validate-state-hook with fp-ts

Introduction

In this article, We will implement a react validate hook that is used in form-item by fp-ts step by step.

Why using the fp-ts?

First, it's supposed to help us to define and resolve the issue about type safety.

Second, actually, it inherits the thoughts of functional programming, helps us to simplify and process complex logic, that is gonnar be readable, improvement in code quality and easy to maintain.

Historical Article

• 函数式编程 - fp-ts 初探
• 函数式编程-入门
• # 函数式编程-进阶

Api Involved

Option

type None = { _tag: 'None' };
type Some<A> = { _tag: 'Some', value: A};
type Option<A> = None | Some<A>

import { Option, some, none, fromNullable, fromPredicate } from 'fp-ts/Option';

function findIndex<A>(arr: Array<A>, predicate: (a: A) => boolean): Option<number> {
	const index = arr.findIndex(predicate);
	return index === -1 ? none : some(index);
}

const arr = [1, 2, 3];
findIndex(arr, n => n === 2); // { _tag: 'Some', value: 1 }
findIndex(arr, n => n === 4); // { _tag: 'None' }

fromNullable(undefined); // { _tag: 'None' }
fromNullable(null); // { _tag: 'None' }
fromNullable(0); // { _tag: 'Some', value: 0 }

const isNumber = <T>(a: T) => !isNaN(Number(a));
const getOptionNumber = fromPredicate(isNumber);
getOptionNumber('a'); // {_tag: 'None'}
getOptionNumber('10'); // { _tag: 'Some', value: '10' }
getOptionNumber(1); // { _tag: 'Some', value: 1 }

pipe

// 1 --- add1 ---> 2 ---- add2 ---> 4 --- add3 ---> 7
const add = (a: number) => (b: number) => a + b;
const add1 = add(1);
const add2 = add(2);
const add3 = add(3);
add3(add2(add1(1))); // 7

// pipe(param, func1, [...func]) ===> func2(func1(1))
// param as the parameter of func1, func1(1) as the paramter of func2, and so on ...
import { pipe } from 'fp-ts/function';

const add = (a: number) => (b: number) => a + b;
const add1 = add(1);
const add2 = add(2);
const add3 = add(3);
pipe(1, add1, add2, add3); // add3(add2(add1(1)))
pipe(1, add1, add2, add3, add3, add3, add3, add3);
// add3(add3(add3(add3(add3(add2(add1(1)))))))

Either

// Either<E, A> => Either functor
//  Either functor has two value: Left and Right,
//  Left: the default value when Right does not exsit，
//  Right:  the normal value
type Left<E> = { _tag: 'Left', left: E };
type Right<A> = { _tag: 'Right', right: A};
type Either<E, A> = Left<E> | Right<A>

import { Either, tryCatch, fromNullable, fromPredicate } from 'fp-ts/Either';

function parse(s: string): Either<Error, unknown> {
  return tryCatch(
     () => JSON.parse(s),
     reason => new Error(String(reason))
  );
}

const success = '{"a": 1, "b": 2}';
const fail = '{"a": a, "b"}';

parse(success); // { _tag: 'Right', right: { a: 1, b: 2 } }
parse(fail); // { _tag: 'Left', left: Error:xxxxx

/******** fromNullable **********/
const getEitherString = fromNullable('defaultValue');
getEitherString(undefined); // { _tag: 'Left', left: 'defaultValue' }
getEitherString(null); // { _tag: 'Left', left: 'defaultValue' }
getEitherString('value'); // { _tag: 'Right', right: 'value' }

/******** fromPredicate **********/
const isEmptyString = (s: string) => s === '';
// fromPredicate(s => boolean, () => defaultValue); // if s => boolean is true, return s, otherwise, return defualtValue
const getEitherString2 = fromPredicate(
	(s: string) => !isEmptyString(s),
	() => 'defaultValue'
);
getEitherString2(''); // { _tag: 'Left', left: 'defaultValue' }
getEitherString2('abc'); // { _tag: 'Right', right: 'abc' }

match

declare const optionNatch: <A, B>(onNone: () => B, onSome: (a: A) => B) => (ma:Option<A>) => B
declare const eitherNatch: <E, A, B>(onNone: (e: E) => B, onSome: (a: A) => B) => (ma: Either<E, A>) => B

