October 2006 - Haskell-Cafe

YAHT: searchAll5
by Magnus Therning 30 Oct '06

30 Oct '06

I can't get searchAll5[1] in Yet Another Haskell Tutorial to run. Ghci complains that it can't find a MonadPlus that satisfies the required type; it needs a MonadPlus. I suspect this is due to the use of 'mzero' and 'mplus', without making StateT a MonadPlus. My thought for this was to "push" mzero into the inner monad and have 'mplus' pass on to the inner monad: instance Monad m => MonadPlus (StateT state m) where mzero = StateT (\s -> return (s, mzero)) (StateT m1) `mplus` (StateT m2) = StateT (\s -> do (s1, a) <- m1 s (s2, b) <- m2 s1 return (s2, a `mplus` b)) What's above isn't accepted by ghci, and I don't even know for sure that my thought makes sense. /M [1]: http://en.wikibooks.org/wiki/Haskell/YAHT/Monads#Monad_Transformers -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. Finagle's Fifth Law: Always draw your curves, then plot your readings.

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Deriving class instances using DrIFT
by Daniel McAllansmith 30 Oct '06

30 Oct '06

Hi. I'm trying to derive some instances using DrIFT, but it will only work for me when I'm deriving for types in the current file or in the prelude. For example, this works: >module Test where > >{-! for Maybe derive : Haskell2Xml !-} this works: >module Test where > >data Foo = Foo > >{-! for Foo derive : Haskell2Xml !-} but this doesn't: >module Test where > >import Data.Word > >{-! for Word32 derive : Haskell2Xml !-} It fails to load the first import, giving an error as follows: {- Generated by DrIFT (Automatic class derivations for Haskell) -} {-# LINE 1 "Test.hs" #-} DrIFT: can't find module Data.Word Any ideas as to why that is? Thanks Daniel

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monadic performance
by Chad Scherrer 30 Oct '06

30 Oct '06

Hello, I've written some code that does a foldl (or scanl, depending on my mood) kind of thing, and builds a huge tree structure as it goes along. I've been careful to make "insert"s as strict (and eager) as possible, since I know all the pieces will be evaluated eventually anyway. Now I'd like to be able to write various output as I traverse the structure, and I was thinking about using the WriterT monad transformer. For this, I could just make the base monad Identity, or I think it could be nice (from a code elegance perspective) to use State and rewrite the insert code to be State-ful. Sometimes when I have used State before, I have been bitten by laziness. Should I expect a monadic version to take a performance hit? What if I use some SPECIALIZE pragmas or somesuch? Is it more efficient to write one from scratch, or do specific type annotations give me the same thing anyway? Thanks, Chad

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mapAccumL - find max in-sequence subsequence
by jim burton 30 Oct '06

30 Oct '06

I need to find the length of the longest in-sequence section of a list of ints...I am guessing something like mapAccumL is the way but not sure how. --this doesn't work Prelude List> mapAccumL (\x y -> if y==(x+1) then (y, x) else (y,0)) 0 $ sort $ nub [9, 1, 2, 7, 3, 4, 5, 6,5] (9,[0,1,2,3,4,5,6,0]) -- View this message in context: http://www.nabble.com/mapAccumL---find-max-in-sequence-subsequence-tf253170… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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More documentation: languages written in Haskell
by dons＠cse.unsw.edu.au 30 Oct '06

30 Oct '06

I noticed today that although we have a list of most applications written in Haskell, nowhere was there collected a page of perhaps our best use case for Haskell: for implementing compilers and interpreters! So here's a new 'libraries and tools' category page: http://haskell.org/haskellwiki/Libraries_and_tools/Compilers_and_interprete… If you know of a compiler or interpreter written in Haskell, (I think at least a few people on this list have written one or two themselves ... ;) please add it to the list. -- Don

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Documenting subcommunities
by dons＠cse.unsw.edu.au 29 Oct '06

