On May 25, 6:47 am, Konrad Hinsen <konrad.hin...@laposte.net> wrote:
> On 24.05.2009, at 07:01, Feng wrote:
>
> > clojure.contrib.types/deftype assumes data to tag are created fresh
> > with no meta data. Would it be useful to have it update meta data if
> > data are already initialized with meta data, e.g. from file or db with
> > other meta data, but :type ::constructor are too program specific to
> > be stored in there?
>
> I considered this, but decided to postpone any action until the
> problem actually occurs. The reason why I prefer the current approach
> wherever it is applicable (i.e. for any values that don't need
> metadata other than for type identification is the associated overhead.
>
> My current implementation creates a single metadata map per type that
> is then attached to each data item. The memory overhead is
> negligible, and the CPU time overhead is just one call to with-meta
> per object creation. The modification that you propose (as well as
> the variant I had considered, using vary-meta) creates a metadata map
> per object, resulting in potentially lots of metadata maps, even
> though many could be identical, and requiring map operations of non-
> negligible cost for each object creation.
>
> Several solutions are envisageble:
>
> 1) Check if the object has metadata, and put on a predefined map if
> not. This leads to an increased CPU overhead for each object
> creation, to to a memory overhead only for types that actually
> require it.
>
> 2) Make the metadata handling an option specified by a parameter to
> deftype.
>
> 3) Delegate metadata handling to the constructor, at least optionally.
>
> It's hard to choose among these solutions without having some real-
> life use cases.
>
Thanks for your feedbacks, Konrad. I'll do some experiment with option
1.
> Konrad.
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