ludo@gnu.org (Ludovic Courtès) writes:

> Hello!
>
> Chris Marusich <cmmarusich@gmail.com> skribis:
>
>> Is the description of "compose" correct?  The manual claims that it
>> "must return a value that is a valid parameter value for the service
>> instance," but I do not think that is actually true.  Judging by the
>> implementation of fold-services in (gnu services), it may return any
>> single value as long as the extend procedure knows how to handle it.  In
>> fact, it is the "extend" procedure which must return a valid parameter
>> value for the service.
>
> Yes, you’re right!  Good catch.

Thank you for confirming my understanding!

>> If this is true, then I suggest we change the documentation as follows:
>>
>>      ‘compose’ (default: ‘#f’)
>>           If this is ‘#f’, then the service type denotes services that
>>           cannot be extended—i.e., services that do not receive “values”
>>           from other services.
>>
>>           Otherwise, it must be a one-argument procedure.  The procedure
>>           is called by ‘fold-services’ and is passed a list of values
>>           collected from extensions.  It may return any single value.
>>
>>      ‘extend’ (default: ‘#f’)
>>           If this is ‘#f’, services of this type cannot be extended.
>>
>>           Otherwise, it must be a two-argument procedure:
>>           ‘fold-services’ calls it, passing it the initial value of the
>>           service as the first argument and the result of applying
>>           ‘compose’ to the extension values as the second argument.  It
>>           must return a value that is a valid parameter value for the
>>           service instance.
>>
>> What do you think?
>
> LGTM!

I've committed this as 881c61d06222a30dbffbf9d039eaca2abd3d22b3.

-- 
Chris