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Review of OCaml-derived language features

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Évaluation

Difficulté
5/5
Temps estimé
Plus d'une semaine
Accessibilité débutants
35/100
Type d'issue
Bug
Clarté
Plutôt claire
Activité
Active
Stack technique
ocaml
Domaine
compilers

Piste de recherche

Start by reading the feature review and its test checklist in this issue, then inspect the linked test work in #9076 and the individual bug reports #9068–#9073 and #9095. The review checked runtime behavior, diagnostics, formatting, editor tooling and interface generation; tests should cover the listed language features, with regression tests added for each fix.

Rédigé par le modèle d'indexation à partir du texte de l'issue.

Description

ReScript inherited a number of language features from OCaml that most code never touches: GADTs, existential types, locally abstract types, polymorphic annotations, functors, recursive and first-class modules, extensible variants, private types, constraint, variance annotations, destructive substitution, module type of and #...t patterns. Several of them had little or no test coverage. This issue tracks a review of these features and the bugs it found.

Method
  • Usage: counted each feature in 21 public ReScript repositories (about 550k lines, including rescript-lang.org, rescript-relay, sury, rescript-nodejs and the two large Juspay apps), in the stdlib and in our test suites.
  • Behaviour: wrote small programs for each feature and checked what they do at run time, their error messages, the formatter, and editor hover, completion and interface generation.
Findings

Run-time behaviour was correct throughout, including GADTs with @as, @tag and @unboxed, existentials packed with first-class modules, and higher-order and generative functors. Every bug was in diagnostics, the formatter or the editor tooling.

Usage outside our own test fixtures:

Feature Use
GADTs, locally abstract types rare (rescript-urql, a few bindings)
Polymorphic annotations ('a.) used in sury
constraint rescript-nodejs (Stream, Net)
open! rescript-lang.org, rescript-pages, rescript-nodejs
module type of include module type of in a .resi (rescript-urql)
#...t patterns rescript-lang.org playground
User-defined open types none; only type exn +=
module rec, with type := / with module := none
Functors, first-class modules, private types, coercion common

None of these is a removal candidate. module rec and with type := / with module := have no uses in the corpus, but the corpus is mostly libraries and misses private application code. module rec is the only way to write mutually recursive modules. with type := / with module := amounts to a few branches in the type checker and parser, so removing it would save little. The remaining features are either used or depend on machinery that stays anyway (for example, exn is itself an open type).

Tests
  • Tests for GADTs, existentials, polymorphic fields, constraint, private and indexed extensible types, destructive substitution and typed first-class modules (#9076); each fix PR adds its own regression tests
Bugs
  • #9065
  • #9066
  • #9067
  • #9068
  • #9069
  • #9070
  • #9071
  • #9072
  • #9073
  • #9063 (reported separately)
  • #9095 (found while reviewing #9077)
Known limitations of the fixes
  • #9084: across files, Find References and rename don't connect a use of an applied functor's member (StrSet.add) to its declaration in the functor's file; within a file they do.
  • #9085: an extension declared inside a module whose signature hides it is still offered by completion.
  • #9083: a functor whose result is a module type identifier ((O: Ord): Set => …) hovers without its parameters.
Not bugs, noted
  • Refutation cases (| C => .) can't be written. This came up in #6987 together with empty variants; that issue was closed as stale without a decision.
  • exception X followed by a line starting with ( parses as exception X(...), the same newline sensitivity as f\n(a).
Langage dominant
OCaml
Étoiles
7.5k
Forks
483
Merge moyen
20 h 19 min
PR mergées (30 j)
242

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