Given the definition const x _ = x in the standard prelude, I have always assumed that the value of y was not important when determining the value of (const x y). What is the value of (f [] []) where f is defined by: > f x y = const (show x == show y) w > where > w = ... By equational reasoning, (f [] []) should be True, probably. Of course, [] == [] gives a type error when compiled, so maybe (f [] []) could be undefined or raise a type error. At least for the instances of == that are defined in the Haskell standard prelude, it seems reasonable to assume that expr1 == expr2 should never be False if expr1 and expr2 are equivalent by equational reasoning. If w is defined by > w = x ++ y ++ "" then (f [] []) = True, but if > w = (x ++ [5], y ++ "") then (f [] []) = False. Now in a procedural program, wouldn't we all complain if one expression had the side effect of changing the value of another expression that did not refer to the value of the first. The expression (show x == show y) doesn't refer to w, but this is the expression whose value may be True or False or undefined. Warren Burton Simon Fraser University
