On Tuesday, 2003-08-12, 13:10, Alistair Bayley wrote:
From: Wolfgang Jeltsch [mailto:wolfgang@jeltsch.net]
For example, the function readFile is pure. For a specific string s the expression readFile s always yields the same result: an I/O action which searches for a file named s, reads its content and takes this content as the result of the *action* (not the expression).
What about getChar? This is a function which takes no arguments, yet returns a (potentially) different value each time. I know, I know: it returns an IO action which reads a single char from the terminal and returns it.
Actually, getChar is not a function at all but it *is* the action you describe. ;-)
Is the IO monad the only one (currently) in which you would say that it "returns an action", which is then executed by the runtime system? I would have thought that monads that are not involved in IO (e.g. State) would be pure in the sense that Van Roy was thinking. You wouldn't need to describe functions in them as "returning an action".
State is similar to I/O. For example, take the function gets :: (state -> val) -> State state val. gets doesn't return a value of type val the same way readFile doesn't return a string. Instead, gets returns some kind of "state-transforming action" of type State state val which is internally represented by a function. The difference to the IO type is that you can implement useful (pure) functions with a type like ... -> State state val -> ... -> result where result doesn't involve the State type. So you can "execute" a state transformer as part of an expression evaluation and without special support by the runtime system. State is pure in the sense that pure "execution" is possible. Wolfgang