Map over the optional payload, preserving failure:
mapMaybe :: (a -> b) -> Maybe a -> Maybe b
mapMaybe _ Nothing = Nothing
mapMaybe f (Just x) = Just (f x)
This is the optional-value mapping operation, equivalent to fmap for Maybe. The question's local name is distinct from the library function Data.Maybe.mapMaybe, which filters and maps an entire list. The algebraic data type keeps absence separate from every ordinary payload value.

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