add package is

5 files changed, 175 insertions, 0 deletions

diff --git a/is/comparable.go b/is/comparable.go
new file mode 100644
index 0000000..7d639d8
--- /dev/null
+++ b/is/comparable.go
@@ -0,0 +1,18 @@
+// SPDX-FileCopyrightText: 2025 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package is
+
+// EqualTo(b)(a) is the same as a == b.
+func EqualTo[T comparable](rhs T) func(T) bool {
+	return func(lhs T) bool {
+		return lhs == rhs
+	}
+}
+
+// DifferentFrom(b)(a) is the same as a != b.
+func DifferentFrom[T comparable](rhs T) func(T) bool {
+	return func(lhs T) bool {
+		return lhs != rhs
+	}
+}
diff --git a/is/doc.go b/is/doc.go
new file mode 100644
index 0000000..42af01e
--- /dev/null
+++ b/is/doc.go
@@ -0,0 +1,28 @@
+// SPDX-FileCopyrightText: 2025 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+// Package is contains functions to express binary operations in a curried style.
+// For example, "foo < bar" can be rewritten as "is.LessThan(bar)(foo)".
+//
+// This is not useful on its own, but may significantly improve readability
+// when replacing function literals. Consider the following example:
+//
+//	import . "github.com/majewsky/gg/option"
+//
+//	func checkNewVolumeSize(size, usage uint64, maxSize Option[uint64]) error {
+//		switch {
+//		case size < usage:
+//			return errors.New("size cannot be smaller than usage")
+//		case maxSize.IsSomeAnd(func(value uint64) bool { return maxSize < size }):
+//			return errors.New("size cannot be larger than maximum")
+//		}
+//	}
+//
+// The IsSomeAnd() check is difficult to read because function literals in Go are clunky.
+// This can be rewritten in a clearer way using is.LessThan():
+//
+//	// original
+//	case maxSize.IsSomeAnd(func(value uint64) bool { return maxSize < size }):
+//	// rewritten
+//	case maxSize.IsSomeAnd(is.LessThan(size)):
+package is
diff --git a/is/is_test.go b/is/is_test.go
new file mode 100644
index 0000000..3c6c7db
--- /dev/null
+++ b/is/is_test.go
@@ -0,0 +1,61 @@
+// SPDX-FileCopyrightText: 2025 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package is_test
+
+import (
+	"testing"
+	"time"
+
+	. "github.com/majewsky/gg/internal/test"
+	"github.com/majewsky/gg/is"
+	. "github.com/majewsky/gg/option"
+)
+
+func TestComparable(t *testing.T) {
+	AssertEqual(t, Some("foo").IsSomeAnd(is.EqualTo("foo")), true)
+	AssertEqual(t, Some("bar").IsSomeAnd(is.EqualTo("foo")), false)
+
+	AssertEqual(t, Some("foo").IsSomeAnd(is.DifferentFrom("foo")), false)
+	AssertEqual(t, Some("bar").IsSomeAnd(is.DifferentFrom("foo")), true)
+}
+
+func TestOrdered(t *testing.T) {
+	AssertEqual(t, Some(3).IsSomeAnd(is.Above(4)), false)
+	AssertEqual(t, Some(4).IsSomeAnd(is.Above(4)), false)
+	AssertEqual(t, Some(5).IsSomeAnd(is.Above(4)), true)
+
+	AssertEqual(t, Some(3).IsSomeAnd(is.Below(4)), true)
+	AssertEqual(t, Some(4).IsSomeAnd(is.Below(4)), false)
+	AssertEqual(t, Some(5).IsSomeAnd(is.Below(4)), false)
+
