4 The Context-Free Level

The context-free level has explicit knowledge about the grammar for the language. For each nonterminal A of the grammar, a corresponding pattern is automatically generated, depending on the defining rule for A. For each rule type described in section 2, the list below describes the corresponding generated patterns.

1. Alternation: A pattern of the following form is generated:

   A: P(# #)

   where P is the pattern corresponding to the super-category of A. The pattern P is thus the super-pattern for A.
2. Constructor: A pattern of the following form is generated:

   A: P (# getT1: getson1(# #) putT1: putson1(# #) getT2: getson2(# #) putT2: putson2(# #) ... getTn: getsonn(# #) putTn: putsonn(# #) #)

   where P is the super-category of A. There is an attribute corresponding to each nonterminal on the right side of the rule. The suffix of the get- and put- attributes (Ti) is the same as the corresponding tag-name.
   If T is an instance of A, the i'th sub-AST can be accessed and changed through the get- and putTi operations. The put- and getsoni patterns have enter (respectively exit) parameters such that an AST can be inserted as the i'th sub-AST by ... Æ T.putTi and will be delivered by T.getTi Æ ...
3. List: A pattern of the following form is generated:

   A: List(# sonCat::< B #)

   where B is the name of the nonterminal on the right side of the rule. The super-pattern is List as the super-category of A is List.

Constructor rules are thus mapped into a composition hierarchy and alternation rules into a classification hierarchy.

By using the context-free level it is not possible for a programmer to construct an AST that violates the context-free syntax.