问题
Say I have a mean function defined like so:
mean xs = sum xs / (fromIntegral $ length xs)
but I want it in some tacit form, like this:
mean = sum / (fromIntegral . length)
Is there a built-in Haskell way to do something along these lines without having to build up my own tacit function (something like this):
tacit :: (a -> b -> c) -> (d -> a) -> (d -> b) -> d -> c
tacit a b c i = a (b i) (c i)
In this form, the function looks like this:
mean = tacit (/) sum (fromIntegral . length)
but it feels like there might be a way to avoid having to use an explicit function such as this. I'm just wondering; is there some way to do this that is built in to Haskell?
回答1:
Applicative functors work pretty well here.
import Control.Applicative
mean = (/) <$> sum <*> (fromIntegral . length)
回答2:
Yes, your tacit function is liftM2 in the (->) r monad (liftM2 is in Control.Monad, and the function instance of Monad is in Control.Monad.Instances).
I found this using the pointfree program (you can install it via cabal install pointfree), invoked as:
$ pointfree '\xs -> sum xs / (fromIntegral $ length xs)'
(in a Unix terminal)
来源：https://stackoverflow.com/questions/12061176/tacit-function-composition-in-haskell