fn: add transpositions for Option and Result

fn/option.go

@@ -251,3 +251,15 @@ func (o Option[A]) SomeToOkf(errString string, args ...interface{}) Result[A] {
 		OptionToLeft[A, A, error](o, fmt.Errorf(errString, args...)),
 	}
 }
+
+// TransposeOptRes transposes the Option[Result[A]] into a Result[Option[A]].
+// This has the effect of leaving an A value alone while inverting the Option
+// and Result layers. If there is no internal A value, it will convert the
+// non-success value to the proper one in the transposition.
+func TransposeOptRes[A any](o Option[Result[A]]) Result[Option[A]] {
+	if o.IsNone() {
+		return Ok(None[A]())
+	}
+
+	return Result[Option[A]]{MapLeft[A, error](Some[A])(o.some.Either)}
+}

fn/option_test.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"testing"
+	"testing/quick"
 
 	"github.com/stretchr/testify/require"
 )
@@ -26,3 +27,27 @@ func TestSomeToOkf(t *testing.T) {
 		Err[uint8](fmt.Errorf(errStr)),
 	)
 }
+
+func TestPropTransposeOptResInverts(t *testing.T) {
+	f := func(i uint) bool {
+		var o Option[Result[uint]]
+		switch i % 3 {
+		case 0:
+			o = Some(Ok(i))
+		case 1:
+			o = Some(Errf[uint]("error"))
+		case 2:
+			o = None[Result[uint]]()
+		default:
+			return false
+		}
+
+		odd := TransposeOptRes(o) ==
+			TransposeOptRes(TransposeResOpt(TransposeOptRes(o)))
+		even := TransposeResOpt(TransposeOptRes(o)) == o
+
+		return odd && even
+	}
+
+	require.NoError(t, quick.Check(f, nil))
+}

fn/result.go

@@ -223,3 +223,15 @@ func AndThen2[A, B, C any](ra Result[A], rb Result[B],
 		})
 	})
 }
+
+// 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
+// non-success value to the proper one in the transposition.
+func TransposeResOpt[A any](r Result[Option[A]]) Option[Result[A]] {
+	if r.IsErr() {
+		return Some(Err[A](r.right))
+	}
+
+	return MapOption(Ok[A])(r.left)
+}

fn/result_test.go

@@ -45,3 +45,28 @@ func TestFlattenResult(t *testing.T) {
 
 	require.NoError(t, quick.Check(f, nil))
 }
+
+func TestPropTransposeResOptInverts(t *testing.T) {
+	f := func(i uint) bool {
+		var r Result[Option[uint]]
+		switch i % 3 {
+		case 0:
+			r = Ok(Some(i))
+		case 1:
+			r = Ok(None[uint]())
+		case 2:
+			r = Errf[Option[uint]]("error")
+		default:
+			return false
+		}
+
+		odd := TransposeResOpt(TransposeOptRes(TransposeResOpt(r))) ==
+			TransposeResOpt(r)
+
+		even := TransposeOptRes(TransposeResOpt(r)) == r
+
+		return odd && even
+	}
+
+	require.NoError(t, quick.Check(f, nil))
+}