[Haskell-cafe] Re: Can we come out of a monad?
Anton van Straaten
anton at appsolutions.com
Fri Jul 30 05:55:47 EDT 2010
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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