gleam-lang/gleam

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

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#4,215 opened on Feb 4, 2025

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help wanted

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

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.
4
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding of Gleam compiler internalsErlang pattern matching optimization
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Examine the Gleam compiler's code generation for pattern matching on lists, particularly how it emits Erlang code for non empty list patterns. Look for existing optimizations in the Erlang compiler's efficiency guide to ensure the emitted code uses the '[ | ]=Xs' pattern to improve branch reordering. The implementation likely involves modifying the compilation pass that handles list patterns in the Erlang target.