Re: [PATCH v5 02/14] trailer: process trailers from file and arguments

[Junio C Hamano <gitster@pobox.com>]

Christian Couder <chriscool@tuxfamily.org> writes:

> Many entries with the same key but distinct values can be configured
> using:
>
> if_exists = add_if_different
> if_missing = add
>
> Many entries with the same key but values that can be the same can be
> configured using:
>
> if_exists = add
> if_missing = add

While the above certainly can express the combinations, I am still
puzzled about the value of having "under this condition" (i.e.
if-exists/if-missing) and "do this" (i.e. add/add-if-different) as
two separate concepts.

If you do not have an existing entry with the same key, no new entry
can be the same as an existing one---therefore, the former "allow
only distinct values for the same key" can just say

  trailer."Fixes".action = add_if_different

or something, without any if_exists/if_missing.  In a similar way,
the latter "duplicated values allowed for the same key" can say

  trailer."Sob".action = add

You can throw into the mix other actions like "add_if_missing", you
can also express "only one value allowed for the same key---the
first one wins", "replace" to mean "replace if there is one with the
same key", and "replace_or_add" to mean "same as 'replace', but add
if there is no existing entries with the same key".  Will we lose
expressiveness by simplifying the semantics, getting rid of this
"under this condition" part and instead making "do this" part more
intelligent?

Even if we assume, for the sake of discussion, that it *is* a good
idea to separate "under this condition" part and "do this" part, I
do not think the above is the only or the best way to express
"distinct values allowed for the same key".  How do we argue that
this from your example

  if_exists = add_if_different
  if_missing = add

is a better design than this, for example?

  if_key_value_exists = ignore ; exact matching <key,val> exists
  if_key_exists = add          ; otherwise
  if_missing = add

The latter splits remaining conditional logic from "do this" part
(no more "add_if_different" that causes "add" action to see if there
is the same key and act differently) and moves it to "under this
condition" part.

I am trying to illustrate that the line to split the "under this
condition" and "do this" looks arbitrary and fuzzy here, and because
of this arbitrary-ness and fuzziness, it is not very obvious to me
why it is a good idea to have "under this condition" as a separate
concept from "do this" settings.

What is the advantage of having such an extra axis?  Does it make it
easier to manage?  Does it allow us to express more?

I can see that the location where a new entry (or a duplicated one)
is added (i.e. do we prepend? do we append?) can be orthogonal to
any of the above; no need to bring up 'where' in the discussion.