[Caml-list] Using Pango with Cairo in Ocaml

[Hendrik Boom <hendrik@topoi.pooq.com>]

I'm writing here because I seem to have exhausted the wisdom of the 
ocam-beginners mailing list.  If there's a better place for me to have 
sent this, and there probably is, please let me know.


I'm trying to use Pango with Cairo, and to do it in OCaml.

I have an example written in C, that draws a multicoloured wheel of 
copies of the word "text".  I got it from
https://developer.gnome.org/pango/stable/pango-Cairo-Rendering.html

It works fine in C.

But I'm running into a snag with the OCaml conversion.

To make a layout, Pango wants to use a value of type Pango.context, and 
all I have from Cairo is a value of type Cairo.context.

There seems to be no way to obtain a Pango context from a Cairo 
context.

The obvious thing would seem to be to look at the C code to see what it 
did, and look for the corresponding function in the OCaml library 
source.

But it turns out the C code just uses the Cairo context as a Pango 
context without any conversion at all.  Pango, apparently, will just 
talk to a Cairo context as if it were a Pango context.  Presumably 
that's the normal way to use Cairo in C.

Now I'm not just saying that in the working C code, the Pango context 
and the Cairo context are the same type; that would be meaningless 
because both aree of the general-purpose gobject type.  I'm saying they 
are the same *value*, i.e., the same pointer, pointing to the same 
thing.

My immediate question is thus, how to I get Pango.context from a 
Cairo.context.  I seem to want a type change in OCaml without a change 
in value.

Pango.context would appear to be defined in pango.ml:28

type context = [`pangocontext] obj


and for cairo.context I can merely find the line

type context

in both cairo.ml1 and cairo.ml.

(I didn't think type definitions could be this abbreviated, but there 
it is.  Is this some way of mentioning a type definition elsewhere tht 
I haven't found yet?)


In the earlier cairo, the one before cairo2, there was a function fo 
convert a Cairo context to a Pango context.  It is not present in 
cairo2.

Now it is possibile to obtain a Cairo context from a gtk window, and 
also a Pango context.  But since cairo can be used independently of gtk 
windows (for example, to prooduce a pdf), there should be a mechanism 
to obtain a pango context from a cairo context independent of any use 
of gtk.

Or is there some deep secret about OCaml that I haven't divined yet (I 
am an OCaml beginner) that there's some way of using the fact that the 
two types are the same in C?

Or is it a bug in Cairo or Pango that this conversion is absent?

-- hendrik