[Haskell-cafe] lists as instances of a class?

[Spencer Janssen]

The problem isn't with lists specifically, but  with any instance that
applies types (rather than type variables) to a type constructor

>From section 4.3.2 of The Haskell 98 Report: "The type (T u1 ... uk)
must take the form of a type constructor T applied to simple type
variables u1, ... uk".  I've run into this restriction several times
myself, and I'm also curious whether this will change in Haskell'.


Spencer Janssen

On 7/10/06, David Roundy <droundy at darcs.net> wrote:
> (This email is a literate haskell program that fails to compile
> without -fglasgow-exts.)
>
> I'm sure I'm missing something lame here, but can someone tell me why
> we apparently can't declare a list to be an instance of a class in
> Haskell 98? Or is there perhaps some other syntax by which I'd declare
> this instance? If so, is this slated for fixing in Haskell'?
>
> $ ghc Test.lhs
>
> Test.lhs:6:1:
>     Illegal instance declaration for `Vec [Double]'
>         (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 `Vec [Double]'
>
> > module Vec where
>
> > class Vec v where
> >    (.+.) :: v -> v -> v
>
> > instance Vec [Double] where
> >    xs .+. ys = zipWith (+) xs ys
>
> > instance Vec Double where
> >    x .+. y = x + y
>
> feeling very stupid,
> David
>
> P.S. This is with ghc 6.4.1.  And oddly enough, if you make the instance
>
> instance Num a => Vec [a] where
>    xs .+. ys = zipWith (+) xs ys
>
> it works fine, but this strikes me as quite an ugly hack.  I really
> want only Doubles to be instances of this class (which I've
> abbreviated for this email).
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