Re: New conditional assignment facility

[Alejandro Colomar]

On Thu, Jan 11, 2024 at 02:22:19PM +0100, Alejandro Colomar wrote:
> Hi Paul!
> 
> On Thu, Jan 11, 2024 at 01:44:19AM -0500, Paul Smith wrote:
> > I've implemented a new capability for conditional assignments (not
> > pushed yet).
> > 
> > After these changes, a "?" can precede any type of assignment
> > operation, not just "=", and make it conditional (that is, it only
> > takes effect if the variable is not already set).
> > 
> > So for example, in addition to "?=" which creates a recursive variable
> > if the variable is not set yet, GNU Make will support "?:=" which
> > creates a simple variable if the variable is not set yet.
> 
> Thanks!!
> 
> >  Of course if
> > it's already set then the right-hand side of the assignment is not
> > expanded.
> > 
> > Similarly the assignments "?::=" and "?:::=" are also supported.
> > 
> > 
> > There is one question I wanted to ask for opinions on: what should we
> > do with the "?+=" operator?
> 
> There's a use for it:  appending to a variable if it wasn't set in the
> environment (ignoring what happened in the Makefile).
> 
> Here's what += does:
> 
>       alx@debian:~/tmp$ cat Makefile 
>       var ?= foo
>       var += bar
>       $(info $(var))
>       alx@debian:~/tmp$ make
>       foo bar
>       make: *** No targets.  Stop.
>       alx@debian:~/tmp$ make var=foo
>       foo
>       make: *** No targets.  Stop.
>       alx@debian:~/tmp$ var=foo make
>       foo bar
>       make: *** No targets.  Stop.

Sorry for reusing "foo".  I'll rewrite with quux, which will be clearer.

        alx@debian:~/tmp$ cat Makefile 
        var ?= foo
        var += bar
        $(info $(var))
        alx@debian:~/tmp$ make 
        foo bar
        make: *** No targets.  Stop.
        alx@debian:~/tmp$ make var=quux
        quux
        make: *** No targets.  Stop.
        alx@debian:~/tmp$ var=quux make
        quux bar
        make: *** No targets.  Stop.

> 
> And here's what ?+= could do:

        alx@debian:~/tmp$ cat Makefile 
        var ?= foo
        var += bar
        $(info $(var))
        alx@debian:~/tmp$ make 
        foo bar
        make: *** No targets.  Stop.
        alx@debian:~/tmp$ make var=quux
        quux
        make: *** No targets.  Stop.
        alx@debian:~/tmp$ var=quux make
        quux
        make: *** No targets.  Stop.

>       alx@debian:~/tmp$ cat Makefile 
>       var ?= foo
>       var ?+= bar
>       $(info $(var))
>       alx@debian:~/tmp$ make-9000
>       foo bar
>       make: *** No targets.  Stop.
>       alx@debian:~/tmp$ make-9000 var=foo
>       foo
>       make: *** No targets.  Stop.
>       alx@debian:~/tmp$ var=foo make-9000
>       foo
>       make: *** No targets.  Stop.
> 
> In the last case (environment variable), since var was set, ?+= would do
> nothing, unlike +=.
> 

-- 
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.

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