Re: [Caml-list] choosing modules at runtime

[Andreas Rossberg <rossberg@ps.uni-sb.de>]

Henri Dubois-Ferriere wrote:
> 
> let run_algo =
>   if (use algorithm 1) then
>   M_1.run_algo
>   else if (use algorithm 2) then
>   M_2.run_algo ...
> 
> and then simply call run_algo from then on. (But this needs to be done for
> every function/value that the signature M exports).

It's not too bad, just do:

 let (algoA, algoB, algoC) =
   if (use algorithm 1) then
   (M_1.algoA, M_1.algoB, M_1.algoC)
   else if (use algorithm 2) then
   (M_2.algoA, M_2,algoB, M_2.algoC) ...

But admittedly it still is somewhat annoying.

> Am I missing something obvious here? Of course what I would conceptually
> like to do would be something like
> 
> let Mod = if (use algorithm 1) then M_1 else if (use alg 2) then M_2 ..
> 
> and then be able to call Mod.run_algo(), but that probably belies that I
> am still thinking in an OO manner...

No, you are right. Unfortunately, this either requires 1st-class
modules, or if-then-else on the module level. (The latter can be
partially faked by heroic use of higher-order functors, but you don't
want to see that and it most likely does not apply to your particular
problem, since the condition itself had to be expressible in the module
language). Both is not (yet?) available in OCaml, and in very few other
languages, for that matter (Moscow ML currently being the only MLish
exception). Such a feature is not impossible to add, but represents a
significant complication of the language that may not be considered
worthwhile.

	- Andreas

-- 
Andreas Rossberg, rossberg@ps.uni-sb.de

"Computer games don't affect kids; I mean if Pac Man affected us
 as kids, we would all be running around in darkened rooms, munching
 magic pills, and listening to repetitive electronic music."
 - Kristian Wilson, Nintendo Inc.
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