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. 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". ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************