package olint

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# olint

Think clippy for OCaml — which is only fair, given that Rust's original compiler was written in OCaml. We're simply returning the favour.

A linter for OCaml source files. Catches antipatterns, partial function misuse, naming violations, and the kind of code that works perfectly well until a Tuesday in March at 3am when it very much doesn't. Uses compiler-libs directly,no custom parser, no PPX dependency, no build step required. Just very firmly held opinions.

Install

opam pin add olint git+https://github.com/Zaneham/Olint

Or, for those who enjoy the journey:

git clone https://github.com/Zaneham/Olint
cd olint
opam install cmdliner
dune build
dune install

Usage

Point it at things. It will find fault.

olint src/
olint lib/ bin/ test/
olint --severity warn src/   # only the serious stuff
olint --format json src/     # for machines, who judge silently
olint --list-rules           # know thy accuser
olint --lang lang/fr.txt src/

Example Output

src/parser.ml:42:3 [W001] unused open: `open Printf` is not referenced
  hint: remove the open or use a qualified name
src/eval.ml:87:5 [W003] List.length in comparison is O(n); compare against [] instead
  hint: use `xs <> []` or `xs = []` for emptiness checks
src/main.ml:12:1 [W005] List.hd may raise; consider a safe alternative
  hint: use List.hd_opt or pattern matching
src/util.ml:31:9 [W008] `if x then true else false` is redundant; use `x` directly
  hint: remove the if/else and use the condition directly

Exit codes: 0 clean, 1 warnings, 2 errors. An escalating scale of disappointment.

Rules

ID

Name

Sev

What it objects to

W001

unused-open

W

open Foo where Foo contributes nothing to proceedings

W002

redundant-match

W

Identity match — every arm returns its pattern, achieving nothing with great ceremony

W003

list-length-compare

W

List.length xs > 0 is O(n); traversing the entire list to check if it exists

W004

list-nth

W

List.nth — indexing into a linked list, the data structure equivalent of sending a letter to your neighbour

W005

partial-function

W

List.hd without a guard — works fine until the list is empty, at which point: scenes

W006

naming-convention

I

Enforces snake_case for values and PascalCase for modules, because convention is all that stands between us and barbarism

W008

bool-redundancy

W

if x then true else false — the boolean identity crisis

W009

eta-reduce

I

fun x -> f x — the unnecessary introduction. "Let me introduce you to someone you've already met"

W010

exn-control-flow

I

try Hashtbl.find ... with Not_found when find_opt is right there

Run olint --list-rules for the full catalogue of grievances.

Inline Suppression

Because the difference between a useful tool and an annoying one is the ability to say "yes, I know, shut up about it."

(* olint:disable W001 *)
open Printf  (* no warning — you've made your choice *)
(* olint:enable W001 *)

(* olint:disable-next-line W005 *)
let first = List.hd xs  (* just this once *)

JSON Output

For CI pipelines, editors, and other systems that prefer their criticism in machine-readable form:

olint --format json src/
[
  {
    "file": "src/parser.ml",
    "line": 42,
    "col": 3,
    "rule": "W001",
    "severity": "W",
    "message": "unused open: `open Printf` is not referenced",
    "hint": "remove the open or use a qualified name"
  }
]

Localisation

Every diagnostic has a stable ID (W001, W003). A developer in Osaka can Google "olint W003" regardless of what language the message is in. Architecture lifted wholesale from BarraCUDA's bc_err.c, because good ideas deserve to be stolen.

olint --lang lang/fr.txt src/

Translation file format (one insult per line):

W001=open inutilisé: `open %s` n'est pas référencé
W001.hint=supprimez le open ou utilisez un nom qualifié

English is compiled in as the default, largely because the errors were written in English first and we haven't got round to the others yet. PR's are accepted for other languages. Spanish and French speakers are warned that once my Duolingo hits 60, badly translated error messages are coming your way (so get in before then)

Architecture

  • Parsetree-based: Ast_iterator on the untyped AST. No .cmt files, no build step. olint src/ just works.
  • compiler-libs: Zero parsing code. The OCaml compiler parses; we merely pass judgement.
  • OCaml 5.4.0: Adapted for the 5.4 Longident.Ldot API change, which wrapped both arguments in Location.loc for reasons best known to the compiler team.
  • Minimal deps: compiler-libs.common + cmdliner.

License

Apache 2.0 — see LICENSE.