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Chapter 4 Syntax extensions

This chapter introduces the way to extend the syntax of OCaml, and how to redefine the whole syntax of the language.

4.1 The abstract syntax tree

A syntax extension or redefinition holds grammar rules which must return abstract syntax tree nodes. The provided file ``q_MLast.cmo'' holds quotations expanders allowing to create such nodes using a concrete syntax: the description of the abstract syntax tree is not necessary. The complete description of these quotations is given in appendix A.

4.2 Extending the language

The language predefined syntaxes, ``pa_o.cmo'' and ``pa_r.cmo'' use the Camlp4 grammar system: so they are extensible.

The module ``Pcaml'' holds grammars and entries variables used by the language predefined syntaxes files. Syntax extensions can be done by extensions of the appropriate entries.

In some entries, ``pa_o.cmo'' and ``pa_r.cmo'' define labels allowing to locate some levels and to add new rules relative to these levels. The entries and their labels are:

• ``Pcaml.expr'', for expressions, with the labels:
  • ``let'' corresponding to the level of the instruction ``let...in''.
  • ``||'', ``&&'', ``<'', ``+'', ``*'', ``**'' corresponding to the so named operator.
  • ``apply'' corresponding to the curryfied application.
  • ``simple'' corresponding to simple expressions.
• ``Pcaml.patt'', for patterns, with the label ``simple'' corresponding to simple patterns.
• ``Pcaml.ctyp'', for types, with the label ``simple'' corresponding to simple types.
• ``Pcaml.sig_item'' and ``Pcaml.str_item'', for signature and structure items, with only one level labelled ``top''.
• ``Pcaml.module_type'' and ``Pcaml.module_expr'', for module types and expressions.
• Some other labels exist but are not documented.

4.3 Examples of language extensions

All examples must be compiled using ``camlp4''. As they use the ``EXTEND'' instruction, Camlp4 must load ``pa_extend.cmo''. A