[GHC] #9562: Type families + hs-boot files = unsafeCoerce

[GHC]

#9562: Type families + hs-boot files = unsafeCoerce
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                   Owner:  ezyang
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #10270            |  Differential Revisions:
-------------------------------------+-------------------------------------

Comment (by ezyang):

 > There does not seem to be a way to feed GHC a list of object files to
 link together and coax it to skip linking

 Actually, this is not true:

 {{{
 ezyang at sabre:~$ ghc -c A.hs
 ezyang at sabre:~$ ghc -c B.hs-boot
 ezyang at sabre:~$ ghc -c C.hs
 ezyang at sabre:~$ ghc -c B.hs
 ezyang at sabre:~$ ghc -c D.hs
 ezyang at sabre:~$ ghc -c Main.hs
 ezyang at sabre:~$ ghc Main.o A.o B.o C.o D.o -o Unsafe
 ezyang at sabre:~$ ./Unsafe
 -5692549928996289131
 }}}

 Unfortunately, GHC has no idea about the dependency structure of the
 object files; for all it knows, it could have been produced by another
 compiler. So there actually is no way for GHC to figure out that `B.o` is
 type-unsafe in this case. This would imply the only way to safely link
 Haskell objects is to use `ghc Main.hs` (which will do dependency
 resolution.)

 Note that Richard's original proposed fix will work in this case, since
 `B.hs` will be refused as `B.hs-boot` doesn't report enough type families.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9562#comment:9>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler