[GHC] #16115: Missing associated type instance not reported with error

31 Dec 2018

      #16115: Missing associated type instance not reported with error
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.10.1
          Component:  Compiler       |           Version:  8.6.3
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I noticed [https://stackoverflow.com/questions/53987924/haskell-couldnt-
 match-expected-type-item-nat-with-actual-type this SO question] was caused
 by a warning disappearing as a result of the error it caused.

 {{{#!hs
 {-# language TypeFamilies, DataKinds #-}
 module NoWarning where
 data Nat = Zero | Succ Nat deriving Show
 class FromList a where
   type Item a :: *
   fromList :: [Item a] -> a

 instance FromList Nat where
   fromList [] = Zero
   fromList (a:as) = Succ (fromList as :: Nat)

 fish :: Nat
 fish = fromList [(),(),()]
 }}}

 If you delete `fish`, you get a nice warning:

 {{{
 NoWarning.hs:8:1: warning: [-Wmissing-methods]
     • No explicit associated type or default declaration for ‘Item’
     • In the instance declaration for ‘FromList Nat’
   |
 8 | instance FromList Nat where
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^...

 }}}

 But with `fish`, all you get is

 {{{
 NoWarning.hs:13:18: error:
     • Couldn't match expected type ‘Item Nat’ with actual type ‘()’
     • In the expression: ()
       In the first argument of ‘fromList’, namely ‘[(), (), ()]’
       In the expression: fromList [(), (), ()]
    |
 13 | fish = fromList [(),(),()]
    |
 }}}

 That warning is the proper explanation of the problem, and it's just
 missing!

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16115
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