[GHC] #8875: Track Exceptions

[GHC]

#8875: Track Exceptions
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        Reporter:  yokto             |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 While you might be able to track this for synchronous exceptions, tracking
 asynchronous exceptions would be nearly impossible given that anyone might
 throw you anything at any time.

 Even if you are okay with this limitation, identifying which exceptions a
 function might throw is a non-trivial task which would require some sort
 of inter-procedure analysis, especially when you account for the fact that
 an exception handler may dynamically choose not to handle a given
 exception.

 To make matters worse, the only exception handling constructs that the
 compiler currently knows about are `throw#` and `catch#` (and some
 variants), which know nothing of exception types (they merely expect some
 boxed Haskell value, which is typically a `SomeException` which packages
 an exception value together with a `Typeable` dictionary so we can
 reconstruct the type later).

 So, while it might be possible to implement some approximation of what is
 requested here, it doesn't seem to me like the complexity of such an
 implementation would pull its wait. Moreover, you can get much of what you
 have requested in the type system, either by encoding errors in the result
 type of your computation or some other encoding of checked exceptions
 (e.g. http://www.well-typed.com/blog/2015/07/checked-exceptions/).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8875#comment:3>
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