Hi Ludo,

On Thu, 2 Sept 2021 at 12:06, Ludovic Courtès <ludo@gnu.org> wrote:

> Unfortunately we cannot do that: rewriting happens lazily, when the
> various inputs fields (which are thunked) are accessed.  When PROC
> returns P, we still need to recurse into its inputs, until CUT? says we
> can stop.  (I’m surprised this change triggers only one test failure
> actually.)

Yeah.  After sending the email, I tried to fix stuff and I get the point. :-)


However, from my opinion, it is easy to check if the package-target is
a package or not, i.e.

  $ guix build foo --<transform>=package-target=new
  guix build: error: package-target: unknown package

For instance by using 'specification->package'; see attached patch.
But then, the test suite fails; I guess because 'dummy-package' and I
have not found the time to investigate.  From my point of view, this
kind of patch will fix one part of the initial issue reported by Ryan.

The other issue is to list if the transformation is applied or not.  I
think it is possible by traversing again the graph and check if a
property appears at least once; well it should be better to warn if
the 'mapping-property' is not found at least once.  I had some
headaches to implement it... and I moved to other "urgent" stuff. :-)


Last, speaking about transformations, the graph is walked too much
when several transformations is applied:

   guix build hello --with-latest=foo --with-input=bar=baz --with-latest=chouib

then the graph is walked 3 times, IIUC.  The options needs a rewrite
to pass a list of specs to 'package-with-latest-upstream' and not
twice a list with only one element.  This would reduce to 2 walks.
Then it could be nice to compose the transformation and then walk only
once (apply 'package-mapping' only once).
Well, maybe I miss something.

Cheers,
simon