+	AssertEqual(t, Some(3).IsSomeAnd(is.NotAbove(4)), true)
+	AssertEqual(t, Some(4).IsSomeAnd(is.NotAbove(4)), true)
+	AssertEqual(t, Some(5).IsSomeAnd(is.NotAbove(4)), false)
+
+	AssertEqual(t, Some(3).IsSomeAnd(is.NotBelow(4)), false)
+	AssertEqual(t, Some(4).IsSomeAnd(is.NotBelow(4)), true)
+	AssertEqual(t, Some(5).IsSomeAnd(is.NotBelow(4)), true)
+}
+
+func TestTime(t *testing.T) {
+	t1 := time.Now()
+	t2 := t1.Add(time.Second)
+	t3 := t2.Add(time.Second)
+
+	AssertEqual(t, Some(t1).IsSomeAnd(is.After(t2)), false)
+	AssertEqual(t, Some(t2).IsSomeAnd(is.After(t2)), false)
+	AssertEqual(t, Some(t3).IsSomeAnd(is.After(t2)), true)
+
+	AssertEqual(t, Some(t1).IsSomeAnd(is.Before(t2)), true)
+	AssertEqual(t, Some(t2).IsSomeAnd(is.Before(t2)), false)
+	AssertEqual(t, Some(t3).IsSomeAnd(is.Before(t2)), false)
+
+	AssertEqual(t, Some(t1).IsSomeAnd(is.NotAfter(t2)), true)
+	AssertEqual(t, Some(t2).IsSomeAnd(is.NotAfter(t2)), true)
+	AssertEqual(t, Some(t3).IsSomeAnd(is.NotAfter(t2)), false)
+
+	AssertEqual(t, Some(t1).IsSomeAnd(is.NotBefore(t2)), false)
+	AssertEqual(t, Some(t2).IsSomeAnd(is.NotBefore(t2)), true)
+	AssertEqual(t, Some(t3).IsSomeAnd(is.NotBefore(t2)), true)
+}
diff --git a/is/ordered.go b/is/ordered.go
new file mode 100644
index 0000000..79fada7
--- /dev/null
+++ b/is/ordered.go
@@ -0,0 +1,34 @@
+// SPDX-FileCopyrightText: 2025 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package is
+
+import "cmp"
+
+// Above(b)(a) is the same as a > b.
+func Above[T cmp.Ordered](rhs T) func(T) bool {
+	return func(lhs T) bool {
+		return lhs > rhs
+	}
+}
+
+// Below(b)(a) is the same as a < b.
+func Below[T cmp.Ordered](rhs T) func(T) bool {
+	return func(lhs T) bool {
+		return lhs < rhs
+	}
+}
+
+// NotAbove(b)(a) is the same as a <= b.
+func NotAbove[T cmp.Ordered](rhs T) func(T) bool {
+	return func(lhs T) bool {
+		return lhs <= rhs
+	}
+}
+
+// NotBelow(b)(a) is the same as a >= b.
+func NotBelow[T cmp.Ordered](rhs T) func(T) bool {
+	return func(lhs T) bool {
+		return lhs >= rhs
+	}
+}
diff --git a/is/time.go b/is/time.go
new file mode 100644
index 0000000..3aadaf1
--- /dev/null
+++ b/is/time.go
@@ -0,0 +1,34 @@
+// SPDX-FileCopyrightText: 2025 Stefan Majewsky <majewsky@gmx.net>
+// SPDX-License-Identifier: Apache-2.0
+
+package is
+
+import "time"
+
+// After(b)(a) is the same as a .After(b).
+func After(rhs time.Time) func(time.Time) bool {
+	return func(lhs time.Time) bool {
+		return lhs.After(rhs)
+	}
+}
+
+// Before(b)(a) is the same as a.Before(b).
+func Before(rhs time.Time) func(time.Time) bool {
+	return func(lhs time.Time) bool {
+		return lhs.Before(rhs)
+	}
+}
+
+// NotAfter(b)(a) is the same as !a.After(b).
+func NotAfter(rhs time.Time) func(time.Time) bool {
+	return func(lhs time.Time) bool {
+		return !lhs.After(rhs)
+	}
+}
+
+// NotBefore(b)(a) is the same as !a.Before(b).
+func NotBefore(rhs time.Time) func(time.Time) bool {
+	return func(lhs time.Time) bool {
+		return !lhs.Before(rhs)
+	}
+}