# Conventions¶

WebAssembly is a programming language that has multiple concrete representations (its binary format and the text format). Both map to a common structure. For conciseness, this structure is described in the form of an abstract syntax. All parts of this specification are defined in terms of this abstract syntax.

## Grammar Notation¶

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 $$\mathit{sym} ::= A_1 ~|~ \dots ~|~ A_n$$.
• 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.

## Auxiliary Notation¶

When dealing with syntactic constructs the following notation is also used:

• $$\epsilon$$ denotes the empty sequence.
• $$|s|$$ denotes the length of a sequence $$s$$.
• $$s[i]$$ denotes the $$i$$-th element of a sequence $$s$$, starting from $$0$$.
• $$s[i \href{../syntax/conventions.html#notation-slice}{\mathrel{\mathbf{:}}} n]$$ denotes the sub-sequence $$s[i]~\dots~s[i+n-1]$$ of a sequence $$s$$.
• $$s \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} [i] = A$$ denotes the same sequence as $$s$$, except that the $$i$$-the element is replaced with $$A$$.
• $$s \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} [i \href{../syntax/conventions.html#notation-slice}{\mathrel{\mathbf{:}}} n] = A^n$$ denotes the same sequence as $$s$$, except that the sub-sequence $$s[i \href{../syntax/conventions.html#notation-slice}{\mathrel{\mathbf{:}}} n]$$ is replaced with $$A^n$$.
• $$\href{../syntax/conventions.html#notation-concat}{\mathrm{concat}}(s^\ast)$$ denotes the flat sequence formed by concatenating all sequences $$s_i$$ in $$s^\ast$$.

Moreover, the following conventions are employed:

• The notation $$x^n$$, where $$x$$ is a non-terminal symbol, is treated as a meta variable ranging over respective sequences of $$x$$ (similarly for $$x^\ast$$, $$x^+$$, $$x^?$$).
• When given a sequence $$x^n$$, then the occurrences of $$x$$ in a sequence written $$(A_1~x~A_2)^n$$ are assumed to be in point-wise correspondence with $$x^n$$ (similarly for $$x^\ast$$, $$x^+$$, $$x^?$$). This implicitly expresses a form of mapping syntactic constructions over a sequence.

Productions of the following form are interpreted as records that map a fixed set of fields $$\mathsf{field}_i$$ to “values” $$A_i$$, respectively:

$\mathit{r} ~::=~ \{ \mathsf{field}_1~A_1, \mathsf{field}_2~A_2, \dots \}$

The following notation is adopted for manipulating such records:

• $$r.\mathsf{field}$$ denotes the contents of the $$\mathsf{field}$$ component of $$r$$.

• $$r \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} \mathsf{field} = A$$ denotes the same record as $$r$$, except that the contents of the $$\mathsf{field}$$ component is replaced with $$A$$.

• $$r_1 \href{../syntax/conventions.html#notation-compose}{\oplus} r_2$$ denotes the composition of two records with the same fields of sequences by appending each sequence point-wise:

$\{ \mathsf{field}_1\,A_1^\ast, \mathsf{field}_2\,A_2^\ast, \dots \} \href{../syntax/conventions.html#notation-compose}{\oplus} \{ \mathsf{field}_1\,B_1^\ast, \mathsf{field}_2\,B_2^\ast, \dots \} = \{ \mathsf{field}_1\,A_1^\ast~B_1^\ast, \mathsf{field}_2\,A_2^\ast~B_2^\ast, \dots \}$
• $$\href{../syntax/conventions.html#notation-compose}{\bigoplus} r^\ast$$ denotes the composition of a sequence of records, respectively; if the sequence is empty, then all fields of the resulting record are empty.

The update notation for sequences and records generalizes recursively to nested components accessed by “paths” $$\mathit{pth} ::= ([\dots] \;| \;.\mathsf{field})^+$$:

• $$s \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} [i]\,\mathit{pth} = A$$ is short for $$s \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} [i] = (s[i] \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} \mathit{pth} = A)$$.
• $$r \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} \mathsf{field}\,\mathit{pth} = A$$ is short for $$r \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} \mathsf{field} = (r.\mathsf{field} \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} \mathit{pth} = A)$$.

where $$r \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}}~.\mathsf{field} = A$$ is shortened to $$r \href{../syntax/conventions.html#notation-replace}{\mathrel{\mbox{with}}} \mathsf{field} = A$$.

## Vectors¶

Vectors are bounded sequences of the form $$A^n$$ (or $$A^\ast$$), where the $$A$$ can either be values or complex constructions. A vector can have at most $$2^{32}-1$$ elements.

$\begin{split}\begin{array}{lllll} \def\mathdef1486#1{{}}\mathdef1486{vector} & \href{../syntax/conventions.html#syntax-vec}{\mathit{vec}}(A) &::=& A^n & (\mathrel{\mbox{if}} n < 2^{32})\\ \end{array}\end{split}$