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Re: safe define

From: Eric Blake
Subject: Re: safe define
Date: Mon, 11 May 2015 08:24:47 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

On 05/11/2015 04:08 AM, Werner LEMBERG wrote:
> Folks,
> has someone ever written a `safe define' macro that makes the defined
> macro expands to itself if called without any argument?

Yes, the manual even has a section on how to do it:

>   sdefine({foo}, {bar})
>   foo    =>  foo
>   foo()  =>  bar

Looks like you have done changequote({,}) so I'll keep with that in my

> I tried the following, and I failed (due to lack of experience
> w.r.t. quoting tricks, I think) – I have difficulties to delay the
> expansion of `$#'...
> Below, I've marked the problematic spots with (nonexisting)
> pseudo-functions.
>   define({sdefine},
>     {define({$1},
>        {ifelse(delay_expansion_of({$#}),
>                {0},
>                {{$1}},
>                {expand_at_definition_time(shift($@))})})})

define({sdefine}, {_$0({$1}, {$2}, {$}{#}, {$}{0})})
define({_sdefine}, {define({$1}, {ifelse({$3}, 0, {{$4}}, {$2})})})

The trick is realizing you need two functions: the outer sdefine that
uses quoting tricks to pass a literal $# and $0 to the inner function,
and the inner function that then does the desired define using the
literal strings in the right place.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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