Heinrich Apfelmus wrote:
However, in a sense, one can interpret the pure in purely functional as a property of the type constructor (->) , i.e. a function of type (a -> b) is guaranteed to not have side effects. In this light, ML and Scheme are lacking a very important type: they only have functions with side effects.
Or more precisely, their type system does not distinguish between functions with and without side effects.
that's not actually more precise! Haskell doesn't entirely do that either. consider: f :: Int -> IO Int f x = return (x + 1) No side effects! But other ->IO typed values do yield side-effects. It is exactly as if ML and Scheme contain only the type of (->) composed with IO. (Well, actually Scheme is dynamically typed so it's a bit silly to say so..) (and Scheme allows functions with zero arguments, which are different from its non-side-effecting values.. not sure about ML) -Isaac