There are several implementation issues that must be handled for this extension. A more complete discussion may be found in our GPCE paper, Reflection in Attribute Grammars
Due to the modular nature of Silver programs, it is possible to reuse the entire concrete syntax of ableC as an "embedded DSL" by exporting it from within the extension. Concrete "bridge productions" from Silver to ableC are supplied, defining the various new Silver constructs for embedding ableC. These productions demand the ast of the wrapped ableC concrete syntax, forwarding to abstract counterparts that perform the actual translation.
The antiquote constructs are effectively an extension to ableC, in turn treating Silver as an embedded DSL. For each antiquote there is a corresponding concrete and abstract ableC-to-Silver bridge production. Note that the abstract bridge productions have no real forward, as they will be translated away in the undecorated tree, and thus forward to an error message to avoid errors with the modular well-definedness analysis.
Once an ableC AST has been produced, we must somehow generate a translation into the Silver Expr AST that, when evaluated at run-time, will construct this ableC AST. To do this, Silver has a general purpouse reflection library that can transform any tree into a generic tree representation, the nonterminal AST:
nonterminal AST;
abstract production nonterminalAST
top::AST ::= prodName::String children::ASTs annotations::NamedASTs
{}
abstract production listAST
top::AST ::= vals::ASTs
{}
abstract production stringAST
top::AST ::= s::String
{}
abstract production integerAST
top::AST ::= i::Integer
{}
...
The library also supplies a (foreign) function reflect :: (AST ::= a) that transform a value of an arbitrary type, in this case an ableC AST nonterminal, into its corresponding AST representation. A new analysis (synthesized attribute) to compute the translation into a Silver Expr constructing this tree is defined on AST, using aspect productions.
Translating antiquote bridge productions from ableC back to Silver present a challenge: we must somehow embed the antiquoted Silver Expr directly within the Silver Expr translation, but reflecting the entire ableC AST will also reflect contained Silver Exprs into ASTs. Attempting to extract these Exprs from the ableC tree prior to reflection is unfeasible, as some set of attributes would need to be written for the entire abstract syntax of ableC. Thus, a method of "reifying" an AST back into an actual value is needed; to do this, a function reify :: (Either<a String> ::= AST) is built into Silver, that either constructs a value of its result type, or returns an error if the AST is not valid for that type. Thus when translating an AST into a Silver Expr and an antiquote production is encountered, its child can be reifyed back to a Silver Expr and used in the translation. Variations on this basic idea are also possible for more complex types of antiquote productions.
Early design discussion may be found in the following github issues: