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49 lines
1.0 KiB
Idris
49 lines
1.0 KiB
Idris
data Maybe a = Nothing
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| Just a
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infixl 1 >>=
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(>>=) : Maybe a -> (a -> Maybe b) -> Maybe b
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(>>=) Nothing k = Nothing
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(>>=) (Just x) k = k x
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data Nat : Type where
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Z : Nat
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S : Nat -> Nat
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plus : Nat -> Nat -> Nat
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plus Z y = y
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plus (S k) y = S (plus k y)
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maybeAdd' : Maybe Nat -> Maybe Nat -> Maybe Nat
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maybeAdd' x y
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= x >>= \x' =>
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y >>= \y' =>
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Just (plus x' y')
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maybeAdd : Maybe Nat -> Maybe Nat -> Maybe Nat
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maybeAdd x y
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= do x' <- x
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y' <- y
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Just (plus x' y')
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data Either : Type -> Type -> Type where
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Left : a -> Either a b
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Right : b -> Either a b
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mEmbed : Maybe a -> Maybe (Either String a)
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mEmbed Nothing = Just (Left "FAIL")
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mEmbed (Just x) = Just (Right x)
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mPatBind : Maybe Nat -> Maybe Nat -> Maybe Nat
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mPatBind x y
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= do Right res <- mEmbed (maybeAdd x y)
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| Left err => Just Z
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Just res
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mLetBind : Maybe Nat -> Maybe Nat -> Maybe Nat
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mLetBind x y
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= do let Just res = maybeAdd x y
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| Nothing => Just Z
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Just res
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