Conor McBride writes:
Jan-Willem Maessen - Sun Labs East wrote:
Tomasz Zielonka wrote:
I found it useful recently, when I needed zip functions for Trees - this way I didn't have to define functions for 3 trees, 4 trees, and so on.
Note also that:
repeat f `zwApply` xs = map f xs
When cooking up my own collection-y things (including splittable supplies, for example), I generally provide fmap and an equivalent of zwApply (a generic repeat is not quite so simple or useful). It's a nice little idiom, and a recommend it highly. ^^^^^
Funny you should choose that word:
http://www.mail-archive.com/haskell@haskell.org/msg15073.html
saves me banging the same old drum.
Now that I think about it, you can generalize the trick I mentioned elsewhere to work over any Idiom/Sequence/more-than-a-functor-not-yet-a-monad thingy.
class Sequence f where unit :: a -> f a (<*>) :: f (a -> b) -> f a -> f b
liftN :: Sequence f => (f a -> b) -> a -> b liftN d f = d (unit f)
suc :: Sequence f => (f b -> c) -> f (a -> b) -> f a -> c suc d f x = d (f <*> x)
zero = id
one :: Sequence f => f (a -> b) -> f a -> f b one = suc zero
two :: Sequence f => f (a -> b -> c) -> f a -> f b -> f c two = suc one
newtype L1 a = L1 { unL1 :: [a] } newtype L2 a = L2 { unL2 :: [a] }
instance Idiom L1 where unit x = L1 [x] L1 fs <*> L1 xs = L1 [ f x | f <- fs, x <- xs ]
instance Idiom L2 where unit x = L2 (repeat x) L2 fs <*> L2 xs = L2 (zipWith ($) fs xs)
*Main> unL1 $ liftN two (,) (L1 [1,2,3]) (L1 "abc")
[(1,'a'),(1,'b'),(1,'c'),(2,'a'),(2,'b'),(2,'c'),(3,'a'),(3,'b'),(3,'c')
]
*Main> unL2 $ liftN two (,) (L2 [1,2,3]) (L2 "abc")
[(1,'a'),(2,'b'),(3,'c')]
--
David Menendez