Move Increment/Decrement from math.go into misc.go.

pkg/mapper/misc.go

@@ -1,5 +1,7 @@
 package mapper
 
+import "github.com/jpfourny/papaya/pkg/constraint"
+
 // Identity returns a function that accepts a value of any type E and returns that value.
 func Identity[E any]() func(E) E {
 	return func(e E) E {
@@ -13,3 +15,17 @@ func Constant[E any, F any](c F) func(E) F {
 		return c
 	}
 }
+
+// Increment returns a function that accepts a value of real number type E and returns the result of adding the provided `step` value to it.
+func Increment[E constraint.RealNumber](step E) func(E) E {
+	return func(e E) E {
+		return e + step
+	}
+}
+
+// Decrement returns a function that accepts a value of real number type E and returns the result of subtracting the provided `step` value from it.
+func Decrement[E constraint.RealNumber](step E) func(E) E {
+	return func(e E) E {
+		return e - step
+	}
+}

pkg/mapper/misc_test.go

@@ -21,3 +21,21 @@ func TestIdentity(t *testing.T) {
 		t.Errorf("Identity()(42) = %#v; want %#v", got, want)
 	}
 }
+
+func TestIncrement(t *testing.T) {
+	m := Increment[int](2)
+	got := m(42)
+	want := 44
+	if got != want {
+		t.Errorf("Increment(2)(42) = %#v; want %#v", got, want)
+	}
+}
+
+func TestDecrement(t *testing.T) {
+	m := Decrement[int](2)
+	got := m(42)
+	want := 40
+	if got != want {
+		t.Errorf("Decrement(2)(42) = %#v; want %#v", got, want)
+	}
+}