io-ts decoding and custom types example

io-ts-playground.ts

import { either, isLeft, isRight } from 'fp-ts/lib/Either'
import * as it from 'io-ts'
import { Int, Type, TypeOf } from 'io-ts'
import { PathReporter } from 'io-ts/lib/PathReporter'

// custom type: integer string

const IntegerString = new Type<number, string, unknown>(
    'IntegerString',
    Int.is,
    (u, c) =>
        either.chain(it.string.validate(u, c), src => {
            const out = +src
            return Int.is(out) ? it.success(out) : it.failure(u, c, `${src} is not a valid integer`)
        }),
    String
)

// container type

const Container = it.interface({
    integerString: IntegerString,
    plainString: it.string
})

// decoding and error reporting

const failedDecode = Container.decode(JSON.parse(JSON.stringify({
    integerString: '114.514',
    plainString: 'this should fail'
})))

const error = PathReporter.report(failedDecode)[0]

if (isLeft(failedDecode)) {
    console.error(error)
} else {
    throw new Error('Assertion failed')
}

// static type extraction

const successDecode = Container.decode(JSON.parse(JSON.stringify({
    integerString: '-1919810',
    plainString: 'this should succeed'
})))

type ContainerT = TypeOf<typeof Container>

function printResult(result: ContainerT) {
    console.log(result)
}

if (isLeft(successDecode)) {
    throw new Error('Assertion failed')
} else {
    printResult(successDecode.right)
}