From: Intransition <transfire@...>
Date: 2012-01-21T03:49:57+09:00
Subject: What defines an Error?

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I wonder how we are to know the appropriate error to raise or to subclass? 

Presently, we simply make a pretty good subjective guess. But I wonder if 
it could be more concrete. Might we be able to define a test for each error 
by which it could be determined? For example, maybe:

   class TypeError

      def self.applies?(target, criterion)
        return false unless Exception === target
        return false unless Exception === criterion
        true
      end

      def raise?(target, criterion)
        return false unless applies?(target, criterion)
        return true unless criterion == target
      end

      def self.apply!(target, criterion)
        raise ArgumentError unless applies?(target, criterion)
        raise self if raise?(target, criterion)
      end
    end

So #applies? would return false if the given arguments do not make any 
sense for the error. And we could use #raise? to see if it would raise the 
error and #apply! to do so.

Does narrowly defining the purpose of errors this way make sense?


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I wonder how we are to know the appropriate error to raise or to subclass? =
<br><br>Presently, we simply make a pretty good subjective guess. But I won=
der if it could be more concrete. Might we be able to define a test for eac=
h error by which it could be determined? For example, maybe:<br><br> &nbsp;=
&nbsp; class TypeError<br><br>&nbsp; &nbsp; &nbsp; def self.applies?(target=
, criterion)<br>&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp; return false unless E=
xception =3D=3D=3D target<br>&nbsp;&nbsp; &nbsp;&nbsp; &nbsp; return false =
unless Exception =3D=3D=3D criterion<br>&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbs=
p; true<br>&nbsp; &nbsp; &nbsp; end<br><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; d=
ef raise?(target, criterion)<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; =
return false unless applies?(target, criterion)<br>&nbsp;&nbsp;&nbsp;&nbsp;=
&nbsp;&nbsp;&nbsp; return true unless criterion =3D=3D target<br>&nbsp;&nbs=
p;&nbsp;&nbsp;&nbsp; end<br><br>&nbsp; &nbsp; &nbsp; def self.apply!(target=
, criterion)<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; raise ArgumentEr=
ror unless applies?(target, criterion)<br>&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&n=
bsp; raise self if raise?(target, criterion)<br>
&nbsp; &nbsp; &nbsp; end<br>
&nbsp; &nbsp; end<br><br>So #applies? would return false if the given argum=
ents do not make any sense for the error. And we could use #raise? to see i=
f it would raise the error and #apply! to do so.<br><br>Does narrowly defin=
ing the purpose of errors this way make sense?<br><br>
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