feat: add State monad (#164)

Author: patrickmichalina

package-lock.json

@@ -50,7 +50,7 @@
     "@typescript-eslint/eslint-plugin": "^4.22.0",
     "@typescript-eslint/parser": "^4.22.0",
     "codecov": "^3.8.1",
-    "eslint": "^7.24.0",
+    "eslint": "^7.25.0",
     "eslint-plugin-promise": "^5.1.0",
     "eslint-plugin-rxjs": "3.1.5",
     "fast-check": "^2.14.0",
@@ -75,7 +75,8 @@
       "src/**/*.ts"
     ],
     "coverageReporters": [
-      "lcov"
+      "lcov",
+      "text"
     ],
     "coverageThreshold": {
       "global": {

package.json

@@ -4,3 +4,4 @@ export * from './either/public_api'
 export * from './result/public_api'
 export * from './monad/public_api'
 export * from './list/public_api'
+export * from './state/public_api'

src/index.ts

@@ -0,0 +1,7 @@
+import { State } from './public_api'
+
+describe('state api', () => {
+  it('should export', () => {
+    expect(new State(a => [a, ''])).toBeInstanceOf(State)
+  })
+})

src/state/pubcli_api.spec.ts

@@ -0,0 +1 @@
+export * from './state'

src/state/public_api.ts

@@ -0,0 +1,40 @@
+import { State } from './state'
+
+describe(State.name, () => {
+
+  it('should construct', () => {
+    const sut = new State<string, string>(state => [state, state + '_test'])
+      .run('starting state')
+
+    expect(sut.state).toEqual('starting state')
+    expect(sut.value).toEqual('starting state_test')
+  })
+
+  it('should of', () => {
+    const sut = new State<string, string>(state => [state, state + '_test'])
+      .of(state => [state, 'other'])
+      .run('starting state')
+
+    expect(sut.state).toEqual('starting state')
+    expect(sut.value).toEqual('other')
+  })
+
+  it('should map', () => {
+    const sut = new State<string, string>(state => [state, state + '_phase1_'])
+      .map<number>(pair => [pair.state + '_ran_x1_3', 3])
+      .run('start_str')
+
+    expect(sut.state).toEqual('start_str_ran_x1_3')
+    expect(sut.value).toEqual(3)
+  })
+
+  it('should flat map', () => {
+    const sut = new State<string, string>(state => [state, 'v1'])
+      .flatMap(pair => new State<string, string>(state => [pair.state + state, pair.value]))
+      .run('start')
+
+    expect(sut.state).toEqual('startstart')
+    expect(sut.value).toEqual('v1')
+  })
+
+})

src/state/state.spec.ts

@@ -0,0 +1,33 @@
+
+export type StateTupple<TState, TValue> = [TState, TValue]
+
+export class StatePair<TState, TValue> {
+  constructor(public readonly state: TState, public readonly value: TValue) { }
+}
+
+export class State<TState, TValue> {
+
+  constructor(private readonly fn: (state: TState) => StateTupple<TState, TValue>) { }
+
+  public of(fn: (state: TState) => StateTupple<TState, TValue>): State<TState, TValue> {
+    return new State(fn)
+  }
+
+  public map<TValueB>(fn: (state: StatePair<TState, TValue>) => StateTupple<TState, TValueB>): State<TState, TValueB> {
+    return new State<TState, TValueB>(c => fn(this.run(c)))
+  }
+
+  public flatMap<TValueB>(fn: (state: StatePair<TState, TValue>) => State<TState, TValueB>): State<TState, TValueB> {
+    return new State<TState, TValueB>(c => {
+      const pair = fn(this.run(c)).run(c)
+      return [pair.state, pair.value]
+    })
+  }
+
+  public run(config: TState): StatePair<TState, TValue> {
+    const tupple = this.fn(config)
+
+    return new StatePair(tupple[0], tupple[1])
+  }
+
+}