Re: Deferred->immediate variable reassignment trick.

[Paul Smith]

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.