[Caml-list] toplevel printing of polymorphic values

[Caml-list] toplevel printing of polymorphic values

[Jeff Henrikson]

-- 
Hello,

I have written a double ended queue and I would like to install a pretty printer in the toplevel.  Just being able to print an 'a Dequeue.t like an 'a list would be fine.

I understand how to use the Format module to get the box, print the delimeters, etc.  The part I don't understand is how to get back to the generic 'a printer.  I cannot see any polypmorphic methods in all of the Format module.  What I want to access seems to be implemented in:

ocaml_source/toplevel/genprintval.ml:

    (* The main printing function *)

    let outval_of_value max_steps max_depth check_depth env obj ty =

Though if it is exposed, I'm sure it is in some other form.  Is what I'm trying to do possible?


Jeff Henrikson

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Re: [Caml-list] toplevel printing of polymorphic values

[Francisco Valverde]

Jeff Henrikson wrote:

Hi,

what I've been using are polymorphic versions of the pp_print_<type> in 
Format. For lists I use these two flavours (of course you can rip them 
from their modules):

    module Poly :
      sig

        module type PPRINTER =
          sig
        type 'a p

        (** A pretty printer in the spirit of Format.pp_print(ers)
           for singly polymorphic types *)
        val pp_print :
            pp:(Format.formatter -> 'a -> unit) ->
              Format.formatter -> 'a p -> unit
        (* [pp_print ~ppa ~ppb fter t]
          A formatter for polymorphic objects, it formats the data
          [t] to [fter] with the aid of formatter [pp].
          @param pp A pretty printer for the parameter type.
          @param fter A Format.formatter.
          @param t The data to be formatted.
          @return Done entirely for side-effects.
         *)
          end

        module type DELIMITED_PPRINTER =
          sig
        type 'a p

        val pp_print :
            ?pre:string -> ?pos:string -> ?sep:string ->
              pp:(Format.formatter -> 'a -> unit) ->
            Format.formatter -> 'a p -> unit
           (* [pp_print ?pre ?pos ?sep ~pp fter t]
          A formatter for polymorphic objects, it formats the data
          [t] to [fter] with the aid of formatter [pp]. It
            will first prefix with [pos], postfix with [pos]
            and intersperse all items with [sep] if they are
            provided.
          @param pre A prefix to enclose the string
          @param pos A postfix to enclose the string
          @param sep A separator for substructures in the string.
          @param pp A pretty printer for the parameter type.
          @param fter A Format.formatter.
          @param t The data to be formatted.
          @return Done entirely for side-effects.
        *)
          end

So that for pretty-printing lists I would use:

(** Defaults for pre, pos and sep print more or less in the standard way *)
(** The breakpoints are as tricky as ever. These aren't right *)
let pp_print ?(pre="[") ?(pos="]") ?(sep=";") ~pp fter =

  let rec plist fter = function
    | a::[] ->   fprintf fter "%a" pp a
    | a::rest -> fprintf fter "%a%s@ %a" pp a sep plist rest

  in function
      [] -> fprintf fter "@[<hov 2>%s%s@]" pre pos
    |  l -> fprintf fter "@[<hov 2>%s%a%s@]" pre plist l pos

So for printing integer lists you  can use,

# let pp_print_int_list = pp_print ~pp:Format.pp_print_int;;
val pp_print_int_list :
  ?pre:string ->
  ?pos:string -> ?sep:string -> Format.formatter -> int L.p -> unit = <fun>
#pp_print_int_list Format.std_formatter [1;2;3];;
[1; 2; 3]- : unit = ()
------------------------------------
Hope it helps!

    F. Valverde



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