[closer-devel] WITH-ACTIVE-LAYERS does not eval its args. Alternatives?

[Pascal Costanza]

Hi Kilian,

On 1 Dec 2013, at 10:58, Kilian Sprotte <kilian.sprotte at gmail.com> wrote:

> Hi,
> 
> I have run into a situation, where I would want WITH-ACTIVE-LAYERS to
> evaluate its layer arguments or to have an alternative construct which
> does so.
> 
> My questions are:
> 
> 1. Is there a reason for WITH-ACTIVE-LAYERS to be designed this way?

Yes, the macro version can be implemented more efficiently, because you already know at macro-expansion time what layers to activate.

> 2. Is there an alternative that allows me to achieve the desired
> effect?

Yes. adjoin-layer and remove-layer allow you to programmatically create new combinations of layers. You need to pass one layer and a “layer context”, which is an already composed set of layers. You need to call adjoin-layer and remove-layer several times if you want to compose several layers, because composition ordering matters. current-layer-context gives you a context of the currently active layers. So adjoin-layer, remove-layer, and current-layer-context in combination are the functions to compute new sets of layers.

Once you have a new layer context, you can use funcall-with-layer-context or apply-with-layer-context to execute a block of code within that context.

Such contexts are first-class, so you can store them and pass them around.

> The background is that I am using layers to represent different
> backends in a database API. I would need WITH-ACTIVE-LAYERS to
> evaluate its argument in the following example: A user wants to
> connect to the database, so he specifies some connection settings,
> including the backend to be used:
> 
> (defun open-db (spec)
>  (destructuring-bind (backend &rest args) spec
>    (with-active-layers (backend)
>      (apply #'%open-db args))))
> 
> Suggestions or Remarks?

This is exactly the kind of case for which the first-class contexts are designed, and I have used them in similar situations for some demos. Please let me know if you run into any problems with this.

I hope this helps,
Pascal

--
Pascal Costanza