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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Pragma directive