A Proper GUI for OCaml (Part One)

Some time ago, I wrote a GUI for a large OCaml project in C++ using WxWidgets, linked to OCaml in a single executable. It was hard and slow to write and crashed all over the place.

So I’ve redone it in a particularly low-tech way. Consider the following:

  • A GUI layer in pure Python, using the wxPython library
  • A GUI library written in pure OCaml

We start the two in separate processes, and have them talk via a socket. Sounds horrible – works fine.


Advantages of this as a long-term approach to the building of a proper OCaml GUI library:

  • Pure. No FFI involved
  • Wxwidgets is very good. It’s a whole platform and a mature one
  • Cross-platform by default. Native appearance on Windows, Unix, OS X
  • Effort required by OCaml community lower than for any other option
  • Build single executables for release with py2exe and py2app
  • Write OCaml interfaces to hundreds of python libraries

Disadvantages (please add more in the comments!)

  • Python and OCaml processes can fail independently
  • Might need to work on getting executable sizes down
  • Asynchronous event loops need care: work required to ensure mouse events don’t get out of sync etc – but this is fairly easy to overcome.

Here is a screenshot of our rendering engine using this approach on the Mac, on Windows and under Ubuntu.


I don’t have the time to write a generic OCaml / wxpython library at the moment, so I’m just going to stick with my special purpose implementation in our software for now.

In Part Two, I’ll release the basic libraries:

  • mltalk.py, pytalk.ml (Establishing a connection between Ocaml and Python processes)
  • camlpy.ml pycaml.py (Marshalling and Unmarshalling Ocaml and Python data)
  • Wxgui.ml (Event polling and synchronous events)
  • main.ml, main.py (Main programs)
  • Scripts for py2exe and py2app

Hopefully someone will have the time to build a proper library from this example.

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