Data commit

Task/Monads-Writer-monad/Python/monads-writer-monad.py

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+"""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)