[Caml-list] Custom prettyprinter for polymorphic datatype

[Caml-list] Custom prettyprinter for polymorphic datatype

[John R Harrison]

In CAML Light and OCaml, how do I install a custom printer for a
polymorphic datatype so that the subcomponents print in the usual way?

For example, suppose I decided to print lists with the elements
separated by "," instead of ";". (This is a bad idea of course, but
serves as a simple example.) Obviously I can write a recursive
function that passes over a list and does X at each element, but how
do I make X work on arbitrary types? Preferably as part of "X" the
custom printer would itself be called recursively if the elements
contain more lists.

If I could just get hold of a "standard printer" function

  print_value : 'a->unit

then presumably I could use that. Is this possible? Or is there a
better way?

John.
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Re: [Caml-list] Custom prettyprinter for polymorphic datatype

[Xavier Leroy]

> In CAML Light and OCaml, how do I install a custom printer for a
> polymorphic datatype so that the subcomponents print in the usual way?
> If I could just get hold of a "standard printer" function
>   print_value : 'a->unit
> then presumably I could use that. Is this possible? Or is there a
> better way?

It is not currently possible.  The system printer works by traversing
both the value to be printed and its type.  In the toplevel loop, the
type is provided by the type-checker.  But user-defined printers have
no convenient way to recover and manipulate this type information, so
they can't call back the system printer.

There is no really good solution to this problem.  The workaround that
I use is to write custom printers for polymorphic datatypes that are
parameterized by printers for the type parameters, e.g.

        print_list : ('a -> unit) -> 'a list -> unit
or
        print_hashtable : ('a -> unit) -> ('b -> unit) ->
                                ('a, 'b) Hashtbl.t -> unit

Then, I partially apply these printers to the (monomorphic) types of
interest and install the partial applications as custom printers:

        let print_int_list = print_list Format.print_int;;
        #install_printer print_int_list;;
        let print_string_list = print_list Format.print_string;;
        #install_printer print_string_list;;

Provided there are only a few list types you're interested in, this
can work.  But I agree a more general solution would be nicer.

- Xavier Leroy
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[Caml-list] Does ocamldebug work for the non-cygwin version of Ocaml 3.01 for Windows?

[Mattias Waldau]

If it doesn't it is a good reason to say it somewhere on the download
page. ocamldebug is really good, and makes debugging very simple.

/mattias

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Re: [Caml-list] Does ocamldebug work for the non-cygwin version of Ocaml 3.01 for Windows?

[Xavier Leroy]

> If it doesn't it is a good reason to say it somewhere on the download
> page. ocamldebug is really good, and makes debugging very simple.

No, ocamldebug doesn't work on the "pure Win32" port of OCaml, while
it works (albeit a bit slowly) on the Cygwin port.  And, yes, the Web
site should explain better the differences between these two ports.

Best regards,

- Xavier Leroy
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