Types

Most types are universally valid. However, restrictions apply to function types as well as the limits of table types and memory types, which must be checked during validation.

Limits

Limits must have meaningful bounds that are within a given range.

\(\{ \href{../syntax/types.html#syntax-limits}{\mathsf{min}}~n, \href{../syntax/types.html#syntax-limits}{\mathsf{max}}~m^? \}\)

  • The value of \(n\) must not be larger than \(k\).
  • If the maximum \(m^?\) is not empty, then:
    • Its value must not be larger than \(k\).
    • Its value must not be smaller than \(n\).
  • Then the limit is valid within range \(k\).
\[\frac{ n \leq k \qquad (m \leq k)^? \qquad (n \leq m)^? }{ \href{../valid/types.html#valid-limits}{\vdash} \{ \href{../syntax/types.html#syntax-limits}{\mathsf{min}}~n, \href{../syntax/types.html#syntax-limits}{\mathsf{max}}~m^? \} : k }\]

Function Types

Function types may not specify more than one result.

\([t_1^n] \href{../syntax/types.html#syntax-functype}{\rightarrow} [t_2^m]\)

  • The arity \(m\) must not be larger than \(1\).
  • Then the function type is valid.
\[\frac{ }{ \href{../valid/types.html#valid-functype}{\vdash} [t_1^\ast] \href{../syntax/types.html#syntax-functype}{\rightarrow} [t_2^?] \mathrel{\mbox{ok}} }\]

Note

The restriction to at most one result may be removed in future versions of WebAssembly.

Table Types

\(\href{../syntax/types.html#syntax-limits}{\mathit{limits}}~\href{../syntax/types.html#syntax-elemtype}{\mathit{elemtype}}\)

  • The limits \(\href{../syntax/types.html#syntax-limits}{\mathit{limits}}\) must be valid within range \(2^{32}\).
  • Then the table type is valid.
\[\frac{ \href{../valid/types.html#valid-limits}{\vdash} \href{../syntax/types.html#syntax-limits}{\mathit{limits}} : 2^{32} }{ \href{../valid/types.html#valid-tabletype}{\vdash} \href{../syntax/types.html#syntax-limits}{\mathit{limits}}~\href{../syntax/types.html#syntax-elemtype}{\mathit{elemtype}} \mathrel{\mbox{ok}} }\]

Memory Types

\(\href{../syntax/types.html#syntax-limits}{\mathit{limits}}\)

  • The limits \(\href{../syntax/types.html#syntax-limits}{\mathit{limits}}\) must be valid within range \(2^{16}\).
  • Then the memory type is valid.
\[\frac{ \href{../valid/types.html#valid-limits}{\vdash} \href{../syntax/types.html#syntax-limits}{\mathit{limits}} : 2^{16} }{ \href{../valid/types.html#valid-memtype}{\vdash} \href{../syntax/types.html#syntax-limits}{\mathit{limits}} \mathrel{\mbox{ok}} }\]

Global Types

\(\href{../syntax/types.html#syntax-mut}{\mathit{mut}}~\href{../syntax/types.html#syntax-valtype}{\mathit{valtype}}\)

  • The global type is valid.
\[\frac{ }{ \href{../valid/types.html#valid-globaltype}{\vdash} \href{../syntax/types.html#syntax-mut}{\mathit{mut}}~\href{../syntax/types.html#syntax-valtype}{\mathit{valtype}} \mathrel{\mbox{ok}} }\]