On Tue, Jul 2, 2013 at 5:06 PM, Ben Wolfson <wolf...@gmail.com> wrote:
> On Tue, Jul 2, 2013 at 4:33 PM, Dragan Djuric <draga...@gmail.com> wrote: > > >> Regarding monadic laws, which one exactly demands that you cannot change >> the monad (not counting the fact that haskell's implementation does it that >> way)? Here are the laws, in Haskell: >> >> return a >>= k = k a >> m >>= return = m >> m >>= (\x -> k x >>= h) = (m >>= k) >>= h >> >> It seems to me the laws are still satisfied if you keep changing monads >> in each bind (if compiler is not restricting it, as is the case with >> Haskell but not with Clojure). >> > > I suppose that may be right: you're supposed to verify that the laws > obtain for a putative monad; they don't come for free just by calling > something a monad. Allowing >>= to have the type m a -> (a -> n b) -> n b > just means that you can't verify that yours obeys the laws. If you get to > choose the type of "return", even the second one is up for grabs! It does > seem somewhat odd to me to advertise the package as being familiar to > Haskellers and to employ category-theoretic concepts and then to be so > blasé about the definition of a monad. (I wonder if you can get away with > this changing of type at all if you define bind in terms of fmap and join). > How are you even supposed to implement bind, in fact? (Never mind reasoning about what's going on in your program if you can't be certain that the code won't switch out the monad you think you're working in, when it does matter to you that you're in a specific one.) Generally for some specific monad you need to do something specific with the return of f. For instance, your seq-bind is implemented in terms of mapcat---meaning that the f that's the second argument of mapcat had better return a seqable. This doesn't work: (mapcat (comp atom inc) '(1 2 3)). -- Ben Wolfson "Human kind has used its intelligence to vary the flavour of drinks, which may be sweet, aromatic, fermented or spirit-based. ... Family and social life also offer numerous other occasions to consume drinks for pleasure." [Larousse, "Drink" entry] -- -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
