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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