The Maybe Monad

monad.types.maybe - The Maybe Monad.

monad.types.maybe.Just

alias of monad.types.maybe.Maybe

class monad.types.maybe.Maybe(value)

Bases: monad.types.monadplus.MonadPlus, monad.mixins.ContextManager, monad.mixins.Ord

The Maybe Monad.

Representing values/computations that may fail.

>>> Just(42)
Just(42)
>>> Just([1, 2, 3])
Just([1, 2, 3])
>>> Just(Nothing)
Just(Nothing)
>>> Just(Just(2))
Just(Just(2))
>>> isinstance(Just(1), Maybe)
True
>>> isinstance(Nothing, Maybe)
True
>>> saving = 100
>>> spend = lambda cost: Nothing if cost > saving else Just(saving - cost)
>>> spend(90)
Just(10)
>>> spend(120)
Nothing
>>> safe_div = lambda a, b: Nothing if b == 0 else Just(a / b)
>>> safe_div(12.0, 6)
Just(2.0)
>>> safe_div(12.0, 0)
Nothing

Bind operation with >>

>>> inc = lambda n: Just(n + 1) if isinstance(n, int) else Nothing
>>> Just(0)
Just(0)
>>> Just(0) >> inc
Just(1)
>>> Just(0) >> inc >> inc
Just(2)
>>> Just('zero') >> inc
Nothing

Comparison with ==, as long as what’s wrapped inside are comparable.

>>> Just(42) == Just(42)
True
>>> Just(42) == Nothing
False
>>> Nothing == Nothing
True

bind(function)

The bind operation of Maybe.

Applies function to the value if and only if this is a Just.

classmethod from_value(value)

Wraps value in a Maybe monad.

Returns a Just if the value is evaluated as true. Nothing otherwise.

plus(monad)

The Associative operation.

monad.types.maybe.Nothing = Nothing

The Maybe that represents nothing, a singleton, like None.