On Mon, Oct 12, 2009 at 10:42 AM, muad
Is it possible to prove correctness of a functions by testing it? I think the tests would have to be constructed by inspecting the shape of the function definition.
not True==False not False==True Done. Tested :-) Less trivially, consider a function of signature swap :: (a,b) -> (b,a) We don't need to test it at all, it can only do one thing, swap its arguments. (Assuming it terminates.) But consider: swap :: (a,a) -> (a,a) If I find that swap (1,2) == (2,1) then I know that swap (x,y)==(y,x) for all types a and b. We only need one test. The reason is that we have a free theorem that says that for all functions, f, of type (a,a) -> (a,a) this holds: f (g a,g b) == let (x,y) = f (a,b) in (g x',g y') For any x and y define g 1 = x g 2 = y Then f(x,y) == f (g 1,g 2) == let (x',y') == f(1,2) in (g x',g y') == let (x',y') == (2,1) in (g x',g y') == (g 2,g 1) == (y,x) In other words, free theorems can turn an infinite amount of testing into a finite test. (Assuming termination.) -- Dan