Re: Deferred->immediate variable reassignment trick.
Paul Smith <[email protected]>
| Newsgroups | gmane.comp.gnu.make.general |
|---|---|
| Organization | GNU's Not UNIX! |
| Message-ID | <[email protected]> |
On Tue, 2023-06-20 at 11:46 -0700, Kaz Kylheku wrote:
> I used a trick in a makefile along the following pattern. It works
> fine. I'm just circulating it for any comments/discussion.
>
> # Delayed evaluation on purpose:
>
> IMPORTANT_VAR = $(shell $CODE_TO_CALCULATE_IMPORTANT_VAR)
>
> # Force lazy variable to be eager:
>
> IMPORTANT_VAR := $(IMPORTANT_VAR)
>
> The requirements being hit here are:
>
> 1. We want the user to be able to override IMPORTANT_VAR from the
> command line. When the user overrides IMPORTANT_VAR, the shell
> command must not be executed.
>
> 2. We don't want every substitution of $(IMPORTANT_VAR) to be running
> the shell command.
I don't understand how this is different than just:
IMPORTANT_VAR := $(IMPORTANT_VAR)
If the user specifies an override of the variable on the command line
then the value in the makefile will never be used, so the shell command
will never be invoked.