Other extensions

Chapter 6

Other extensions

6.1

Extensible functions

Camlp4 provides a system of functions defined by pattern matching, which are extensible.

The library module is Extfun and the syntax to be loaded is "pa_extfun.cmo". The empty function is Extfun.empty. You can extend a function using the statement extfun whose syntax is:

extfun expression with

[ pattern-1 -> expression-1

| pattern-2 -> expression-2

...

| pattern-n -> expression-n ]

The patterns are ordered in lexicographic order (for example, in a tuple, first comparing the first elements of the tuple). Variables are inserted after constructors. In an extension, the patterns do not need to be in a ``good'' order, since they are sorted. Non exhaustive pattern matching do not generate a warning.

``Or'' patterns can be used only at the first level. In this case:

          pat1 | pat2 -> expr

The binding is split into two cases (the expr is duplicated):

            pat1 -> expr
          | pat2 -> expr

Internal ``or'' patterns inside patterns are not accepted.

The statement extfun returns another extensible function. The type of extensible functions is ('a, 'b) Extfun.t. To use an extensible function, one must use the function Extfun.apply which transforms it in a function of type 'a -> 'b. If matching failed, such a function raises the exception Extfun.Failure.

The contents (patterns) of an extensible function can be displayed using Extfun.print.

Remark: extensible functions are not efficient: when applied, all patterns are tested, one by one, until one of them matches.

Extensible functions are used in Camlp4 extensible pretty printing.

6.2

Functional streams

The functional streams are another implementation of streams. Like normal streams, their contents can be accessible only one element at a time. But the elements are not removed. A functional stream parser returns the couple of a result and the remaining stream.

The library module is Fstream and the syntax to be loaded is "pa_fstream.cmo". The syntax of a functional