Isaac, Isaac Dupree wrote:
Heinrich Apfelmus wrote:
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.
All Haskell functions are pure without exception. For example: greet :: String -> IO () greet name = putStrLn $ "Hello, "++name This is a pure function from String to IO (). This function (like all Haskell functions) has no side effects. Its return value of type IO () merely _represents_ an IO action. The runtime system knows how to act on this representation. This also means that there is no such thing in Haskell as marking a function as side-effecting. This distinction may be subtle, but it's important. Steve