ClojureDocs - clojure.core/alter

;; alter is a way to change the value of a reference. ;; Here we're defining a ref named 'names' and setting its value to ;; an empty vector. user=> (def names (ref [])) #'user/names ;; A function to add a name to the vector (notice the meat's wrapped ;; in a dosync user=> (defn add-name [name] (dosync (alter names conj name))) #'user/add-name user=> (add-name "zack") ["zack"] user=> (add-name "shelley") ["zack" "shelley"] ;; Notice that the var 'names' points to the reference that we created user=> (println names) #<Ref@658ba380: [zack shelley]> ;; To get the actual value of the ref, you use the '@' symbol, or deref user=> (println @names) [zack shelley] user=> (println (deref names)) [zack shelley]

(defn alter "Must be called in a transaction. Sets the in-transaction-value of ref to: (apply fun in-transaction-value-of-ref args) and returns the in-transaction-value of ref." {:added "1.0" :static true} [^clojure.lang.Ref ref fun & args] (. ref (alter fun args)))

I was fooling around with how exactly ref works with maps. Since the example here uses a vector, I thought maybe some of you might want to see a short example using a map.

In an aggregator I'm working on, I want to keep a record of how many sources and how many articles I've aggregated. Instead of using an atom for each, I'll reference a map called "counts." Here's a simple little function that increments and returns the new value of the counter stored in the map:

 (def counts (ref {:articles 0 :sources 0}))
(defn inc-ref [ref type]
"increment a map value with key type stored in ref"
    (dosync
     (alter ref assoc type (inc (type @ref)))
     (type @ref)))
user> (inc-ref counts :sources)
=>1
user> counts
=>{:articles 0, :sources 1}

and if you wanted to be able to add counters dynamically (one of the advantages of using a map in this context) you could redefine the function employ an optional argument, which if present instructs the function to create a new key-value pair using the name and initial value provided:

(defn inc-ref [ref type & [init-value]]
  (if init-value
    (dosync
     (alter ref assoc type init-value)
     (type @ref))
    (dosync
     (alter ref assoc type (inc (name @ref)))
     (type @ref))))

user> (inc-ref counts :articles)
=>1
user> (inc-ref counts :magazines 1)
=>1
user> (:magazines @counts)
=>1

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2 comment(s) for alter.

borg

I was fooling around with how exactly ref works with maps. Since the example here uses a vector, I thought maybe some of you might want to see a short example using a map. In an aggregator I'm working on, I want to keep a record of how many sources and how many articles I've aggregated. Instead of using an atom for each, I'll reference a map called "counts." Here's a simple little function that increments and returns the new value of the counter stored in the map:

grayzone

In the previous example at row 07 has a reference to 'name @ref':

(alter ref assoc type (inc (name @ref)))

maybe is incorrect and the correct mode is:

 (alter ref assoc type (inc (type @ref)))

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alter

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Source clojure/core.clj:2180 top

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2 comment(s) for alter.