Fwd: [Haskell-cafe] Class

[Slavomir Kaslev]

---------- Forwarded message ----------
From: Slavomir Kaslev <slavomir.kaslev at gmail.com>
Date: Nov 2, 2006 10:47 AM
Subject: Re: [Haskell-cafe] Class
To: Daniel Fischer <daniel.is.fischer at web.de>


On 11/2/06, Daniel Fischer <daniel.is.fischer at web.de> wrote:
> Am Donnerstag, 2. November 2006 00:06 schrieb Slavomir Kaslev:
> > Hello.
> >
> > I am new to Haskell and I am going through "Haskell: The craft of
> > functional programming". I am trying to grasp haskell's classes and
> > instances, so here is slightly modified code from the book:
> >
> > class Show a => Visible a where
> >     toString :: a -> String
> >     toString = show
> >     size :: a -> Int
> >     size = length . show
> >
> > instance Visible a => Visible [a] where
> >     toString = concat . map toString
> >     size = foldl (+) 0 . map size
> >
> > vSort :: (Visible a, Ord a) => [a] -> String
> > vSort = toString . List.sort
>                      ^^^^^^^
> my ghc complained that List.sort is not in scope, did you import Data.List as
> List?

Yes, I did imported Data.List. My mistake for forgetting to add it in the post.

> >
> > s = vSort [1..3]
> >
> > Unfortunetly in ghc it gives the following type error:
> >     Ambiguous type variable `a' in the constraints:
> >       `Visible a' arising from use of `vSort' at d:/tmp.hs:83:4-8
> >       `Enum a' arising from the arithmetic sequence `1 .. 3' at
> > d:/tmp.hs:83:10-15
> >       `Num a' arising from the literal `3' at d:/tmp.hs:83:14
> >       `Ord a' arising from use of `vSort' at d:/tmp.hs:83:4-8
> >     Probable fix: add a type signature that fixes these type variable(s)
> > Failed, modules loaded: none.
> >
> > As you can see, Visible is nothing more than an adapter to the Show
> > class. How I got thing so far, [1..3] :: (Num a, Enum a) => [a], has a
> > Show instance so does class Num (which 'subclasses' Show). Therefore,
> > I can't see any reason why toString function can't call show from
> > those instances.
>
> First problem: class Visible has no instances yet, so even if you disambiguate
> the type by writing e.g.
> s = vSort [1 :: Int .. 3], you'll get an error message:
> Visible.hs:20:4:
>     No instance for (Visible Int)
>       arising from use of `vSort' at Visible.hs:20:4-8
>     Probable fix: add an instance declaration for (Visible Int)
>     In the definition of `s': s = vSort ([1 :: Int .. 3])

Visible 'subclasses' Show. Doesn't this mean that Visible should be
defined for all types with Show instances? I don't want to write
Visible Int, if ghc has already defined a Show Int instance.

>
> And the second problem:
> The typechecker has no means of determining which type 1 should have.
> By virtue of the fact that numeric literals are overloaded in Haskell, it has
> type Num a => a. The use of enumFromTo adds the Enum a constraint and vSort
> adds Ord and Visible, however there may be many types satisfying these
> constraints and ghc says it's up to you to select one.
> And even if there is only one instance of Visible declared, ghc (nor, as far
> as I know, any other Haskell implementation) won't select that because there
> might, somewhere in a long-forgotten directory, lie a module dormant in which
> another instance satisfying all constraints is declared.
> To sum up: instance selection is left to the user, the compiler does only type
> _inference_. Often that determines which instance fits, but sometimes you
> have to give an expression type signature to tell the compiler what to
> choose.

I agree with second problem you pointed out. Thanks for mentioning it.

--
Slavomir Kaslev


-- 
Slavomir Kaslev