Review of OCaml-derived language features #9074

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:

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 (Test GADTs and other OCaml-derived type and module features #9076); each fix PR adds its own regression tests

Bugs

• Create interface breaks types of applied functors and include F(...) #9065
• Signature mismatch errors use OCaml wording #9066
• Non-recursive GADT gets a different error depending on which constructor is a GADT #9067
• Existential types are shown as $Any_'a in errors and hover #9068
• Destructuring an existential with let reports "this switch" #9069
• Formatter breaks let f: type a. ... = headers when the body spans several lines #9070
• Hover on a functor drops its parameter #9071
• Hover and completion on an applied functor show the functor body's types #9072
• No completion for extensible variant constructors #9073
• Record coercion can turn an immutable field into a mutable one #9063 (reported separately)
• Create interface drops attributes in front of type declarations #9095 (found while reviewing Print types from applied functors in created interfaces #9077)

Known limitations of the fixes

• Use the applied signature for modules created by functor application #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.
• Complete the constructors of extensible variants #9085: an extension declared inside a module whose signature hides it is still offered by completion.

Not bugs, noted

• Refutation cases (| C => .) can't be written. This came up in Empty variant type #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).