Steve Horne wrote:
Heinrich Apfelmus wrote:
Maybe it helps to try to find an example of a function f :: A -> B for some cleverly chosen types A,B that is not pure, i.e. does not return the same values for equal arguments.
[..] For your specific challenge, place that as a left-hand argument in a bind...
f :: Int -> IO Int f = getAnIntFromTheUser >>= \i -> return (i+1)
Well, the value of i isn't decidable until runtime. The value of i+1 is not decidable until runtime. The value of return (i+1) is not decidable until runtime and so on. It can only be partially evaluated at compile-time, but when it is fully evaluated, you get a different IO action returned by f depending on what Int you got from the user.
The function f :: Int -> IO Int f x = getAnIntFromTheUser >>= \i -> return (i+x) is pure according to the common definition of "pure" in the context of purely functional programming. That's because f 42 = f (43-1) = etc. Put differently, the function always returns the same IO action, i.e. the same value (of type IO Int) when given the same parameter. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com