Should case expressions compile to an if for each case?

[devinalvaro]

This Gleam code today:

  case model {
    NotFound -> todo
    Home(_) -> todo
    Upload(_) -> todo
  }

compiles to the following JS code.

  if (model instanceof NotFound) {
    ...
  } else if (model instanceof Home) {
    ...
  } else {
    ...
  }

Should it compiles to something like the following instead?

  if (model instanceof NotFound) {
    ...
  } else if (model instanceof Home) {
    ...
  } else if (model instanceof Upload) {
    ...
  } else {
    ...
  }

The former is correct for pure Gleam, but with FFI code not strictly typed it can lead to unpredictable behaviors.

The latter can more easily surface FFI bugs, but it is perhaps not clear what should run inside the else.

[lpil]

We don't do defensive programming to guard against mistakes from outside of Gleam code as it would have a notable impact on code size and runtime performance. Like with any JS code you must make sure you pass in the correct type of value otherwise anything unexpected can happen.

If you wish to have these checks yourself you can add them using the Dynamic data type.

[devinalvaro]

Since those concerns more relevant to production, then technically we could have a "development" build mode that does defensive programming? Although understandable if this is not a good idea due to the maintenance burden!

---

This suggestion came up because I was encountering a weird behavior in my project, and it took me some time to even realize that it was due to an FFI bug. After the realization it was straightforward to track down the bug. But it's probably a good idea to surface bugs like this as soon as possible in the development.

[lpil]

That would be a very large amount of work and have a large ongoing maintenance cost. We don't have the people or the money to take on expensive low value work, unfortunately.