On 11 July 2010 15:18, Jeffrey Schwab <[email protected]> wrote:
> Which instance in time? Just once, up front, or could the deref happen anew
> on each pass? Is there any way to know, in general, whether a deref is
> guaranteed to happen only once, or might a macro like doseq move the
> expression @a into the middle of a loop (or elide the deref altogether)?
I think these two snippets might be useful:
user=> (def foo (atom #{1 2 3 4 5}))
#'user/foo
user=> (doseq [x (range 2) y @foo] (println x y) (swap! foo disj y))
0 1
0 2
0 3
0 4
0 5
nil
...whereas...
user=> (doseq [x (range 2) y #{1 2 3 4 5}] (println x y))
0 1
0 2
0 3
0 4
0 5
1 1
1 2
1 3
1 4
1 5
nil
On 11 July 2010 15:52, Stuart Halloway <[email protected]> wrote:
> Help me understand your confusion so I can improve the docstrings.
I think it's not so much about the order of traversal of the seqs, but
about the time when the seq expressions are evaluated. As displayed
above, all seq expressions except the leftmost one are evaluated once
*per iteration* (of their particular loop level), so their values
might change between iterations.
Sincerely,
Michał
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