The Fortress Language Specification - CiteSeerX

36.5 Expanders for Fortress
As the above examples demonstrate, it is often useful to denote Fortress abstract syntax using Fortress concrete syntax.
A special set of syntax expanders are defined in the API Fortress.Syntax for every nonterminal in Fortress grammar,
defined in Appendix G. The name of each expander consists of the name of the nonterminal in lowercase. The
terminating symbol for each nonterminal is the special reserved word end . For example:
expr
x + y
end
expands to the SyntaxTree:
Call(Empty,
VarRef(Identifier(“+”)),
VarRef(Identifier(“x”)),
VarRef(Identifier(“y”)))
When one of these syntax expanders parses a binding construct, the bound identifier is replaced with an identifier
resulting from a call to the function gensym , which yields an identifier distinct from all other identifiers bound in
any component installed in the same fortress, or in any other fortress, anywhere, throughout all time past, present, and
future. All variable references captured by the original identifier are replaced with references to the new identifier.
For convenience, a syntax expander with opening delimiter ≪ and terminating delimiter ≫ behaves identically to
the expr expander when Fortress.Syntax is imported.
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