Kevin Jardine wrote:
I think that these are therefore the responses to the original questions:
I am of the understanding that once you into a monad, you cant get out of it?
You can run monadic functions and get pure results.
Some clarifications: First, many monads (including State) are completely pure in a referential transparency sense, so the issue we're discussing is not a question of whether results are pure (in general) but rather whether they're monadic or not, i.e. whether the type of a result is something like "Monad m => m a", or just "a". Second, what I was calling a "monadic function" is a function of type: Monad m => a -> m b These are the functions that bind (>>=) composes. When you apply these functions to a value of type a, you always get a monadic value back of type "m b", because the type says so. These functions therefore *cannot* do anything to "escape the monad", and by the same token, a chain of functions composed with bind, or the equivalent sequence of statements in a 'do' expression, cannot escape the monad. It is only the monadic values (a.k.a. actions) of type "m b" that you can usually "run" using a runner function specific to the monad in question, such as execState (or unsafePerformIO). (Note that as Lyndon Maydwell pointed out, you cannot escape a monad using only Monad type class functions.)
So it looks like in that sense you can "get out of it".
At this level, you can think of a monad like a function (which it often is, in fact). After you've applied a function to a value and got the result, you don't need the function any more. Ditto for a monad, except that for monads, the applying is usually done by a monad-specific runner function. Anton