Gleam integration tests with large API IDs: gleeunit suite behaves differently across Erlang and JavaScript targets

Answer to the question

Yes, the differing representation of Int is the most likely cause of the divergence you’re seeing. On the Erlang target Int is an arbitrary‑precision integer, but on the JavaScript target it becomes a Number, which can only represent integers safely up to 2^53‑1 (~9 × 10¹⁵). If your API returns IDs larger than that, decoding them into an Int on the JS target will silently lose precision, causing equality checks in your tests to fail while the Erlang run passes.

Typical idiomatic handling of external IDs

• Store the ID as a String (or a thin wrapper type) in the public API and in the test suite. Strings are safe on both targets and preserve the exact value.
• If you need numeric operations on the ID, validate the string against 2^53‑1 before converting to Int on the JS target, or use gleam/js’s js.BigInt for arithmetic.
• Prefer the wrapper approach: define a type like pub type Id = String and use it throughout your code. This keeps the semantics clear and avoids accidental integer overflows.

Gating target‑specific tests in gleeunit

Gleeunit has no built‑in tagging. Common patterns are:

• Separate files/directories: put JS‑only tests in tests/js/ and Erlang‑only tests in tests/erlang/. Run gleam test --target javascript tests/js/ or the Erlang equivalent.
• Compile‑time flag: pass --define IS_JS=true when building for JS. In your test code write

  let IS_JS = @define IS_JS
  if IS_JS {
    // JS‑specific assertions
  } else {
    // Erlang‑specific assertions
  }
  

• Environment variable: read process.env.GLEAM_TARGET (available in JS) or a custom env var in Erlang. Conditionally skip or adapt tests.

Practical verification steps

1. Write a test that decodes a JSON payload containing an ID larger than 2^53‑1 into a field of type Int. Assert that the value equals the literal.
2. Run gleam test --target erlang – it should pass.
3. Run gleam test --target javascript – it should fail, showing the loss of precision.
4. Replace the Int field with String (or a wrapper). Repeat the runs; both should now pass.
5. Add a compile‑time flag --define IS_JS=true and a conditional test that only triggers on the JS target. Verify that the test is excluded when the flag is absent.

What to ask next

To tailor the recommendation further, could you confirm the exact size of the IDs you receive from the external API? Knowing whether they exceed 2^53‑1 will determine if you need the wrapper approach or if a simple numeric check suffices.