[Caml-list] Polymorphic pretty printing

[Andrej Bauer <Andrej.Bauer@andrej.com>]

This is a question for gurus.

I am contemplating writing an enhanced toplevel that could display
graphics as we well as text (the evil plan is to replace Mathematica).
The first step seems to be: how to install a pretty printer for a
_polymorphic_ type.

As an example, consider this:

  type 'a set = { elements : 'a list }

I want a value of this type to print out as

  {a, b, c, ... d}

instead of

  {elements = [a; b; c; ...; d]}

The trouble is, how to print out the elements a, b, c, ..., d
since they are of a polymorphic type.

According to the somewhat old message at

http://caml.inria.fr/archives/200201/msg00234.html

I should use Toplevel.print_out_value to do this. But, print_out_value
expects an arguments of type Outcometree.out_value, and it is not
clear to me where I will get it. Is there another function that converts
an arbitrary value (of an arbitrary type!) to an Outcometree.out_value?

Am I supposed to rewrite half of toplevel.ml to get this working?

It would be helpful if there were, somewhere in the world, a minimal and
_complete_ example of how one can actually write a polymorphic pretty
printer. This _must_ be an FAQ.

Best regards,

Andrej

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