On Thu, Jan 21, 2010 at 11:20 AM, Stephen Blackheath [to
Haskell-Beginners]
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.
Coming from an imperative background, I have found that distinction to be confusing. I like to understand how things work. For example, haskell's lazyness confused me a lot until I heard about thunks. Back to IO. What exactly would be the representation of an IO action, if not an abstract notion ? Hopefully that is optimised out by the compiler. Indeed, If I look at the compiled output of a simple program, it looks to me like the effects are executed within the function, and no special structure is returned. David.