Codec.md

• Codec interface

Codec interface

export interface Codec<I, O, A> extends D.Decoder<I, A>, E.Encoder<O, A> {}

A codec is just a decoder and an encoder packed together.

The following laws must hold

1. pipe(codec.decode(u), E.fold(() => u, codec.encode)) = u for all u in unknown
2. codec.decode(codec.encode(a)) = E.right(a) for all a in A

You can build a new codec using the make helper

Example

import * as C from 'io-ts/Codec'
import * as D from 'io-ts/Decoder'
import * as E from 'io-ts/Encoder'
import { pipe } from 'fp-ts/function'

const decoder: D.Decoder<unknown, number> = pipe(
  D.string,
  D.parse((s) => {
    const n = parseFloat(s)
    return isNaN(n)
      ? D.failure(s, `cannot decode ${JSON.stringify(s)}, should be parsable into a number`)
      : D.success(n)
  })
)

const encoder: E.Encoder<string, unknown> = {
  encode: String
}

export const NumberFromString: C.Codec<unknown, string, number> = C.make(decoder, encoder)