[Caml-list] Macro expansion for logging with call site (file & line)

[Viet Le <vietlq85@gmail.com>]

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Hi,

I would like to write a logger in OCaml. In C++ it's pretty straightforward
with macro expansion that gives correct call site (file & line):

$ cat -n cpp-logger.cpp
     1 #include <iostream>
     2
     3 #define LOG_START do {\
     4    std::cerr << __FILE__ << ":" << __LINE__ << ": "
     5 #define LOG_END std::endl; } while (0)
     6
     7 int main()
     8 {
     9    LOG_START << "Got problem!" << LOG_END;
    10    return 0;
    11 }

$ ./a.out
cpp-logger.cpp:9: Got problem!

In OCaml we have __LOC__ and __LOC_OF__:

http://caml.inria.fr/pub/docs/manual-ocaml/libref/Pervasives.html

However it's not trivial to get correct call site:

$ cat -n yahoo.ml
     1  print_string "yahoo\n";;
     2
     3  Printf.printf "__LOC__ = %s\n" __LOC__;;
     4
     5  let report s =
     6      let loc, _ = __LOC_OF__ s in
     7      Printf.printf "LOC of the expression (%s): %s\n" s loc ;;
     8
     9  report "bad bug" ;;

$ ./a.outyahoo
__LOC__ = File "yahoo.ml", line 3, characters 31-38
LOC of the expression (bad bug): File "yahoo.ml", line 6, characters 28-29

I expect line 9 but I get line 6 in OCaml. I would appreciate some help to
get this working. Does it involve campl4/5 / ppx?

Thanks.

-- 
Kind regards,
Viet

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