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Maybe 也能使用 Function Composition

Sam Xiao's Avatar 2019-06-01

之前都使用 Maybe 自帶的 Method,以 OOP 的 Method Chaining 處理 Maybe,事實上也能完全以 FP 的 Function Composition 處理。

Version

VS Code 1.34.0
Quokka 1.0.216
Ramda 0.26.1
Crocks 0.11.1

OOP

import { isString, and, not, isEmpty, prop, safe, Maybe } from 'crocks';
import { compose, join, split, toLower, concat } from 'ramda';

let { of } = Maybe;

// isNonEmptyString :: a -> Boolean
let isNonEmptyString = and(not(isEmpty), isString);

// createUrlSlug :: String -> String
let createSlug = compose(join('-'), split(' '), toLower);

// appendSlug :: String -> String
let appendSlug = concat('https://oomusou.io/crocks/');

// createUrl :: String -> String
let createUrl = compose(appendSlug, createSlug);

let data = {
  title: '',
  price: 100,
};

// fn :: Object -> Maybe String
let fn = obj => prop('title', obj)
  .chain(safe(isNonEmptyString))
  .alt(of('page-not-found'))
  .map(createUrl);

fn(data).option('https://oomusou.io'); // ?

23 行

// fn :: Object -> Maybe String
let fn = obj => prop('title', obj)
  .chain(safe(isNonEmptyString))
  .alt(of('page-not-found'))
  .map(createUrl);

使用 alt() 提早處理 Nothing 一文中,由於 prop() 回傳 Maybe,我們使用了一系列的 chain()alt()map() 處理 Maybe,這些都是 Maybe 自帶 method,呈現出 OOP 風格的 method chaining,我們能否使用 FP 的 function composition 處理 Maybe 呢 ?

compose000

FP

import { isString, and, not, isEmpty, prop, safe, Maybe, chain, alt, map, option } from 'crocks';
import { compose, join, split, toLower, concat, pipe } from 'ramda';

let { of } = Maybe;

// isNonEmptyString :: a -> Boolean
let isNonEmptyString = and(not(isEmpty), isString);

// createUrlSlug :: String -> String
let createSlug = compose(join('-'), split(' '), toLower);

// appendSlug :: String -> String
let appendSlug = concat('https://oomusou.io/crocks/');

// createUrl :: String -> String
let createUrl = compose(appendSlug, createSlug);

let data = {
  title: '',
  price: 100,
};

// fn :: Object -> Maybe String
let fn = pipe(
  prop('title'),
  chain(safe(isNonEmptyString)),
  alt(of('page-not-found')),
  map(createUrl)
);

option('https://oomusou.io')(fn(data)); // ?

第 1 行

import { isString, and, not, isEmpty, prop, safe, Maybe, chain, alt, map, option } from 'crocks';

從 Crocks import 進 chain()alt()option(),改用 function 操作 Maybe

第 2 行

import { compose, join, split, toLower, concat, pipe } from 'ramda';

從 Ramda import 進 pipe()

也可以使用 Crocks 的 compose()pipe()

23 行

// fn :: Object -> Maybe String
let fn = pipe(
  prop('title'),
  chain(safe(isNonEmptyString)),
  alt(of('page-not-found')),
  map(createUrl)
);

pipe()obj parameter 給 point-free 了,且完全不使用 method chaining。

31 行

option('https://oomusou.io')(fn(data)); // ?

option() 也可改用 function 方式。

compose001

Function Composition

import { isString, and, not, isEmpty, prop, safe, Maybe, chain, alt, map, option } from 'crocks';
import { compose, join, split, toLower, concat, pipe } from 'ramda';

let { of } = Maybe;

// isNonEmptyString :: a -> Boolean
let isNonEmptyString = and(not(isEmpty), isString);

// createUrlSlug :: String -> String
let createSlug = compose(join('-'), split(' '), toLower);

// appendSlug :: String -> String
let appendSlug = concat('https://oomusou.io/crocks/');

// getSlugIfEmptyString :: Maybe String -> Maybe String
let getSlugIfEmptyString = pipe(
  chain(safe(isNonEmptyString)),
  alt(of('page-not-found')),
);

// createUrl :: String -> String
let createUrl = compose(appendSlug, createSlug);

let data = {
  title: '',
  price: 100,
};

// fn :: Object -> Maybe String
let fn = pipe(
  prop('title'),
  getSlugIfEmptyString,
  map(createUrl)
);

option('https://oomusou.io')(fn(data)); // ?

29 行

// fn :: Object -> Maybe String
let fn = pipe(
  prop('title'),
  getSlugIfEmptyString,
  map(createUrl)
);

Q:從 method chaining 改成 function composition 只是語法差異,還有什麼實際用途嗎 ?

chain()alt() 是為了處理 empty string 特別邏輯,可將這部分單獨抽出成 getSlugIfEmptyString(),其語義更為清楚。

15 行

// getSlugIfEmptyString :: Maybe String -> Maybe String
let getSlugIfEmptyString = pipe(
  chain(safe(isNonEmptyString)),
  alt(of('page-not-found')),
);

反之若使用 method chaining,則 chain()alt() 很難單獨抽出來,因為都綁在 Maybe 上了,這就是 FP 將 data 與 function 分離的優勢。。

compose002

Conclusion

See Also

使用 alt() 提早處理 Nothing
使用 coalesce() 提早提供 Function 處理 Nothing

Reference

Andy Van Slaars, Compose Function for Reusability with the Maybe Type
Crocks, chain()
Crocks, alt()
Crocks, coalesce()
Crocks, option()