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)))
Steve Miner
[email protected]
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