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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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

Research direction
Investigate the Erlang code generation for case guards in the Gleam compiler. The bug occurs when a custom type containing a function field is compared using the equality operator within a case guard. The generated Erlang code places a function reference directly in a guard expression, which is illegal. The fix likely involves either preventing the use of such comparisons in guards (by moving the equality check outside the guard) or generating a different Erlang construct (like a separate if expression) that avoids the guard restriction. Look at the code generation for case expressions, specifically the pattern compilation for guards involving custom types with function fields.
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.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
RustGleam
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65