[Haskell-cafe] Typeclass question
I'm sure there's a way to do this, but it's escaping me at present. I want to do something like this: data Foo = Bar a = Foo a Bool ... That is, I want to create a new type, Foo, whose constructor takes both a Boolean and a value of a type of class Bar. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Typeclass question
On Sat, Dec 27, 2008 at 2:24 PM, Andrew Wagner wagner.and...@gmail.com wrote:
I'm sure there's a way to do this, but it's escaping me at present. I want
to do something like this:
data Foo = Bar a = Foo a Bool ...
That is, I want to create a new type, Foo, whose constructor takes both a
Boolean and a value of a type of class Bar.
Try this:
{-# LANGUAGE ExistentialQuantification #-}
data Foo = forall a. Bar a = Foo a Bool
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
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Re: [Haskell-cafe] Typeclass question
On Sat, Dec 27, 2008 at 12:44 PM, David Menendez d...@zednenem.com wrote:
On Sat, Dec 27, 2008 at 2:24 PM, Andrew Wagner wagner.and...@gmail.com
wrote:
I'm sure there's a way to do this, but it's escaping me at present. I
want
to do something like this:
data Foo = Bar a = Foo a Bool ...
That is, I want to create a new type, Foo, whose constructor takes both a
Boolean and a value of a type of class Bar.
Try this:
{-# LANGUAGE ExistentialQuantification #-}
data Foo = forall a. Bar a = Foo a Bool
Though for existentials, I find GADT more natural (actually I find GADT more
natural in most cases):
data Foo where
Foo :: Bar a = a - Foo
Luke
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Re: [Haskell-cafe] Typeclass question
Seems like you want an existential type: data Foo = forall a. Bar a = Foo a Bool On 27 Dec 2008, at 22:24, Andrew Wagner wrote: I'm sure there's a way to do this, but it's escaping me at present. I want to do something like this: data Foo = Bar a = Foo a Bool ... That is, I want to create a new type, Foo, whose constructor takes both a Boolean and a value of a type of class Bar. ___ 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] Typeclass question
There is a disadvantage in GADTs. They don't work in Hugs.
On 27 Dec 2008, at 22:49, Luke Palmer wrote:
On Sat, Dec 27, 2008 at 12:44 PM, David Menendez d...@zednenem.com
wrote:
On Sat, Dec 27, 2008 at 2:24 PM, Andrew Wagner wagner.and...@gmail.com
wrote:
I'm sure there's a way to do this, but it's escaping me at
present. I want
to do something like this:
data Foo = Bar a = Foo a Bool ...
That is, I want to create a new type, Foo, whose constructor takes
both a
Boolean and a value of a type of class Bar.
Try this:
{-# LANGUAGE ExistentialQuantification #-}
data Foo = forall a. Bar a = Foo a Bool
Though for existentials, I find GADT more natural (actually I find
GADT more natural in most cases):
data Foo where
Foo :: Bar a = a - Foo
Luke
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Re: [Haskell-cafe] Typeclass question
Hmm, I actually simplified my problem too much. What I actually want is:
data Foo a = forall a. Bar a = Foo a Bool
...except I want the 'a' on the left to match the 'a' on the right, so
that you can only construct values out of values of the parameterized
type, which also must be of the Bar class.
On Dec 27, 2008, at 1:44 PM, David Menendez d...@zednenem.com wrote:
On Sat, Dec 27, 2008 at 2:24 PM, Andrew Wagner wagner.and...@gmail.com
wrote:
I'm sure there's a way to do this, but it's escaping me at present.
I want
to do something like this:
data Foo = Bar a = Foo a Bool ...
That is, I want to create a new type, Foo, whose constructor takes
both a
Boolean and a value of a type of class Bar.
Try this:
{-# LANGUAGE ExistentialQuantification #-}
data Foo = forall a. Bar a = Foo a Bool
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
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Re: [Haskell-cafe] Typeclass question
Oh! That's much simplier:
data Bar a = Foo a = Foo a Bool
On 27 Dec 2008, at 23:09, Andrew Wagner wrote:
Hmm, I actually simplified my problem too much. What I actually want
is:
data Foo a = forall a. Bar a = Foo a Bool
...except I want the 'a' on the left to match the 'a' on the right,
so that you can only construct values out of values of the
parameterized type, which also must be of the Bar class.
On Dec 27, 2008, at 1:44 PM, David Menendez d...@zednenem.com
wrote:
On Sat, Dec 27, 2008 at 2:24 PM, Andrew Wagner wagner.and...@gmail.com
wrote:
I'm sure there's a way to do this, but it's escaping me at
present. I want
to do something like this:
data Foo = Bar a = Foo a Bool ...
That is, I want to create a new type, Foo, whose constructor takes
both a
Boolean and a value of a type of class Bar.
Try this:
{-# LANGUAGE ExistentialQuantification #-}
data Foo = forall a. Bar a = Foo a Bool
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
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Re: [Haskell-cafe] Typeclass question
Andrew Wagner wrote: Hmm, I actually simplified my problem too much. What I actually want is: data Foo a = forall a. Bar a = Foo a Bool ...except I want the 'a' on the left to match the 'a' on the right, so that you can only construct values out of values of the parameterized type, which also must be of the Bar class. Well, you can ignore my previous contribution to this thread anyway. I failed to see the numerous other responses suggesting the same thing. I recommend against what you are wanting to do. It is probably nicer to have something like this: data Foo a = Foo a Bool -- don't export this foo :: Bar a = a - Bool - Foo a -- export this foo = Foo You can also use GHC's new ViewPatterns extension if you would still like to be able to pattern match on Foo values in other modules and don't mind being restricted to more recent versions of GHC. - Jake ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Typeclass question
On Sat, Dec 27, 2008 at 3:09 PM, Andrew Wagner wagner.and...@gmail.com wrote:
Hmm, I actually simplified my problem too much. What I actually want is:
data Foo a = forall a. Bar a = Foo a Bool
...except I want the 'a' on the left to match the 'a' on the right, so that
you can only construct values out of values of the parameterized type, which
also must be of the Bar class.
Something like this?
{-# LANGUAGE ExistentialQuantification #-}
class Bar a where
bar :: a - a
data Foo a = (Bar a) = Foo a Bool
baz :: Foo a - a
baz (Foo a _) = bar a
This works fine for me with GHC 6.8, but I'd expect Hugs and earlier
versions of GHC to reject it.
See section 8.4.5 of the GHC manual.
http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#gadt-style
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
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[Haskell-cafe] typeclass question
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] typeclass question
| 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. Sounds like a functional dependency (class ToReadable a b | a - b ) Also, MPTC take me out of the world of haskell 98, which I was trying to avoid. Why. Everyone does it, and MPTC will be in Haskell-Prime (but FD may be not) http://hackage.haskell.org/trac/haskell-prime/wiki/MultiParamTypeClassesDilemma J.W. signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] typeclass question
On Thu, 11 Sep 2008, Tim Docker wrote: 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: I think that type classes are not for keystroke reduction, but for writing generic algorithms. If there is no algorithm that becomes more generic by the use of a type class, I would not use a type class, but stick to labelToRenderable and friends. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] typeclass question
Also, MPTC take me out of the world of haskell 98, which I was trying to avoid. Why. Everyone does it, Well, it's a library that others might use, so I would prefer to avoid using language extensions, especially functional deps which I don't understand, and which seem to have an uncertain future. Tim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] typeclass question
(Henning:) If there is no algorithm that becomes more generic by the use of a type class, I would not use a type class, but stick to labelToRenderable [...] The problem with function names as labelToRenderable is that they have type information as part of the name. Consistency of that information cannot be enforced by the language, which makes it dangerous. If you want type information (e.g. to resolve overloading, for the compiler - and for the reader!) use the language, and write a type annotation. J.W. signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] typeclass question
Well, it's a library that others might use, so I would prefer to avoid using language extensions, especially functional deps which I don't understand, and which seem to have an uncertain future. I think there will be a storm of protest if support for this simple shape of dependencies ( ... | a - b ) would be dropped from the major Haskell implementations. (There used to be some status page on what compiler supports what extension, anyone know the current location for that?) J.W. signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] typeclass question
On Thu, 2008-09-11 at 13:23 +0200, Johannes Waldmann wrote: Well, it's a library that others might use, so I would prefer to avoid using language extensions, especially functional deps which I don't understand, and which seem to have an uncertain future. I think there will be a storm of protest if support for this simple shape of dependencies ( ... | a - b ) would be dropped from the major Haskell implementations. For backwards-compatibility reasons, or because you think they're better than type families? Personally, I am quite enthusiastic about type families, although that is influenced by a (somewhat abandoned) project of mine that ended up with a 3 parameter type class (5 for the sub-class created for quickCheck support) with one-to-one relations every way. And multiple `global' variables implemented with dynamic parameters (they would have needed to be thread-local, eventually, anyway) with types parameterized on the afore-mentioned 3 parameters plus two more to allow the choice between ST and STM. When you get types like this: -- | Wait for another thread to change the buffer contents. displayWaitRedisplay :: (Buffer b d mk, ?currentBuffer :: BufferState b d mk STM TVar, ?currentWindow :: Window b d mk c STM TVar) = b TVar - STM () types like this: -- | Wait for another thread to change the buffer contents. displayWaitRedisplay :: (Buffer b, ?currentBuffer :: BufferState b STM, ?currentWindow :: Window b c STM) = b TVar - STM () look like heaven. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] typeclass question
if support for this simple shape of dependencies ( ... | a - b ) ... For backwards-compatibility reasons, Yes. or because you think they're better than type families? Don't know (haven't used them). Concrete example: I have this class Partial p i b | p i - b http://dfa.imn.htwk-leipzig.de/cgi-bin/cvsweb/tool/src/Challenger/Partial.hs?rev=1.28 What would type families buy me here? In my code, this class has tons of instances (I count 80). How much would I need to change them? Could this be automated? Thanks - J.W. signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] typeclass question
On Thu, 2008-09-11 at 18:34 +0200, Johannes Waldmann wrote: if support for this simple shape of dependencies ( ... | a - b ) ... For backwards-compatibility reasons, Yes. This gives point, then, to my concerns about letting Haskell become a practical language. At some point, production systems always seem to be end-of-lifed by backwards compatibility. or because you think they're better than type families? Don't know (haven't used them). Concrete example: I have this class Partial p i b | p i - b http://dfa.imn.htwk-leipzig.de/cgi-bin/cvsweb/tool/src/Challenger/Partial.hs?rev=1.28 What would type families buy me here? I can't figure out what b is. I could, of course, argue that it would force you to come up with a name for `b', so people reading the code could understand what it does. In my code, this class has tons of instances (I count 80). How much would I need to change them? instance Partial p i b where = instance Partial p i type B p i = b And type signatures involving Partial would have to change. Could this be automated? To a certain extent. Finding the places that need to change could be automated, which is always the first step :) jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] typeclass question
Hi Tim, Your example seems like a perfect fit for functional dependencies. On Thu, Sep 11, 2008 at 3:36 AM, Tim Docker [EMAIL PROTECTED] wrote: Well, it's a library that others might use, so I would prefer to avoid using language extensions, especially functional deps which I don't understand, and which seem to have an uncertain future. I completely agree with you that it is a good idea to stick to Haskell'98 when you can, especially in library code, so you'll have to decide if you really want to use the class. As for not understanding functional dependencies, it sounds like you are not giving yourself enough credit. Your previous comment basically contains the definition of a functional dependency: | 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. This is all there is to a fun. dep., from a programmer's perspective---it adds a constraint on the instances one can declare for a given multi-parameter type class. Hope this helps, -Iavor ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Typeclass question
I've got what's probably a beginner's question, but I'm out of ideas for solving it. It looks like it tripped me up in Write Yourself a Scheme... too, since the code there seems like it's arranged so I never ran into it... I've got a couple functions: binop :: (AmbrosiaData - AmbrosiaData - AmbrosiaM AmbrosiaData) - AmbrosiaM () binop operator = do second - popStack first - popStack result - operator first second pushStack result numNumOp :: Num a = (a - a - a) - AmbrosiaData - AmbrosiaData - AmbrosiaM AmbrosiaData numNumOp op (Number val1) (Number val2) = return $ Number $ op val1 val2 numNumOp op (Float val1) (Float val2) = return $ Float $ op val1 val2 numNumOp op _ _ = throwError . TypeMismatch $ Number the intention being that I can define primitive operations for an interpreter with (+, binop $ numNumOp (+)). In the Number case, val1 val2 are Integers, while in the Float case, they're Floats. However, when I try to compile, I get: ./Primitives.hs:82: Cannot unify the type-signature variable `a' with the type `Integer' Expected type: Integer Inferred type: a In the application `op val1 val2' In the second argument of `($)', namely `op val1 val2' This makes no sense to me, since Integer is an instance of Num and op is defined as type Num a = a - a - a. Shouldn't the typechecker be able to instantiate 'a' with Integer and give the resulting type Integer - Integer - Integer, and then pass the final Integer to Number? Is my understanding of typeclasses off somehow? Any help would be appreciated. Regards, Jonathan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Typeclass question
On Sun, Dec 03, 2006 at 12:26:30PM -0500, Jonathan Tang wrote: I've got what's probably a beginner's question, but I'm out of ideas for solving it. It looks like it tripped me up in Write Yourself a Scheme... too, since the code there seems like it's arranged so I never ran into it... I've got a couple functions: binop :: (AmbrosiaData - AmbrosiaData - AmbrosiaM AmbrosiaData) - AmbrosiaM () binop operator = do second - popStack first - popStack result - operator first second pushStack result numNumOp :: Num a = (a - a - a) - AmbrosiaData - AmbrosiaData - AmbrosiaM AmbrosiaData numNumOp op (Number val1) (Number val2) = return $ Number $ op val1 val2 numNumOp op (Float val1) (Float val2) = return $ Float $ op val1 val2 numNumOp op _ _ = throwError . TypeMismatch $ Number When you say Num a = (a - a - a) - ..., what that means is: numNumOp :: forall a. Num a = ((a - a - a) - AmbrosiaData - AmbrosiaData - AmbrosiaM AmbrosiaData) That is, what you pass must be a single type. It looks like what you want is: numNumOp :: (forall a. Num a = (a - a - a)) - AmbrosiaData - AmbrosiaData - AmbrosiaM AmbrosiaData Notice that forall is in the argument - thus numNumOp must recieve a polymorphic argument. This is called rank-2 polymorphism, and numNumOp is a rank-2 polymorphic function. Unfortunately, it turns out that allowing foralls inside function arguments makes typechecking much harder, in general impossible. GHC allows it but requires explicit type annotations on all rank-2 polymorphic functions. Haskell98 forbids rank-2 polymorphism altogether (and explicit foralls, for that matter). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
