[Bug] Builtin composite decoders cannot take decoders with non-`unknown` input types

Version

2.0.0-beta9

Steps to reproduce

1. Define a Decoder<O, I> where I is a type other than unknown
2. Attempt to pass the defined Decoder<O, I> to any of the builtin composite decoders

E.g.

import { Decoder, DecodeResult, optional } from 'decoders';
import { unknown } from 'decoders/annotate';
import { err, ok } from 'decoders/result';
import * as uuid from 'uuid';

const binaryUUIDFromStringUUID: Decoder<Uint8Array, string> = (uuidString: string): DecodeResult<Uint8Array> => {
  try {
    return ok(Uint8Array.from(uuid.parse(uuidString)));
  } catch (error) {
    return err(unknown(error));
  }
};

const optionalBinaryUUIDFromStringUUID = optional(binaryUUIDFromStringUUID);

Expected result

No type errors.

Actual result

TS2345: Argument of type 'Decoder<Uint8Array, string>' is not assignable to parameter of type 'Decoder<Uint8Array, unknown>'. 
  Type 'unknown' is not assignable to type 'string'.

Affected composite decoders/functions:

• guard
• compose
• predicate
• transform
• array
• nonEmptyArray
• set
• describe
• dict
• exact
• inexact
• mapping
• object
• either
• lazy
• maybe
• nullable
• optional
• taggedUnion
• tuple

Motivating use-cases

• The binary UUID example from above
• Decoding arrays with a known element type without having to re-assert the known type:

  const bigIntFromString: Decoder<bigint, string> = ...;
  const bigIntArrayFromStringArray: Decoder<bigint[], string[]> = array(bigIntFromString);
  

• Decoding objects with known value types without having to re-assert the known types:

  const booleanFromString: Decoder<boolean, string> = ...;
  const bigIntFromString: Decoder<bigint, string> = ...;
  const recordFromStringifiedValueRecord: Decoder<{ foo: boolean, bar: bigint }, { foo: string, bar: string }> =
    object({ foo: booleanFromString, bar: bigIntFromString });
  

• Piece-wise composition of decoders:

  const bigIntFromString: Decoder<bigint, string> = ...;
  const bigIntFromJSON: Decoder<bigint> = compose(compose(json, string), bigIntFromString);
  

[nvie]

Yeah, I confess the beta iteration cycles have been a bit wild 🙈 But I think the end result is much nicer and more future-proof so it’s done mostly to avoid another major breaking change soon.

Btw, regarding your specific use of compose, there also is the .then() method now which most closely resembles the old compose helper. (But I still think for the aforementioned cases you won’t need it.)

Please do let me know if you need help converting anything in particular that isn’t clearly documented enough – I much appreciate the early feedback and I’d be happy to help you out with anything.

[nvie]

Hey @treykasada – I just published 2.0.0-beta10 which enables the examples above. Would you like to give it a spin? Much appreciated! 🙏