2.2 Additional Syntactic Specifications

Grammar extensions

In addition to the rules of the standard Tiger grammar (see Syntactic Specifications), extensions adds the following productions.

# A list of decs metavariable
decs ::= _decs ( integer ) decs

exp ::=
  # Cast of an expression to a given type
   _cast ( exp , ty )
  # An expression metavariable
 | _exp ( integer )

lvalue ::=
  # Cast of a l-value to a given type
   _cast ( lvalue , ty )
  # A l-value metavariable
 | _lvalue ( integer )

# A type name metavariable
type-id ::= _namety ( integer )

Metavariables

The ‘_decs’, ‘_exp’, ‘_lvalue’, ‘_namety’ keywords are used as metavariables, i.e., they are names attached to an (already built) AST. They don’t create new AST nodes, but are used to retrieve existing nodes, stored previously. For instance, upon an _exp(51) statement, the parser fetches the tree attached to the metavariable 51 (an expression) from the parsing context (see the implementation for details).