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Custom Annotation Syntax for the Wasm Text Format




  • This proposal adds the ability to decorate a module in textual notarion with arbitrary annotations of the form (@id ...).

  • Neither the syntactic shape nor the semantics is prescribed by the Wasm specification, though the Appendix might include a description of optional support for name section annotations and generic custom sections.

  • This proposal only affects the text format, nothing else.


Extend the Text Format as follows:

  • Anywhere where white space is allowed, allow annotations of the following form:

    annot ::= "(@"idchar+  annotelem* ")"
    annotelem ::= keyword | reserved | uN | sN | fN | string | id | "(" annotelem* ")" | "(@"idchar+ annotelem* ")"

    In other words, an annotation can contain any sequence of tokens, as long as it is well-bracketed. No white space is allowed as part of the initial (@idchar+ delimiter.

  • The initial idchar+ is meant to be an identifier categorising the extension, and plays a role similar to the name of a custom section. By convention, annotations corresponding to a custom section should use the same id.

Extend the Appendix on the Custom Sections:

  • Define annotations reflecting the Name section, which take the form of annotations (@name "name"). They may be placed after the binder for any construct that can be named by the name section.

  • Define annotation syntax expressing arbitrary custom sections; cf. As with any matter concerning annotations, it is up to implementations how they handle the case where an explicit custom section overlaps with individual annotations that are associated with the same custom section.


Expressing generic custom sections (cf.

  (@custom "my-fancy-section" (after function) "contents-bytes")

Expressing names

(module (@name "Gümüsü")
  (func $lambda (@name "λ") (param $x (@name "α βγ δ") i32) (result i32) (get_local $x))

Web IDL bindings (cf.

  (func (export "f") (param i32 (@js unsigned)) ...)                        ;; argument converted as unsigned
  (func (export "method") (param $x anyref (@js this)) (param $y i32) ...)  ;; maps this to first arg
  (func (import "m" "constructor") (@js new) (param i32) (result anyref)    ;; is called as a constructor
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