[GHC] #14327: Type error in program caused by unrelated definition

[GHC]

#14327: Type error in program caused by unrelated definition
-------------------------------------+-------------------------------------
        Reporter:  lexi.lambda       |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:  duplicate         |             Keywords:  FunDeps
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13506            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * keywords:   => FunDeps
 * status:  new => closed
 * resolution:   => duplicate
 * related:   => #13506


Comment:

 OK, it turns out this is actually a duplicate of #13506. First, here is
 the program that I tested, for the sake of posterity:

 {{{#!hs
 {-# language DeriveFunctor, FunctionalDependencies
       , MultiParamTypeClasses, FlexibleInstances #-}
 module T14327 where
 import Prelude hiding (readFile, writeFile)
 import Data.Functor.Sum

 data Free f a = Pure a | Free (f (Free f a)) deriving (Functor)

 class Monad m => MonadFree f m | m -> f where
   wrap :: f (m a) -> m a

 instance Functor f => Applicative (Free f)
 instance Functor f => Monad (Free f)

 instance Functor f => MonadFree f (Free f) where
   wrap = Free

 liftF :: (Functor f, MonadFree f m) => f a -> m a
 liftF fa = wrap (return <$> fa)

 data FileSystemF a
   = ReadFile FilePath (String -> a)
   | WriteFile FilePath String a
   deriving (Functor)

 data ConsoleF a
   = WriteLine String a
   deriving (Functor)

 data CloudF a
   = GetStackInfo String (String -> a)
   deriving (Functor)

 type App = Free (Sum FileSystemF (Sum ConsoleF CloudF))

 writeLine :: String -> App ()
 writeLine line = liftF (InR (WriteLine line ()))

 readFile :: FilePath -> App String
 readFile path = liftF (InL (ReadFile path id))

 writeFile :: FilePath -> String -> App ()
 writeFile path contents = liftF (InL (WriteFile path contents ()))
 }}}

 On GHC 8.2.1 and earlier, this does indeed spuriously give more errors
 than it should:

 {{{
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling T14327           (
 /u/rgscott/Documents/Hacking/Haskell/Bug.hs, interpreted )

 /u/rgscott/Documents/Hacking/Haskell/Bug.hs:37:18: error:
     • Couldn't match type ‘Sum ConsoleF CloudF’ with ‘ConsoleF’
         arising from a functional dependency between:
           constraint ‘MonadFree
                         (Sum FileSystemF ConsoleF)
                         (Free (Sum FileSystemF (Sum ConsoleF CloudF)))’
             arising from a use of ‘liftF’
           instance ‘MonadFree f (Free f)’
             at /u/rgscott/Documents/Hacking/Haskell/Bug.hs:15:10-42
     • In the expression: liftF (InR (WriteLine line ()))
       In an equation for ‘writeLine’:
           writeLine line = liftF (InR (WriteLine line ()))
    |
 37 | writeLine line = liftF (InR (WriteLine line ()))
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 /u/rgscott/Documents/Hacking/Haskell/Bug.hs:40:17: error:
     • Couldn't match type ‘ConsoleF’ with ‘Sum ConsoleF CloudF’
         arising from a functional dependency between constraints:
           ‘MonadFree
              (Sum FileSystemF (Sum ConsoleF CloudF))
              (Free (Sum FileSystemF (Sum ConsoleF CloudF)))’
             arising from a use of ‘liftF’
             at /u/rgscott/Documents/Hacking/Haskell/Bug.hs:40:17-46
           ‘MonadFree
              (Sum FileSystemF ConsoleF)
              (Free (Sum FileSystemF (Sum ConsoleF CloudF)))’
             arising from a use of ‘liftF’
             at /u/rgscott/Documents/Hacking/Haskell/Bug.hs:37:18-48
     • In the expression: liftF (InL (ReadFile path id))
       In an equation for ‘readFile’:
           readFile path = liftF (InL (ReadFile path id))
    |
 40 | readFile path = liftF (InL (ReadFile path id))
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 But on GHC HEAD, it doesn't!

 {{{
 GHCi, version 8.3.20171004: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling T14327           (
 /u/rgscott/Documents/Hacking/Haskell/Bug.hs, interpreted )

 /u/rgscott/Documents/Hacking/Haskell/Bug.hs:37:18: error:
     • Couldn't match type ‘Sum ConsoleF CloudF’ with ‘ConsoleF’
         arising from a functional dependency between:
           constraint ‘MonadFree
                         (Sum FileSystemF ConsoleF)
                         (Free (Sum FileSystemF (Sum ConsoleF CloudF)))’
             arising from a use of ‘liftF’
           instance ‘MonadFree f (Free f)’
             at /u/rgscott/Documents/Hacking/Haskell/Bug.hs:15:10-42
     • In the expression: liftF (InR (WriteLine line ()))
       In an equation for ‘writeLine’:
           writeLine line = liftF (InR (WriteLine line ()))
    |
 37 | writeLine line = liftF (InR (WriteLine line ()))
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 It turns out that commit 48daaaf0bba279b6e362ee5c632de69ed31ab65d (`Don't
 report fundep wanted/wanted errors`) fixed this problem. This led me to
 realize that this entire ticket is simply a more involved version of the
 program in #13506 (which concerns error cascades with functional
 dependencies), the ticket that the aforementioned commit originally fixed.
 So I'll close this as a duplicate.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14327#comment:4>
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