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Refine Collection Checkers

Collection checkers can be combined with primitive checkers to refine more complex values.

array()

Validates a value as a $ReadOnlyArray<T>, given a value checker of type Checker<T>.

// define checker
const check = array(number());

// test a value
const result = check([1,2,3]);
assert(result.type === 'success');

// result should typecheck
const value: $ReadOnlyArray<number> = result.value;

// test an invalid value
assert(check('test').type === 'failure');
assert(check(['test', 'other']).type === 'failure');

tuple()

Validates a value as a strictly typed tuple.

// define checker
const check = tuple(number(), string(), bool());

// test a value
const result = check([1,'2', false]);
assert(result.type === 'success');

// result should typecheck
const value: $ReadOnly<[number, string, boolean]> = result.value;

// test an invalid value
assert(check('test').type === 'failure');
assert(check(['test', 'other']).type === 'failure');

dict()

Validates a value as a $ReadOnly<{[key: string]: T}>, given a value checker of type Checker<T>.

// define checker
const check = dict(number());

// test a value
const result = check({a: 1, b: 2, c: 3});
assert(result.type === 'success');

// result should typecheck
const value: $ReadOnly<{[key: string]: number}> = result.value;

// test an invalid value
assert(check('test').type === 'failure');
assert(check({a: 'test', b: 'other', c: 3}).type === 'failure');

object()

Validates a value as a $ReadOnly<{[key: K]: T}>, given an object of checkers of type {[key: K]: Checker<T>}.

// define checker
const check = object({
a: number(),
b: string(),
c: optional(string()) // use `optional` for optional properties
});

// test a value
const result = check({a: 1, b: 'test'});
assert(result.type === 'success');

// result should typecheck
const value: $ReadOnly<{a: number, b: string}> = result.value;

// test an invalid value
assert(check('test').type === 'failure');
assert(check({a: 'test', b: 1}).type === 'failure');
assert(check({a: 1, c: 'test'}).type === 'failure');

set()

Checker to assert if a mixed value is a Set type with valid values.

// define checker
const check = set(number());

// test a value
const result = check(new Set([1, 2]));
assert(result.type === 'success');

// result should typecheck
const value: $ReadOnlySet<number> = result.value;

// test an invalid value
assert(check('test').type === 'failure');
assert(check({a: 'test', b: 'other', c: 3}).type === 'failure');

map()

Checker to assert if a mixed value is a Map type with valid keys and values.

// define checker
const check = map(date(), number());

// test a value
const result = check(new Map([[new Date(), 1], [new Date(2), 2]]));
assert(result.type === 'success');

// result should typecheck
const value: $ReadOnlyMap<Date, number> = result.value;

// test an invalid value
assert(check('test').type === 'failure');
assert(check({a: 'test', b: 'other', c: 3}).type === 'failure');

writableArray()

Identical to array(), but the returned type is a Array<> instead of $ReadOnlyArray<>

const coerce = writableArray(number());
const result = coerce([1, 2, 3]);

assert(result.type === 'success', 'should succeed');
result.value[0] = 3;

writableObject()

Identical to object, but the returned type is a writable object instead of $ReadOnly<>

const coerce = writableObject({
name: string(),
job: object({
years: number(),
title: string(),
}),
});

const result = coerce({name: 'Elsa', job: {title: 'Engineer', years: 3}});
assert(result.type === 'success', 'should succeed');

// should Flow check as writable
result.value.name = 'MechaElsa';

writableDict()

Identical to dict(), but the returned type is a writable object instead of $ReadOnly<>

const coerce = writableDict(number());
const result = coerce({a: 1, b: 2});
assert(result.type === 'success', 'should succeed');

// should Flow check as writable
result.value.a = 3;