Re: [Haskell-cafe] Haskell purity and printing

Cristian Baboi wrote:

This is what I "understand" so far ...

Suppose we have these two values: a) \x->x + x b) \x->2 * x Because these to values are equal, all functions definable in Haskell must preserve this.

Oh but you can distinguish these functions. Consider

a x = x+x b x = 2*x

data T = A | B deriving (Show, Eq)

instance Num T where _ + _ = A _ * _ = B

f :: (T -> T) -> T f y = y undefined

main = print (f a) >> print (f b)

which prints A, then B. The key point here is that a and b have type (Num a => a -> a) and while well behaved Num instances certainly can not distinguish a and b, artificial ones like above can. Enjoy, Bertram