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doc:plans [2014/01/13 10:07] – [with-policy] winklerdoc:plans [2015/05/11 17:06] (current) – gkazhoya
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 +//**Note:** this page is deprecated. Please use [[tutorials:advanced:plans|this one]] instead.//
 +
 ====== Writing Plans ====== ====== Writing Plans ======
  
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 <code lisp> <code lisp>
 (define-policy my-policy (max-num match-num) (define-policy my-policy (max-num match-num)
-  (:description "This is an example policy.")+  "This is an example policy."
   (:init (format t "Initializing policy~%")   (:init (format t "Initializing policy~%")
          t)          t)
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              (sleep 2))))              (sleep 2))))
 </code> </code>
 +
 +=== Exception handling ===
 +When policies are used, multiple failures can be signalled. The most meaningful of those are
 +  * ''policy-not-found'': Signalled when a named policy is used that was not defined before.
 +  * ''policy-init-failed'': Signalled when initialization of a policy went wrong (i.e. '':init'' returned ''nil'').
 +  * ''policy-check-condition-met'': The '':check'' condition of the policy returned a non-''nil'' value, '':recover'' was executed and the ''body'' code was interrupted before it could complete execution.
 +
 +If one wants to monitor the triggering of a policy's '':check'' condition, this can be achieved like this:
 +<code lisp>
 +(top-level
 +  (with-failure-handling
 +      ((policy-check-condition-met (f)
 +         (declare (ignore f))is available that the given
 +         (do-custom-handling-here)
 +         (retry))) ;; Or whatever seems appropriate in your use-case e.g. (return)
 +    (with-named-policy 'my-policy (10 5)
 +      (loop do (format t "Main loop cycle.~%")
 +               (sleep 2)))))
 +</code>
 +
 +=== Using multiple policies at once ===
 +When multiple policies are to be used (either a mix of different policies, or the same policy multiple times, each with different parameters), two helpful macros can be used: ''with-policies'' and ''with-named-policies''. Both of these take lists of policies, together with their respective instantiation parameters, as arguments.
 +
 +When policy instances by the names ''my-policy-object'' and ''my-other-policy-object'' should be used, the following code snippet reflects this behaviour. The policy ''my-policy-object'' takes two ''int''s as parameters, while ''is available that the givenmy-other-policy-object'' takes a string as an argument.
 +<code lisp>
 +(with-policies
 +    ((my-policy-object (3 1))
 +     (my-policy-object (100 4))
 +     (my-other-policy-object ("Test")))
 +  (body-code))
 +</code>
 +In this example, the resulting code will be equivalent to the following:
 +<code lisp>
 +(with-policy my-policy-object (3 1)
 +  (with-policy my-policy-object (100 4)
 +    (with-policy my-other-policy-object ("Test")
 +      (body-code))))
 +</code>
 +
 +The same princple applies to ''with-named-policies'', with the only difference being that it does not take policy instances, but policy name symbols as parameters:
 +<code lisp>
 +(with-named-policies
 +    (('my-policy (3 1))
 +     ('my-policy (100 4))
 +     ('my-other-policy ("Test")))
 +  (body-code))
 +</code>
 +This results in the same behavior as:
 +<code lisp>
 +(with-named-policy 'my-policy (3 1)
 +  (with-named-policy 'my-policy (100 4)
 +    (with-named-policy 'my-other-policy ("Test")
 +      (body-code))))
 +</code>
 +
 +=== Built-in Policies ===
 +== timeout-policy ==
 +When a piece of code only has a limited maximum amount of time for execution (and must be aborted after that duration), the ''timeout-policy'' comes in handy.
 +
 +Use it like this:
 +<code lisp>
 +(with-policy cpl:timeout-policy (5.0) ; Timeout after 5.0 seconds (fractions may be used)
 +  (body-code-goes-here))
 +</code>
 +And for catching the check condition when the timeout actually happens:
 +<code lisp>
 +(with-failure-handling
 +    ((policy-check-condition-met (f)
 +      (declare (ignore f))
 +      (handle-error-here-and-maybe-retry)))
 +  (with-policy cpl:timeout-policy (5.0)
 +    (body-code-goes-here)))
 +</code>
 +The ''timeout-policy'' stops the given ''body'' code after a given amount of time (in seconds) if it hasn't finished by then. This helps to add a ''timeout'' functionality to functions that do not inherently support a timeout mechanism (blocking function calls).