Data.Either.Linear

The Either type represents values with two possibilities: a value of type Either a b is either Left a or Right b.

The Either type is sometimes used to represent a value which is either correct or an error; by convention, the Left constructor is used to hold an error value and the Right constructor is used to hold a correct value (mnemonic: "right" also means "correct").

Examples

The type Either String Int is the type of values which can be either a String or an Int. The Left constructor can be used only on Strings, and the Right constructor can be used only on Ints:

>>> let s = Left "foo" :: Either String Int>>> sLeft "foo">>> let n = Right 3 :: Either String Int>>> nRight 3>>> :type ss :: Either String Int>>> :type nn :: Either String Int

The fmap from our Functor instance will ignore Left values, but will apply the supplied function to values contained in a Right:

>>> let s = Left "foo" :: Either String Int>>> let n = Right 3 :: Either String Int>>> fmap (*2) sLeft "foo">>> fmap (*2) nRight 6

The Monad instance for Either allows us to chain together multiple actions which may fail, and fail overall if any of the individual steps failed. First we'll write a function that can either parse an Int from a Char, or fail.

>>> import Data.Char ( digitToInt, isDigit )>>> :{    let parseEither :: Char -> Either String Int        parseEither c          | isDigit c = Right (digitToInt c)          | otherwise = Left "parse error">>> :}

The following should work, since both '1' and '2' can be parsed as Ints.

>>> :{    let parseMultiple :: Either String Int        parseMultiple = do          x <- parseEither '1'          y <- parseEither '2'          return (x + y)>>> :}

>>> parseMultipleRight 3

But the following should fail overall, since the first operation where we attempt to parse 'm' as an Int will fail:

>>> :{    let parseMultiple :: Either String Int        parseMultiple = do          x <- parseEither 'm'          y <- parseEither '2'          return (x + y)>>> :}

>>> parseMultipleLeft "parse error"

Linearly consume an Either by applying the first linear function on a value constructed with Left and the second linear function on a value constructed with Right.

Get all the left elements in order, and consume the right ones.

Get all the right elements in order, and consume the left ones.

Get the left element of a consumable Either with a default

Get the right element of a consumable Either with a default

Partition and consume a list of Eithers into two lists with all the lefts in one and the rights in the second, in the order they appeared in the initial list.