Haskell point-free

I have this:


ops :: [[Char] -> [Char]]
...
fun :: [Char] -> [[Char]]
fun str = zipWith ($) ops $ words str

How do I write fun in a point-free style? Dropping the argument gives me type errors.

Help me, LaTeX's also making me use faggot quotes!

>>41
That's why I have a strict policy of writing papers only in Microsoft Word.

>>41
Those are ``expert quotes''. Stop diluting this board with your newfag antics.

>>37
I love how everyone ignores you're post because it clashes with there beliefs.

    +1 Anonymous likes this.

>>44
Ihibbit :(

>>43
0/10.
Back to /b/, please.

OP back here, still learning Haskell, and Im stuck at the technical aspects of monads.

liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r
Right?
So, why doesnt liftM elem (Just 'd') (Just "dongz") work? m is Maybe, a1 is Char and r is [Char]. What's wrong?

>>47
elem :: (Eq a) => a -> [a] -> Bool

How is that going to match a1 -> r?

>>47
I chortl'd.
elem :: Eq a => a -> [a] -> Bool
Protip: liftM2

>>48,49
FUCK

>>47
tl;dr: use liftM2

liftM :: Monad m => (a -> b) -> (m a -> m b)
elem :: (a -> ([a] -> Bool))
liftM elem :: Monad m => m a -> m ([a] -> Bool)
liftM elem (Just 'd') :: Monad m => Maybe ([a] -> Bool)

Can you see why that doesn't work? You can't apply that to Just "dongz", because it's not a function: it is a nomad with the function in his pocket. What you really want is liftM2:

liftM2 :: Monad m => (a -> b -> c) -> (m a -> m b -> m c)
liftM2 elem :: Monad m => m a -> m [a] -> m Bool
liftM2 elem (Just 'd') (Just "dongz") :: Maybe Bool

But I won't stop here, because there's another interesting way, namely the Applicative type class. I recommend the the Typeclassopedia¹ for understanding this and its relationship to the others, but here's an introduction.

class Functor f => Applicative f where
    pure :: a -> f a -- For a monad, pure = return

    -- This is why it is called an applicative functor.
    --
    -- For any monad, you can define <*> as such:
    --   f <*> x = liftM2 ($) f x
    --
    -- There are applicatives which are not monads, such as the ZipList
    -- applicative (as far as I know).
    --
    -- E.g. in the Maybe monad this is
    --   Maybe (a -> b) -> (Maybe a -> Maybe b)
    (<*>) :: f (a -> b) -> (f a -> f b)

-- This is already defined in Control.Applicative.
instance Applicative Maybe where
    pure = Just
    Just f <*> Just x = Just (f x)
    _ <*> _ = Nothing

-- Just succ <*> Just 3 = Just 4
-- [succ, (*2)] <*> [7,8] = [8,9,14,16]
-- (zip <*> tail) [1..10] = [(1,2),(2,3),(3,4),(4,5),(5,6),(6,7),(7,8),(8,9),(9,10)]

liftM elem :: Monad m => m a -> m ([a] -> Bool)
liftM elem (Just 'd') :: Maybe ([a] -> Bool)
(<*>) liftM elem (Just 'd') :: Maybe [a] -> Maybe Bool
liftM elem (Just 'd') <*> Just "dongz" :: Maybe Bool

Now, if you define <$> = liftM², you get a nice result:

elem <$> (Just 'd') <*> Just "dongz" :: Maybe Bool

Alternative, but equivalent answers:

pure elem <*> (Just 'd') <*> Just "dongz" :: Maybe Bool
elem `liftM` (Just 'd') `ap` Just "dongz" :: Maybe Bool

¹ http://haskell.org/sitewiki/images/8/85/TMR-Issue13.pdf
² Or <$> = fmap, it's the same since liftM = fmap.

>>47
import Control.Applicative
liftM elem (Just 'd') <*> (Just "dongz")

>>51
Enterprise HASKAL Programmer Detected

Now you're stuck with defining a special operator for every flavour of liftM and have several problems.

>>53
Wait, I thought you meant liftM2 but it was actually a reference. A++ nice bbcode would read again

>>51
it is a nomad with the function in his pocket.
I LOVE YOU! I LOVE YOUR POST! I READ IT FIVE TIMES! KEEP POSTING!

Awarded For: the most intuitive explanation of monads ever
ｇｏｏｄ　ｊｏｂ！

bampu

>>>51

>>51
This leads me to believe that Haskell is currently solving problems that will affect society in ten years or so. I predict there's a major telecom blackout because Erlang fails to do parallel garbage collection. Maybe major bank systems will crash because of a mistake in taking the wrong locks during transactions. Or perhaps an epic fail of Amazon's databases after an incorrect typecast.
Or perhaps an epic fail
/b/tards program in Haskell. I've been living a lie.

>>58
OH MY GOD!

they´re everywhere! what if they´re in my house!

HELP ME /prog/ ! please !

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Cartesian product in Haskell:

(<*>) . ((,) <$>)

Point-free + Applicative = awesome.

Haskell is constipated shit.

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