[GHC] #8947: Depending on hint/ghc API fixes the binary version I can use

[GHC]

#8947: Depending on hint/ghc API fixes the binary version I can use
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       Reporter:  nh2       |             Owner:
           Type:  bug       |            Status:  new
       Priority:  normal    |         Milestone:
      Component:  GHC API   |           Version:  7.6.3
       Keywords:            |  Operating System:  Unknown/Multiple
   Architecture:            |   Type of failure:  GHC rejects valid program
  Unknown/Multiple          |         Test Case:
     Difficulty:  Unknown   |          Blocking:
     Blocked By:            |
Related Tickets:            |
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 When I `build-depend` on hint-0.4.0.0 with GHC 7.6.3, I get


 {{{
 % cabal configure
 Resolving dependencies...
 cabal: Could not resolve dependencies:
 trying: pointcloudviewer-0.1.0 (user goal)
 trying: hint-0.4.0.0/installed-cf3... (dependency of
 pointcloudviewer-0.1.0)
 trying: ghc-7.6.3/installed-494... (dependency of
 hint-0.4.0.0/installed-cf3...)
 trying: bin-package-db-0.0.0.0/installed-608... (dependency of
 ghc-7.6.3/installed-494...)
 trying: GLUtil-0.7.4/installed-223... (dependency of
 pointcloudviewer-0.1.0)
 next goal: linear (dependency of GLUtil-0.7.4/installed-223...)
 rejecting: linear-1.6/installed-300... (conflict: bin-package-db =>
 binary==0.5.1.1/installed-72e..., linear =>
 binary==0.7.1.0/installed-a33...)
 }}}

 This fails because `linear` uses a `binary > 0.5.1.1`, but through my use
 of `hint` which depends on `ghc` (`hint` does not directly depend on
 `binary`), I pull in a constraint `binary == 0.5.1.1`.

 I believe this is bad, because depending on the GHC API in any way fixes
 the binary version I can use, which immediately makes a range of newer
 libraries unavailable to my program.

 Probably GHC should somehow hide the fact that binary is used inside, and
 let the rest of my program use whatever binary it likes.


 ----


 I am aware that this might not be immediately easy, since the GHC API
 might expose some `binary` types to my program that I could try use with
 my newer binary version, which of course would be illegal.

 Maybe it is possible to implement such a check into actual typechecking,
 and let it pass if I don't actually use types from the old version with
 functions from the new library. I actually believe I have seen compiler
 errors in the past where two identical types where not unified because one
 was from a different version, so maybe infrastructure for this is already
 in place?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8947>
GHC <http://www.haskell.org/ghc/>
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