Data.List.EitherFunctions

Functions involving lists of Either.

Map

partlyMap :: (a -> Maybe b) -> [a] -> [Either a b]

>>> import Prelude (even, show)

>>> partlyMap (\x -> if even x then Just (show x) else Nothing) [1..5]
[Left 1,Right "2",Left 3,Right "4",Left 5]

Group

groupEither :: [Either a b] -> [Either [a] [b]]

>>> groupEither [Left 1, Left 2, Right 'a', Left 3, Right 'b', Right 'c']
[Left [1,2],Right "a",Left [3],Right "bc"]

Partition

partition :: [Either a b] -> ([a], [b])

>>> partition [Left 1, Left 2, Right 'a', Left 3, Right 'b', Right 'c']
([1,2,3],"abc")

Span

spanLeft :: [Either a b] -> ([a], [Either a b])

>>> spanLeft [Left 1, Left 2, Right 'a', Left 3, Right 'b', Right 'c']
([1,2],[Right 'a',Left 3,Right 'b',Right 'c'])

>>> spanLeft [Right 'a', Left 3, Right 'b', Right 'c']
([],[Right 'a',Left 3,Right 'b',Right 'c'])

spanLeft' :: [Either a b] -> ([a], Maybe (b, [Either a b]))

Similar to spanLeft, but preserves a little more information in the return type: if the remainder of the list is non-empty, then it necessarily begins with a Right, and so we can go ahead and unwrap that and return it as a value of type b.

>>> spanLeft' [Left 1, Left 2, Right 'a', Left 3, Right 'b', Right 'c']
([1,2],Just ('a',[Left 3,Right 'b',Right 'c']))

>>> spanLeft' [Right 'a', Left 3, Right 'b', Right 'c']
([],Just ('a',[Left 3,Right 'b',Right 'c']))

>>> spanLeft' [Left 1, Left 2, Left 3]
([1,2,3],Nothing)

spanRight :: [Either a b] -> ([b], [Either a b])

Similar to spanRight, but preserves a little more information in the return type: if the remainder of the list is non-empty, then it necessarily begins with a Left, and so we can go ahead and unwrap that and return it as a value of type a.

>>> spanRight [Left 1, Left 2, Right 'a', Left 3, Right 'b', Right 'c']
("",[Left 1,Left 2,Right 'a',Left 3,Right 'b',Right 'c'])

>>> spanRight [Right 'a', Left 3, Right 'b', Right 'c']
("a",[Left 3,Right 'b',Right 'c'])

spanRight' :: [Either a b] -> ([b], Maybe (a, [Either a b]))

>>> spanRight' [Left 1, Left 2, Right 'a', Left 3, Right 'b', Right 'c']
("",Just (1,[Left 2,Right 'a',Left 3,Right 'b',Right 'c']))

>>> spanRight' [Right 'a', Left 3, Right 'b', Right 'c']
("a",Just (3,[Right 'b',Right 'c']))

>>> spanRight' [Right 'a', Right 'b', Right 'c']
("abc",Nothing)

Lead

leadLeft :: [Either a b] -> ([b], [(a, [b])])

>>> leadLeft [Right 'a', Right 'b', Left 1, Right 'c', Right 'd', Left 2, Right 'e', Right 'f']
("ab",[(1,"cd"),(2,"ef")])

>>> leadLeft [Left 1, Left 2, Right 'a', Left 3, Right 'b', Right 'c']
("",[(1,""),(2,"a"),(3,"bc")])

leadLeft'

Arguments

:: a

Leader to use for the first group in case the list does not begin with a Left.

-> [Either a b]

-> [(a, [b])]

>>> leadLeft' 0 [Right 'a', Right 'b', Left 1, Right 'c', Right 'd', Left 2, Right 'e', Right 'f']
[(0,"ab"),(1,"cd"),(2,"ef")]

>>> leadLeft' 0 [Left 1, Left 2, Right 'a', Left 3, Right 'b', Right 'c']
[(1,""),(2,"a"),(3,"bc")]

leadRight :: [Either a b] -> ([a], [(b, [a])])

>>> leadRight [Left 1, Left 2, Right 'a', Left 3, Left 4, Right 'b', Left 5, Left 6]
([1,2],[('a',[3,4]),('b',[5,6])])

>>> leadRight [Right 'a', Left 3, Left 4, Right 'b', Right 'c', Left 5, Left 6]
([],[('a',[3,4]),('b',[]),('c',[5,6])])

leadRight'

Arguments

:: b

Leader to use for the first group in case the list does not begin with a Right.

-> [Either a b]

-> [(b, [a])]

>>> leadRight' 'z' [Left 1, Left 2, Right 'a', Left 3, Left 4, Right 'b', Left 5, Left 6]
[('z',[1,2]),('a',[3,4]),('b',[5,6])]

>>> leadRight' 'z' [Right 'a', Left 3, Left 4, Right 'b', Right 'c', Left 5, Left 6]
[('a',[3,4]),('b',[]),('c',[5,6])]

Branch

branchLeft :: BranchComparison a -> [Either a b] -> ([b], Forest (a, [b]))

>>> import Prelude

>>> heading level title = Left (level, title)
>>> chapter = heading 1
>>> section = heading 2
>>> p text = Right text

>>> :{
>>> list =
>>> [ p "Copyright"
>>> , p "Preface"
>>> , chapter "Animals"
>>> , p "The kingdom animalia"
>>> , section "Vertebrates"
>>> , p "Cats"
>>> , p "Snakes"
>>> , section "Invertebrates"
>>> , p "Worms"
>>> , p "Jellyfishes"
>>> , chapter "Fungus"
>>> , p "Yeast"
>>> , p "Truffles"
>>> , p "Morels"
>>> ]
>>> :}

>>> import Data.Functor.Contravariant
>>> flipComparison (Comparison f) = Comparison (flip f)
>>> headingComparison = contramap fst (flipComparison defaultComparison)

>>> (frontMatter, mainMatter) = branchLeft headingComparison list

>>> frontMatter
["Copyright","Preface"]

>>> import Data.List
>>> showContent ((_, x), ys) = x ++ ": " ++ intercalate ", " ys

>>> import Data.Tree
>>> putStrLn $ drawForest $ map (fmap showContent) mainMatter
Animals: The kingdom animalia
|
+- Vertebrates: Cats, Snakes
|
`- Invertebrates: Worms, Jellyfishes

Fungus: Yeast, Truffles, Morels

branchRight :: BranchComparison b -> [Either a b] -> ([a], Forest (b, [a]))

Same as branchLeft, but with the types flipped; here, Right is the case that indicates a branch.

type BranchComparison a = Comparison a

The relative significance of branches (greater values are closer to the root).