Typescript 4.7 beta ESM module config messes with io-ts type inference

[florianbepunkt]

🐛 Bug report

Using ESM support added in TS 4.7 causes io-ts to wrongly infer types.

Current Behavior

ESM

import * as COD from "io-ts/lib/Codec.js"
const a = COD.string;
const b = a.decode(1);  // b resolves to Kind2<M, E, A>

CJS

import * as COD from "io-ts/Codec"
const a = COD.string;
const b = a.decode(1);  // b resolves to Either

Expected behavior

b should be inferred as Either<DecodeError, string>

Reproducible example

I created a repro repo here: https://github.com/florianbepunkt/io-ts-esm-repro Main branch uses ESM module, a CJS branch also exists – both use typescript 4.7 beta. The example is super basic (three lines).

Your environment

Software	Version(s)
io-ts	2.2.16
fp-ts	2.12.1
TypeScript	4.7

[florianbepunkt]

Basically this issue is only about type inference. In ESM the TS compiler infers a Decoder as a Kleisli decoder... meaning this line

https://github.com/gcanti/io-ts/blob/1f23c7c4936690506eda3e1f7ad0066e84aa8bb1/src/Decoder.ts#L78

points the compiler to

https://github.com/gcanti/io-ts/blob/1f23c7c4936690506eda3e1f7ad0066e84aa8bb1/src/Kleisli.ts#L32-L34

Which is the inferred return type of DEC.string.decode().

If the decoder/encoder is explicitly typed like so

export interface Decoder<I, A> extends K.Kleisli<E.URI, I, DecodeError, A> {
  decode: (i: I) => E.Either<DecodeError, A>
}

then the issue is gone. So this is a quick fix. But what strikes me as odd... I have no clue what this has to do with ESM module at all. But as you can see in the link repro repo, it only happens in an ESM config.

[florianbepunkt]

I updated the demo repo here: https://github.com/florianbepunkt/io-ts-esm-repro

The issue still exists with TS 4.7.3