I am new to Clojure and have been working through some of the examples
on the PCL -> Clojure blog. Currently I am working through Chapter 6
(variables).
The blog says "Nothing stops multiple functions from closing on the
same variables. Here is a function that returns an incrementer,
decrementer, and accessor, all sharing the same counter:"
(defn counters []
(let [count (ref 0)]
(list #(dosync (alter count inc))
#(dosync (alter count dec))
#(deref count))))
I tried to this example (with Clojure 1.3.0 and 1.2.1) and found that
it does not appear to work, or I am totally missing something. Here
is some output from a REPL session using this function. Could you
help me understand the output or, if there is a mistake in the
example, could you explain what it is?
Question 1: I expected these two statements to produce output 1, 2
user=> ((nth (counters) 0))
1
user=> ((nth (counters) 0))
1
Question 2: So then I did the following:
user=> (def a (nth (counters) 0))
#'user/a
user=> (def b (nth (counters) 2))
#'user/b
user=> (a)
1
user=> (a)
2
user=> (a)
3
user=> (b)
0
Now the incrementer appears to be working, but then when I try to use
the accessor to inspect the value of "count", it is still 0. So it
appears that the 3 anonymous functions in the list are not closing
over "count".
Could somebody help me understand this behavior? Thank you for your
help.
matthew
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