It's to do with the chunking of sequences. they are taken in blocks of 32.
So minimum of 32 will be executed.
(first (remove nil? (map foo (vec (range 1 1000)))))
Exeuting for... 1
Exeuting for... 2
Exeuting for... 3
Exeuting for... 4
Exeuting for... 5
Exeuting for... 6
Exeuting for... 7
Exeuting for... 8
Exeuting for... 9
Exeuting for... 10
Exeuting for... 11
Exeuting for... 12
Exeuting for... 13
Exeuting for... 14
Exeuting for... 15
Exeuting for... 16
Exeuting for... 17
Exeuting for... 18
Exeuting for... 19
Exeuting for... 20
Exeuting for... 21
Exeuting for... 22
Exeuting for... 23
Exeuting for... 24
Exeuting for... 25
Exeuting for... 26
Exeuting for... 27
Exeuting for... 28
Exeuting for... 29
Exeuting for... 30
Exeuting for... 31
Exeuting for... 32
2
D
On Wednesday, 6 March 2013 22:11:43 UTC+11, Achint Sandhu wrote:
>
> Hi,
>
> In the sample code below, I'd like to rerun the first value that returns a
> non-nil result upon the application of a function (foo in the code sample
> below). In the real use case , the computation of the function is
> expensive, so I'd only like to run foo until I find the first non-nil value.
>
> Looking at the documentation for map indicates that it is lazy and does
> what I need.
> user> (doc map)
> -------------------------
> clojure.core/map
> ([f coll] [f c1 c2] [f c1 c2 c3] [f c1 c2 c3 & colls])
> Returns a lazy sequence consisting of the result of applying f to the
> set of first items of each coll, followed by applying f to the set
> of second items in each coll, until any one of the colls is
> exhausted. Any remaining items in other colls are ignored. Function
> f should accept number-of-colls arguments.
> nil
>
> I believe the idiomatic way to write this in clojure would be:
> user> (clojure-version)
> "1.5.0"
> user> (defn foo [x] (println "Exeuting for..." x) (when (even? x) x))
> #'user/foo
> user> (foo 10)
> Exeuting for... 10
> 10
> user> (foo 1)
> Exeuting for... 1
> nil
> user> (first (remove nil? (map foo '(1 3 5 2 7 6))))
> Exeuting for... 1
> Exeuting for... 3
> Exeuting for... 5
> Exeuting for... 2
> 2
> user> (first (remove nil? (map foo #{1 3 5 2 7 6})))
> Exeuting for... 1
> Exeuting for... 2
> 2
>
> This works as expected, however when the collection is a vector, the
> mapping is no longer lazy.
>
> user> (first (remove nil? (map foo [1 3 5 2 7 6])))
> Exeuting for... 1
> Exeuting for... 3
> Exeuting for... 5
> Exeuting for... 2
> Exeuting for... 7
> Exeuting for... 6
> 2
>
> I would have expected map's behaviour to be agnostic of the collection
> type passed in, but it appears not to be the case.
>
> Insight into this would be greatly appreciated.
>
> Thank you.
>
> Cheers,
> Achint
>
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