Nesc-internals/AST

Objects

The abstract syntax tree (AST) is defined using a "pure data" (no methods) object-oriented style:

• a node type is defined by its name, super-type, and fields; there is a root type called node
• the actual fields of a node type are the union of its own fields and of those of its super-type
• all node types have a kind field that identifies the nodes actual type, allowing for up- and down-casts

Because C is not an object-oriented language, these node types are defined in a file called nodetypes.def, and a series of emacs-lisp scripts generate C types, macros and functions for manipulating the AST nodes. Specifically, if X is a node definition with fields f1, ..., fn and super-type Y then

• X and Y are C pointer types to AST_X, AST_Y structures respectively
• AST_X, AST_Y have an initial integer field called kind
• the other fields of AST_X are: the fields of AST_Y, followed by f1, ..., fn
• new_X, new_Y allocate nodes of type X and Y respectively; these constructors take as arguments a subset of the fields of X, Y (the choice is made on a per-field basis in nodetypes.def)
• CAST(T, e) does a checked cast of expression e to type T; T must be a super- or sub-type of the actual (runtime) type of e (a pointer to some node)
• CASTPTR(T, e) does the same as CAST(T, e) except that e must evaluate to a pointer to a pointer to some node (CASTSRPTR(T, e) is the same as CASTPTR(T, e))