September 2008 - Haskell-Cafe

typeclass question
by Tim Docker 11 Sep '08

11 Sep '08

I have a typeclass related question that I have been puzzling over. In a library I am working on, I have a series of functions for converting values to Renderables: | labelToRenderable :: Label -> Renderable | legendToRenderable :: Legend -> Renderable | axisToRenderable :: Axis v -> Renderable | layoutToRenderable :: Layout x y -> Renderable These names are overloaded for convenience via a typeclass: | class ToRenderable a where | toRenderable :: a -> Renderable | | instance ToRenderable Label where | toRenderable = labelToRenderable | ... But some recent changes mean that Renderable needs to become a type constructor, and the functions now product different types: | labelToRenderable :: Label -> Renderable () | legendToRenderable :: Legend -> Renderable String | axisToRenderable :: Axis v -> Renderable () | layoutToRenderable :: Layout x y a -> Renderable a Is there a nice way to overload a "toRenderable" function for these? Something like this is possible: | class ToRenderable a b where | toRenderable :: a -> Renderable b But the above is, I think, too general for my needs. I don't want to be able to generate Renderables of different type b for a single input type a. Also, MPTC take me out of the world of haskell 98, which I was trying to avoid. Am I missing something simple? Any pointers would be much appreciated. Tim

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Haskell as a scripting language.
by David F. Place 11 Sep '08

11 Sep '08

Hi, All. I needed to make a batch of edits to some input files after a big change in my program. Normally, one would choose some scripting language, but I can't bear to program in them. The nasty thing about using Haskell is that giving regexes as string constants sometime requires two levels of quoting. For instance. (mkRegex "\\\\\\\\") matches \\. To get around that, I put the regexes in the head of a literate program and let the program gobble itself up. Works great! I think I'll always turn to Haskell for my scripting needs now. I put the file in the Haskell Pastebin, if you would like to see it. http://hpaste.org/10249 Cheers, David

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Field names
by Daryoush Mehrtash 11 Sep '08

11 Sep '08

In type section of the Gentle Introduction to Haskell http://www.cs.sfu.ca/CC/SW/Haskell/hugs/tutorial-1.4-html/moretypes.htmlthe… is this example: > data Point = Pt {pointx, pointy :: Float} > abs (Pt {pointx = x, pointy = y}) = sqrt (x*x + y+y) Why is it pointx=x and not x=pointx? -- Daryoush Weblog: http://perlustration.blogspot.com/

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Re: [Haskell-cafe] Re: Field names
by Richard A. O'Keefe 11 Sep '08

11 Sep '08

On 11 Sep 2008, at 3:54 am, Brandon S. Allbery KF8NH wrote: > I think that only counts as the origin of the idea; isn't :-prefixed > infix constructors a ghc-ism? Haskell 98 report, page 10: "An operator symbol starting with a colon is a constructor". (I seem to have four copies of the report on my Mac...)

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STM and FFI
by Mauricio 10 Sep '08

10 Sep '08

Hi, Is it possible to use foreign function interface with STMs? If so, where can I find examples? Thanks, Maurício

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Re: [Haskell-cafe] packages and QuickCheck
by Sean Leather 10 Sep '08

10 Sep '08

> How do folks like to package up QuickCheck tests for their libraries? In > the main library? As a separate repo & package? Same repo & separate > package? Keeping tests with the tested code allows testing of non-exported > functionality, but can add quite a lot of clutter. > I have QuickCheck properties plus HUnit tests, but I think the question is the same. For me, it's in the same repository and shipped with the package source. I think that if you ship source (even via Hackage), you should also ship tests. So, if somebody wants to modify the source, they can run the tests. And making it convenient to test is very important, so I have "cabal test" (or "runhaskell Setup.hs test" without cabal-install) configured to run the tests. I don't think tests should (in general) be part of the user-visible API, so I have them external to the module hierarchy. Testing non-exported functionality without exporting the test interface seems difficult in general. Is there a way to hide part of a module interface with Cabal? Then, you could have a 'test' function exported from each module for testing but hidden for release. My current leaning is to split a package "foo" into packages "foo" and > "foo-test" > What benefit does this provide? Thanks, Sean

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Haskell stacktrace
by Pieter Laeremans 10 Sep '08

10 Sep '08

Hello, I've written a cgi script in haskell, it crashes sometimes with the error message Prelude . tail : empty list In Java we would use this approach to log the erro try { } catch (Exception e) { } -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

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User defined annotations for Haskell (Was: [Haskell-cafe] packages and QuickCheck)
by Max Bolingbroke 10 Sep '08

10 Sep '08

2008/9/10 Johannes Waldmann <waldmann(a)imn.htwk-leipzig.de>: > >>> Has there ever been a discussion of typed, user-definable, >>> user-processable source code annotations for Haskell? >> >> afair it was on haskell-prime list > > http://hackage.haskell.org/trac/haskell-prime/ticket/88 > > if you can call that a discussion :-) I had to implement a kind of annotation system for GHC as part of my Summer of Code project for compiler plugins. You can see a discussion at http://hackage.haskell.org/trac/ghc/wiki/Plugins/Annotations, though I was not aware of that Haskell' ticket. The system as I have implemented is quite basic: the absolute minimum I needed for plugins. The syntax is essentially: {-# ANN foo "I am an annotation!" #-} Where "I am an annotation" may be replaced with any Typeable value, and is subject to staging requirements analgous to Template Haskell (e.g. you may not refer to things in the module being compiled other than by Template Haskell Name). As well as values you can annotate types and modules, and annotations are accessible via the GHC API. See that wiki page for full details. This system has not yet been merged into GHC (or even code reviewed by GHC HQ), so if/when it becomes available to end users the design may have changed drastically. Cheres, Max

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trying to understand monad transformers....
by Daryoush Mehrtash 10 Sep '08

10 Sep '08

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Functional references
by Tim Newsham 09 Sep '08

09 Sep '08

I'm playing with functional references and looking for some feedback on a small FRec library I put together: http://www.thenewsh.com/~newsham/FRef.hs Novel (I think) is that the library is applied to some data accesses that are not normally covered by functional references -- ie. extracting words or replacing words in a string or values in an association list. I'm hoping it will provides a useful framework for editing complex values such as data embedded in Base64 cookies in an HTTP header. Tim Newsham http://www.thenewsh.com/~newsham/

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