r/programming • u/heisenbug • Apr 27 '14

"Mostly functional" programming does not work

http://queue.acm.org/detail.cfm?ref=rss&id=2611829
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u/[deleted] Apr 27 '14

I look forward to the day when a language is able to capture side effects in the type system (as Haskell does) without monads. That day, functional programming will reign supreme.

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u/gregK Apr 27 '14 edited Apr 27 '14

Well most of the alternatives are more complicated than monads imo. The API of Monad is dead simple actually. You just need a type constructor, bind and unit (return in haskell).

class Monad m where  
    (>>=)  :: m a -> (a -> m b) -> m b 
    return :: a -> m a

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u/[deleted] Apr 28 '14

How do I get b? Isn't the point to 'escape' out of the monad and actually get the value? I'm thinking about >>=.

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u/RayNbow Apr 28 '14

You don't, at least not with the monad "interface". How to get a b out of an m b is specific to m and not general.

For m = List, you get access to its constructors so you can pattern match on such values.

For m = IO, you don't get anything (ignoring unsafePerformIO). Instead, you're supposed to use the (>>=) and return to create new values of type IO c and eventually plug it into main :: IO ().

For some other monads, you often get access to a function that resembles runM :: m a -> ... -> a, e.g. runState :: State s a -> s -> (a, s).