[r6rs-discuss] [Formal] The components of a compound condition should be independent

---
This message is a formal comment which was submitted to formal-comment_at_r6rs.org, following the requirements described at: http://www.r6rs.org/process.html
---
Submitter: John Cowan
Email address: cowan_at_ccil.org
Issue type: Simplification
Priority: Major
Component: Exceptions
Report version: 5.92
Summary:  The components of a compound condition should be independent.
In R5.92RS, a condition may contain any number of component conditions.
However, if two or more of the components are of the same exact type,
then condition-ref will be able to retrieve the fields only of the first
such component, not the others.  A fortiori, if a condition contains
two components foo and bar of types &foo and &bar respectively, such
that &foo is a supertype of &bar, then asking for a field of &foo will
return the value from whichever component appears first.
In order to simplify the use of conditions, I propose to make it an
error to create a compound condition with two or more components of the
same type, unless that type has no fields.  This means that the order
of component conditions no longer matters.
None of the standard condition types of report section 6.3 conflict in
this way, but it would mean, for example, that the condition created
by the "error" procedure could not be compounded with another such,
nor with any simple condition of type &who, &irritants, or &message.
If this is done, then the acccessor procedures defined by
define-condition-type can be applied to compound as well as simple
conditions of that type, since the result will not be ambiguous or
order-dependent.  (R5.92RS is silent on using such an accessor on a
compound condition.)
-- 
John Cowan   http://ccil.org/~cowan  cowan_at_ccil.org
[P]olice in many lands are now complaining that local arrestees are insisting
on having their Miranda rights read to them, just like perps in American TV
cop shows.  When it's explained to them that they are in a different country,
where those rights do not exist, they become outraged.  --Neal Stephenson