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
(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). _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe