Added Result filter methods (#224)

* Added Result filter methods

* PR update: Use a different error value in the tests

* PR updates

* Added pipeable functions for some methods that were missing them
* Added tests for `Result#map_error`
* Added tests for piping functions

Author: brendanmaguire

expression/core/result.py

@@ -150,6 +150,38 @@ def is_ok(self) -> bool:
         """Return `True` if the result is an `Ok` value."""
         return self.tag == "ok"
 
+    def filter(self, predicate: Callable[[_TSource], bool], default: _TError) -> Result[_TSource, _TError]:
+        """Filter result.
+
+        Returns the input if the predicate evaluates to true, otherwise
+        returns the `default`
+        """
+        match self:
+            case Result(tag="ok", ok=value) if predicate(value):
+                return self
+            case Result(tag="error"):
+                return self
+            case _:
+                return Error(default)
+
+    def filter_with(
+        self,
+        predicate: Callable[[_TSource], bool],
+        default: Callable[[_TSource], _TError],
+    ) -> Result[_TSource, _TError]:
+        """Filter result.
+
+        Returns the input if the predicate evaluates to true, otherwise
+        returns the `default` using the value as input
+        """
+        match self:
+            case Result(tag="ok", ok=value) if predicate(value):
+                return self
+            case Result(tag="ok", ok=value):
+                return Error(default(value))
+            case Result():
+                return self
+
     def dict(self) -> builtins.dict[str, _TSource | _TError | Literal["ok", "error"]]:
         """Return a json serializable representation of the result."""
         match self:
@@ -352,6 +384,11 @@ def map2(
     return x.map2(y, mapper)
 
 
+@curry_flip(1)
+def map_error(result: Result[_TSource, _TError], mapper: Callable[[_TError], _TResult]) -> Result[_TSource, _TResult]:
+    return result.map_error(mapper)
+
+
 @curry_flip(1)
 def bind(
     result: Result[_TSource, _TError],
@@ -374,11 +411,46 @@ def is_error(result: Result[_TSource, _TError]) -> TypeGuard[Result[_TSource, _T
     return result.is_error()
 
 
+@curry_flip(1)
+def filter(
+    result: Result[_TSource, _TError],
+    predicate: Callable[[_TSource], bool],
+    default: _TError,
+) -> Result[_TSource, _TError]:
+    return result.filter(predicate, default)
+
+
+@curry_flip(1)
+def filter_with(
+    result: Result[_TSource, _TError],
+    predicate: Callable[[_TSource], bool],
+    default: Callable[[_TSource], _TError],
+) -> Result[_TSource, _TError]:
+    return result.filter_with(predicate, default)
+
+
 def swap(result: Result[_TSource, _TError]) -> Result[_TError, _TSource]:
     """Swaps the value in the result so an Ok becomes an Error and an Error becomes an Ok."""
     return result.swap()
 
 
+@curry_flip(1)
+def or_else(result: Result[_TSource, _TError], other: Result[_TSource, _TError]) -> Result[_TSource, _TError]:
+    return result.or_else(other)
+
+
+@curry_flip(1)
+def or_else_with(
+    result: Result[_TSource, _TError],
+    other: Callable[[_TError], Result[_TSource, _TError]],
+) -> Result[_TSource, _TError]:
+    return result.or_else_with(other)
+
+
+def merge(result: Result[_TSource, _TSource]) -> _TSource:
+    return result.merge()
+
+
 def to_option(result: Result[_TSource, Any]) -> Option[_TSource]:
     from expression.core.option import Nothing, Some
 
@@ -406,9 +478,17 @@ def of_option_with(value: Option[_TSource], error: Callable[[], _TError]) -> Res
     "map",
     "bind",
     "dict",
+    "filter",
+    "filter_with",
     "is_ok",
     "is_error",
+    "map2",
+    "map_error",
+    "merge",
     "to_option",
     "of_option",
     "of_option_with",
+    "or_else",
+    "or_else_with",
+    "swap",
 ]

tests/test_result.py

@@ -194,6 +194,14 @@ def test_result_error_chained_map(msg: str, y: int):
         case _:
             assert False
 
+@given(st.text())
+def test_map_error(msg: str):
+    assert Error(msg).map_error(lambda x: f"more {x}") == Error("more " + msg)
+
+@given(st.text())
+def test_map_error_piped(msg: str):
+    assert Error(msg).pipe(result.map_error(lambda x: f"more {x}")) == Error(f"more {msg}")
+
 
 @given(st.integers(), st.integers())  # type: ignore
 def test_result_bind_piped(x: int, y: int):
@@ -362,6 +370,54 @@ def test_pipeline_error():
     assert hn(42) == error
 
 
+def test_filter_ok_passing_predicate():
+    xs: Result[int, str] = Ok(42)
+    ys = xs.filter(lambda x: x > 10, "error")
+
+    assert ys == xs
+
+
+def test_filter_ok_failing_predicate():
+    xs: Result[int, str] = Ok(5)
+    ys = xs.filter(lambda x: x > 10, "error")
+
+    assert ys == Error("error")
+
+
+def test_filter_error():
+    error = Error("original error")
+    ys = error.filter(lambda x: x > 10, "error")
+
+    assert ys == error
+
+def test_filter_piped():
+    assert Ok(42).pipe(result.filter(lambda x: x > 10, "error")) == Ok(42)
+
+
+def test_filter_with_ok_passing_predicate():
+    xs: Result[int, str] = Ok(42)
+    ys = xs.filter_with(lambda x: x > 10, lambda value: f"error {value}")
+
+    assert ys == xs
+
+
+def test_filter_with_ok_failing_predicate():
+    xs: Result[int, str] = Ok(5)
+    ys = xs.filter_with(lambda x: x > 10, lambda value: f"error {value}")
+
+    assert ys == Error("error 5")
+
+
+def test_filter_with_error():
+    error = Error("original error")
+    ys = error.filter_with(lambda x: x > 10, lambda value: f"error {value}")
+
+    assert ys == error
+
+def test_filter_with_piped():
+    assert Ok(42).pipe(result.filter_with(lambda x: x > 10, lambda value: f"error {value}")) == Ok(42)
+
+
 class MyError(BaseModel):
     message: str
 
@@ -525,6 +581,8 @@ def test_result_swap_with_error():
     xs = result.swap(error)
     assert xs == Ok(1)
 
+def test_swap_piped():
+    assert Ok(42).pipe(result.swap) == Error(42)
 
 def test_ok_or_else_ok():
     xs: Result[int, str] = Ok(42)
@@ -549,6 +607,8 @@ def test_error_or_else_error():
     ys = xs.or_else(Error("new error"))
     assert ys == Error("new error")
 
+def test_or_else_piped():
+    assert Ok(42).pipe(result.or_else(Ok(0))) == Ok(42)
 
 def test_ok_or_else_with_ok():
     xs: Result[str, str] = Ok("good")
@@ -574,6 +634,10 @@ def test_error_or_else_with_error():
     assert ys == Error("new error from original error")
 
 
+def test_or_else_with_piped():
+    assert Ok(42).pipe(result.or_else_with(lambda _: Ok(0))) == Ok(42)
+
+
 def test_merge_ok():
     assert Result.Ok(42).merge() == 42
 
@@ -601,3 +665,7 @@ class Child2(Parent):
 def test_merge_subclasses():
     xs: Result[Parent, Parent] = Result.Ok(Child1(x=42))
     assert xs.merge() == Child1(x=42)
+
+
+def test_merge_piped():
+    assert Ok(42).pipe(result.merge) == 42