[Haskell] IO, exceptions and error handling

[Alastair Reid]

> I assume the suggested mapException function [sect 5.4] remains
> unproblematic ... is it (or some equivalent) actually implemented? 

http://etudiants.insia.org/~jbobbio/pafp/docs/base/Control.Exception.html#v%
3AmapException

The same page has a host of other useful operations.

Two useful capabilities that are missing in the actual implementation are:

1) Provision for routinely adding filename and line number.
   (Of course, we'd need some kind of compiler support for filling
   that info in.)

  data Exception = ... | Location SrcLoc Exception | ...

2) The ability to build up lists of exceptions perhaps by adding something 
like:

  data Exception = ... | Nested Exception Exception | ...

This would allow handlers to build 'stacks' of exceptions rather like the 
stackdumps Java exceptions produce.  For example:

  foo x = mapException (Nested (ErrorCall "while in foo")) $ bar x
  bar x = mapException (Nested (ErrorCall "while in bar")) $ baz x
  baz x = 1/x

  foo 0
  ==>
  Nested (ErrorCall "while in foo")
         (Nested (ErrorCall "while in bar")
                 (ArithException DivideByZero))

[Alternate names for 'Nested' welcome.  `WhileDoing` is my next best 
attempt :-)]

--
Alastair Reid