Hello! Chris Marusich <cmmarus...@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. > 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! Thank you, Ludo’.
