Error trapping in Dyalog APL forms

An application's GUI looks like a Form with stuff on it. So it is appealing to describe it that way. That is to say, to define the GUI as a class derived from a Dyalog native Form.

:Class MyApp: 'Form'

    ∇ make
      :Access Public
      :Implements Constructor
      Caption←'My Application'

      BTN←⎕NEW'Button'(⊂'Caption' 'Push me')
      BTN.onSelect←'ButtonHandler'
    ∇

    ∇ ButtonHandler(obj xxx)
      obj.Caption←'Ouch'
    ∇

:EndClass

Usage is trivial: (⎕NEW MyApp).Wait. The Wait method is enough to keep the MyApp instance in existence until the Kill button is clicked. Then everything disappears: no objects left, no clean-up to do.

This is an appealing way to code a GUI because of the high degree of encapsulation. Everything the application's GUI needs is contained in the Form. It doesn't even require a name assigned to it.

However, trapping such an application requires a little thought. If your error traps have relied on knowing the names of forms and so on, you will find them difficult to use here.

Let's make the reasonable assumption an error might occur elsewhere in the active workspace, not necessarily in a method of MyApp. Let's suppose that in the event of an error or an interrupt we want to (a) log the event and local environment, and (b) cut back and either restart or resume the application.

One thing we can't do is cut back the stack so far that MyApp's methods are no longer on it. For example, let's insert a domain error into ButtonHandler:

    ∇ ButtonHandler(obj xxx)
      obj.Caption←'Ouch'
      ÷0
    ∇

start it, and push the button.

      (⎕NEW MyApp).Wait
DOMAIN ERROR
ButtonHandler[2] ÷0
                ∧
      )SI
[#.[MyApp]] #.MyApp.ButtonHandler[2]*
⎕DQ
      ⎕THIS
#.[MyApp]

You might think that you could set ⎕TRAP to cut back to the instance and resume, by writing:

      ⎕TRAP←⊂0 'C' '⎕←''TRAPPED'' ⋄ ⎕←↑⎕DM ⋄ Wait'

either in the class script or in its constructor. But it turns out that ⎕TRAP is set in the workspace root, not in the object. When the trap fires, the stack is cut back to immediate execution, and the MyApp instance vanishes.

A more robust approach overrides the Wait method MyApp inherited from the Form class. We can localise ⎕TRAP in Wait, log the error from the environment in which it occurred, then ensure the stack is cut back only to a point at which we can resume.

    ∇ Wait;⎕TRAP;done
      :Access Public
      ⎕TRAP←⊂(⍳500)'E' '⎕←↑¨⎕SI ⎕LC ⋄ ⎕SIGNAL 911'
      :Repeat
          :Trap 911
              done←0∊⍴⎕BASE.Wait
          :Else
              done←0
          :EndTrap
      :Until done
    ∇

Here is a slightly more elaborate version taken from a commercial application. This application's user-interface object needs its error trapping to report what is in its ThisObj slot.

    ∇ Wait;⎕TRAP;done
      :Access Public
      ⎕TRAP←⊂(911~⍨⍳1006)'E' '#.SESSION.Log.Error ⎕DM ⋄ ⎕SIGNAL 911'
      :Repeat
          :Trap 911
              done←0∊⍴⎕BASE.Wait
          :Else
              #.SESSION.Log.Record'Error in ',⍕ThisObj
              CursorObj←#.GUI.CURSOR.Default
              #.GUI.Error'System has trapped and logged an error in ',⍕⊃⊃⎕CLASS ThisObj
              done←0
          :EndTrap
      :Until done
    ∇

See also

• Error trapping with Dyalog APL