Andrea Rossato wrote:
Il Fri, Sep 01, 2006 at 07:22:02AM +0200, Tomasz Zielonka ebbe a scrivere:
On Fri, Sep 01, 2006 at 01:13:14AM +0200, Benjamin Franksen wrote:
So getting the value out of the monad is not a pure function (extract :: Monad m => m a -> a). I think I stated that, already, in my previous post.
The only generic way of "extracting" values from a monadic value is the bind operator. The lack of extract function is a feature :-) But now I know that you are not really claiming such a function exists.
I do not understand this discussion, but I'd like to.
Can you please tell me what you are talking about in terms of this example?
Sure. Your definition of bind (>>=):
newtype M a = TypeConstructor {unpack::(a, String)} deriving (Show)
instance Monad M where return a = (TypeConstructor (a,"")) (>>=) m f = TypeConstructor (a1,b++b1) where (a,b) = unpack m (a1,b1) = unpack (f a)
applies f to something that it has extracted from m, via deconstructor unpack, namely a. Thus, your bind implementation must know how to produce an a from its first argument m. In this case it doesn't even have to use an 'environment', since the value a is really packed together with the monadic value m. In fact for this monad you /do/ have an 'extract' function: extract = fst . unpack However, there are monads like IO for which it is not possible to define such a function (and for IO this is indeed the whole point). However, in order to 'run' (i.e. finally actually use) a monadic value that involves an application of bind, the latter would have to supply some argument to its second argument (which is a function), so there must be some way to 'get an a out of' the first argument m. I struggle to make this 'somehow get an a out of the m' more precise. Cheers Ben