Re: (solved)Can I defer make-var expansion until a command is executed

[Chen Jun （陈军）]

Brian Dessent wrote:
> Chen Jun  wrote:
>
>   
> > Well Brian, the example presented by me is just a fabricated one, not
> > for my real world use. I use $(shell ) just to introduce a make variable
> > or a make function so that my question becomes concrete. My real world
> > case regarding this problem is quite a bit complex so I'd like not to
> > talk about to it directly.
> >     
>
> Regardless, I think the same principle holds: if you need logic that
> considers factors that change during the execution of a recipe, then
> code that logic using the shell rather than make's functions.  I can't
> think of a situation where this would not be possible; you can pass
> through to shell variables the contents of any makefile variables that
> might be required by that logic.  And if all else fails there's always
> the universal hammer of recursively invoking a sub-make.
>
> Brian
>
>   
Well Brian, in theory you're correct -- by using shell commands. But 
there is another important factor that matters: "user convenience". 
Sometimes, using make's internal functions to do some process is more 
convenient than shell commands, imaging make provide some great 
functions like $(patsubst ), $(foreach ) etc. Using shell functions may 
some other times be more powerful, but consider running GNU make on M$ 
Windows, I'd like to avoid shell commands if that can be processed by 
make functions.

Finally, I've come up with an solution to my example question, that is, 
using double colon rule:

========

IsFileExist = $(shell if [ -f $1 ]; then echo 1; fi )

outfile = out.txt


$(outfile):: 
       @echo "STRT: $@ $(if $(call IsFileExist,$@),exist,absent)"
       @touch $@

$(outfile):: 
       @echo "DONE: $@ $(if $(call IsFileExist,$@),exist,absent)"


========

And my real world problem can be solved with that method. Also thanks to 
the great foresight of GNU make. Although the make manual says "the 
cases where double-colon rules really make sense are those where the 
order of executing the commands would not matter", I found in my example 
it really make sense even if the execution order does matter.

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