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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