Вот определение простой, казалось бы, функции на Haskell
routine :: Int -> Maybe Int
routine x = do
y1 <- Just 1
y2 <- Just 2
return (y1 + x + y2)
routine от 3 будет Just 6
> routine 3
Just 6
Слегка изменим функцию
routine :: Int -> Maybe Int
routine x = do
y1 <- Just 1
_ <- Nothing
y2 <- Just 2
(
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