[GHC] #9139: Using lift with StateT and Maybe on GHCI

[GHC]

#9139: Using lift with StateT and Maybe on GHCI
------------------------------------+----------------------------
       Reporter:  Jefffrey          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  low               |         Milestone:
      Component:  GHCi              |           Version:  7.6.3
       Keywords:                    |  Operating System:  MacOS X
   Architecture:  Unknown/Multiple  |   Type of failure:  Other
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+----------------------------
 I was playing around with `StateT` and `Maybe` today and I run the
 following two commands (consider that `TC` == `import qualified
 Control.Monad.Trans.Class as TC`:

 {{{
 ghci> let g = TC.lift Just [3, 4]  :: StateT [Int] Maybe Int
 }}}

 which led to a:

 {{{
 <interactive>:54:9:
     Couldn't match kind `* -> *' with `*'
     Expected type: [t0] -> StateT [Int] Maybe Int
       Actual type: [t0] -> StateT [Int] Maybe Int
     Kind incompatibility when matching types:
       [t_i] :: * -> *
       [t0] :: *
     The function `TC.lift'ghc: panic! (the 'impossible' happened)
   (GHC version 7.6.3 for x86_64-apple-darwin):
         kindFunResult
 <<details unavailable>>

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 and


 {{{
 > let g = TC.lift Just (1, [3, 4])  :: StateT [Int] Maybe Int
 }}}

 which led to:


 {{{
 <interactive>:55:9:
     Couldn't match kind `* -> *' with `*'
     Expected type: (t0, [t1]) -> StateT [Int] Maybe Int
       Actual type: (t0, [t1]) -> StateT [Int] Maybe Int
     Kind incompatibility when matching types:
       (t_i, [t_l]) :: * -> *
       (t0, [t1]) :: *
     The function `TC.lift'ghc: panic! (the 'impossible' happened)
   (GHC version 7.6.3 for x86_64-apple-darwin):
         kindFunResult
 <<details unavailable>>

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 I'm just doing what the last line asked me. :)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9139>
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