Concrete and Abstract Syntax
A Camlp4 preprocessor operates by loading a collection of compiled modules which define a parser as well as a pretty-printer: the parser converts an input program into an internal representation. This internal representation constitutes the abstract syntax tree (AST). It can be output in a binary form, e.g. it can be passed directly to one of the OCaml compilers, or it can be converted back into a clear text program. The notion of concrete syntax refers to the format in which the abstract syntax is represented.
For instance, the OCaml expression (1 + 2) can also be written ((+) 1 2) or (((+) 1) 2). The difference is only at the level of the concrete syntax, since these three versions are equivalent representations of the same abstract syntax tree. As demonstrated by the definition of a revised syntax for OCaml, the same programming language can use different concrete syntaxes. They would all converge to an abstract syntax tree in a unique format that a compiler can handle.
The abstract syntax tree is at the center of the syntax extensions, which are in fact OCaml programs. Although the definition of grammars must be done in OCaml, the parser that is being defined or extended is not necessarily related to OCaml, in which case the syntax tree that is being manipulated is not the one of OCaml. Several libraries are provided which facilitate the specific manipulation of OCaml syntax trees.
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