feat: add predicate

fun/predicate.go

@@ -0,0 +1,18 @@
+package fun
+
+// Predicate is a function with a boolean result type.
+type Predicate[A any] func(A) bool
+
+// IsEqualTo compares two arguments for equality.
+func IsEqualTo[A comparable](a A) Predicate[A] {
+	return func(other A) bool {
+		return a == other
+	}
+}
+
+// Not negates the given predicate.
+func Not[A any](p Predicate[A]) Predicate[A] {
+	return func(a A) bool {
+		return !p(a)
+	}
+}

fun/predicate_test.go

@@ -0,0 +1,13 @@
+package fun_test
+
+import (
+	"testing"
+
+	"github.com/primetalk/goio/fun"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestEqualTo(t *testing.T) {
+	assert.True(t, fun.IsEqualTo(5)(5))
+	assert.False(t, fun.Not(fun.IsEqualTo(5))(5))
+}