#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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler