[mercury-users] Conditional compilation

[Ralph Becket]

Michael Day, Tuesday,  5 February 2002:
> 
> Is there any way to achieve conditional compilation in Mercury without 
> resorting to foreign_code #ifdef hackery?
> 
> More precisely, say you have a program that can do several different 
> things and you wish to specify which of those things should be compiled in 
> without having to edit the code manually each time. Presumably Mercury 
> itself has something along these lines for supporting a particular set of 
> grades, specified by the configure script?

You could have a module with various compilation flags set as simple
predicates:

	:- module compilation_options.
	:- interface.
	:- import_module bool.
	:- pred option1 is det.
	:- pred option2 is det.
	:- pred option3 is failure.
	:- pred option4 is det.
	...
	:- implementation.
	:- pragma inline(option1/0).
	option1.
	:- pragma inline(option2/0).
	option2.
	:- pragma inline(option3/0).
	option3 :- fail.
	:- pragma inline(option4/0).
	option4.
	...

Then you just write code like this:

	...
	( if option1 then <option1 specific code> ),
	( if option2 then <option2 specific code> ),
	( if option3 then <option3 specific code> ),
	( if option4 then <option4 specific code> ),
	...

The optionX predicates should be inlined and the if-thens optimized
away.

It's not hugely pretty, but it is doable.

- Ralph
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