3 ms·

I think for those of us that have a C language family background, parallel examples in both C (or C# or Java or Python) and Haskell would be very beneficial! W

• jmillikin 17y ago
  C and Java can't represent monads at all, since they don't support closures. C# and Python can, sort of, but their syntax and object-oriented semantics make it somewhat difficult. I don't know C# well, and Python needs a lot of magic[1] to get reasonable-looking code. Here's a Python example. It's not a direct translation, but is close enough that you should be able to figure out the original. I've included the Haskell code in comments. import abc # class Monad m where # build :: a -> m a # bind :: m a -> (a -> m b) -> m b class Monad (metaclass = abc.ABCMeta): @classmethod def build (cls, x): raise NotImplementedError def bind (self, f): raise NotImplementedError # data Maybe a = Just a | Nothing # deriving (Show) class Maybe (Monad): def __init__ (self, x): self.__x = x def __repr__ (self): if self is Nothing: return "Nothing" return ("Just %r" % self.__x) # instance Monad Maybe where # build x = Just x # # bind Nothing _ = Nothing # bind (Just x) f = f x @classmethod def build (cls, x): return Just (x) def bind (self, f): if self is Nothing: return Nothing return f (self.__x) def Just (x): return Maybe (x) Nothing = Maybe (None) # clamp limit x = if x > limit then Nothing else Just x def clamp (limit, x): if x > limit: return Nothing return Just (x) # multiclamp2 lim1 lim2 x = bind # (clamp lim1 x) # (\x' -> clamp lim2 x') def multiclamp2 (lim1, lim2, x): return clamp (lim1, x).bind (lambda x2: clamp (lim2, x2)) [1] http://www.valuedlessons.com/2008/01/monads-in-python-with-nice-syntax.html http://www.valuedlessons.com/2008/01/monads-in-python-with-n...
  • eru 17y ago
    You don't need closures for using Monads. It `just' makes them usable in practise.