[GHC] #9049: Expose srcLoc from the assertion architecture to allow better error messages

[GHC]

#9049: Expose srcLoc from the assertion architecture to allow better error
messages
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        Reporter:  nh2               |                   Owner:  gridaphobe
            Type:  feature request   |                  Status:  patch
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.8.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D578
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Comment (by gridaphobe):

 Replying to [comment:26 simonpj]:
 > Re "the second bullet point under Implementation Details", I now
 understand.  Good point.  The issue is this:
 > {{{
 > f :: (?loc :: CallStack) => IO ()
 > f = print (?location :: CallStack)
 > }}}
 > There are two alternatives for what to pass to `print`:
 >   * Pass the call site of `print`, pushed onto the passed-in call stack
 >   * Pass a singleton stack, just the call site of `print`, reflecting
 the change in name.
 > You have implemented the first, but I think the latter would perhaps be
 more intuitive.  Does anyone else have opinions?

 The more I think about this, the more inclined I am to go with the latter
 behavior as well. The former, while useful, is just a bit strange. Also,
 from a maintainability perspective, it would probably be easier to switch
 from the 2nd to the 1st behavior, if needed, than vice-versa (since the
 1st should allow strictly more programs to type-check).

 But I'd also like to hear from others.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9049#comment:30>
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