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Chapter 2 Quotations

This chapter presents the Camlp4 quotation system. A quotation is a part of a program enclosed by special parentheses in position of expression or pattern (exclusively), and whose treatment is done by an user function, called a ``quotation expander''. Several quotations are usable in the same source code.

When Camlp4 encounters such a construction, the appropriate quotation expander is called with the string contents of the quotation.

There are two kinds of quotation expanders:

• Those returning strings. The result of the expander is parsed by Camlp4 afterwards. They are easy to use, but they depend on the language syntax: the returned string must be of OCaml syntax if the language syntax used is the normal syntax, in revised syntax if the language syntax is the revised syntax, etc.
• Those returning syntax trees. They are independent from the language syntax used, but to use them, one muse know how to create syntax tree nodes. See appendix A.

2.1 Syntax

There are two kinds of quotations:

• The anonymous quotations start with ``<<'' and end with ``>>''. The associated expander is the ``default'' expander.
• The named quotations start with ``<:id<'' and end with ``>>'', where ``id'' is any identifier. The associated expander is the expander named ``id''.

The string inside a quotation is any string. It can hold other quotations. The string sequences ``<<'', ``<:id<'' and ``>>'', if not matching, must be prefixed by a backslash and backslashes must be doubled.

2.2 Creating a quotation expander

A quotation expander is a function written in OCaml. A call to the Camlp4 library function ``Quotation.add'' (see section 7.7) adds the quotation expander. The variable ``Quotation.default'' holds the name of the default quotation.

The file holding the quotation expander must be compiled using the OCaml compiler, with the Camlp4 library directory in its directory path (option ``-I''). An object file (ending with ``.cmo'') is created, which is loadable in Camlp4, as parameter of the command ``camlp4'', or in the OCaml toplevel using the directive ``#load''.

2.3