/***********************/
// match fold
// declare const optionNatch: <A, B>(onNone: () => B, onSome: (a: A) => B) => (ma: Option<A>) => B
// declare const eitherNatch: <E, A, B>(onNone: (e: E) => B, onSome: (a: A) => B) => (ma: Either<E, A>) => B
/***********************/
import { fromPredicate, match } from 'fp-ts/Option';
import { pipe } from 'fp-ts/function';

pipe(
  2,
  fromPredicate(value => value !== 0), // Option<number> ==> { _tag: 'Some', value: 2 }
  match(
    () => 0, // onNone callback
    value => 10 / value // onSome callback
  )
); // 5

// if the first parmeter is 0, the final result is 0

Chain

declare const chain: <E, A, B>(f: (a: A) => Either<E, B>) => (ma: Either<E, A>) => Either<E, B>

import { Either, chain, left, right } from 'fp-ts/Either';
import { pipe } from 'fp-ts/function';

const multiplyByTen = <T>(value: T): Either<string, number> =>
	typeof value === 'number' ? right(value * 10) : left('not a number');
        
const increment = (value: number): Either<string, number> => right(value + 1);

const func = <T>(value: T) => pipe(value, multiplyByTen, chain(increment));

Let's look at a react componnet demo

That is a component about phoneNumber validate, we generally code like this...

import { useState, ChangeEvent, FormEvent } from 'react';

const FormTest = () => {
   const [mobleNumber, setMobileNumber] = useState('');
   const [error, setError] = useState('');

   const validateMobileNumber = (value: string): boolean => {
      if (!value) {
        setError('moblie number can not be empty');
        return false;
      }
      if (!/^1[3-9]\d{9}$/.test(value)) {
        setError('moble number does not follow the rule');
        return false;
      }

      setError('');
      return true;
   };

   const anotherValidate = (value: string) => {
      return true;
   };

   const handleNumberChange = (e: ChangeEvent<HTMLInputElement>) => {
      const { value } = e.target;
      validateMobileNumber(value);
      setMobileNumber(value);
   };

   const onSubmit = (e: FormEvent) => {
      e.preventDefault();
      const validations =
        [validateMobileNumber(mobleNumber), anotherValidate(mobleNumber)];
      
      if (validations.some(valid => !valid)) {
	      return;
      }
      alert('submit...');
   };

   return (
     <form onSubmit={onSubmit}>
         <input
            type="text"
            onChange={handleNumberChange}
            value={mobleNumber}
         />
         <span style={{ color: 'red' }}>{error}</span>
         <div>
             <button type="submit">submit</button>
         </div>
     </form>
   );
};

export default FormTest;

Start from fp-ts

1. validate rules

import type { Predicate } from 'fp-ts/Predicate';
// interface Predicate<A> {  
//    (a: A): boolean  
// }

// function currying

const startsWith =
	(search: string): Predicate<string> =>
	(text: string) =>
	   text.startsWith(search);

const minLength =
	(limit: number): Predicate<string> =>
	(text: string) =>
	  text.length >= limit;

const maxLength =
	(limit: number): Predicate<string> =>
	(text: string) =>
	  text.length <= limit;

const testPhoneNumberPattern = (text: string) => !/[^0-9]/gi.test(text);

// test
const myMobileNum = '01012345';
testPhoneNumberPattern(myMobileNum); // true
testPhoneNumberPattern('0101a2345'); // false
startsWith('01')(myMobileNum); // true
maxLength(11)(myMobileNum); // true
minLength(10)(myMobileNum); // false

