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--- tutorials:advanced [2015/05/11 17:04] – created gkazhoya
+++ tutorials:advanced [2015/05/11 17:06] (current) – removed gkazhoya
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-====== Writing Plans ======
-==== with-policy ====
-{{ :doc:with-policy.png?nolink&300|}}
-Policies can be used to define generic monitoring behavior that should run concurrently besides main functionality code. Such a policy might be monitoring
-  * the position of an object inside a robot gripper to see whether it is moving out of the gripper (losing the object)
-  * the position of an object in the real world, in order to keep the robot's head pointing towards it
-  * the collision of a robot joint with objects in the real world
-  * generic events that must definitely interrupt a certain piece of code, but are too specialized to be implemented directly in the monitored code itself
-A policy consists of multiple parts:
-  * A name (to identify it)
-  * A description string (to make its purpose clear)
-  * A parameter list (which can be used to specialize a generic policy for a situation at hand, without generating a new policy every time the situation changes slightly)
-  * Code blocks that need to be evaluated during monitoring
-=== Usage example ===
-An example of how to define a policy is shown here. The policy accepts two custom parameters, one defining a maximum number and one a match number. During execution of the main ''body'' code (which is really just a loop with an output here), it checks a randomly generated number (ranging from ''0'' to ''max-num'') against the match number ''match-num''. If this is the case (and it will be, eventually), it interrupts (and ends) the main ''body'' code and executes the '':recover'' code, followed by the '':clean-up'' code (both just outputting something here).
-<code lisp>
-(define-policy my-policy (max-num match-num)
-  "This is an example policy."
-  (:init (format t "Initializing policy~%")
-         t)
-  (:check (format t "Checking if random number from 0
-                     to ~a equals ~a~%" max-num match-num)
-          (let ((rnd (random max-num)))
-            (format t "Got number ~a~%" rnd)
-            (cond ((eql rnd match-num)
-                   (format t "Match~%")
-                   t)
-                  (t (sleep 1)))))
-  (:recover (format t "Running recovery mechanisms~%"))
-  (:clean-up (format t "Running clean-up~%")))
-</code>
-The calling code for using the policy uses ''with-named-policy'' to refer to the name as specified while defining the policy. The second parameter is a list of parameter values for customizing the policy instance. The rest of the code should be pretty self explanatory.
-<code lisp>
-(top-level
-  (with-named-policy 'my-policy (10 5)
-    (loop do (format t "Main loop cycle.~%")
-             (sleep 2))))
-</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).