I think that we are having a terminology confusion here. For me, a
pure function is one that does not operate inside a monad. Eg. ++,
map, etc.
It was at one point my belief that although code in monads could call
pure functions, code in pure functions could not call functions that
operated inside a monad.
I was then introduced to functions such as execState and
unsafePerformIO which appear to prove that my original belief was
false.
Currently I am in a state of deep confusion, but that is OK, because
it means that I am learning something new!
Kevin
On Jul 30, 11:55 am, Anton van Straaten
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
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