RE: [Caml-list] Build number and date in OCaml?

[David Allsopp <dra-news@metastack.com>]

Julien Signoles wrote:
> 2013/4/9 Étienne André <Etienne.Andre@univ-paris13.fr>
> I've been using OCaml for a couple of years, but without using any advanced feature;
> so my question may be a little naive. Is there any way to insert easily the current 
> date and time of compiling, as well as, e.g., an incremental build number in an OCaml
> program? So that it is printed at runtime, e.g., in the program header.
>
> This kind of information is part of your build process and are not directly accessible
> in OCaml. If you want to access it in your OCaml program, you have to pass them from 
> the build environment to the program environment. As Jeremie Dimino said, the usual way
> is to general a small OCaml file at build time and to link it to your program.
> For instance, if you use 'make', you could have the following lines in your Makefile:
> VERSION=...
> config.ml: Makefile

NB - if you want the build stamp ever to be updated, it'll need to depend on more than Makefile. You can either have it depend on all of your ML source files or, if using GNU make, you could declare config.ml as .PHONY (which means it will be rebuilt at every invocation of make).

<snip>

> CMO_FILES = config.cmo ... (* other cmo files *)

There's a further subtlety which can come into play if your build has more than one output, which is to place config.cmo / config.cmx as an order-only dependency. For example, suppose your build system has two programs whose sources are Foo.ml and Bar.ml both of which depend on Common.ml. If you structure your Makefile as:

Config.ml: Foo.ml Bar.ml Common.ml
	...

foo.exe: Common.cmx Foo.cmx | Config.cmx
	...

bar.exe: Common.cmx Bar.cmx | Config.cmx
	...

then updates to Foo.ml will not cause bar.exe to be rebuilt. The advantage of this approach is that the generating of the build stamp does not interfere with make - i.e. you don't get any additional recompilations. Of course, there are times where you do want all output programs to have the same build stamp but most of the time (i.e. while developing!) you don't want a change in one small part of the system to force recompilation/linking of the whole system... 

HTH,


David