On Tue, Jan 04, 2022 at 05:50:31PM +0100, Ricardo Wurmus wrote:
> Hi Guix,
> 
> does this pattern look familiar to you?
> 
>   (arguments
>     (list
>     #:phases
>     '(modify-phases %standard-phases
>       (add-after 'unpack 'i-dont-care
>         (lambda _
>           (substitute* "this-file"
>             (("^# some unique string, oh, careful! gotta \\(escape\\) this\\." m)
>              (string-append m "\nI ONLY WANTED TO ADD THIS LINE!\n"))))))))
> 
> This is a lot of boilerplate just to add a line to a file.  I’m using
> “substitute*” but actually I don’t want to substitute anything.  I just
> know that I need to add a line somewhere in “this-file”.
> 
> Or maybe it’s a CMakeLists.txt file that inexplicably wants to download
> stuff?  I should patch that file but it’s a multi-line change.  So I’m
> trying to do the same as above with several different anchor strings to
> comment out lines.
> 
> We have a lot of packages like that.  And while this boilerplate pattern
> looks familiar to most of us now, it is really unclear.  It is
> imperative and abuses regular expression matching when really it should
> have been a patch.
> 
> There are a few reasons why we don’t use patches as often:
> 
> 1. the source code is precious and we prefer to modify the original
> sources as little as necessary, so that users can get the source code as
> upstream intended with “guix build -S foo”.  We patch the sources
> primarily to get rid of bundled source code, pre-built binaries, or
> code that encroaches on users’ freedom.
> 
> 2. the (patches …) field uses patch files.  These are annoying and
> inflexible.  They have to be added to dist_patch_DATA in gnu/local.mk,
> and they cannot contain computed store locations.  They are separate
> from the package definition, which is inconvenient.

It also feels wrong to add a 30 line patch, taking into account the
header bits, to make a 3 line change.

> 3. snippets feel like less convenient build phases.  Snippets are not
> thunked, so we can’t do some things that we would do in a build phase
> substitution.  We also can’t access %build-inputs or %outputs.  (I don’t
> know if we can use Gexps there.)

I believe you can leave out the modules line and use a gexp in the
snippet (without the "'(begin" portion )

> I feel that the first point is perhaps a little overvalued.  I have
> often felt annoyed that I had to manually apply all this build phase
> patching to source code obtained with “guix build -S”, but I never felt
> that source code I got from “guix build -S” was too far removed from
> upstream.
> 
> It may not be possible to apply patches with computed store locations —
> because when we compute the source derivation (which is an input to the
> package derivation) we don’t yet know the outputs of the package
> derivation.  But perhaps we can still agree on a more declarative way to
> express patches that are to be applied before the build starts; syntax
> that would be more declarative than a serious of brittle substitute*
> expressions that latch onto hopefully unique strings in the target
> files.
> 
> (We have something remotely related in etc/committer.scm.in, where we
> define a record describing a diff hunk.)
> 
> Here’s a colour sample for the new bikeshed:
> 
>   (arguments
>     (list
>       #:patches
>       #~(patch "the-file"
>          ((line 10)
>           (+ "I ONLY WANTED TO ADD THIS LINE"))
>          ((line 3010)
>           (- "maybe that’s better")
>           (+ (string-append #$guix " is better"))
>           (+ "but what do you think?")))))

I have on at least one occasion stopped myself from trying to use ed (it
IS the standard editor) to apply something that SHOULD BE trivial to
change.

-- 
Efraim Flashner   <efraim@flashner.co.il>   רנשלפ םירפא
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