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erlfmt

Opinionated Erlang code formatter. Built with rebar3, runs on OTP 24+.

Build & Test

rebar3 compile              # Compile
rebar3 ct                   # Run all tests (Common Test)
rebar3 ct --suite=test/erlfmt_SUITE --case=<test_name>  # Run a single test
rebar3 dialyzer             # Static type analysis
make check                  # Full check: compile, test, dialyzer, format check

The getopt dependency is only available under the test and release profiles. Running rebar3 ct automatically triggers escriptize as a pre-hook.

Formatting

make fmt                    # Self-format the project (builds release escript first)
make checkfmt               # Check formatting without writing

src/erlfmt_parse.erl is auto-generated from erlfmt_parse.yrl and excluded from formatting.

Project Structure

Source (src/)

File Role
erlfmt.erl Main module: escript entry (main/1), rebar3 plugin entry (init/1), API (format_file/2, format_string/2, read_nodes/1). Orchestrates the scan-parse-format pipeline. Also handles pragma logic and skip path for --require-pragma.
erlfmt_scan.erl Scanner wrapping erl_scan. Detects shebangs, collects comments, enriches token annotations.
erlfmt_parse.yrl Yecc grammar producing the custom AST (compiled to erlfmt_parse.erl).
erlfmt_recomment.erl Re-inserts comments into the AST as pre_comments/post_comments/pre_dot_comments.
erlfmt_format.erl Converts AST nodes to algebra documents via to_algebra/1. All layout rules live here.
erlfmt_algebra.erl Document algebra engine
erlfmt_cli.erl CLI option parsing, file glob expansion, parallel formatting execution.
rebar3_fmt_prv.erl Rebar3 provider plugin for rebar3 fmt.

Formatting pipeline

Input -> Scanner (erlfmt_scan) -> Parser (erlfmt_parse)
      -> Recommenter (erlfmt_recomment) -> Formatter (erlfmt_format)
      -> Algebra renderer (erlfmt_algebra) -> Output

Tests (test/)

Uses Erlang Common Test. Main suite is erlfmt_SUITE.erl with test data in erlfmt_SUITE_data/.

Test patterns:

  • Parser tests: parse strings and ?assertMatch on AST structure
  • Snapshot tests: snapshot_same/2 verifies idempotent formatting; snapshot_formatted/2 compares .erl against .erl.formatted
  • Pragma tests: test --require-pragma, --insert-pragma, and skip behavior
  • Error tests: verify error handling for invalid directives
  • Range tests: test formatting of file subsets