WIP: Adds validate function

returns/_internal/pipeline/validation.py

@@ -0,0 +1,51 @@
+from typing import Callable, Iterable, Sequence, Type, TypeVar
+
+from returns._internal.iterable import internal_iterable_kind
+from returns.interfaces.specific.result import ResultLikeN
+from returns.primitives.hkt import Kinded, KindN, kinded
+from returns.primitives.iterables import CollectAll
+
+# Definitions:
+_FirstType = TypeVar('_FirstType', contravariant=True)
+_SecondType = TypeVar('_SecondType')
+_ThirdType = TypeVar('_ThirdType')
+
+_ResultKind = TypeVar('_ResultKind', bound=ResultLikeN)
+
+
+def validate(  # noqa: WPS234
+    container_type: Type[_ResultKind],
+    steps: Iterable[
+        Callable[
+            [_FirstType],
+            KindN[_ResultKind, _FirstType, _SecondType, _ThirdType],
+        ]
+    ],
+) -> Kinded[Callable[
+    [_FirstType],
+    KindN[_ResultKind, _FirstType, Sequence[_SecondType], _ThirdType],
+]]:
+    """
+    Functional validation.
+
+    Works with any ``ResultLikeN`` type.
+    """
+
+    @kinded
+    def factory(instance: _FirstType) -> KindN[
+        _ResultKind,
+        _FirstType,
+        Sequence[_SecondType],
+        _ThirdType,
+    ]:
+        if not steps:
+            return container_type.from_value(instance)
+
+        swapped = [step(instance).swap() for step in steps]
+        return internal_iterable_kind(
+            container_type, swapped, CollectAll,
+        ).bind(
+            lambda errors: container_type.from_value(errors)
+            if errors else container_type.from_failure(errors),
+        ).swap()
+    return factory