createLanguageThis inference collapses productions to unknown (Deno 2.6.8 / TS 5.9.2)

[ClaudiuCeia]

Summary

createLanguageThis is advertised as inference-friendly (so callers can avoid writing explicit language types), but on Deno 2.6.8 (TypeScript 5.9.2) the inferred types for productions referenced via this often collapse to Parser<unknown>. This causes downstream combinators like either(...) / seq(...) to yield unknown values and breaks type-checking.

Versions

• Deno: 2.6.8
• TypeScript: 5.9.2 (via Deno)
• combine: jsr:@claudiu-ceia/combine@0.2.6

Repro

Save as repro.ts:

import {
  createLanguageThis,
  either,
  map,
  number,
  regex,
  seq,
} from "jsr:@claudiu-ceia/combine@0.2.6";

export const Lang = createLanguageThis({
  Literal() {
    return map(regex(/k/i, "k"), () => 1000);
  },
  ShortLiteral() {
    return map(regex(/m/i, "m"), () => 1_000_000);
  },
  Inner() {
    return map(
      seq(number(), either(this.Literal, this.ShortLiteral)),
      ([num, lit]) => num * lit,
    );
  },
  parser() {
    return this.Inner;
  },
});

Run:

deno check repro.ts

Actual

Type error:

TS18046 [ERROR]: 'lit' is of type 'unknown'.

Expected

lit should be inferred as number (since both Literal() and ShortLiteral() return Parser<number>).

Workaround

Provide an explicit defs type via the generic parameter:

import type { Parser } from "jsr:@claudiu-ceia/combine@0.2.6";

type L = {
  Literal: () => Parser<number>;
  ShortLiteral: () => Parser<number>;
  Inner: () => Parser<number>;
  parser: () => Parser<number>;
};

export const Lang2 = createLanguageThis<L>({ /* same body */ });

Notes

This seems to be tied to how createLanguageThis’s signature infers T and then computes BoundFromThisDefs<T> (in src/language.ts). Inference appears to lose the specific return type for methods when those methods are used through this inside other methods.