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Tue, 28 Feb 2012

Current State of Exceptions in Rakudo and Perl 6


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It's been a while since my last update on my grant work on exceptions for Perl 6 and Rakudo, and I can report lots of progress.

The work on Rakudo's exception system made me realize that we conflated two concepts in the base exception type: on the one hand the infrastructure for reporting errors and backtraces, and on the other hand holding some sort of error message as an attribute.

As a result, we now have a base class called Exception from which all exception types must inherit. When a non-Exception object is passed to die(), it is wrapped in an object of class X::AdHoc. Other error classes can decide to generate the error message without having an attribute for it (for example hard-coded in a method).

Typed exceptions are now thrown not only from the setting, but also from the compiler itself, namely the grammar and the action methods. In fact the majority of errors from these two parts of the compiler are now handled with dedicated exception types.

The most user-visible change is a new and improved backtrace printer, which produces usually much shorter and more readable backtraces. The old one is still available on demand. Consider the program

sub f {
    g() for 1..10;
}
sub g { die 'OH NOEZ' }
f;

The old backtrace printer produced:

OH NOEZ
  in sub g at /home/moritz/p6/rakudo/ex.pl:4
  in block <anon> at /home/moritz/p6/rakudo/ex.pl:2
  in method reify at src/gen/CORE.setting:4471
  in method reify at src/gen/CORE.setting:4376
  in method reify at src/gen/CORE.setting:4376
  in method gimme at src/gen/CORE.setting:4740
  in method eager at src/gen/CORE.setting:4715
  in method eager at src/gen/CORE.setting:1028
  in sub eager at src/gen/CORE.setting:5000
  in sub f at /home/moritz/p6/rakudo/ex.pl:2
  in block <anon> at /home/moritz/p6/rakudo/ex.pl:5
  in <anon> at /home/moritz/p6/rakudo/ex.pl:1

Where the eager, gimme and reify methods come from the 'for' lop, which is compiled to the equivalent of eager (1..10).map: { g() }.

The new backtrace printer produces

OH NOEZ
  in sub g at ex.pl:4
  in sub f at ex.pl:2
  in block <anon> at ex.pl:5

It is also a special pleasure to report that after a walk through a change to throw a typed exception, we've received a pull request by a new developer which also changes an exception from X::AdHoc to a dedicated type.

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