withHandlersDo

Control-struct: ]withHandlersDo{ ... }

file: job.t
package: muf
status: alpha

The ]withHandlersDo{...} operator is the current Muq MUF surface syntax for executing code under the control of specified event handlers. (See section Handlers.)

(The ]withHandlerDo{...} operator is a synonym for ]withHandlersDo{...}.)

The syntax is

[  event0  handler0
   event1  handler1
   ...
| ]withHandlersDo{ ... }

where:

• Each event must be an instance of Class Event (see section makeEvent).
• Each handler must be a { [] -> [] } function accepting a block describing the event, and returning that block unchanged if it returns at all. (A handler which resolves the event will normally not return.)

To prevent recursive handler disasters, when any of the specified handlers is invoked, it is run in a context in which the given handler bindings are all effectively dis-established. (Handler bindings established by other ]withHandlersDo{ statements are of course completely unaffected by this, even if they specify the same handler functions.)

Example: This calls a function fn with a handler established that counts warnings. Since the handler then returns normally, normal warning handling is not otherwise affected:

0 --> *warnings-seen*
[ .err.warning
  :: { [] -> [] ! } ++ *warnings-seen* ;
| ]withHandlersDo{
  fn
}

Example: This prints a custom message on divide by zero, then (by simply returning) lets the usual handler(s) take over:

[ .err.divisionByZero
  :: { [] -> [] ! }
    "-Real- hackers don't divide by zero!\n" ,
  ;
| ]withHandlersDo{
  1 0 /
}

For a fuller discussion of events, see section Muq Events.

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