2. validate function

import type { Predicate } from 'fp-ts/Predicate';
import { Either, chain, fromPredicate } from 'fp-ts/Either';
import { pipe } from 'fp-ts/function';
import { every, map } from 'fp-ts/Array';

// ... validate rules code

const validate =
	<T>(validators: Array<Predicate<T>>, errorMessage: string) =>
	(value: T) =>
	  pipe(
          value,
          fromPredicate(
          val => 
              pipe(
                  validators,
                  map(fn => fn(val)),
                  every(Boolean)
              ),
              () => errorMessage
         )
     );
     
export const validatePhoneNumber = (phoneNumber: string): Either<string, string> =>
	pipe(
        phoneNumber,
        validate([minLength(1)], 'mobile number can not be empty'),
        chain(
          validate(
            [testPhoneNumberPattern, startsWith('01'), minLength(10), maxLength(11)],
            'mobile number must confirm the rule'
          )
        )
   );

// test
validatePhoneNumber(''); // Left('moblie number can not be empty')
validatePhoneNumber('0123abc'); // Left('moble number must confirm the rule')
validatePhoneNumber('01011234567'); // Right('01011234567')

3.hook - useStateWithValidator.ts

It replaces the feature of useState, involving validate and error-show functionality

import { useState } from 'react';
import { Either, match } from 'fp-ts/Either';
import { empty, isEmpty } from 'fp-ts/string';
import { identity, pipe } from 'fp-ts/function';

type StateValidator = {
	validate: () => boolean;
	error: string;
};

/**
 * hook
 * @param initialState 
 * @param validator 
 * @returns [value, changeValue, error, StateValidator]
 */
const useStateWithValidator = <T>(
	initialState: T,
	validator: (v: T) => Either<string, T>
): [T, (v: T) => void, string, StateValidator] => {
	const [value, setValue] = useState<T>(initialState);
	const [error, setError] = useState('');

	const changeError = (errorMessage: string): string => {
		setError(errorMessage);
		return errorMessage;
	};

        /**
         * match(onLeft, onRight)
         * function identity<A>(a: A): A ==> input: A，output: A
         */
	const changeValue = (v: T): void => {
		pipe(
			v, // input value as the param of setValue
			setValue, // setState
			() => validator(v), // validate value
			match(identity, () => empty), // if the result of validate is left, invalid, then trigger identity, otherwise, it was passed，trigger () => empty
			changeError // modify error. 
		);
	};

	const stateValidator: StateValidator = {
		validate(): boolean {
			return pipe(
				validator(value),
				match(identity, () => empty),
				changeError,
				isEmpty
			);
		},
		get error(): string {
			return error;
		},
	};

	return [value, changeValue, error, stateValidator];
};

export default useStateWithValidator;

4. use hook in component

import { ChangeEvent, FormEvent } from 'react';
import useStateWithValidator from '../../hooks/useStateWithValidator';
import { validatePhoneNumber } from '../utils';

export default () => {
    const [mobileNumber, setMobileNumber, error, mobileNumberValidator] =
        useStateWithValidator('', validatePhoneNumber);
	
    const handleNumberChange = (e: ChangeEvent<HTMLInputElement>) =>
        setMobileNumber(e.target.value);

    const onSubmit = (e: FormEvent) => {
        e.preventDefault();
        const validators = [mobileNumberValidator];
        const isInvalid = validators.map(validator =>
            validator.validate()).some(valid => !valid);

        if (isInvalid) 
            return;
        }
        alert('submit...');
    };

    return (
        <form onSubmit={onSubmit}>
            <input
                type="text"
                onChange={handleNumberChange}
                value={mobileNumber}
            />
            <span style={{ color: 'red' }}>{error}</span>
            <div>
                <button type="submit">submit</button>
            </div>
        </form>
    );
};

Final

We implement a react hook with fp-ts, the primacy is not the hook, but the validate function. It works like magic, looks like complexer than our coding previously.

But more safer function logic and compose the validate function is a better way for programming. At same time, FP lets us organize the logic flow flexiblely, like this: