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70 lines
1.8 KiB
Idris
70 lines
1.8 KiB
Idris
module Data.Maybe
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%default total
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public export
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isNothing : Maybe a -> Bool
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isNothing Nothing = True
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isNothing (Just _) = False
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public export
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isJust : Maybe a -> Bool
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isJust Nothing = False
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isJust (Just _) = True
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||| Proof that some `Maybe` is actually `Just`
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public export
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data IsJust : Maybe a -> Type where
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ItIsJust : IsJust (Just x)
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export
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Uninhabited (IsJust Nothing) where
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uninhabited ItIsJust impossible
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||| Decide whether a 'Maybe' is 'Just'
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public export
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isItJust : (v : Maybe a) -> Dec (IsJust v)
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isItJust (Just _) = Yes ItIsJust
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isItJust Nothing = No absurd
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||| Convert a `Maybe a` value to an `a` value by providing a default `a` value
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||| in the case that the `Maybe` value is `Nothing`.
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public export
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fromMaybe : (Lazy a) -> Maybe a -> a
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fromMaybe def Nothing = def
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fromMaybe _ (Just j) = j
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||| Returns the `a` value of a `Maybe a` which is proved `Just`.
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public export
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fromJust : (v : Maybe a) -> (0 _ : IsJust v) => a
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fromJust (Just x) = x
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fromJust Nothing impossible
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||| Returns `Just` the given value if the conditional is `True`
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||| and `Nothing` if the conditional is `False`.
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public export
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toMaybe : Bool -> Lazy a -> Maybe a
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toMaybe True j = Just j
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toMaybe False _ = Nothing
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export
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justInjective : Just x = Just y -> x = y
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justInjective Refl = Refl
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||| Convert a `Maybe a` value to an `a` value, using `neutral` in the case
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||| that the `Maybe` value is `Nothing`.
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public export
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lowerMaybe : Monoid a => Maybe a -> a
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lowerMaybe Nothing = neutral
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lowerMaybe (Just x) = x
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||| Returns `Nothing` when applied to `neutral`, and `Just` the value otherwise.
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export
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raiseToMaybe : Monoid a => Eq a => a -> Maybe a
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raiseToMaybe x = if x == neutral then Nothing else Just x
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public export
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filter : (a -> Bool) -> Maybe a -> Maybe a
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filter _ Nothing = Nothing
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filter f (Just x) = toMaybe (f x) x
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