> Right now, I'm using constants (let var = ...) or mutable references
> (let var := ...) to encode such things.
> 
> However I'm not satisfied. With constants, I obviously can only modify
> them at compile time. With mutables, I can modify them but potentially
> at any time, meaning potential bugs.

I am not sure I understand your point, but the construction "let var =
..." does not define *compile time* constants: the expression "..." is
computed at *run time*. It can be any computation.

Typically, a program using the Xlib would start with:
    let dpy = xOpenDisplay "" in ...

The computation of xOpenDisplay "" is a very complicated computation of the
Xlib, creating a connection to the X server. But the variable "dpy" is not
mutable and then is never modified nor modifiable in the rest of the program.

-- 
Daniel de RAUGLAUDRE
daniel.de_rauglaudre@inria.fr
http://cristal.inria.fr/~ddr/