[Rd] Best way to implement optional functions?

[Paul Gilbert]

On 10/22/2015 03:55 PM, Duncan Murdoch wrote:
> I'm planning on adding some new WebGL functionality to the rgl package,
> but it will pull in a very large number of dependencies. Since many
> people won't need it, I'd like to make the new parts optional.
>
> The general idea I'm thinking of is to put the new stuff into a separate
> package, and have rgl "Suggest" it.  But I'm not sure whether these
> functions  should only be available in the new package (so users would
> have to attach it to use them), or whether they should be in rgl, but
> fail if the new package is not available for loading.
>
> Can people suggest other packages that solve this kind of problem in a
> good way?

I do something similar in several packages. I would distinguish between 
the situation where the new functions have some functionality without 
all the extra dependencies, and the case where they really do not. In 
the former case it makes sense to put the functions in rgl and then fail 
when the extra functionality is demanded and not available. In the 
latter case, it "feels like" you are trying to defeat Depends: or 
Imports:. That route has usually gotten me in trouble.

Another thing you might want to consider is that, at least for awhile, 
the new functions in rglPlus will probably be less stable then those in 
rgl. Being able to change those and update rglPlus without needing to 
update rgl can be a real advantage (i.e. if the API for the new 
functions is in rgl, and you need to change it, then you are required to 
notify all the package maintainers that depend on rgl, do reverse 
testing, and you have to explain that your update of rgl is going to 
break rglPlus and you have a new version of that but you cannot submit 
that yet because it will not work until the new rgl is in place.)

Paul
>
> Duncan Murdoch
>
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