carbon-lang/executable_semantics/syntax/README.md

The code in this directory is responsible for translating Carbon source code to the AST defined in ast. It consists primarily of a Flex lexer defined in lexer.lpp and a Bison grammar defined in parser.ypp.

It is possible to define and test a new expression syntax without defining its semantics by using the UnimplementedExpression AST node type and the same techniques can be applied to other kinds of AST nodes as needed. See the handling of the UNIMPL_EXAMPLE token for an example of how this is done, and see unimplemented_example_test.cpp for an example of how to test it.