Jack Hill <jackhill@jackhill.us> writes:

> Thanks Marius!
>
> On Wed, 18 Dec 2019, Marius Bakke wrote:
>
>> It's not pretty, but you could create a package that takes "match-theme"
>> as an input and makes the necessary adjustments.
>>
>> See the 'mariadb/fixed-install-db' variant added in
>> 9077cf68ec57c0303ef7746e203c3fe5ed041add for an example.
>
> I tried this approach first, and find the results acceptable for this 
> purpose. This technique seems most applicable when what needs to modified 
> is a text file. I ended up with this package definition:
>
> ```
> (package
>       (inherit matcha-theme)
>       (native-inputs '())
>       (inputs `(("matcha-theme" ,matcha-theme)))
>       (outputs '("out"))
>       (build-system trivial-build-system)
>       (arguments
>        `(#:modules ((guix build utils))
>  	#:builder
>  	(begin
>  	  (use-modules ((guix build utils)))
>  	  (let ((upstream-theme (assoc-ref %build-inputs "matcha-theme"))
>  		(out (assoc-ref %outputs "out")))
>  	    (copy-recursively upstream-theme out)
>  	    (substitute* (find-files out "\\.css$")
>  	      (("abb9b6") "859900"))))))
>       (synopsis "jackhill's version of the matcha-theme"))
> ```
>> Another "cleaner" approach could be to use 'computed-file' to create a
>> patched source tarball, and pass that as the source in your local
>> variant.
>
> I have not yet tried this. I should because, as you point out, this is 
> "cleaner", and would work in situations where what needs to be modified in 
> compiled into an opaque object in the output.
>
> However, without trying this, I don't see how it would work better than 
> using a snippet in the origin definition. Or perhaps you meant to created 
> the patched source tarball from the output of the upstream matcha-theme 
> package?
>
> Either way, I think I'm still left with the question of how can we make 
> modifying packages easier without the need to resort to kludges.

'substitute-keyword-arguments' and 'snippets' do make it pretty easy,
but as you found it does not work with 'trivial-build-system'.

Perhaps we should just discourage the latter entirely: "matcha-theme"
can easily be rewritten to use 'gnu-build-system' instead by deleting or
overriding the unnecessary phases.

Feel free to submit a patch that does just that!  :-)