Yet Another Monad Tutorial

Bayley, Alistair
Tue, 12 Aug 2003 12:10:24 +0100

> From: Wolfgang Jeltsch []
> 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".

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