On 7/11/10 9:52 AM, Stuart Halloway wrote:
Thanks. What I'm having trouble with is not the reference vs. value semantics, but the order of evaluation.
Clojure seems to be all over the map on this; it's probably not as bad as it looks, but I'm accustomed to
either purely strict or purely lazy evaluation. I looked at the documentation for "doseq" and
"for", but they did not say anything about evaluation order; I then looked at the source for
"for", but reading it is a non-trivial task for a lisp newb like myself.
The documentation strings for "for" state that it is lazy and rightmost-fastest. The docs for
"doseq" are similarly specific: non-lazy for side effects, with ordering as per "for".
Help me understand your confusion so I can improve the docstrings.
Thank you.
clojure.core/for
([seq-exprs body-expr])
Macro
List comprehension. Takes a vector of one or more
binding-form/collection-expr pairs, each followed by zero or more
modifiers, and yields a lazy sequence of evaluations of expr.
I did not know whether the sequence expressions (the right halves of the
binding-forms) were evaluated strictly, or re-evaluated on each pass
(like the body-expr). They are apparently evaluated a priori, when the
comprehension is defined:
user=> (def a (atom #{1 2}))
#'user/a
user=> (def f (for [i @a] i))
#'user/f
user=> (swap! a disj 2)
#{1}
user=> f
(1 2)
This was important to know, because I was altering the atom from within
a doseq body.
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