Re: [Haskell-cafe] Partially applied type synonyms in instance heads
Haskell's type system does not directly allow eta expansion, i.e. instance Map (\x y - [(x, y)]) a b where ... is not legal. The straight-forward way to tackle this is with a newtype declaration, i.e. newtype Assoc a b = Assoc [(a, b)] instance Map Assoc a b where ... works as expected. There's an avenue for you to continue with if you like. In regards to what GHC was trying to tell you with its error message: type synonyms (like your Assoc) can never be partially applied. In your instance, Assoc is not applied to a and b (don't let the adjaceny fool you), it's just sitting by itself... the most lonely version of partial application. Hence it croaks. Hope that helps, Nick ps - I've never pondered why it is that type synonyms can't be partially applied. I'm sure someone will pipe up with the answer. On 10/28/06, Mathieu Boespflug [EMAIL PROTECTED] wrote: 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mapAccumL - find max in-sequence subsequence
Sebastian Sylvan-2 wrote: I'm not sure I completely understand what you want, and if it needs to be cute (i.e. some clever one liner usage of a library function). But here's my get-the-job-done-solution (assuming I understood what you want): import Data.List import Data.Ord longestInSequence :: (Enum a) = [a] - Int longestInSequence = maximum . map (length . takeInSeq) . tails takeInSeq [] = [] takeInSeq [x] = [x] takeInSeq (x:y:xs) | fromEnum (succ x) == fromEnum y = x : takeInSeq (y:xs) | otherwise = takeInSeq (x:xs) /S Thanks, that's what I was looking for - and it doesn't need to be 'cute'! -- View this message in context: http://www.nabble.com/mapAccumL---find-max-in-sequence-subsequence-tf2531704.html#a7059817 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The Read Class
Yes pleaseI wold like some examples (no importance how interesting) Maybe i can work my way from there.On 28 Oct 2006, at 23:32, Daniel Fischer wrote:Hi,probably the best way is to write a parser using ReadP/ReadPrec and theninstance Read SomeType where readPrec = yourParserIf you wish, I could send you a few examples (not very interesting, but they might help getting you going).Cheers,DanielAm Samstag, 28. Oktober 2006 23:49 schrieb Tiago Veloso: Hi,I am trying to find out how to work with the Read Class for a schoolproject, 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 knowhow to build a Read instance, i do know about it for the Show Class.Can some one help?ThanksTiago Veloso Tiago Veloso ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Rank N type tutorial?
David House [EMAIL PROTECTED] writes: On 27/10/06, Greg Buchholz [EMAIL PROTECTED] wrote: I thought exists was spelled forall in Haskell? There is some confusion here, It's the notation for data declarations that causes the confusion. To rearrange your text a bit: So, for example: [forall a. a] -- the list whose elements all have the type forall a. a, i.e. a list of bottom. list of bottoms. forall a. [a] -- all elements have some (the same) type a, which can be assumed to be any type at all by a callee. and consequently also must be a list of bottoms. This: data T = forall a. MkT a is where the confusion comes in. This means that: MkT :: forall a. a - T If the standard syntax were data T where MkT :: forall a. a - T (as for GADTs), there's be less of a problem. (it's still misleading, because T looks like it has no variables in it, which might lead one to conclude that MkT throws away its argument) Equally, the natural way of writing it with the old syntax would be: data T = MkT (exists a. a) ie a T is the constructor MkT applied to some unknown type. So people seeing the first form tend to think it means this last. -- which is what you said, but I think this highlights the source of confusion better. -- Jón Fairbairn [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] YAHT: Ex 9.2
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) [EMAIL PROTECTED] Jabber: [EMAIL PROTECTED] 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 pgpuAU81gQzEb.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] YAHT: Ex 9.2
Quick fix: add this line to the top of your file
{-# OPTIONS -fglasgow-exts #-}
Having String instead of a free type variable is beyond the basic
limitations of Haskell's type classses.
