On Tue, Apr 26, 2011 at 1:06 PM, Steve Miner <stevemi...@gmail.com> wrote:
> Creating your own "Closed Map" type would be the object-oriented approach.
> One downside is that a closed-map feels like a normal map, but can't be
> safely substituted for most maps because of the booby-trap when using assoc
> (etc.) with a new key. It doesn't really fulfill the map contract anymore.
> You'll have to do some defensive copying if you need a normal map.
>
> I would like to suggest a functional approach as an alternative. It seems to
> me that the concept of "closedness" is a matter of interpretation. It could
> be determined by the functions manipulating the data. Sometimes you might
> want to treat the data (map) as closed and other times you might not care, so
> you just need to use the appropriate functions.
>
> If you create your closed maps with all the allowed keys (nil values as
> appropriate), you just need to call contains? on any new key to make sure
> it's allowed. You could do that in a pre-condition. For example:
>
> (defn assert-key [m k] {:pre [(contains? m k)]} k)
>
> (defn assoc-closed [m k v]
> (assoc m (assert-key m k) v))
>
> (defn get-closed [m k]
> (get m (assert-key m k)))
+1
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