add getMonoid to Ord

src/Ord.ts

@@ -16,6 +16,7 @@ import { Semigroup } from './Semigroup'
 import { Eq } from './Eq'
 import { Contravariant1 } from './Contravariant'
 import { pipeable } from './pipeable'
+import { Monoid } from './Monoid'
 
 declare module './HKT' {
   interface URItoKind<A> {
@@ -226,6 +227,18 @@ export function getSemigroup<A = never>(): Semigroup<Ord<A>> {
   }
 }
 
+const empty = fromCompare(() => 0)
+
+/**
+ * @since 2.4.0
+ */
+export function getMonoid<A = never>(): Monoid<Ord<A>> {
+  return {
+    empty,
+    concat: getSemigroup<A>().concat
+  }
+}
+
 /**
  * Given a tuple of `Ord`s returns an `Ord` for the tuple
  *

test/Ord.ts

@@ -6,6 +6,7 @@ import {
   clamp,
   getDualOrd,
   getSemigroup,
+  getMonoid,
   ordDate,
   ordNumber,
   ordString,
@@ -15,6 +16,7 @@ import {
   getTupleOrd,
   ordBoolean
 } from '../src/Ord'
+import { fold } from '../src/Monoid'
 
 describe('Ord', () => {
   it('getTupleOrd', () => {
@@ -51,6 +53,33 @@ describe('Ord', () => {
     ])
   })
 
+  it('getMonoid', () => {
+    interface Person {
+      name: string
+      age: number
+    }
+    const people: Person[] = [
+      { name: 'John', age: 42 },
+      { name: 'Dorothy', age: 37 },
+      { name: 'John', age: 37 }
+    ]
+
+    const sortByName = ord.contramap(ordString, (p: Person) => p.name)
+    const sortByAge = ord.contramap(ordNumber, (p: Person) => p.age)
+
+    const monoid = getMonoid<Person>()
+    const sortByNameByAge = fold(monoid)([sortByName, sortByAge])
+    const dontSort = fold(monoid)([])
+
+    assert.deepStrictEqual(sort(sortByNameByAge)(people), [
+      { name: 'Dorothy', age: 37 },
+      { name: 'John', age: 37 },
+      { name: 'John', age: 42 }
+    ])
+
+    assert.deepStrictEqual(sort(dontSort)(people), people)
+  })
+
   it('ordNumber', () => {
     assert.strictEqual(ordNumber.compare(1, 2), -1)
     assert.strictEqual(ordNumber.compare(2, 1), 1)