Subject: bug#38529: Make --ad-hoc the default for guix environment proposed deprecation mechanism

From: Konrad Hinsen <[email-masked]>
Date: Tue, 17 Dec 2019 07:49:21 +0100

On 16/12/2019 23:09, Ludovic Courtès wrote:
> So in a more algorithmic manner:
>> 1. if ad-hoc and inputs-of is present at the same invocation: fail
>> hard. (With an error like incompatible options present)
>> 2. if only ad-hoc is present, then print a deprecation warning (yes,
>> we could make this suspendable with an environment variable, like you
>> described)
>> 3. if only inputs-of present, then do the new behaviour.
>> 4. if neither ad-hoc nor inputs-of present then
>>    a. if GUIX_ENVIRONMENT_DEPRECATED is 1: do the current behaviour,
>>    b. if GUIX_ENVIRONMENT_DEPRECATED is undefined, or is not 1: do the
>> new behaviour.
> That sounds like a good plan to me.
>
> #4 is the trickiest, and I think it’d be good to give users a bit of
> time so they can start adjusting before deprecation is in effect.

#4 is trickiest for another reason: there is no future-proof use of 
"guix environment" that works right now and will continue to work. Nor 
is there any way to see, when looking at a command line, whether it's 
old-style or new-style, if neither --ad-hoc nor --inputs-of are present. 
This means that all existing documentation (tutorials etc.) will become 
misleading in the future. Worse, even documentation written today, in 
full awareness of a coming change, can't do better than saying "watch 
out, this will do something else in the future".

The first rule of backwards-compatibility is: never change the meaning 
of an existing valid command/API. Add new valid syntax, deprecate old 
valid syntax, but don't change the meaning of something that was and 
will be valid.

How about a more drastic measure: deprecate "guix environment" and 
introduce a new subcommand with the desired new behaviour?

Cheers,

   Konrad.