[GHC] #8634: Relax functional dependency coherence check ("liberal coverage condition") (was: Code valid in GHC 7.6 is impossible to move over GHC 7.7 (because of liberal coverage condition))

[GHC]

#8634: Relax functional dependency coherence check ("liberal coverage condition")
-----------------------------------+---------------------------------------
        Reporter:  danilo2         |            Owner:
            Type:  feature         |           Status:  new
  request                          |        Milestone:
        Priority:  high            |          Version:  7.7
       Component:  Compiler        |         Keywords:
      Resolution:                  |     Architecture:  Unknown/Multiple
Operating System:                  |       Difficulty:  Unknown
  Unknown/Multiple                 |       Blocked By:
 Type of failure:  None/Unknown    |  Related Tickets:  #1241, #2247, #8356
       Test Case:                  |
        Blocking:                  |
-----------------------------------+---------------------------------------
Changes (by goldfire):

 * type:  bug => feature request


Comment:

 I have to say I like the idea of `-XDysfunctionalDependencies`. An error
 in functional dependencies can never make a program "go wrong". Why do I
 claim this? Because the type safety of GHC Haskell is based on the type
 safety of Core, GHC's internal, typed language.... and functional
 dependencies don't exist, at all, in Core. So, perhaps functional
 dependencies can change exactly what Core is produced, but they can only
 go from one type-safe Core program to another type-safe Core program.

 I see something like `-XDysfunctionalDependencies` as quite like
 `-XIncoherentInstances`. These threaten coherence (the notion that the
 same instance of a class will be used for the same type(s) in different
 places) but not type safety. The a power user wants to ignore coherence,
 maybe that's OK.

 `-XDysfunctionalDependencies` has another nice benefit: it gives users a
 quick and dirty way to arbitrarily nudge the type inference engine.
 Currently, one of the tenets of type inference is that it makes no guesses
 and performs no search. But with `-XDysfunctionalDependencies` users could
 provide (perhaps incoherent) hints to the type inference engine, which
 would allow more programs to type check.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:7>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler