CODEWITHSUNDEEP
haskell
piggyback
piggyback

Reputation: 9254

Monads passing variable in haskell

Is there a possible way to rewrite this in a single line through the use of Monads?

input <- getLine
let parsed = reads input :: [(Int,String)]

I could not understand how I can pass input, my attemp would be thinking about lambda notation.

Upvotes: 3

Views: 134

Answers (2)

AndrewC
AndrewC

Reputation: 32455

Another place for fmap ! Use fmap to change a value or values inside something else - in this case, inside getLine, but it's more widely useful.

parsed <- fmap reads getLine :: IO [(Int,String)]

or with import Data.Functor or import Control.Applicative you could write this as

parsed <- reads <$> getLine :: IO [(Int,String)]

The compiler might well be able to deduce you mean Int from elsewhere, in which case you could omit the explicit type signature.

fmap is a very very useful function indeed to know. You could read more in this other answer about fmap where I ended up writing a mini tutorial.

Actually, if you're interested in clean ways of writing one-liners, then you should find out more about Applicative Functors.

Upvotes: 7

Daniel Fischer
Daniel Fischer

Reputation: 183888

Sure, you can use liftM (or fmap, since IO also has a Functor instance) to have it on one line,

parsed <- (reads :: ReadS Int) `liftM` getLine

But I would argue that that rarely increases readability.

Upvotes: 4

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