Re: [Haskell-cafe] Proposal: Applicative => Monad: Call for consensus

On Tue, 2011-01-25 at 12:17 +0100, Gábor Lehel wrote:

On Tue, Jan 25, 2011 at 10:20 AM, Ketil Malde wrote:

...

Erik Hesselink writes:

...

...

...

importing Control.Applicative

...

...

main = print =<< liftM2 (+) readLn (return 3) [...] line noise

...

Why not just:

main = print . (+3) =<< readLn

Or using applicative:

print =<< (+3) <$> readLn

?

(Which separates the printing from the addition.)

-k

IMHO, all these proposed solutions just serve to further illustrate the problem. :-)

Even SHE? main = (| print (| readLn + ~5 |) @|) int main () { print ("%d\n", readLn () + 5); } Looks rather similar (except noise of both languages).

Personally I don't mind having to use explicit combinators to interact with monadic values -- forces me to think things through, and all that -- but it's true that having automatic lifting would be convenient, and look less syntaxy.

class Debug m where debug :: Show a => m a -> m a instance Debug (Writer [String]) where debug x = tell (show x) instance Debug IO where debug = print instance (Show w, Show a) => Show (Writer w) where -- Yes I'm using old mtl to illustrate the problem show (Writer (a, w)) = "Writer (" ++ show a ++ ", " ++ show w ++ ")" main = debug (return (return ())) *> return () What does it do? - In case of no lifting it prints "Writer ((), [])" - In case of lifting it may mean "debug <$> return (return ())" which would not print anything Regards