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.

<source lang=apl>

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

</syntaxhighlight>

Usage is trivial: <source lang=apl inline>(⎕NEW MyApp).Wait</syntaxhighlight>. The <source lang=apl inline>Wait</syntaxhighlight> method is enough to keep the <source lang=apl inline>MyApp</syntaxhighlight> instance in existence until the <source lang=apl inline>Kill</syntaxhighlight> 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 <source lang=apl inline>MyApp</syntaxhighlight>. 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 <source lang=apl inline>MyApp</syntaxhighlight>'s methods are no longer on it. For example, let's insert a domain error into <source lang=apl inline>ButtonHandler</syntaxhighlight>:

<source lang=apl>

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

</syntaxhighlight>

start it, and push the button.

<source lang=apl>

     (⎕NEW MyApp).Wait

DOMAIN ERROR ButtonHandler[2] ÷0

               ∧
     )SI

[#.[MyApp]] #.MyApp.ButtonHandler[2]* ⎕DQ

     ⎕THIS

1. .[MyApp]

</syntaxhighlight>

You might think that you could set <source lang=apl inline>⎕TRAP</syntaxhighlight> to cut back to the instance and resume, by writing:

<source lang=apl>

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

</syntaxhighlight>

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

A more robust approach overrides the <source lang=apl inline>Wait</syntaxhighlight> method <source lang=apl inline>MyApp</syntaxhighlight> inherited from the <source lang=apl inline>Form</syntaxhighlight> class. We can localise <source lang=apl inline>⎕TRAP</syntaxhighlight> in <source lang=apl inline>Wait</syntaxhighlight>, 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.

<source lang=apl>

   ∇ 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
   ∇

</syntaxhighlight>

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 <source lang=apl inline>ThisObj</syntaxhighlight> slot.

<source lang=apl>

   ∇ 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
   ∇

</syntaxhighlight>

See also

• Error trapping with Dyalog APL