Re: [Haskell-cafe] Principal type in Haskell

william kim wrote:

I am confused by the notion of principal type in Haskell with type classes. Consider a simple example:

f x y = [x] == [y]

GHCi yields type f :: (Eq [a]) => a -> a -> Bool.

But according to the paper "Type classes: an exploration of the design space", predicate Eq [a] should be reduced to Eq a.

Does it mean that nobody is entitled to override the standard instance, and, say, declare: instance Eq [a] where x==y = length x == length y ? Jerzy Karczmarczuk