29 Oct '06

As the Haskell community grows and spreads its lambda-tipped tentacles into new domains, I've noticed that some distinct sub-communities are emerging. To try to document and collect information relevant to these groups, some new wiki pages have been created. Alongside the 'traditional' areas of: http://haskell.org/haskellwiki/Haskell_in_research http://haskell.org/haskellwiki/Haskell_in_education There is also a growing number of users of Haskell: http://haskell.org/haskellwiki/Haskell_in_industry http://haskell.org/haskellwiki/Haskell_and_mathematics If you're using Haskell for maths or in industry, and know of some relevant resources, please feel free to add the material to the above pages. Alternatively, if there are subcommunities that are missing a page (hmm, the open source community, perhaps?), then dive in and create a page documenting this. -- Don

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YAHT: Ex 9.2
by Magnus Therning 29 Oct '06

29 Oct '06

I've been slowly making my way through Yet Another Haskell Tutorial. As a first introduction to the language it doesn't seem bad at all. However, I'm wondering about the proposed solution to exercise 9.2. The text itself suggests using "instance Monad (Either String) where", so I arrived at instance Monad (Either String) where return a = Right a fail a = Left a Right a >>= f = f a Left a >>= _ = Left a However, when loading it in ghci 6.6 I get the following error message: Illegal instance declaration for `Monad (Either String)' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Monad (Either String)' The solution, according to the author is instance Monad (Either String) where return x = Right x fail s = Left s Right x >>= f = f x Left s >>= _ = Left s Which results in exactly the same error message. I'm suspecting this is a result of my limited grasp of Haskell's syntax (an area where YAHT is sorely lacking). Any tips/pointers appreciated. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. As far as the laws of mathematics refer to reality, they are not certain, and as far as they are certain, they do not refer to reality. -- Albert Einstein

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Rank N type tutorial?
by Greg Buchholz 29 Oct '06

29 Oct '06

Anyone know of a good source for learning about higher ranked types? I'm not quite sure why this is illegal... > foo :: Integer -> (forall a. Show a => a) > foo 2 = ["foo"] > foo x = x ...while this is just fine... > bar :: Integer -> (forall a. Show a => a->b) -> b > bar 2 k = k ["bar"] > bar x k = k x Thanks, Greg Buchholz

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The Read Class
by Tiago Veloso 29 Oct '06

29 Oct '06

Hi, I am trying to find out how to work with the Read Class for a school project, i need to declare instances of Read for a few data types. My problem is that i do not know how to do it, i mean i do not know how to build a Read instance, i do know about it for the Show Class. Can some one help? Thanks Tiago Veloso

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Partially applied type synonyms in instance heads
by Mathieu Boespflug 29 Oct '06

29 Oct '06

Hi everyone, I'm running into trouble with type synonyms in instance heads and I can't figure out what the resulting error message means. The case I'm considering is as follows: -- hoist a and b to class variables so that instances declarations may -- constrain them. class Map m a b where toAssoc :: m a b -> [(a, b)] fromAssoc :: [(a, b)] -> m a b type Assoc a b = [(a, b)] instance Map Assoc a b where toAssoc = id fromAssoc = id The class Map is used to allow translation from one map type to another (FiniteMap, arrays, etc...) by means of expressing the map as an association list. Useful for defining isomorphisms and so on. Now I'd like to define an association list as itself a trivial instance of Map class, but I cannot do so without wrapping the type of an association list, [(a, b)], behind a type synonym, as I'm not aware of any way of writing a type constructor of kind (* -> * -> *) constructing the type [(a, b)]. But when compiling the above code with GHC I get the following error: Map.hs:9:0: Type synonym `Assoc' should have 2 arguments, but has been given 0 In the instance declaration for `Map Assoc a b' Any idea what this means? Also, is there any other way of declaring an instance for [(a, b)] without using type synonyms? Many thanks, Mathieu

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