I run: cabal haddock I get the error: haddock: parse error in doc string so: cabal haddock -v Doesn't really provide any extra information, it gives me the exact haddock command-line used but there is still no way (I can see) of obtaining the source file containing the parse error. So I downloaded via darcs the latest haddock source code and installed that but I get the exact same error message. I tried grepping in the haddock source code for that particular error message (and portions of it) but I couldn't find it. Finally I tried running haddock on a single source file: haddock Language/Haskell/Exts.hs and even this gets the exact same error message, which is somewhat bizarre since that file does not contain any documentation strings. Anyone have any ideas? regards allan -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

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