Re: [Help-smalltalk] exceptions

[Paolo Bonzini]

Stephen Compall wrote:
> Robin Redeker wrote:
>   
> > So basically the object 1 doesn't understand some exception handling
> > message and throws an exception itself, which seems to trigger the
> > handler again. - I would expect that a exception thrown while catching
> > one (where the catching process fails) would throw that exception so
> > that it doesn't trigger the same catching process. But maybe i'm just
> > plain wrong with my expectations.
> >     
>
> I spotted a workaround that doesn't mess with the current handler stack:
>
> !Object methodsFor: 'exception handling'!
> goodness: anException
>     "Answer an integer indicating whether a handler with me as
>     the 'on:' argument can handle anException, where a negative
>     number means it can't."
>     ^-1
> ! !
>   
Even better:

Object: 1 error: did not understand #goodness:
MessageNotUnderstood(Exception)>>#signal
SmallInteger(Object)>>#doesNotUnderstand:
optimized [] in BlockClosure class>>#exceptionHandlerSearchBlock
[] in CoreException>>#instantiateNextHandler:
MethodContext(ContextPart)>>#scanBacktraceForAttribute:do:
CoreException>>#instantiateNextHandler:
Error(Exception)>>#signal
Error(Exception)>>#signal:
UndefinedObject(Object)>>#error:
optimized [] in UndefinedObject>>#executeStatements
BlockClosure>>#on:do:
UndefinedObject>>#executeStatements
nil

See attached patch (huge because of whitespace changes).

Paolo

--- orig/kernel/BlkClosure.st
+++ mod/kernel/BlkClosure.st
@@ -63,22 +63,26 @@ exceptionHandlerSearchBlock
         | best bestGoodness goodness activeHandlers nested |
         bestGoodness := -1.
         activeHandlers := context at: context numArgs + 1.
-       nested := false.
+        context at: context numArgs + 1 put: -1.
+       nested := activeHandlers = -1.
 
-        1 to: context numArgs - 1 by: 2 do: [ :i |
-            goodness := (context at: i) goodness: signal exception.
-            goodness > -1 ifTrue: [
-                (activeHandlers bitAt: i) = 1 ifTrue: [
-                    "Sorry, this handler is already active..."
-                    nested := true.
-                    goodness := -1.
-                ]
-            ].
-            goodness > bestGoodness ifTrue: [
-                best := i.
-                bestGoodness := goodness.
-            ]
-        ].
+       nested
+           ifFalse: [
+               1 to: context numArgs - 1 by: 2 do: [ :i |
+                   goodness := (context at: i) goodness: signal exception.
+                   goodness > -1 ifTrue: [
+                       (activeHandlers bitAt: i) = 1 ifTrue: [
+                           "Sorry, this handler is already active..."
+                           nested := true.
+                           goodness := -1.
+                       ]
+                   ].
+                   goodness > bestGoodness ifTrue: [
+                       best := i.
+                       bestGoodness := goodness.
+                   ]
+               ]
+           ].
 
         "Now instantiate the best handler we found"
         best isNil
@@ -96,6 +100,7 @@ exceptionHandlerSearchBlock
                #found
             ]
            ifTrue: [
+               context at: context numArgs + 1 put: activeHandlers.
                nested ifTrue: [ #skip ] ifFalse: [ nil ]
            ]
     ]