doc: Improve <service-type> documentation.

* doc/guix.texi (Service Reference): Correct and clarify some statements
  regarding the "compose" and "extend" procedures of <service-type>.
master
Chris Marusich 2018-03-29 07:29:49 +02:00
parent 3cd4447f56
commit 881c61d062
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@ -21090,16 +21090,16 @@ services.
Otherwise, it must be a one-argument procedure. The procedure is called
by @code{fold-services} and is passed a list of values collected from
extensions. It must return a value that is a valid parameter value for
the service instance.
extensions. It may return any single value.
@item @code{extend} (default: @code{#f})
If this is @code{#f}, services of this type cannot be extended.
Otherwise, it must be a two-argument procedure: @code{fold-services}
calls it, passing it the initial value of the service as the first argument
and the result of applying @code{compose} to the extension values as the
second argument.
calls it, passing it the initial value of the service as the first
argument and the result of applying @code{compose} to the extension
values as the second argument. It must return a value that is a valid
parameter value for the service instance.
@end table
@xref{Service Types and Services}, for examples.