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 <[EMAIL PROTECTED]> 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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