fn: add Sink to Result

fn/result.go

@@ -224,6 +224,16 @@ func AndThen2[A, B, C any](ra Result[A], rb Result[B],
 	})
 }
 
+// Sink consumes a Result, either propagating its error or processing its
+// success value with a function that can fail.
+func (r Result[A]) Sink(f func(A) error) error {
+	if r.IsErr() {
+		return r.right
+	}
+
+	return f(r.left)
+}
+
 // TransposeResOpt transposes the Result[Option[A]] into a Option[Result[A]].
 // This has the effect of leaving an A value alone while inverting the Result
 // and Option layers. If there is no internal A value, it will convert the

fn/result_test.go

@@ -70,3 +70,29 @@ func TestPropTransposeResOptInverts(t *testing.T) {
 
 	require.NoError(t, quick.Check(f, nil))
 }
+
+func TestSinkOnErrNoContinutationCall(t *testing.T) {
+	called := false
+	res := Err[uint8](errors.New("err")).Sink(
+		func(a uint8) error {
+			called = true
+			return nil
+		},
+	)
+
+	require.False(t, called)
+	require.NotNil(t, res)
+}
+
+func TestSinkOnOkContinuationCall(t *testing.T) {
+	called := false
+	res := Ok(uint8(1)).Sink(
+		func(a uint8) error {
+			called = true
+			return nil
+		},
+	)
+
+	require.True(t, called)
+	require.Nil(t, res)
+}