Maybe -base -package
The MaybeT monad transformer adds the ability to fail to a monad.
A sequence of actions succeeds, producing a value, only if all the actions in the sequence are successful. If one fails, the rest of the sequence is skipped and the composite action fails.
For a variant allowing a range of error values, see Control.Monad.Trans.Error.
The parameterizable maybe monad, obtained by composing an arbitrary monad with the Maybe monad.
Computations are actions that may produce a value or fail.
The return function yields a successful computation, while >>= sequences two subcomputations, failing on the first error.
O(n). Map values and collect the Just results.
> let f x = if x == "a" then Just "new a" else Nothing
> mapMaybe f (fromList [(5,"a"), (3,"b")]) == singleton 5 "new a"
O(n). Map values and collect the Just results.
> let f x = if x == "a" then Just "new a" else Nothing
> mapMaybe f (fromList [(5,"a"), (3,"b")]) == singleton 5 "new a"
O(n). Map keys/values and collect the Just results.
> let f k _ = if k < 5 then Just ("key (:) " ++ (show k)) else Nothing
> mapMaybeWithKey f (fromList [(5,"a"), (3,"b")]) == singleton 3 "key (:) 3"
O(n). Map keys/values and collect the Just results.
> let f k _ = if k < 5 then Just ("key (:) " ++ (show k)) else Nothing
> mapMaybeWithKey f (fromList [(5,"a"), (3,"b")]) == singleton 3 "key (:) 3"
optionMaybe p tries to apply parser p. If p fails without consuming input, it return Nothing, otherwise it returns Just the value returned by p.
Tries to generate a value that satisfies a predicate.