From mboxrd@z Thu Jan  1 00:00:00 1970
From: Myles English <mylesenglish@gmail.com>
Subject: Re: REPL example session for overriding a package?
Date: Tue, 24 Jan 2017 15:05:08 +0000
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To: Ludovic =?utf-8?Q?Court=C3=A8s?= <ludo@gnu.org>
Cc: help-guix <help-guix@gnu.org>



on [2017-01-24] at 14:56 Ludovic Courtès writes:

> Myles English <mylesenglish@gmail.com> skribis:
>
>> I am trying to make a package that inherits from an existing package and
>> overrides a *minimum* number of elements of the original package
>> definition.
>>
>> Probably the most efficient way to do this would be to try out some
>> things in the REPL using tools from (gnu packages).  Having read a bit
>> about it I am finding it difficult to get my head around working in the
>> REPL, does anyone have an example session?
>>
>>
>> - get an element of a package definition
>> - substitute a nested element
>>
>> Specifically, for the package eigen in (gnu packages algebra) I'd like
>> to remove, or maybe add to, the snippet.
>
> Maybe something like this:
>
> --8<---------------cut here---------------start------------->8---
> scheme@(guile-user)> ,use(gnu packages algebra)
> scheme@(guile-user)> ,use(guix)
> scheme@(guile-user)> (define my-eigen
> 		       (package
> 			 (inherit eigen)
> 			 (source (origin
> 				   (inherit (package-source eigen))
> 				   (snippet
> 				    `(begin
> 				       ,(origin-snippet (package-source eigen))
> 				       (display "hey there!\n")))))))
> scheme@(guile-user)> (origin-snippet (package-source my-eigen))
> $3 = (begin (substitute* "CMakeLists.txt" …))
> --8<---------------cut here---------------end--------------->8---
>
> The snippet here is really pasting together two pieces of code
> (imperative code), which can be brittle (“unhygienic” in macro
> parlance.)

Thank you Ludo' that's just what I am looking for.

Myles