fn: reimplement internals of either, add tests

fn/either.go

@@ -2,97 +2,119 @@ package fn
 
 // Either is a type that can be either left or right.
 type Either[L any, R any] struct {
-	left  Option[L]
-	right Option[R]
+	isRight bool
+	left    L
+	right   R
 }
 
 // NewLeft returns an Either with a left value.
 func NewLeft[L any, R any](l L) Either[L, R] {
-	return Either[L, R]{left: Some(l), right: None[R]()}
+	return Either[L, R]{left: l}
 }
 
 // NewRight returns an Either with a right value.
 func NewRight[L any, R any](r R) Either[L, R] {
-	return Either[L, R]{left: None[L](), right: Some(r)}
+	return Either[L, R]{isRight: true, right: r}
 }
 
 // ElimEither is the universal Either eliminator. It can be used to safely
 // handle all possible values inside the Either by supplying two continuations,
 // one for each side of the Either.
 func ElimEither[L, R, O any](f func(L) O, g func(R) O, e Either[L, R]) O {
-	if e.left.IsSome() {
-		return f(e.left.some)
+	if !e.isRight {
+		return f(e.left)
 	}
 
-	return g(e.right.some)
+	return g(e.right)
 }
 
 // WhenLeft executes the given function if the Either is left.
 func (e Either[L, R]) WhenLeft(f func(L)) {
-	e.left.WhenSome(f)
+	if !e.isRight {
+		f(e.left)
+	}
 }
 
 // WhenRight executes the given function if the Either is right.
 func (e Either[L, R]) WhenRight(f func(R)) {
-	e.right.WhenSome(f)
+	if e.isRight {
+		f(e.right)
+	}
 }
 
 // IsLeft returns true if the Either is left.
 func (e Either[L, R]) IsLeft() bool {
-	return e.left.IsSome()
+	return !e.isRight
 }
 
 // IsRight returns true if the Either is right.
 func (e Either[L, R]) IsRight() bool {
-	return e.right.IsSome()
+	return e.isRight
 }
 
 // LeftToOption converts a Left value to an Option, returning None if the inner
 // Either value is a Right value.
 func (e Either[L, R]) LeftToOption() Option[L] {
-	return e.left
+	if e.isRight {
+		return None[L]()
+	}
+
+	return Some(e.left)
 }
 
 // RightToOption converts a Right value to an Option, returning None if the
 // inner Either value is a Left value.
 func (e Either[L, R]) RightToOption() Option[R] {
-	return e.right
+	if !e.isRight {
+		return None[R]()
+	}
+
+	return Some(e.right)
 }
 
 // UnwrapLeftOr will extract the Left value from the Either if it is present
 // returning the supplied default if it is not.
 func (e Either[L, R]) UnwrapLeftOr(l L) L {
-	return e.left.UnwrapOr(l)
+	if e.isRight {
+		return l
+	}
+
+	return e.left
 }
 
 // UnwrapRightOr will extract the Right value from the Either if it is present
 // returning the supplied default if it is not.
 func (e Either[L, R]) UnwrapRightOr(r R) R {
-	return e.right.UnwrapOr(r)
+	if !e.isRight {
+		return r
+	}
+
+	return e.right
 }
 
 // Swap reverses the type argument order. This can be useful as an adapter
 // between APIs.
 func (e Either[L, R]) Swap() Either[R, L] {
 	return Either[R, L]{
-		left:  e.right,
-		right: e.left,
+		isRight: !e.isRight,
+		left:    e.right,
+		right:   e.left,
 	}
 }
 
 // MapLeft maps the left value of the Either to a new value.
 func MapLeft[L, R, O any](f func(L) O) func(Either[L, R]) Either[O, R] {
 	return func(e Either[L, R]) Either[O, R] {
-		if e.IsLeft() {
+		if !e.isRight {
 			return Either[O, R]{
-				left:  MapOption(f)(e.left),
-				right: None[R](),
+				isRight: false,
+				left:    f(e.left),
 			}
 		}
 
 		return Either[O, R]{
-			left:  None[O](),
-			right: e.right,
+			isRight: true,
+			right:   e.right,
 		}
 	}
 }
@@ -100,16 +122,16 @@ func MapLeft[L, R, O any](f func(L) O) func(Either[L, R]) Either[O, R] {
 // MapRight maps the right value of the Either to a new value.
 func MapRight[L, R, O any](f func(R) O) func(Either[L, R]) Either[L, O] {
 	return func(e Either[L, R]) Either[L, O] {
-		if e.IsRight() {
+		if e.isRight {
 			return Either[L, O]{
-				left:  None[L](),
-				right: MapOption(f)(e.right),
+				isRight: true,
+				right:   f(e.right),
 			}
 		}
 
