Monads/Writer monad: Difference between revisions
A Python implementation
(added Perl programming solution) |
(A Python implementation) |
||
Line 815:
add one: 3.2360679774998
half: 1.6180339887499
</pre>
=={{header|Python}}==
<lang python>"""A Writer Monad. Requires Python >= 3.7 for type hints."""
from __future__ import annotations
import functools
import math
import os
from typing import Any
from typing import Callable
from typing import Generic
from typing import List
from typing import TypeVar
from typing import Union
T = TypeVar("T")
class Writer(Generic[T]):
def __init__(self, value: Union[T, Writer[T]], *msgs: str):
if isinstance(value, Writer):
self.value: T = value.value
self.msgs: List[str] = value.msgs + list(msgs)
else:
self.value = value
self.msgs = list(f"{msg}: {self.value}" for msg in msgs)
def bind(self, func: Callable[[T], Writer[Any]]) -> Writer[Any]:
writer = func(self.value)
return Writer(writer, *self.msgs)
def __rshift__(self, func: Callable[[T], Writer[Any]]) -> Writer[Any]:
return self.bind(func)
def __str__(self):
return f"{self.value}\n{os.linesep.join(reversed(self.msgs))}"
def __repr__(self):
return f"Writer({self.value}, \"{', '.join(reversed(self.msgs))}\")"
def lift(func: Callable, msg: str) -> Callable[[Any], Writer[Any]]:
"""Return a writer monad version of the simple function `func`."""
@functools.wraps(func)
def wrapped(value):
return Writer(func(value), msg)
return wrapped
if __name__ == "__main__":
square_root = lift(math.sqrt, "square root")
add_one = lift(lambda x: x + 1, "add one")
half = lift(lambda x: x / 2, "div two")
print(Writer(5, "initial") >> square_root >> add_one >> half)
</lang>
{{out}}
<pre>
1.618033988749895
initial: 5
square root: 2.23606797749979
add one: 3.23606797749979
div two: 1.618033988749895
</pre>
| |||