Add Monad.sequence

monads/monad.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
-from typing import Any, Callable, Generic, TypeVar
+import functools
+from typing import Any, Callable, Generic, Iterable, List, TypeVar
 
 from .applicative import Applicative
 from .functor import Functor
@@ -16,4 +17,17 @@ class Monad(Applicative[T]):
     def bind(self, function: Callable[[T], Any]) -> Monad[S]:  # pragma: no cover
         raise NotImplementedError
 
+    @classmethod
+    def pure(cls, value: T) -> Monad[T]:
+        raise NotImplementedError
+
+    @classmethod
+    def sequence(cls, xs: Iterable[Monad[T]]) -> Monad[List[T]]:
+        """Evaluate monadic actions in sequence, collecting results."""
+
+        def reducer(acc: Monad[List[T]], x: Monad[T]) -> Monad[List[T]]:
+            return acc.bind(lambda acc_: x.map(lambda x_: acc_ + [x_]))
+
+        return functools.reduce(reducer, xs, cls.pure([]))
+
     __rshift__ = bind

tests/test_monads.py

@@ -41,3 +41,9 @@ def test_associativity(monad) -> None:
         return monad.pure(n + 5)
 
     assert m.bind(f).bind(g) == m.bind(lambda x: f(x).bind(g))
+
+
+def test_sequence(monad) -> None:
+    assert monad.pure([1, 2, 3]) == monad.sequence(
+        [monad.pure(1), monad.pure(2), monad.pure(3)]
+    )