 		return Either[L, O]{
-			left:  e.left,
-			right: None[O](),
+			isRight: false,
+			left:    e.left,
 		}
 	}
 }

fn/either_test.go

@@ -0,0 +1,139 @@
+package fn
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+func TestPropConstructorEliminatorDuality(t *testing.T) {
+	f := func(i int, s string, isRight bool) bool {
+		Len := func(s string) int { return len(s) } // smh
+		if isRight {
+			v := ElimEither(
+				Iden[int],
+				Len,
+				NewRight[int, string](s),
+			)
+			return v == Len(s)
+		}
+
+		v := ElimEither(
+			Iden[int],
+			Len,
+			NewLeft[int, string](i),
+		)
+		return v == i
+	}
+	if err := quick.Check(f, nil); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestPropWhenClauseExclusivity(t *testing.T) {
+	f := func(i int, isRight bool) bool {
+		var e Either[int, int]
+		if isRight {
+			e = NewRight[int, int](i)
+		} else {
+			e = NewLeft[int, int](i)
+		}
+		z := 0
+		e.WhenLeft(func(x int) { z += x })
+		e.WhenRight(func(x int) { z += x })
+
+		return z != 2*i && e.IsLeft() != e.IsRight()
+	}
+	if err := quick.Check(f, nil); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestPropSwapEitherSelfInverting(t *testing.T) {
+	f := func(i int, s string, isRight bool) bool {
+		var e Either[int, string]
+		if isRight {
+			e = NewRight[int, string](s)
+		} else {
+			e = NewLeft[int, string](i)
+		}
+		return e.Swap().Swap() == e
+	}
+	if err := quick.Check(f, nil); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestPropMapLeftIdentity(t *testing.T) {
+	f := func(i int, s string, isRight bool) bool {
+		var e Either[int, string]
+		if isRight {
+			e = NewRight[int, string](s)
+		} else {
+			e = NewLeft[int, string](i)
+		}
+		return MapLeft[int, string, int](Iden[int])(e) == e
+	}
+	if err := quick.Check(f, nil); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestPropMapRightIdentity(t *testing.T) {
+	f := func(i int, s string, isRight bool) bool {
+		var e Either[int, string]
+		if isRight {
+			e = NewRight[int, string](s)
+		} else {
+			e = NewLeft[int, string](i)
+		}
+		return MapRight[int, string, string](Iden[string])(e) == e
+	}
+	if err := quick.Check(f, nil); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestPropToOptionIdentities(t *testing.T) {
+	f := func(i int, s string, isRight bool) bool {
+		var e Either[int, string]
+		if isRight {
+			e = NewRight[int, string](s)
+
+			r2O := e.RightToOption() == Some(s)
+			o2R := e == OptionToRight[string, int, string](
+				Some(s), i,
+			)
+			l2O := e.LeftToOption() == None[int]()
+
+			return r2O && o2R && l2O
+		} else {
+			e = NewLeft[int, string](i)
+			l2O := e.LeftToOption() == Some(i)
+			o2L := e == OptionToLeft[int, int](Some(i), s)
+			r2O := e.RightToOption() == None[string]()
+
+			return l2O && o2L && r2O
+		}
+	}
+	if err := quick.Check(f, nil); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestPropUnwrapIdentities(t *testing.T) {
+	f := func(i int, s string, isRight bool) bool {
+		var e Either[int, string]
+		if isRight {
+			e = NewRight[int, string](s)
+			return e.UnwrapRightOr("") == s &&
+				e.UnwrapLeftOr(0) == 0
+		} else {
+			e = NewLeft[int, string](i)
+			return e.UnwrapLeftOr(0) == i &&
+				e.UnwrapRightOr("") == ""
+		}
+	}
+	if err := quick.Check(f, nil); err != nil {
+		t.Fatal(err)
+	}
+}

fn/option.go

@@ -218,30 +218,18 @@ func (o Option[A]) UnsafeFromSome() A {
 // providing the Right value that should be used if the Option value is None.
 func OptionToLeft[O, L, R any](o Option[O], r R) Either[O, R] {
 	if o.IsSome() {
-		return Either[O, R]{
-			left:  o,
-			right: None[R](),
-		}
+		return NewLeft[O, R](o.some)
 	}
 
