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      set-validator!

      clojure.core

      Available since 1.0 (source)
      • (set-validator! iref validator-fn)
      Sets the validator-fn for a var/ref/agent/atom. validator-fn must be nil or a
side-effect-free fn of one argument, which will be passed the intended
new state on any state change. If the new state is unacceptable, the
validator-fn should return false or throw an exception. If the current state (root
value if var) is not acceptable to the new validator, an exception
will be thrown and the validator will not be changed.
      © Rich Hickey. All rights reserved. Eclipse Public License 1.0
      2 Examples
      user=> (def atm (atom [2]))
#'user/atm

user=> (set-validator! atm #(every? even? %))
nil

user=> (swap! atm into [5])
#<CompilerException java.lang.IllegalStateException: Invalid reference state (NO_SOURCE_FILE:0)>

user=> (set-validator! atm nil)
nil

user=> (swap! atm into [5]))
[2 5]
      user=> (def a 1)
#'user/a

user=> (set-validator! (var a) (fn [update-result] (< update-result 3)))
nil

user=> (alter-var-root (var a) inc)
2

user=> (alter-var-root (var a) inc)
IllegalStateException Invalid reference state  clojure.lang.ARef.validate (ARef.java:33)

user=> a
2
      See Also
      clojure.core/atom

      Creates and returns an Atom with an initial value of x and zero or more options (in any order): ...

      Added by kumarshantanu
      clojure.core/agent

      Creates and returns an agent with an initial value of state and zero or more options (in any order...

      Added by kumarshantanu
      clojure.core/ref

      Creates and returns a Ref with an initial value of x and zero or more options (in any order): :...

      Added by kumarshantanu
      1 Note
        By , created 13.1 years ago

        If you want your validator to throw an exception with a useful message, make sure it is a RuntimeException (or subclass), otherwise ARef#validate will throw an IllegalStateException with a generic message.