Hi, Am 01.10.2009 um 19:08 schrieb Rich Hickey:
It simply doesn't compose or flow. Making the nil env special (e.g. non-replacing) just moves the problem into higher-order functions that use the construct: (defn needs-x [] (use-env-x)) (defn needs-y [] (use-env-y)) (defn foo [] (with-env x (fn [f] (needs-x) (f)))) (let [f (foo)] (with-env y (f needs-y))) needs-y isn't going to get it.
This is "simply" solved by having a chain of environments. The algorithm works like this:
* The desired value is contained in the current map. => Use it. ("Younger" bindings override "older" ones) * The desired value is not contained go up one step in the chain of env maps and repeat. * If there is no further step in the chain (ie. we arrived at nil), use the root binding of the Var.
"simply" with quotes, because I have no clue about the performance impact of that. It should fix your example, though.
Elk, an rather old, embeddable Scheme interpreter, constructs its environment like that.
Sincerely Meikel
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