Consider the following function:
foo =
[1,2,3] >>=
return . (*2) . (+1)
For better readability and logic, I would like to move my pure functions (*2)
and (+1)
to the left of the return. I could achieve this like this:
infixr 9 <.
(<.) :: (a -> b) -> (b -> c) -> (a -> c)
(<.) f g = g . f
bar =
[1,2,3] >>=
(+1) <.
(*2) <.
return
However, I don't like the right-associativity of (<.)
.
Let's introduce a function leftLift
:
leftLift :: Monad m => (a -> b) -> a -> m b
leftLift f = return . f
baz =
[1,2,3] >>=
leftLift (+1) >>=
leftLift (*2) >>=
return
I quite like this. Another possibility would be to define a variant of bind
:
infixl 1 >>$
(>>$) :: Monad m => m a -> (a -> b) -> m b
(>>$) m f = m >>= return . f
qux =
[1,2,3] >>$
(+1) >>$
(*2) >>=
return
I am not sure whether that is a good idea, since it would not allow me to use do
notation should I want that. leftLift
I can use with do
:
bazDo = do
x <- [1,2,3]
y <- leftLift (+1) x
z <- leftLift (*2) y
return z
I didn't find a function on Hoogle with the signature of leftLift
. Does such a function exist, and, if, what is it called? If not, what should I call it? And what would be the most idiomatic way of doing what I am trying to do?
Edit: Here's a version inspired by @dunlop's answer below:
infixl 4 <&>
(<&>) :: Functor f => f a -> (a -> b) -> f b
(<&>) = flip fmap
blah =
[1,2,3] <&>
(+1) <&>
(*2) >>=
return
I should also add that I was after a bind
-variant, because I wanted to write my code in point-free style. For do
-notation, I guess I don't need to "pretend" that I'm doing anything monadic, so I can use let
s.
Every Monad
is a Functor
(and an Applicative
too). Your (>>$)
is (flipped) fmap
.
GHCi> :t fmap
fmap :: Functor f => (a -> b) -> f a -> f b
GHCi> :t (<$>) -- Infix synonym for 'fmap'
(<$>) -- Infix synonym for 'fmap'
:: Functor f => (a -> b) -> f a -> f b
GHCi> fmap ((*2) . (+1)) [1,2,3]
[4,6,8]
GHCi> (*2) . (+1) <$> ([1,2,3] >>= \x -> [1..x])
[4,4,6,4,6,8]
(By the way, a common name for flipped fmap
is (<&>)
. That is, for instance, what lens calls it.)
If you are using do-notation, there is little reason to use any variant of fmap
explicitly for this kind of transformation. Just switch your <-
monadic bindings for let-bindings:
bazDo = do
x <- [1,2,3]
let y = (+1) x
z = (*2) y
return z
bazDo = do
x <- [1,2,3]
let y = (+1) x
return ((*2) z)