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.

I mean, I got this;

fun = (zipWith ($) ops).words

But it's ugly with all these parens. Is there a cleaner way?

godmaint i hate haskell

1000 posts

>>2
Remove them, duh.

>>4
That's one post.  Lrn2count.

What >>5 said. Also, can't you just write String to make your code clearer?

1000 posts

>>6
Thats not elitist enough.

This thread has over 8 replies.
You can't reply anymore.

1000 posts

>>3-4,7,9-10
Why so hateful on OP? This thread is surely better than more of the half of the threads on /prog/'s front page. I don't even use Haskell, and I find it baffling that you'd want to prevent actual programming discussion.

Might I suggest you go back to /b/, and take those "sex vs programming" threads there with you too!

As long as you dont return a fucking pointer to the stack... its fine.

Have fun but its better to use pointers.

Each time some fucktarded c++ "programmer" goes arround passing huge classes as value a copy constructor dies.

As long as you dont return a fucking pointer to the stack... its fine.

Have fun but its better to use pointers.

Each time some fucktarded c++ "programmer" goes arround passing huge classes as value a copy constructor dies.

>>2
So fun = zipWith id ops . words

Remember, any time you pass ($) to a function that expects another function, you can substitute id and it will still work

>>12-13
Your dumb.

>>15
What about it?

>>14
don't get it

>>17
The fact that id x = x implies that (id x) y = (x) y, which is just a goofier (and syntactically illegal) way of saying id x y = x y.

FUCK OFF ALL YOU "HELPFUL" ASSHOLEE

>>19
Please. We are civilized here. You clearly meant: "HELPFUL" ANUSE

>>13,18
MIND=BLOWN

>>14
Why would you? ($) is just one character more and makes your intent clearer.

>>2
Function application has a higher precedence than any user-defined operator. Just remove the parens. Write sin x ^ 2, not (sin x) ^ 2. Also, please always use spaces around all binary operators, so that words . lines is consistent with Just . lines (since Just.lines means "the function lines in the module Just")

>>11
LOL!!!!!!

>>18
if it's syntactically illegal then what does it mean?

>>26
I'm no haskeller, but you I don't think you can compose type constructors with functions.

>>27
Type constructors are functions.

>>27
Wait, I fucked up there. Data constructors are functions. But you're still talking crap.

>>29
Yes, you're right. For some reason I thought he was referring to >>24, instead of >>18 and drew my conclusions based on that(with the presumption that it was wrong) Perhaps I should get my eyes tested (and learn Haskell while I'm at it)

>>18
GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
Prelude> let f = (+ 2)
Prelude> f 1
3
Prelude> (id f) 1
3
What do you mean "syntactically illegal"?

>>31
It think it's syntactically illegal to define the function as (f x) y = x y, as f would have to be a data constructor.

Prelude> let (f x) y = x y
Prelude> :t f
f :: (t -> t1) -> t -> t1
Prelude> let (f x) y = x y
Prelude> :t f
f :: (t -> t1) -> t -> t1
Prelude> :t let (f *** g) x y = (f x, g y) in (***)
let (f *** g) x y = (f x, g y) in (***)
  :: (t -> t1) -> (t2 -> t3) -> t -> t2 -> (t1, t3)
Prelude>

>>30
Learn Haskell? Have fun with that, professor...

Did you ever notice that GHCi uses faggot quotes?

>>35
I thought that was the original reason that people used them in Prgaue...

>>36
http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-5.html#%_chap_Temp_2

Educators, generals, dieticians, psychologists, and parents program. Armies, students, and some societies are programmed. An assault on large problems employs a succession of programs, most of which spring into existence en route. These programs are rife with issues that appear to be particular to the problem at hand. To appreciate programming as an intellectual activity in its own right you must turn to computer programming; you must read and write computer programs -- many of them. It doesn't matter much what the programs are about or what applications they serve. What does matter is how well they perform and how smoothly they fit with other programs in the creation of still greater programs. The programmer must seek both perfection of part and adequacy of collection. In this book the use of ``program'' is focused on the creation, execution, and study of programs written in a dialect of Lisp for execution on a digital computer. Using Lisp we restrict or limit not what we may program, but only the notation for our program descriptions.

>>36
IIRC info pages use singular faggot quotes

>>38
Who the fuck uses info pages?

>>39
Emacs users

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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