- Haskell-Cafe - Haskell.org

GADTs, type classes, existentials
by Mike Hamburg 07 May '07

07 May '07

Hello all, I'm trying to build a variation on Maps which supports a fast "concat-like" operation, for a library I'm writing. I'd rather not re-implement Data.Map, so I'm having a try with GADTs. The relevant part of my source file: ------------------------------------------------------------------------- data MetaMap k v where Map :: Ord k => Map.Map k v -> MetaMap k v Cat :: MetaMap k1 (MetaMap k2 v) -> MetaMap (k1,k2) v {- other, similar constructors -} lookup :: Monad m => k -> MetaMap k v -> m v lookup k (Map m) = Map.lookup k m lookup k (Cat m) = case k of (k1,k2) -> lookup k1 m >>= lookup k2 ------------------------------------------------------------------------- Unfortunately, this doesn't work: Data/MetaMap.hs:45:20: Could not deduce (Ord k) from the context (Monad m) arising from use of `Map.lookup' at Data/MetaMap.hs:45:20-33 Possible fix: add (Ord k) to the type signature(s) for `lookup' In the expression: Map.lookup k m In the definition of `lookup': lookup k (Map m) = Map.lookup k m This error happens because the Ord constraint isn't propagated into the function body. Of course, I can't add Ord k to the type signature for lookup, because one can't deduce Ord k from Ord (k1,k2). Is there a clean way around this error? Thanks for your time, Mike Hamburg

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Mounting haskell.org wiki under WikipediaFS?
by dons＠cse.unsw.edu.au 07 May '07

07 May '07

Anyone tried editing haskell.org's wiki as text, using: http://wikipediafs.sourceforge.net/ "WikipediaFS is a mountable Linux virtual file system that enables you to deal with Wikipedia (or any Mediawiki-based site) articles as if they were real files. It is thus possible to use a real text editor to view and edit articles." Seems like this might make some of the larger edit passes a lot easier. -- Don

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Is Excel a FP language?
by Tony Morris 07 May '07

07 May '07

In a debate I proposed "Excel is a functional language". It was refuted and I'd like to know what some of you clever Haskellers might think :) My opposition proposed (after some "weeding out") that there is a distinction between Excel, the application, the GUI and Excel, the language (which we eventually agreed (I think) manifested itself as a .xls file). Similarly, VB is both a language and a development environment and referring to VB is a potential ambiguity. I disagree with this analogy on the grounds that the very definition of Excel (proposed by Microsoft) makes no distinction. Further, it is impossible to draw a boundary around one and not the other. I also pointed to the paper by Simon Peyton-Jones titled, "Improving the world's most popular functional language: user-defined functions in Excel", which quite clearly refers to Excel as a [popular] functional language. The debate started when I referred to the fact that financial institutions change their functional language from Excel to something like OCaml or Haskell. Of course, there is no doubting that these companies can replace their entire use of Excel with a functional language, which I think is almost enough to fully support my position (emphasis on "almost"). -- Tony Morris http://tmorris.net/

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Silly mail client
by Andrew Coppin 06 May '07

06 May '07

OK, this is hacking me off now... Does ANYBODY know how I can convince Thunderbird to send replies to Haskell Cafe rather than sending them to the original poster? This is really becoming tiresome...

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Topoi, the categorical Analysis of Logic
by Philippe de Rochambeau 06 May '07

06 May '07

Hello, has anybody on this list read Goldblatt's "Topoi, the categorical Analysis of Logic"? Did reading that book make you a better Haskell programmer? If so, how? Cheers, phiroc

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Any Haskellers in St Louis, MO?
by Aditya Siram 06 May '07

06 May '07

Hi there.. Are there any Haskellers in St Louis , MO? Is there an active group here? Want to start one? Thanks... Deech _________________________________________________________________ Exercise your brain! Try Flexicon. http://games.msn.com/en/flexicon/default.htm?icid=flexicon_hmemailtaglineap…

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Re: [Haskell-cafe] Chaos
by Andrew Coppin 06 May '07

06 May '07

Just so people know... The version in Darcs now has strict Colour and Vector constructors. (Makes it run a few percent faster.)

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[IO Int] -> IO [Int]
by Phlex 06 May '07

06 May '07

Hello all, I'm trying to learn haskell, so here's is my first newbie question. I hope this list is appropriate for such help requests. I'm trying to write a function with the signature [IO Int] -> IO [Int] Here is my first attempt : conv :: [IO Int] -> IO [Int] conv l = do val <- (head l) return (val : (conv (tail l))) This does not work as I'm consing an Int to an (IO [Int]). So I tried this : conv2 :: [IO Int] -> IO [Int] conv2 l = do val <- (head l) rest <- (conv2 (tail l)) return (val : rest) That works, but it won't work for infinite lists. How could I achieve the desired result ? Thanks in advance, Sacha

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The Functional Pearls
by dons＠cse.unsw.edu.au 05 May '07

05 May '07

I've created a wiki page collecting the 'functional pearl' papers that have appeared in JFP and ICFP and other places over the last 20 odd years. http://haskell.org/haskellwiki/Research_papers/Functional_pearls Lots of lovely functional programs there. There's also a list on that page of pearls that don't appear to be online. If you know where they live, please add the links! Cheers, Don

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Displaying infered type signature of 'offside' functions
by Georg Sauthoff 05 May '07

05 May '07

Hi, I like the strong static type system of Haskell for various reasons. One reason is, that it makes easier to understand new code. I.e. when I read code I type ':t foo' in ghci/hugs from time to time, to check my own idea of the type signature, if it is not included in the source code. Well, ':t ' doesn't work withwith non-top-level/local functions (or does it?). Thus I searched for a way to get this information for functions, which are defined offside (of the main indentation level). So, what tools do you use to get the inferred type signature of local functions? I first looked at the GHC-API, but I found this comment in http://darcs.haskell.org/ghc/compiler/main/GHC.hs : - NOTE: -- - things that aren't in the output of the typechecker right now: [..] -- - type signatures Hm, is this comment in sync with the current GHC source? Then I looked at the ghc options. '-ddump-types' only dumps the signatures of global functions ... But '-ddump-tc' includes type signatures of offside defined functions. I.e. something like ghc -ddump-tc Foo.lhs 2>&1 \ | awk '/^[A-Z].+lhs/, ! // { print $0; }' \ | awk '/ :: / , /\[\]/ { if (!($0 ~ /\[\]/)) print $0; }' \ | less does the trick. But as the haskell/ghc wiki mentions, the -ddump-* output is optimized for human reading and not for some kind of 'automatic use'. For automatic use one should look at the GHC-API ... Well, I mention this, because I would like to integrate some lookup feature (for type signatures) into vim (if it doesn't exist yet). Best regards Georg Sauthoff

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