#9136 Keep @tag and runtime annotations when printing a type declaration

acdf563eef100e5b8d34a9803830c81bd1771b76..pr-9136

08279c285 · cknitt + Claude · Oct 30, 2026, 07:14 UTC

Keep @tag and runtime annotations when printing a type declaration

Why

The printer of type declarations kept only @immediate and @unboxed. @tag, @notUndefined, @instanceOf and @typeOf were dropped, so a signature mismatch about them printed two declarations that looked the same, hover hid them, and createInterface wrote an interface for an applied functor that does not match its implementation (#9134).

modifiedDiagnostic @tag mismatch

// Foo.res@tag("type")type t =  | @as("a") A  | B// Foo.resi@tag("kind")type t =  | @as("a") A  | B

diagnostic

Before

After

Actual declaration (Foo.res:2:1-4:5):Actual declaration (Foo.res:2:1-4:5): type t = @as("a") A | B @tag("type") type t = @as("a") A | B Expected declaration (Foo.resi:2:1-4:5):Expected declaration (Foo.resi:2:1-4:5): type t = @as("a") A | B @tag("kind") type t = @as("a") A | B Their @tag annotations differ.Their @tag annotations differ.

modifiedDiagnostic @notUndefined functor parameter

module Make = (  X: {    @notUndefined type t  },) => {  type u = X.t}module Applied = Make({  type t = option<int>})

diagnostic

Before

After

Actual declaration (not_undefined_functor_argument.res:10:3-22):Actual declaration (not_undefined_functor_argument.res:10:3-22): type t = option<int> type t = option<int> Expected declaration (not_undefined_functor_argument.res:3:19-24):Expected declaration (not_undefined_functor_argument.res:3:19-24): type t @notUndefined type t Their runtime representations differ:Their runtime representations differ: Actual declaration: can hold undefined Actual declaration: can hold undefined Expected declaration: @notUndefined Expected declaration: @notUndefined

modifiedFormat createInterface for an applied functor

module Tagged = (X: Type) => {  @tag("kind") type wrapped = Value({value: X.t}) | Empty({reason: string})}module IntTagged = Tagged({  type t = int})

rescript

Before the interface does not compile against the implementation

After

module IntTagged: {module IntTagged: { type wrapped = Value({value: int}) | Empty({reason: string}) @tag("kind") type wrapped = Value({value: int}) | Empty({reason: string})}}

modifiedRepresentation printed declaration annotations, Printtyp.tree_of_type_decl

representation

Before

After

otype_immediate: bool ↦ @immediateotype_unboxed: bool ↦ @unboxedotype_attributes: string list ↦ @immediate @unboxed @tag("…") @notUndefined | @instanceOf("…") | @typeOf("…")

sourcewhat the declaration itself states, not what it inherits from an abbreviated type

Review

Why read the attributes rather than the declaration's runtime shape? The shape of an abbreviation can come from the type it abbreviates; the printed declaration should show what it states itself, as the source does.
Can printing raise for a malformed annotation? No. The annotations are parsed by the functions that typing uses, which raise on malformed payloads; those were reported when the declaration was typed, and the printer then leaves the annotation out.

922b66364 · cknitt + Claude · Oct 30, 2026, 15:50 UTC

Say when declarations that differ read the same

Why

With the annotations printed, two declarations can still read the same and not match, when a type they mention differs between the two sides, e.g. a base class that the interface leaves abstract. The error then showed the same text twice, which reads as a bug in the compiler. The reasons now start with a note saying so, which replaces the lead-in of a difference in runtime representation.

modifiedDiagnostic declarations that print the same

// Classes.res@instanceOf("Animal") type animal@instanceOf("Dog") type dog = private animal// Classes.resitype animal@instanceOf("Dog") type dog = private animal

diagnostic

Before

After

Actual declaration (Classes.res:2:20-44):Actual declaration (Classes.res:2:20-44): @instanceOf("Dog") type dog = private animal @instanceOf("Dog") type dog = private animal Expected declaration (Classes.resi:2:20-44):Expected declaration (Classes.resi:2:20-44): @instanceOf("Dog") type dog = private animal @instanceOf("Dog") type dog = private animal Their runtime representations differ:The declarations read the same, but a type they mention differs: Actual declaration: instances of Dog, a subclass of Animal Actual declaration: instances of Dog, a subclass of Animal Expected declaration: instances of Dog Expected declaration: instances of Dog

Review

Why compare the printed text? It is what the reader sees, and the case to catch is exactly two declarations that look the same.

6176c4d28 · cknitt + Claude · Oct 30, 2026, 16:06 UTC

Escape a @tag name when printing it

Codex review of #9136: the printed @tag put the decoded name between quotes as it is, so a tag with a quote printed as invalid source and a backslash changed its value when reparsed, also in interfaces written by createInterface. It is now encoded as a string literal, which keeps non-ASCII letters as they are.

modifiedDiagnostic @tag names with a quote, a backslash and a non-ASCII letter

module M: {  @tag("k\"ind") type t = A({x: int}) | B({y: int})} = {  @tag("typ\\é") type t = A({x: int}) | B({y: int})}

diagnostic

Before

After

Actual declaration (tag_name_escaped.res:6:18-51):Actual declaration (tag_name_escaped.res:6:18-51): @tag("typ\é") type t = A({x: int}) | B({y: int}) @tag("typ\\é") type t = A({x: int}) | B({y: int}) Expected declaration (tag_name_escaped.res:4:18-51):Expected declaration (tag_name_escaped.res:4:18-51): @tag("k"ind") type t = A({x: int}) | B({y: int}) @tag("k\"ind") type t = A({x: int}) | B({y: int})