initial checkin

Signed-off-by: Dr. Carsten Leue <carsten.leue@de.ibm.com>

eq/contramap.go

@@ -0,0 +1,11 @@
+package eq
+
+// Contramap implements an Equals predicate based on a mapping
+func Contramap[A, B any](f func(b B) A) func(Eq[A]) Eq[B] {
+	return func(fa Eq[A]) Eq[B] {
+		equals := fa.Equals
+		return FromEquals(func(x, y B) bool {
+			return equals(f(x), f(y))
+		})
+	}
+}

eq/eq.go

@@ -0,0 +1,43 @@
+package eq
+
+import (
+	F "github.com/ibm/fp-go/function"
+)
+
+type Eq[T any] interface {
+	Equals(x, y T) bool
+}
+
+type eq[T any] struct {
+	c func(x, y T) bool
+}
+
+func (self eq[T]) Equals(x, y T) bool {
+	return self.c(x, y)
+}
+
+func strictEq[A comparable](a, b A) bool {
+	return a == b
+}
+
+// FromStrictEquals constructs an `Eq` from the canonical comparison function
+func FromStrictEquals[T comparable]() Eq[T] {
+	return FromEquals(strictEq[T])
+}
+
+// FromEquals constructs an `Eq` from the comparison function
+func FromEquals[T any](c func(x, y T) bool) Eq[T] {
+	return eq[T]{c: c}
+}
+
+// Empty returns the equals predicate that is always true
+func Empty[T any]() Eq[T] {
+	return FromEquals(F.Constant2[T, T](true))
+}
+
+// Equals returns a predicate to test if one value equals the other under an equals predicate
+func Equals[T any](eq Eq[T]) func(T) func(T) bool {
+	return func(other T) func(T) bool {
+		return F.Bind2nd(eq.Equals, other)
+	}
+}

eq/monoid.go

@@ -0,0 +1,18 @@
+package eq
+
+import (
+	M "github.com/ibm/fp-go/monoid"
+	S "github.com/ibm/fp-go/semigroup"
+)
+
+func Semigroup[A any]() S.Semigroup[Eq[A]] {
+	return S.MakeSemigroup(func(x, y Eq[A]) Eq[A] {
+		return FromEquals(func(a, b A) bool {
+			return x.Equals(a, b) && y.Equals(a, b)
+		})
+	})
+}
+
+func Monoid[A any]() M.Monoid[Eq[A]] {
+	return M.MakeMonoid(Semigroup[A]().Concat, Empty[A]())
+}

eq/testing/eq.go

@@ -0,0 +1,13 @@
+package testing
+
+import (
+	EQ "github.com/ibm/fp-go/eq"
+	"github.com/stretchr/testify/assert"
+)
+
+// Eq implements the equal operation based on `ObjectsAreEqualValues` from the assertion library
+func Eq[A any]() EQ.Eq[A] {
+	return EQ.FromEquals(func(l, r A) bool {
+		return assert.ObjectsAreEqualValues(l, r)
+	})
+}