However, enabling the very common Glasgow extensions expands the rules
to admit your definition. If you for some reason do not wish to add
the Glasgow extensions, the following also works.
newtype EitherStr a = ES (Either String a)
instance Monad EitherStr where
return a = ES (Right a)
fail a = ES (Left undefined)
ES (Right a) = f = f a
ES (Left a) = _ = ES (Left a)
On 10/29/06, Magnus Therning [EMAIL PROTECTED] wrote:
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)
[EMAIL PROTECTED] Jabber: [EMAIL PROTECTED]
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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[Haskell-cafe] Deriving class instances using DrIFT
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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[Haskell-cafe] More documentation: how to create a Haskell project
There's been a bit of discussion on irc, lists and privately about about documenting publically the best practice for creating a new Haskell project -- be that a library or an application. Some advice is now available here: http://haskell.org/haskellwiki/How_to_write_a_Haskell_program Suggestions include: * use darcs * use cabal But we could do with more information on: * where to host a haskell project * integration of testsuites * anything about Hackage? * portability issues? So have a look at the page and make some suggestions, so it will be easier for newcomers in the future to create and contribute new projects, that will be readily accesible, useable and adopted by the community. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] More documentation: how to create a Haskell project
Tony Morris http://tmorris.net/ Donald Bruce Stewart wrote: There's been a bit of discussion on irc, lists and privately about about documenting publically the best practice for creating a new Haskell project -- be that a library or an application. Agreed. Some advice is now available here: http://haskell.org/haskellwiki/How_to_write_a_Haskell_program Suggestions include: * use darcs * use cabal But we could do with more information on: * where to host a haskell project * integration of testsuites * anything about Hackage? * portability issues? So have a look at the page and make some suggestions, so it will be easier for newcomers in the future to create and contribute new projects, that will be readily accesible, useable and adopted by the community. My suggestion: The steps of reasoning as you start with a blank directory. For example: 1) We are about to write a package called foo using Haskell so we start with setting up a foo.cabal file (assuming you have answer what is cabal?) and Setup.hs. 2) Then we create a src directory and set hs-source-dirs: src 3) Annotate our source with these comments for haddock. 4) If you want links to base libraries in your haddock output, do such and such (how do you do that anyway?) 5) If this and that, then so and so... ... Finally, there should be a bunch of files that represent the artifacts of following these steps from a blank directory (hnop is a bit scant). -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Documenting subcommunities
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Deriving class instances using DrIFT
On Mon, Oct 30, 2006 at 01:24:58PM +1300, Daniel McAllansmith wrote: 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. Since DrIFT can only understand haskell source code, it can't derive instances for anything you don't have the original source to. such as things in the pre-compiled libraries that come with ghc. you will likely have to write out those instances by hand. Another possibility is that you could replicate just the data declarations by hand, and use DrIFT -r to just spit out the derivations and put those in a file on their own. a long time ago DrIFT used to be able to read ghc 'hi' files, but that has not worked in a while, it is possible not all the documentation has been updated to reflect that. John -- John Meacham - ⑆repetae.net⑆john⑈ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: function types as instances of Num
The problem seems equivalent to the following:
http://pobox.com/~oleg/ftp/Haskell/typecast.html#local-fd
That is, the inferred type is too general to chose the appropriate
instance. The solution is also the same: either add type annotations
to restrict the inferred type (and so make it _match_ the desired
instance) -- or use local type inference. Here's the working code of
your Haskell embedding of the Forth-like stack language:
{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-undecidable-instances #-}
main = print $ test ()
test = square . 4
dup (a, b) = (a, (a, b))
mult (a, (b, c)) = (b*a, c)
square = mult . dup
instance Num c = Eq (a - (c, b))
instance Num c = Show (a - (c, b))
instance (Num c, TypeCast a b) = Num (a - (c, b)) where
fromInteger val stack = (fromInteger val,typeCast stack)
[TypeCast elided]
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[Haskell-cafe] More documentation: languages written in Haskell
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_interpreters 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: More documentation: languages written in Haskell
Donald == Donald Bruce Stewart [EMAIL PROTECTED] writes: Donald I noticed today that although we have a list of most Donald applications written in Haskell, nowhere was there Donald collected a page of perhaps our best use case for Haskell: Donald for implementing compilers and interpreters! Donald So here's a new 'libraries and tools' category page: Donald http://haskell.org/haskellwiki/Libraries_and_tools/Compilers_and_interpreters Donald If you know of a compiler or interpreter written in Donald Haskell, (I think at least a few people on this list have Donald written one or two themselves ... ;) please add it to the Donald list. I have no login at haskell.org, so I post here. There is a Curry compiler implemented in Haskell: The Münster Curry Compiler (http://danae.uni-muenster.de/~lux/curry/). -- WBR, Max Vasin. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
