Re: Evaluating a variable only once after a recipe has run

[R. Diez]

> Oh.  Well, you should just ask :)

Many thanks, you are a star.  ;-)


> [...]
> Deferred = $1 = $$(eval $1 := $$$$(shell $2))$$($1)

Is there a way to allow variable number of arguments? This is so that all of 
the following work:

  $(eval $(call Deferred,OUTPUT,some-command))
  $(eval $(call Deferred,OUTPUT,some-command arg1))
  $(eval $(call Deferred,OUTPUT,some-command arg1 arg2))

I could not see in the GNU Make documentation an indication about what happens 
if the function uses 2 arguments and you pass 3.


I have yet another question: why is $(eval $(call ...)) necessary? Is it not 
possible to write it so that the user only needs a $(call ) ?


> There are issues with this.  If the command you want to run has
> unmatched ")" you have problems.

I am guessing that escaping a ')' is not possible. But I can live with:

CLOSING_PARENTHESIS := )

  $(eval $(call Deferred,OUTPUT,some-command $(CLOSING_PARENTHESIS)))

Or will the nested $(eval) invocations break the usual trick with 
CLOSING_PARENTHESIS ?


> Also if you want use a literal
>'$' in some-command you'll have problems getting enough
> escaping although it can be done.

You mean that $$ is not enough in this example?

  $(eval $(call Deferred,OUTPUT,some-command arg1 $$ arg2))


> Personally I'm not convinced the eval/call version is more
> readable but YMMV.

I believe that this is important. You have written "Whoa! What is going on here?" in your blog. I would like to avoid that very effect every time 
somebody sees the trick inside a makefile. Having a function with an informative name avoids most questions. If there are any left, the function is 
the single point where you can find all documentation and warnings about itr.

I would like to put together in the comments of that function a few examples of how to use it, what pitfalls there are with escaping, and yet more 
examples about how to work-around those escaping pitfalls. Or maybe with examples of what is not possible. Then I would have a function that you can 
use 95 % of the time without having to think too much about it.

Thanks again,
  rdiez