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Date: 2001-06-12 (14:53)
From: jeanmarc.eber@l...
I just began to play seriously with Ocaml's toplevel and have the following 
remarks and suggestions:

1. Standard use of the .ocamlinit file:
You have the possibility of "customising" the toplevel by a suitable 
'.ocamlinit' file that is "#use"d before going in interactive mode. 
Unfortunately, the '.ocamlinit' file isn't "#use"d in script mode, making it 
impossible to create a *symmetric* environment for interactive and script mode. 
I would suggest to "#use" also the ".ocamlinit" file in script mode (the result 
being that it is put in front of your script file).

2. Type of user-defined printers:
The printing formatter system under OCaml is now everywhere in the compiler and 
the standard libraries unified toward the "Format.formatter -> ('a, 
Format.formatter, unit) format -> 'a type" approach, where all output functions 
receive a formatter and a format string, giving everywhere these nice "fprintf 
ppf "@[...@]" printo o ..." calls. This of course abstracts away the 
destination of your output, that has to be parameterised by the choice of a 
suitable formatter.
Unfortunately, user defined printers aren't of type Format.formatter -> ('a, 
Format.formatter, unit) format -> 'a, but must be of type ('a, 
Format.formatter, unit) format -> 'a. It seems (its not very clear from the 
manual) that user-defined printers always have to print on 
Format.std_formatter. Why ? I don't see any other explanation than historical 
reasons... :-) I nethertheless suggest to unify also this last part of the 
formatting system in OCaml. I don't think *many* OCaml users are using 
industrially user defined printers and those who do could, I think, accept this 
minor (and type tracked by the compiler!) change in their source.
Look at the relevant code in genprintval.ml (comments are mine):

let install_printer path ty fn = (* fn is the user defined printer *)
      let print_val ppf obj = (* we have a ppf here *)
        try fn obj with (* but we don't use it here !!!! *)
        | exn ->
           fprintf ppf "<printer %a raised an exception>" Printtyp.path path in
      printers := (path, ty, print_val) :: !printers

3. Consequence: a minor subtle bug in OCaml's toplevel:
The following case isn't treated correctly under OCamls toplevel:
- you use "script" mode.
- your script raises an exception.
- your exception type contains one or more values that are printed by a custom 

In this case, OCaml uses Format.err_formatter for its output (only for 
exceptions, as other results aren't printed, of course), look at topmain.ml:

let file_argument name =
  exit (if Toploop.run_script Format.err_formatter name Sys.argv then 0 else 2)

the exception will be printed on err_formatter, but calls recursively your 
custom formatter that outputs on std_formatter... resulting in a not beautiful 
"error message".

This shows how urgent it is to standardise *everywhere* on the same (powerful 
and practical) formatting technology :-).

Jean-Marc Eber
LexiFi Technologies

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