How can I use context from Cairo library

[bigos]

Context

Ignoring all the nonsense around I have problem with context.
https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-t

I have no idea how to correctly express the type, because the ocaml bindings give me no clues.

https://github.com/Chris00/ocaml-cairo

How can I modify my code so the context works? Here I use ptr long and get the error.

Error: This expression has type widget -> context -> 'a -> 'b -> 'c -> unit
but an expression was expected of type
widget ->
Signed.long Ctypes_static.ptr -> int -> int -> gpointer -> unit
Type context is not compatible with type
Signed.long Ctypes_static.ptr = (Signed.long, [ `C ]) pointer

let drawing_area_set_draw_func =
  foreign ~from:libgtk "gtk_drawing_area_set_draw_func"
    ( widget
    @-> funptr
          (widget @-> ptr long @-> int @-> int @-> gpointer @-> returning void)
    @-> gpointer @-> gpointer @-> returning void )

Proposal

No response

Tasks and updates

No response

[yallop]

The Cairo documentation has this:

typedef struct _cairo cairo_t;

i.e. cairo_t is an incomplete struct type. With ctypes the structure function is used to define struct types. Here's what the documentation for structure says:

val structure : string -> 's Ctypes_static structure typ

Construct a new structure type. The type value returned is incomplete and can be updated using field until it is passed to seal, at which point the set of fields is fixed.

The type ('_s structure typ) of the expression returned by the call structure tag includes a weak type variable, which can be explicitly instantiated to ensure that the OCaml values representing different C structure types have incompatible types. Typical usage is as follows:

type tagname
let tagname : tagname structure typ = structure "tagname"

Following the last part ("Typical usage is as follows"), you can define the cairo_t type like this (assuming you have Ctypes open):

type cairo_t
let cairo_t : cairo_t structure typ = structure "_cairo"

[bigos]

Sorry, I could not make sense of your answer and somebody suggested to do it manually. I got stuct with the type of cr. I am a noob and I do not know how to use your example in this context.

https://github.com/bigos/gtk_minimal/blob/8265e7d47b771c1cb434ca582c84a8c62b2a9ecf/bin/main.ml#L112

If you had time to look again and had an idea how to help I would appreciate.

[yallop]

(* Is it possible to use cairo2 library here, or I need to use my own bindings *)

Mixing your ctypes-based bindings with the cairo2 bindings won't work, because the two sets of bindings use different types. You'll have to bind everything using ctypes, or switch over entirely to using cairo2.

[bigos]

Thank you for clarification

[bigos]

Thank you very much!
I have been trying to learn OCaml for a few months, and this is the best answer I have seen so far. I like your clarity.