-	return Either[O, R]{
-		left:  None[O](),
-		right: Some(r),
-	}
+	return NewRight[O, R](r)
 }
 
 // OptionToRight can be used to convert an Option value into an Either, by
 // providing the Left value that should be used if the Option value is None.
 func OptionToRight[O, L, R any](o Option[O], l L) Either[L, O] {
 	if o.IsSome() {
-		return Either[L, O]{
-			left:  None[L](),
-			right: o,
-		}
+		return NewRight[L, O](o.some)
 	}
 
-	return Either[L, O]{
-		left:  Some(l),
-		right: None[O](),
-	}
+	return NewLeft[L, O](l)
 }

fn/result.go

@@ -39,12 +39,17 @@ func Errf[T any](errString string, args ...any) Result[T] {
 // Unpack extracts the value or error from the Result.
 func (r Result[T]) Unpack() (T, error) {
 	var zero T
-	return r.left.UnwrapOr(zero), r.right.UnwrapOr(nil)
+
+	if r.IsErr() {
+		return zero, r.right
+	}
+
+	return r.left, nil
 }
 
 // Err exposes the underlying error of the result type as a normal error type.
 func (r Result[T]) Err() error {
-	return r.right.some
+	return r.right
 }
 
 // IsOk returns true if the Result is a success value.
@@ -59,66 +64,73 @@ func (r Result[T]) IsErr() bool {
 
 // Map applies a function to the success value if it exists.
 func (r Result[T]) Map(f func(T) T) Result[T] {
-	if r.IsOk() {
-		return Ok(f(r.left.some))
+	return Result[T]{
+		MapLeft[T, error](f)(r.Either),
 	}
-
-	return r
 }
 
 // MapErr applies a function to the error value if it exists.
 func (r Result[T]) MapErr(f func(error) error) Result[T] {
-	if r.IsErr() {
-		return Err[T](f(r.right.some))
+	return Result[T]{
+		MapRight[T](f)(r.Either),
 	}
-
-	return r
 }
 
 // Option returns the success value as an Option.
 func (r Result[T]) Option() Option[T] {
-	return r.left
+	return r.Either.LeftToOption()
 }
 
 // WhenResult executes the given function if the Result is a success.
 func (r Result[T]) WhenResult(f func(T)) {
-	r.left.WhenSome(func(t T) {
-		f(t)
-	})
+	r.WhenLeft(f)
 }
 
 // WhenErr executes the given function if the Result is an error.
 func (r Result[T]) WhenErr(f func(error)) {
-	r.right.WhenSome(func(e error) {
-		f(e)
-	})
+	r.WhenRight(f)
 }
 
 // UnwrapOr returns the success value or a default value if it's an error.
 func (r Result[T]) UnwrapOr(defaultValue T) T {
-	return r.left.UnwrapOr(defaultValue)
+	if r.IsErr() {
+		return defaultValue
+	}
+
+	return r.left
 }
 
 // UnwrapOrElse returns the success value or computes a value from a function
 // if it's an error.
 func (r Result[T]) UnwrapOrElse(f func() T) T {
-	return r.left.UnwrapOrFunc(f)
+	if r.IsErr() {
+		return f()
+	}
+
+	return r.left
 }
 
 // UnwrapOrFail returns the success value or fails the test if it's an error.
 func (r Result[T]) UnwrapOrFail(t *testing.T) T {
 	t.Helper()
 
-	return r.left.UnwrapOrFail(t)
+	if r.IsErr() {
+		t.Fatalf("Result[%T] contained error: %v", r.left, r.right)
+	}
+
+	var zero T
+
+	return zero
 }
 
 // FlatMap applies a function that returns a Result to the success value if it
 // exists.
 func (r Result[T]) FlatMap(f func(T) Result[T]) Result[T] {
 	if r.IsOk() {
-		return f(r.left.some)
+		return r
 	}
-	return r
+
+	return f(r.left)
 }
 
 // AndThen is an alias for FlatMap. This along with OrElse can be used to
@@ -143,10 +155,10 @@ func (r Result[T]) OrElse(f func() Result[T]) Result[T] {
 // it exists.
 func FlatMap[A, B any](r Result[A], f func(A) Result[B]) Result[B] {
 	if r.IsOk() {
-		return f(r.left.some)
+		return f(r.left)
 	}
 
-	return Err[B](r.right.some)
+	return Err[B](r.right)
 }
 
 // AndThen is an alias for FlatMap. This along with OrElse can be used to