[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 simonpj):

 Thanks.  I've added some comments to Phab:D578.

 Here are some other thoughts:

  * Could you modify [wiki:ExplicitCallStack/ImplicitLocations] to describe
 the exact specification of what you propose?   For example, the design
 means that there are two new types in the `base` library, defined in
 `GHC.Location`:
    * `SrcLoc` (for a source location)
    * `Location` (for a stack of source locations)
    Both are abstract, so you can't make a new `SrcLoc` or `Location`; only
 GHC can do that. Is that what we want?

  * You can add a section on open questions too, for things you aren't sure
 about.

  * I'm very keen that `SrcLoc` is used also by the `Typeable` library (and
 any other reflection libraries too).  Currently `Data.Typeable.Internals`
 defines `TyCon` which contains package/module/name information, but it
 should just use a `SrcLoc`.  We are proposing to redesign `Typeable`
 anyway (see [wiki:Typeable]) so we can do this `SrcLoc` stuff at the same
 time

  * Given this broader use, is `Location` the right name?  It's really more
 like a `SrcLocStack`.

  * The stack would be even better if it said something like "foo called at
 <location>" rather than just "<location>".  That would be easy enough to
 implement (the information is in the `CtOrigin` of the constraint), but
 again the data type would need to be elaborated a bit, something like
 {{{
 data Location = Location [(String, SrcLoc)]
 }}}

  * At a grungy level, I wonder about all the copies of the same module-
 name and file-name strings.  Maybe the desugarer should just generate one
 copy and use it?  Or maybe CSE should gather them up (I don't think it
 does so at the moment.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9049#comment:23>
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