add getFunctionSemigroup

package.json

@@ -7,7 +7,7 @@
  "typings": "lib/index.d.ts",
  "scripts": {
  "lint": "tslint src/**/*.ts test/**/*.ts examples/**/*.ts exercises/**/*.ts",
- "mocha": "mocha -r ts-node/register test/*.ts",
+ "mocha": "TS_NODE_CACHE=false mocha -r ts-node/register test/*.ts",
  "prettier":
  "prettier --no-semi --single-quote --print-width 120 --parser typescript --list-different \"{src,test,examples,exercises}/**/*.ts\"",
  "fix-prettier":

src/Monoid.ts

@@ -10,7 +10,8 @@ import {
  semigroupSum,
  semigroupAny,
  getArraySemigroup,
- semigroupVoid
+ semigroupVoid,
+ getFunctionSemigroup
 } from './Semigroup'
 import { Endomorphism, identity, compose } from './function'
 
@@ -105,7 +106,7 @@ export const monoidVoid: Monoid<void> = {
 /** @function */
 export const getFunctionMonoid = <M>(M: Monoid<M>) => <A = never>(): Monoid<(a: A) => M> => {
  return {
- concat: (f, g) => a => M.concat(f(a), g(a)),
+ ...getFunctionSemigroup(M)<A>(),
  empty: () => M.empty
  }
 }

src/Semigroup.ts

@@ -35,6 +35,13 @@ export const getDualSemigroup = <A>(S: Semigroup<A>): Semigroup<A> => {
  }
 }
 
+/** @function */
+export const getFunctionSemigroup = <S>(S: Semigroup<S>) => <A = never>(): Semigroup<(a: A) => S> => {
+ return {
+ concat: (f, g) => a => S.concat(f(a), g(a))
+ }
+}
+
 /** @function */
 export const getRecordSemigroup = <O extends { [key: string]: any }>(
  semigroups: { [K in keyof O]: Semigroup<O[K]> }