Erlang external function with union return type

[qmg-tmay]

I've been playing around with Gleam and enjoying it so far! However, I've run into some pain points when attempting interop with Erlang.

My understanding from the docs is that Erlang atoms and tagged types can be modelled using Gleam's type variants but I'm not sure how to deal with scenarios where Erlang can return several different types with different structures.

For example, io:get_line can return either a string, an eof atom, or a tagged error {error, term}.

I've attempted to model this as:

import gleam/erlang/charlist.{type Charlist}

pub fn main() -> Nil {
    get_line(charlist.from_string("Hello: "))
}

@external(erlang, "io", "get_line")
fn get_line(prompt: Charlist) -> GetLineResponse

type GetLineResponse {
   Data(Charlist)
   Eof
   Error(ErlangAny)
}

type ErlangAny

This doesn't work and I end up with an error like:

runtime error: Erlang error

No pattern matched in an Erlang case expression.

unmatched value:
  "10\n"

Any thoughts on what I'm doing wrong?

[GearsDatapacks]

Data(Charlist) would be represented as {data, "..."}, which is not what the code is actually returning. Instead what you should do is write an erlang wrapper which calls get_line and returns something which can be represented as a Gleam type. The externals documentation may be of help: https://gleam.run/documentation/externals/

I would also recommend converting from a charlist to a binary string so you can perform regular Gleam string operations on the output.

If you just want to get user input, you can also use the input package which has written this FFI for you.

[qmg-tmay]

In this case, the input package does just what I need - thanks!

More generally, the erlang wrapper makes sense - I'll keep that in mind for the future