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[bug#27155] [PATCH 0/2] Support service extensions on the "final" servic

From: 宋文武
Subject: [bug#27155] [PATCH 0/2] Support service extensions on the "final" service values
Date: Fri, 16 Jun 2017 01:12:15 +0800
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.2 (gnu/linux)

address@hidden (Ludovic Courtès) writes:

> Hi Ricardo,
> Ricardo Wurmus <address@hidden> skribis:
>> I think it is useful to have the ability to add rewriters at the end of
>> service composition.  In my opinion it is always good to have an escape
>> hatch, and this seems to fit the bill.  But I agree that it is not
>> an elegant solution, and I wouldn’t want to advocate using it.
> Right.  As discussed on IRC, one problem is ordering: if there are
> several users of this features for a given service, you can’t really
> tell what’s going to happen, unless the modifications happen to be
> commutable.
>> As to your second idea: it seems tedious for service writers to have to
>> anticipate the ways in which services could be extended (here given by
>> providing extension points).
> Boilerplate aside, I’m not sure it would be this tedious.
>> Would it make more sense to allow *extensions* to specify how they
>> should be applied rather than letting services define extension points?
>> This would shift the burden away from services to service extensions.
>> Extensions would still need to provide a way of extending the parent
>> service, but this could be optional.
> What would it look like?

Maybe allow a service to override extensions specified by its

It can be:
--8<---------------cut here---------------start------------->8---
(define etc-service-type
    (name 'etc)
    (default-extensions (list ...))
    (extension-points (list ...))))

(define builtin-etc-service
  (... %base-services))

(define my-etc-service
  (service etc-service-type
    (service-value builtin-etc-service)
    (list (service-extension
--8<---------------cut here---------------end--------------->8---

So we can change what service actually do, this is really powerful!

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