Add filter method to Maybe

perhaps/maybe.py

@@ -107,6 +107,22 @@ def unwrap_or_else(self, f: Callable[[], T]) -> T:
         """
         ...
 
+    @abstractmethod
+    def filter(self, f: Callable[[T], bool]) -> "Maybe[T]":
+        """
+        Filter the value of the Maybe, returning Nothing if the predicate returns False.
+
+        Analogous to `Option::filter` in Rust.
+
+        >>> Just(1).filter(lambda x: x > 1)
+        Nothing()
+        >>> Just(1).filter(lambda x: x < 1)
+        Nothing()
+        >>> Just(1).filter(lambda x: x == 1)
+        Just(1)
+        """
+        ...
+
     @abstractmethod
     def to_optional(self) -> Optional[T]:
         """
@@ -201,6 +217,9 @@ def unwrap_or(self, default: T) -> T:
     def unwrap_or_else(self, f: Callable[[], T]) -> T:
         return self.value
 
+    def filter(self, f: Callable[[T], bool]) -> Maybe[T]:
+        return self if f(self.value) else Nothing()
+
     def to_optional(self) -> T:
         return self.value
 
@@ -276,6 +295,9 @@ def unwrap_or(self, default: T) -> T:
     def unwrap_or_else(self, f: Callable[[], T]) -> T:
         return f()
 
+    def filter(self, f: Callable[[T], bool]) -> "Nothing[T]":
+        return Nothing()
+
     def to_optional(self) -> None:
         return None
 

tests/test_maybe.py

@@ -71,6 +71,12 @@ def test_unwrap_or():
     assert Nothing().unwrap_or(2) == 2
 
 
+def test_filter():
+    assert Just(1).filter(lambda x: x > 1) == Nothing()
+    assert Just(1).filter(lambda x: x < 1) == Nothing()
+    assert Just(1).filter(lambda x: x == 1) == Just(1)
+
+
 def test_from_try():
     assert Maybe.from_try(lambda: 1 / 1, ZeroDivisionError) == Just(1)
     assert Maybe.from_try(lambda: 1 / 0, ZeroDivisionError) == Nothing()