90 lines
2.3 KiB
Elm
90 lines
2.3 KiB
Elm
module Either where
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{-| Represents any data that can take two different types.
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# Type and Constructors
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@docs Either
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# Taking Eithers apart
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@docs either, isLeft, isRight
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# Eithers and Lists
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@docs lefts, rights, partition
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-}
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import List
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{-| Represents any data may take two forms. For example, a user ID may be
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either an `Int` or a `String`.
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This can also be used for error handling `(Either String a)` where
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error messages are stored on the left, and the correct values
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(“right” values) are stored on the right.
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-}
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data Either a b = Left a | Right b
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{-| Apply the first function to a `Left` and the second function to a `Right`.
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This allows the extraction of a value from an `Either`.
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either (\n -> n + 1) sqrt (Left 4) == 5
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either (\n -> n + 1) sqrt (Right 4) == 2
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map : (a -> b) -> Either err a -> Either err b
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map f e = either Left (\x -> Right (f x)) e
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-}
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either : (a -> c) -> (b -> c) -> Either a b -> c
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either f g e = case e of { Left x -> f x ; Right y -> g y }
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{-| True if the value is a `Left`.
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isLeft (Left "Cat") == True
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isLeft (Right 1123) == False
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-}
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isLeft : Either a b -> Bool
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isLeft e = case e of { Left _ -> True ; _ -> False }
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{-| True if the value is a `Right`.
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isRight (Left "Cat") == False
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isRight (Right 1123) == True
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-}
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isRight : Either a b -> Bool
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isRight e = case e of { Right _ -> True ; _ -> False }
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{-| Keep only the values held in `Left` values.
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lefts [Left 3, Right 'a', Left 5, Right "eight"] == [3,5]
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-}
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lefts : [Either a b] -> [a]
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lefts es = List.foldr consLeft [] es
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{-| Keep only the values held in `Right` values.
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rights [Left 3, Right 'a', Left 5, Right 'b'] == ['a','b']
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-}
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rights : [Either a b] -> [b]
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rights es = List.foldr consRight [] es
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{-| Split into two lists, lefts on the left and rights on the right. So we
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have the equivalence: `(partition es == (lefts es, rights es))`
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partition [Left 3, Right 'a', Left 5, Right 'b'] == ([3,5],['a','b'])
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-}
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partition : [Either a b] -> ([a],[b])
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partition es = List.foldr consEither ([],[]) es
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consLeft e vs =
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case e of
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Left v -> v::vs
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Right _ -> vs
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consRight e vs =
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case e of
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Left _ -> vs
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Right v -> v::vs
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consEither e (ls,rs) =
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case e of
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Left l -> (l::ls,rs)
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Right r -> (ls,r::rs)
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