fn: reimplement internals of either, add tests

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)
+	}
+}