In this lesson, we’ll create a safe function that gives us a flexible way to create Maybes based on a value and a predicate function that we supply. We’ll verify its behavior with values that satisfy the predicate, and values that do not.
We can write more functional approach, for example write predicate functions:
const isNumber = n => typeof n === ‘number‘ ? Maybe.Just(n) : Maybe.Nothing(); const isString = s => typeof s === ‘string‘ ? Maybe.Just(s) : Maybe.Nothing();
High order function:
const safe = pred => val => pred(val); const safeNum = safe(isNumber); const safeStr = safe(isString);
Those functions are useful when we want use in large scale application, because those are composable.
Full code demo:
const {inc, upper} = require(‘./utils‘);
const Maybe = require(‘crocks/Maybe‘);
const isNumber = n => typeof n === ‘number‘ ? Maybe.Just(n) : Maybe.Nothing();
const isString = s => typeof s === ‘string‘ ? Maybe.Just(s) : Maybe.Nothing();
const safe = pred => val => pred(val);
const safeNum = safe(isNumber);
const safeStr = safe(isString);
const inputN = safeNum(2); // Just 3 -> 3
const inputS = safeStr(‘test‘); // Just TEST -> TEST
const input = safeStr(undefined); // Nothing -> 0
const result = inputS
.map(upper)
.option("");
console.log(result);
Crocks lib also provides those functions, you actually don‘t need to write it by yourself.
https://evilsoft.github.io/crocks/docs/functions/predicate-functions.html
const {inc, upper} = require(‘./utils‘);
const Maybe = require(‘crocks/Maybe‘);
const safe = require(‘crocks/Maybe/safe‘);
const { isNumber, isString} = require(‘crocks/predicates‘);
/*
const isNumber = n => typeof n === ‘number‘ ? Maybe.Just(n) : Maybe.Nothing();
const isString = s => typeof s === ‘string‘ ? Maybe.Just(s) : Maybe.Nothing();
const safe = pred => val => pred(val);
*/
const safeNum = safe(isNumber);
const safeStr = safe(isString);
const inputN = safeNum(2); // Just 3 -> 3
const inputS = safeStr(‘test‘); // Just TEST -> TEST
const input = safeStr(undefined); // Nothing -> 0
const result = inputS
.map(upper)
.option("");
console.log(result);
[Javascript Crocks] Create a Maybe with a `safe` Utility Function
原文：https://www.cnblogs.com/Answer1215/p/9026265.html