Haskell knowledge.md

Haskell knowledge

mapMaybe

Prelude Data.Maybe> Data.Maybe.mapMaybe (\i -> if i==1 then Nothing else Just i) [1,2,3]
[2,3]

sequence

sequence :: [Maybe a] -> Maybe [a]

Prelude Data.Maybe> sequence [Just 1, Just 2, Nothing]
Nothing
Prelude Data.Maybe> sequence [Just 1, Just 2]
Just [1,2]

mapM

mapM f is equivalent to sequence . map f:

Prelude Data.Maybe> sequence (map (\a -> if a==1 then Just (a+1) else Nothing) [1,2,3])
Nothing
Prelude Data.Maybe> sequence (map (\a -> if a==1 then Just (a+1) else Just a) [1,2,3])
Just [2,2,3]
Prelude Data.Maybe> mapM (\a -> if a==1 then Just (a+1) else Just a) [1,2,3]
Just [2,2,3]
Prelude Data.Maybe> mapM (\a -> if a==1 then Just (a+1) else Nothing) [1,2,3]
Nothing

stack ghci libraries

stack ghci [--no-load]
Prelude> :set -package hspec
Prelude> import Test.Hspec

ghci multiline input

:set +m

ghci language features

:set -XOverloadedStrings