Behavior changes and types are incorrect based on `intersection` array order

Author: Ethan826Created Jan 24, 2023Updated Jul 29, 2024

Bug report

When using t.intersection, the correctness of the types/behavior is affected by the order of the codecs in the array.

Current Behavior

I get no type error in circumstances in which the code fails at runtime (see the example below).

Expected behavior

I expected that if it's intentional that the order of elements in intersection is meaningful, that would be reflected in the types, or if it isn't, that the order of arguments wouldn't matter in runtime behavior.

Reproducible example

typescript

import * as E from "fp-ts/Either";
import { pipe } from "fp-ts/function";
import * as O from "fp-ts/Option";
import * as t from "io-ts";
import { DateFromUnixTime, optionFromNullable } from "io-ts-types";

describe("bug with types", () => {
  it("infers types incorrectly", () => {
    const SomeCodec = t.intersection([
      t.type({ date: optionFromNullable(DateFromUnixTime) }),
      t.record(t.string, optionFromNullable(t.unknown)),
    ]);

    const result = SomeCodec.decode({ date: 367722000, dog: "cat" });
    
    // No type error but it throws
    expect(() =>
      pipe(
        result,
        E.map(({ date }) =>
          pipe(
            date,
            // `d` isn't actually a date here. It's a number (like the input)
            O.map((d) => d.toISOString())
          )
        )
      )
    ).toThrow(TypeError);
  });

  it("infers types correctly with a different order", () => {
    const SomeCodec = t.intersection([
      // Move this up
      t.record(t.string, optionFromNullable(t.unknown)),
      t.type({ date: optionFromNullable(DateFromUnixTime) }),
    ]);

    const result = SomeCodec.decode({ date: 367722000, dog: "cat" });

    expect(() =>
      pipe(
        result,
        E.map(({ date }) =>
          pipe(
            date,
            O.map((d) => d.toISOString())
          )
        )
      )
    ).not.toThrow(TypeError);
  });

  it("infers types correctly without `optionFromNullable`", () => {
    const SomeCodec = t.intersection([
      t.type({ date: optionFromNullable(DateFromUnixTime) }),
      t.record(t.string, t.unknown),
    ]);

    const result = SomeCodec.decode({ date: 367722000, dog: "cat" });

    expect(() =>
      pipe(
        result,
        E.map(({ date }) =>
          pipe(
            date,
            O.map((d) => d.toISOString())
          )
        )
      )
    ).not.toThrow(TypeError);
  });
});

Your environment

I just wrote the code that does this today so haven't seen it fail or succeed with other io-ts versions.

Software	Version(s)
io-ts	2.2.20
fp-ts	2.13.1
io-ts-types	0.5.19
TypeScript	4.7.4

Source: gcanti/io-ts