# 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.

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Copyright 2007-2017 Daniel de Rauglaudre (INRIA)