57c4b79eb3
Based on very valid observation here (https://groups.google.com/forum/#!topic/elm-discuss/_vvE0OWNKDU) that these docs were bad before.
65 lines
1.6 KiB
Elm
65 lines
1.6 KiB
Elm
module Maybe where
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{-| Represents an optional value. Maybe it is there, maybe it is not.
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# Type and Constructors
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@docs Maybe
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# Taking Maybes apart
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@docs maybe, isJust, isNothing
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# Maybes and Lists
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@docs justs
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-}
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import Basics (not, (.))
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import List (foldr)
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{-| The Maybe datatype. Useful when a computation may or may not
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result in a value (e.g. logarithm is defined only for positive
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numbers).
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-}
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data Maybe a = Just a | Nothing
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{-| Provide a default value and a function to extract the contents of a `Maybe`.
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When given `Nothing` you get the default, when given a `Just` you apply the
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function to the associated value.
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isPositive : Maybe Int -> Bool
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isPositive maybeInt = maybe False (\n -> n > 0) maybeInt
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map : (a -> b) -> Maybe a -> Maybe b
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map f m = maybe Nothing (\x -> Just (f x)) m
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-}
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maybe : b -> (a -> b) -> Maybe a -> b
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maybe b f m = case m of
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Just v -> f v
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Nothing -> b
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{-| Check if a maybe happens to be a `Just`.
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isJust (Just 42) == True
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isJust (Just []) == True
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isJust Nothing == False
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-}
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isJust : Maybe a -> Bool
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isJust = maybe False (\_ -> True)
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{-| Check if constructed with `Nothing`.
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isNothing (Just 42) == False
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isNothing (Just []) == False
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isNothing Nothing == True
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-}
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isNothing : Maybe a -> Bool
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isNothing = not . isJust
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cons : Maybe a -> [a] -> [a]
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cons mx xs = maybe xs (\x -> x :: xs) mx
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{-| Filters out Nothings and extracts the remaining values.
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justs [Just 0, Nothing, Just 5, Just 7] == [0,5,7]
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-}
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justs : [Maybe a] -> [a]
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justs = foldr cons []
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