Conventions¶
The WebAssembly component specification defines a language for specifying components, which, like the WebAssembly core language, may be represented by multiple complete representations (e.g. the binary format and the text format). In order to avoid duplication, the static and dynamic semantics of the WebAssembly component model are instead defined over an abstract syntax.
The following conventions are adopted in defining grammar rules for abstract syntax.
Terminal symbols (atoms) are written in sans-serif font: \(\mathsf{i32}, \mathsf{end}\).
Nonterminal symbols are written in italic font: \(\mathit{valtype}, \mathit{instr}\).
\(A^n\) is a sequence of \(n\geq 0\) iterations of \(A\).
\(A^\ast\) is a possibly empty sequence of iterations of \(A\). (This is a shorthand for \(A^n\) used where \(n\) is not relevant.)
\(A^+\) is a non-empty sequence of iterations of \(A\). (This is a shorthand for \(A^n\) where \(n \geq 1\).)
\(A^?\) is an optional occurrence of \(A\). (This is a shorthand for \(A^n\) where \(n \leq 1\).)
Productions are written \(\mathit{sym} ::= A_1 ~|~ \dots ~|~ A_n\).
Large productions may be split into multiple definitions, indicated by ending the first one with explicit ellipses, \(\mathit{sym} ::= A_1 ~|~ \dots\), and starting continuations with ellipses, \(\mathit{sym} ::= \dots ~|~ A_2\).
Some productions are augmented with side conditions in parentheses, “\((\mathrel{\mbox{if}} \mathit{condition})\)”, that provide a shorthand for a combinatorial expansion of the production into many separate cases.
If the same meta variable or non-terminal symbol appears multiple times in a production, then all those occurrences must have the same instantiation. (This is a shorthand for a side condition requiring multiple different variables to be equal.)