Pragma directive

Pragma directive

The directive “#pragma” allows to evaluate expressions at parse time, useful, for example, to test syntax extensions by the statement EXTEND without having to compile it in a separate file.

To use it, add the syntax extension “pa_pragma.cmo” in the Camlp5 command line. It adds the ability to use this directive.

As an example, let’s add syntax for the statement ‘repeat’ and use it immediately:

#pragma
  EXTEND
    GLOBAL: expr;
    expr: LEVEL "top"
      [ [ "repeat"; e1 = sequence; "until"; e2 = SELF ->
            <:expr< do { $e1$; while not $e2$ do { $e1$ } } >> ] ]
    ;
    sequence:
      [ [ el = LIST1 expr_semi -> <:expr< do { $list:el$ } >> ] ]
    ;
    expr_semi:
      [ [ e = expr; ";" -> e ] ]
    ;
  END;

let i = ref 1 in
repeat print_int i.val; print_endline ""; incr i; until i.val = 10;

The compilation of this example (naming it “foo.ml”) can be done with the command:

ocamlc -pp "camlp5r q_MLast.cmo pa_extend.cmo pa_pragma.cmo" -I +camlp5 foo.ml

Notice that it is still experimental and probably incomplete, for the moment.