[GHC] #10009: type inference regression when faking injective type families

[GHC]

#10009: type inference regression when faking injective type families
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        Reporter:  aavogt            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:  7.10.1
       Component:  Compiler (Type    |                 Version:  7.10.1-rc1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:                    |
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Comment (by aavogt):

 It affects both: the library has a number of definitions that don't
 compile with 7.10RC1 right now. It affects how clients can put library
 functions together. For example the library defines:

 {{{
 -- possible type signatures:
 -- hUntagSelf :: HList '[Tagged x x, Tagged y y] -> HList '[x,y]
 -- hTagSelf   :: HList '[x,y] -> HList '[Tagged x x, Tagged y y]

 -- hZipTIP :: TIP '[Tagged x x] -> TIP '[Tagged y y]
 --                 -> TIP '[Tagged (x,y) (x,y)]
 hZipTIP (TIP x) (TIP y) = TIP (hTagSelf (hZipList (hUntagSelf x)
 (hUntagSelf y)))
 }}}

 If I figure out where to annotate to get rid of the 7.10RC1 type error:

 {{{
 Data/HList/TIP.hs:251:1: Warning:
     Could not deduce (TagR a0 ~ TagR a)
     from the context (HZipList x y a,
                       TagUntag a,
                       TagUntag x,
                       TagUntag y)
       bound by the inferred type for ‘hZipTIP’:
                  (HZipList x y a, TagUntag a, TagUntag x, TagUntag y) =>
                  TIP (TagR x) -> TIP (TagR y) -> TIP (TagR a)
       at Data/HList/TIP.hs:251:1-81
     NB: ‘TagR’ is a type function, and may not be injective
     The type variable ‘a0’ is ambiguous
     Expected type: TIP (TagR x) -> TIP (TagR y) -> TIP (TagR a)
       Actual type: TIP (TagR x0) -> TIP (TagR y0) -> TIP (TagR a0)
     When checking that ‘hZipTIP’ has the inferred type
       hZipTIP :: forall (a :: [*]) (x :: [*]) (y :: [*]).
                  (HZipList x y a, TagUntag a, TagUntag x, TagUntag y) =>
                  TIP (TagR x) -> TIP (TagR y) -> TIP (TagR a)
     Probable cause: the inferred type is ambiguous

 Data/HList/TIP.hs:251:1: Warning:
     Could not deduce (HZipList x0 y0 a0)
     from the context (HZipList x y a,
                       TagUntag a,
                       TagUntag x,
                       TagUntag y)
       bound by the inferred type for ‘hZipTIP’:
                  (HZipList x y a, TagUntag a, TagUntag x, TagUntag y) =>
                  TIP (TagR x) -> TIP (TagR y) -> TIP (TagR a)
       at Data/HList/TIP.hs:251:1-81
     The type variables ‘a0’, ‘x0’, ‘y0’ are ambiguous
     When checking that ‘hZipTIP’ has the inferred type
       hZipTIP :: forall (a :: [*]) (x :: [*]) (y :: [*]).
                  (HZipList x y a, TagUntag a, TagUntag x, TagUntag y) =>
                  TIP (TagR x) -> TIP (TagR y) -> TIP (TagR a)
     Probable cause: the inferred type is ambiguous
 }}}

 clients probably want to define functions like this too, and requiring a
 type annotation here probably doubles the amount of code that has to be
 written.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10009#comment:7>
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