Re: (format "Hello %1$S%2$s" 'world "!")

[Philipp Stephani <p.stephani2@gmail.com>]

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Clément Pit-Claudel <cpitclaudel@gmail.com> schrieb am So., 2. Juli 2017 um
18:34 Uhr:

> On 2017-07-02 11:03, Noam Postavsky wrote:
>
> > Why would we need a new function since %n$ would be an error for the old
> format anyway?
>
> This in itself is a good reason to have a new function.  A new function
> lets you migrate existing code, including plugins, to the new syntax, and
> load the polyfill in older Emacsen.  A new syntax that causes errors in old
> Emacsen isn't usable by anyone outside of Emacs core, for fear of
> compatibility problems.
>

We extend functions all the time. Here's a random entry from NEWS:

*** Time conversion functions now accept an optional ZONE argument
that specifies the time zone rules for conversion.  ZONE is omitted or
nil for Emacs local time, t for Universal Time, 'wall' for system wall
clock time, or a string as in the TZ environment variable.  The
affected functions are 'current-time-string', 'current-time-zone',
'decode-time', and 'format-time-string'.  The function 'encode-time',
which already accepted a simple time zone rule argument, has been
extended to accept all the new forms.

There's no difference between those changes and this one: the new syntax
was an error before, but now has a specified meaning.


>
> Also, the syntax is very heavy.  Was it inspired by another programming
> language?  The ones I'm familiar with use $1, {1}, or similar syntax, but
> I've never seen %1$s :/
>
>
As said, it comes from GNU printf.
We need to use a syntax that's backwards-compatible, that rules out $1 or
{1}.

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