[Haskell-cafe] Principal type in Haskell

Jerzy Karczmarczuk jerzy.karczmarczuk at info.unicaen.fr
Thu Jun 22 06:16:51 EDT 2006

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

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