Examples of language extensions

All examples must be compiled using ``camlp4''. As they use the ``EXTEND'' instruction, Camlp4 must load ``pa_extend.cmo''. As they use the quotations to create abstract syntax tree nodes, Camlp4 must load ``q_MLast.cmo''. The compilation command is therefore:

    ocamlc -pp "camlp4o pa_extend.cmo q_MLast.cmo" -I camlp4-lib-dir -c file.ml

After this, the syntax extension takes place in Camlp4 when ``file.cmo'' is loaded.

Infix

This is an example to add the infix operator ``o'', composition of two functions. For the meaning of the quotation expr used here, see appendix A.

       open Pcaml;;
       EXTEND
         expr: AFTER "apply"
           [[ f = expr; "o"; g = expr -> <:expr< fun x -> $f$ ($g$ x) >> ]];
       END;;

Repeat until à la Pascal

The ``repeat...until'' loop of Pascal is closed to the ``while'' loop except that it is executed at least once. We can implement it like this:

       open Pcaml;;
       EXTEND
         expr: LEVEL "let"
           [[ "repeat"; e1 = expr; "until"; e2 = expr ->
                 <:expr< do $e1$; return while not $e2$ do $e1$; done >> ]];
       END;;