52 lines
1.5 KiB
Elm
52 lines
1.5 KiB
Elm
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module Either where
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import List
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-- Represents any data that can take two different types.
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--
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-- This can also be used for error handling `(Either String a)` where error
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-- messages are stored on the left, and the correct values (“right”
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-- values) are stored on the right.
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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 : (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 : 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 : 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 : [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 : [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 : [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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