gleam-lang/gleam

Explicitly add list pattern when matching on non-empty list on the Erlang target

Open

#4,215 opened on Feb 4, 2025

View on GitHub
 (1 comment) (0 reactions) (0 assignees)Rust (960 forks)batch import
help wanted

Repository metrics

Stars
 (21,417 stars)
PR merge metrics
 (Avg merge 10d 19h) (69 merged PRs in 30d)

Description

From Erlang's efficiency guide:

Don't:

map_pairs1(_Map, [], Ys) -> Ys;
map_pairs1(_Map, Xs, []) -> Xs;
map_pairs1(Map, [X|Xs], [Y|Ys]) -> todo.

Do:

map_pairs2(_Map, [], Ys) -> Ys;
map_pairs2(_Map, [_|_]=Xs, [] ) -> Xs;
map_pairs2(Map, [X|Xs], [Y|Ys]) -> todo.

In the second example, since Xs can no longer match with anything, the compiler is free to reorder the branches and make it as efficient as possible. I wonder if the Gleam compiler could emit code that plays nicely with this when it can tell you're matching on a non-empty list.

Contributor guide

Research direction
Investigate how Gleam's compiler generates pattern matching for non empty lists when targeting Erlang, and propose a change to emit the optimized pattern as recommended by Erlang's efficiency guide (using `[ | ]=Xs` instead of `Xs`).
Tech stack
rust
Domain
backendperformance
Issue type
Performance
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
RustErlangcompiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50