पहले नीचे समाधान के लिए कुछ के उपयोग का उदाहरण reduce कहा जाता है (जब तक आप एक बेहतर नाम का सुझाव):
> reduce [(["ab", "c"], "12")] :: [(String, String)]
[("ab","12"),("c","12")]
> reduce [(["ab", "c"], "12")] :: [(Char, Char)]
[('a','1'),('a','2'),('b','1'),('b','2'),('c','1'),('c','2')]
> reduce [("ab", "12"), ("cd", "3")] :: [(Char, Char)]
[('a','1'),('a','2'),('b','1'),('b','2'),('c','3'),('d','3')]
आपका उदाहरण भी इसके साथ हल किया जाता है:
complexReduce :: Monad m => m (m (a, b, m [m (c, m d)])) -> m (a, b, [(c, d)])
complexReduce = reduce
और reduce के कार्यान्वयन :
{-# LANGUAGE FlexibleContexts, FlexibleInstances, IncoherentInstances, MultiParamTypeClasses, UndecidableInstances #-}
import Control.Monad
-- reduce reduces types to simpler types,
-- when the reduction is in one of the following forms:
-- * make a Monad disappear, like join
-- * move a Monad out, like sequence
-- the whole magic of Reduce is all in its instances
class Reduce s d where
reduce :: s -> d
-- Box is used only for DRY in Reduce instance definitions.
-- Without it we, a Reduce instance would need
-- to be tripled for each variable:
-- Once for a pure value, once for a monadic value,
-- and once for a reducable value
newtype Box a = Box { runBox :: a }
instance Monad m => Reduce (Box a) (m a) where
reduce = return . runBox
instance Reduce a b => Reduce (Box a) b where
reduce = reduce . runBox
redBox :: Reduce (Box a) b => a -> b
redBox = reduce . Box
-- we can join
instance (Monad m
, Reduce (Box a) (m b)
) => Reduce (m a) (m b) where
reduce = join . liftM redBox
-- we can sequence
-- * instance isnt "Reduce [a] (m [b])" so type is always reduced,
-- and thus we avoid overlapping instances.
-- * we cant make it general for any Traversable because then
-- the type system wont find the right patterns.
instance (Monad m
, Reduce (Box a) (m b)
) => Reduce (m [a]) (m [b]) where
reduce = join . liftM (sequence . fmap redBox)
instance (Monad m
, Reduce (Box a) (m c)
, Reduce (Box b) (m d)
) => Reduce (a, b) (m (c, d)) where
reduce (a, b) = liftM2 (,) (redBox a) (redBox b)
instance (Monad m
, Reduce (Box a) (m d)
, Reduce (Box b) (m e)
, Reduce (Box c) (m f)
) => Reduce (a, b, c) (m (d, e, f)) where
reduce (a, b, c) =
liftM3 (,,) (redBox a) (redBox b) (redBox c)
उत्कृष्ट उत्तरों के लिए धन्यवाद दोस्तों! आप बिल्कुल सहायक थे! – Jay