On 2010-04-14 11:12, John Meacham wrote:
On Wed, Apr 14, 2010 at 02:07:52AM -0700, Ashley Yakeley wrote:
So the facts that (1) f == g (2) f undefined = 6 (3) g undefined = undefined is not a problem?
This is not a problem. f and g represent the same moral function, they are just implemented differently. f is smart enough to know that its argument doesn't matter, so it doesn't need to evaluate it. g waits forever trying to evaluate its function, not knowing it doesn't need it.
Hence they are distinct functions,
They are distinct Haskell functions, but they represent the same moral function.
and should not be determined to be equal by an equality instance.
I don't see why not. It doesn't break the expected Eq laws of reflexivity, symmetry, transitivity. Also, it supports this law: if f == g = True, then f x == g x = True ... in exactly the same way that it supports reflexivity, that is, "fast and loose" ignoring bottom.
A compiler will not transform g into f because said distinction is important and part of the definition of a function.
I'm not seeing this implication as part of the semantics of (==). -- Ashley Yakeley