Wolfgang Jeltsch
Am Donnerstag, 27. Dezember 2007 16:34 schrieb Cristian Baboi:
I'll have to trust you, because I cannot test it.
let x=(1:x); y=(1:y) in x==y .
I also cannot test this:
let x=(1:x); y=1:1:y in x==y
In these examples, x and y denote the same value but the result of x == y is _|_ (undefined) in both cases. So (==) is not really equality in Haskell but a kind of weak equality: If x doesn’t equal y, x == y is False, but if x equals y, x == y might be True or undefined.
[1..] == [1..] certainly isn't undefined, it always evaluates to True, just like [2..] == [2..]. It just takes ghci eternity to prove, as its runtime system doesn't even think of trying to put the axiom forall n: if x == y then x + n == y + n together with its knowledge about the equal stepping of the lists and short-circuit to True.