gleam-lang/gleam

Invalid Erlang code generated when comparing type containing a function in case guard

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#3,069 opened on Apr 29, 2024

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help wanted

Description

The following Gleam code compiles to invalid Erlang code. The problem is due to the guard expression and the presence of a function in the type being compared.

The comparison works as expected if the equality check is done directly, i.e. foo == hello_greeting_wrapper.

Note that everything works correctly when targeting JavaScript, this is an Erlang-specific issue.

type GreetingWrapper {
  GreetingWrapper(greeting_fn: fn() -> String)
}

fn hello_greeting() {
  "Hello!"
}

const hello_greeting_wrapper = GreetingWrapper(hello_greeting)

fn is_hello(foo: GreetingWrapper) {
  case foo {
    f if f == hello_greeting_wrapper -> True
    _ -> False
  }
}

pub fn main() {
  is_hello(hello_greeting_wrapper)
}

The output from gleam run is:

Compiling bug_repro
/bug_repro/build/dev/erlang/bug_repro/_gleam_artefacts/bug_repro.erl:16:41: illegal guard expression
%   16|         F when F =:= {greeting_wrapper, fun hello_greeting/0} ->
%     |                                         ^

error: Shell command failure

There was a problem when running the shell command `escript`.

Tested on Gleam v1.1.0 on macOS.

Contributor guide

Tech stack
rust
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Gleam language basicsRust programmingCompiler architecture understanding
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Examine the Gleam compiler source code, particularly the Erlang code generation for case guards. The error indicates that a fun reference is illegal in a guard expression on the Erlang side. Investigate the relevant code in `src/codegen/erlang.rs` or similar files to understand how comparisons in guards are compiled. Look for existing tests related to guard expressions with function references. The fix likely involves ensuring that comparisons involving functions are not placed directly in guards, or that the fun reference is appropriately wrapped.