The Try Monad

smonad.types.ftry - The Try Monad.

class smonad.types.ftry.Failure(value)

Bases: smonad.types.ftry.Try

Failure of Try.

class smonad.types.ftry.Success(value)

Bases: smonad.types.ftry.Try

Success of Try.

class smonad.types.ftry.Try(value)

Bases: smonad.types.monad.Monad, smonad.mixins.ContextManager, smonad.mixins.Ord

The Either Monad by a different name

Represents values/computations with two possibilities.

>>> Success(42)
Success(42)
>>> Success([1, 2, 3])
Success([1, 2, 3])
>>> Failure('Error')
Failure('Error')
>>> Success(Failure('Error'))
Success(Failure('Error'))
>>> isinstance(Success(1), Try)
True
>>> isinstance(Failure(None), Try)
True
>>> saving = 100
>>> broke = Failure('I am broke')
>>> spend = lambda cost: broke if cost > saving else Success(saving - cost)
>>> spend(90)
Success(10)
>>> spend(120)
Failure('I am broke')
>>> safe_div = lambda a, b: Failure(str(a) + '/0') if b == 0 else Success(a / b)
>>> safe_div(12.0, 6)
Success(2.0)
>>> safe_div(12.0, 0)
Failure('12.0/0')

Bind operation with >>

>>> inc = lambda n: Success(n + 1) if type(n) is int else Failure('Type error')
>>> Success(0)
Success(0)
>>> Success(0) >> inc
Success(1)
>>> Success(0) >> inc >> inc
Success(2)
>>> Success('zero') >> inc
Failure('Type error')

Comparison with ==, as long as they are the same type and what’s wrapped inside are comparable.

>>> Failure(42) == Failure(42)
True
>>> Success(42) == Success(42)
True
>>> Failure(42) == Success(42)
False

A Failure is less than a Success, or compare the two by the values inside if thay are of the same type.

>>> Failure(42) < Success(42)
True
>>> Success(0) > Failure(100)
True
>>> Failure('Error message') > Success(42)
False
>>> Failure(100) > Failure(42)
True
>>> Success(-2) < Success(-1)
True

bind(function)

The bind operation of Try.

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

match(failure_handler, right_handler=None)

Given two mapping functions (one from an Failure to a value, one from a Success to a Value), unwrap the value stored in this Try, apply the appropriate mapping function, and return the result.

If the right_handler is None and self is an instance of Success, the wrapped value is returned

recover(recovery_fn)

“Recover” from a left value by applying a recovery_fn to the wrapped value and returning it in the case of a left value; otherwise, return the wrapped right value.

unit = <smonad.types.monadic.Monadic object>