fn: implement ResultOpt type for operations with optional values

.gitignore

@@ -80,3 +80,4 @@ coverage.txt
 # Release build directory (to avoid build.vcs.modified Golang build tag to be
 # set to true by having untracked files in the working directory).
 /lnd-*/
+.aider*

fn/result_opt.go

@@ -0,0 +1,64 @@
+package fn
+
+// ResultOpt represents an operation that may either fail (with an error) or
+// succeed with an optional final value.
+type ResultOpt[T any] struct {
+	Result[Option[T]]
+}
+
+// OkOpt constructs a successful ResultOpt with a present value.
+func OkOpt[T any](val T) ResultOpt[T] {
+	return ResultOpt[T]{Ok(Some(val))}
+}
+
+// NoneOpt constructs a successful ResultOpt with no final value.
+func NoneOpt[T any]() ResultOpt[T] {
+	return ResultOpt[T]{Ok(None[T]())}
+}
+
+// ErrOpt constructs a failed ResultOpt with the provided error.
+func ErrOpt[T any](err error) ResultOpt[T] {
+	return ResultOpt[T]{Err[Option[T]](err)}
+}
+
+// MapResultOpt applies a function to the final value of a successful operation.
+func MapResultOpt[T, U any](ro ResultOpt[T], f func(T) U) ResultOpt[U] {
+	if ro.IsErr() {
+		return ErrOpt[U](ro.Err())
+	}
+	opt, _ := ro.Unpack()
+	return ResultOpt[U]{Ok(MapOption(f)(opt))}
+}
+
+// AndThenResultOpt applies a function to the final value of a successful
+// operation.
+func AndThenResultOpt[T, U any](ro ResultOpt[T], f func(T) ResultOpt[U],
+) ResultOpt[U] {
+
+	if ro.IsErr() {
+		return ErrOpt[U](ro.Err())
+	}
+	opt, _ := ro.Unpack()
+	if opt.IsNone() {
+		return NoneOpt[U]()
+	}
+	return f(opt.some)
+}
+
+// IsSome returns true if the operation succeeded and contains a final value.
+func (ro ResultOpt[T]) IsSome() bool {
+	if ro.IsErr() {
+		return false
+	}
+	opt, _ := ro.Unpack()
+	return opt.IsSome()
+}
+
+// IsNone returns true if the operation succeeded but no final value is present.
+func (ro ResultOpt[T]) IsNone() bool {
+	if ro.IsErr() {
+		return false
+	}
+	opt, _ := ro.Unpack()
+	return opt.IsNone()
+}

fn/result_opt_test.go

@@ -0,0 +1,136 @@
+package fn
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestOkOpt(t *testing.T) {
+	value := 42
+	resOpt := OkOpt(value)
+	opt, err := resOpt.Unpack()
+	require.NoError(t, err)
+	require.True(t, opt.IsSome(), "expected Option to be Some")
+	require.Equal(t, value, opt.UnsafeFromSome())
+	require.True(t, resOpt.IsSome())
+	require.False(t, resOpt.IsNone())
+}
+
+func TestNoneOpt(t *testing.T) {
+	resOpt := NoneOpt[int]()
+	opt, err := resOpt.Unpack()
+	require.NoError(t, err)
+	require.True(t, opt.IsNone(), "expected Option to be None")
+	require.True(t, resOpt.IsNone())
+	require.False(t, resOpt.IsSome())
+}
+
+func TestErrOpt(t *testing.T) {
+	errMsg := "some error"
+	resOpt := ErrOpt[int](errors.New(errMsg))
+	_, err := resOpt.Unpack()
+	require.Error(t, err)
+	require.EqualError(t, err, errMsg)
+	require.False(t, resOpt.IsSome())
+	require.False(t, resOpt.IsNone())
+}
+
+func TestMapResultOptOk(t *testing.T) {
+	value := 10
+	resOpt := OkOpt(value)
+	mapped := MapResultOpt(resOpt, func(i int) int {
+		return i * 3
+	})
+	opt, err := mapped.Unpack()
+	require.NoError(t, err)
+	require.True(t, opt.IsSome(), "expected mapped Option to be Some")
+	require.Equal(t, 30, opt.UnsafeFromSome())
+}
+
+func TestMapResultOptNone(t *testing.T) {
+	resOpt := NoneOpt[int]()
+	mapped := MapResultOpt(resOpt, func(i int) int {
+		return i * 3
+	})
+	opt, err := mapped.Unpack()
+	require.NoError(t, err)
+	require.True(t, opt.IsNone(), "expected mapped Option to remain None")
+}
+
+func TestMapResultOptErr(t *testing.T) {
+	errMsg := "error mapping"
+	resOpt := ErrOpt[int](errors.New(errMsg))
+	mapped := MapResultOpt(resOpt, func(i int) int {
+		return i * 3
+	})
+	_, err := mapped.Unpack()
+	require.Error(t, err)
+	require.EqualError(t, err, errMsg)
+}
+
+func incrementOpt(x int) ResultOpt[int] {
+	return OkOpt(x + 1)
+}
+
+func TestAndThenResultOptOk(t *testing.T) {
+	resOpt := OkOpt(5)
+	chained := AndThenResultOpt(resOpt, incrementOpt)
+	opt, err := chained.Unpack()
+	require.NoError(t, err)
+	require.True(t, opt.IsSome(), "expected chained Option to be Some")
+	require.Equal(t, 6, opt.UnsafeFromSome())
+}
+
+func TestAndThenResultOptNone(t *testing.T) {
+	resOpt := NoneOpt[int]()
+	chained := AndThenResultOpt(resOpt, incrementOpt)
+	opt, err := chained.Unpack()
+	require.NoError(t, err)
+	require.True(t, opt.IsNone(), "expected chained result to remain None")
+}
+
+func TestAndThenResultOptErr(t *testing.T) {
+	errMsg := "error in initial result"
+	resOpt := ErrOpt[int](errors.New(errMsg))
+	chained := AndThenResultOpt(resOpt, incrementOpt)
+	_, err := chained.Unpack()
+	require.Error(t, err)
+	require.EqualError(t, err, errMsg)
+}
+
+func maybeEvenOpt(x int) ResultOpt[int] {
+	if x%2 == 0 {
+		return OkOpt(x / 2)
+	}
+	return NoneOpt[int]()
+}
+
+func TestAndThenResultOptProducesNone(t *testing.T) {
+	// Given an odd number, maybeEvenOpt returns None.
+	resOpt := OkOpt(5)
+	chained := AndThenResultOpt(resOpt, maybeEvenOpt)
+	opt, err := chained.Unpack()
+	require.NoError(t, err)
+	require.True(t, opt.IsNone(), "expected chained result to be None")
+}
+
+func TestMapAndThenIntegration(t *testing.T) {
+	resOpt := OkOpt(2)
+	chained := MapResultOpt(
+		AndThenResultOpt(resOpt, func(x int) ResultOpt[int] {
+			return OkOpt(x + 3)
+		}),
+		func(y int) int {
+			return y * 2
+		},
+	)
+	opt, err := chained.Unpack()
+	require.NoError(t, err)
+	require.True(
+		t, opt.IsSome(), "expected integrated mapping and "+
+			"chaining to produce Some",
+	)
+	require.Equal(t, 10, opt.UnsafeFromSome())
+}