TJ <[EMAIL PROTECTED]> said: > In the wikibook article here: > http://en.wikibooks.org/wiki/Haskell/Understanding_monads, which > really does an excellent job explaining things (nuclear waste > woohoo!), I am stuck at the following code snippet: > > container >>= fn = > \st -> let (a, st2) = container st > container2 = fn a > in container2 st2 > > What stumps me is that (>>=) is supposed to return a container, but if > we return (container2 st2) as in the code, then what we're really > returning is the contents of the container!
Note the lambda abstraction (\st -> ...) at the beginning of the definition. This means that (container >>= fn) returns a *function* that maps an input state to the result of (container2 st2). It doesn't return the result of (container st2) directly. > So what would happen if we do this: > > nuclearWasteInContainer >>= processTheWaste >>= thoroughProcessTheWaste > > It seems to me that the second (>>=) in the above expression would > have the arguments (nuclearWaste) and (nuclearWasteProcessor), when > what it really expects are (Container nuclearWaste) and > (nuclearWasteProcessor). So isn't something wrong with the definition > of (>>=) above? Or am I missing something? > > (I know the article says that the type for their supposed State monad > at that point is not actually correct, and will be clarified further > on, but that seems to be irrelevant to my question.) > > TJ the forever noobie. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
