#16234: Unable to resolve type families -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: #16211 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code {{{ {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} import Control.Monad.Classes (MonadReader) --import Control.Monad.Primitive () import Control.Monad.Trans.State.Lazy (StateT) main :: (n ~ StateT () IO, MonadReader () n) => IO () main = undefined }}} produces the error (cleaned up for readability) {{{ Main.hs:9:1: error: * No instance for (monad-classes.MonadReaderN (monad-classes.FindTrue '[monad-classes.CanDo (StateT () IO) (monad-classes.EffReader ()), monad-classes.CanDo IO (monad-classes.EffReader ())]) () (StateT () IO)) arising from a use of `main' * In the expression: main When checking the type of the IO action `main' | 9 | main = undefined | ^ }}} This is the same error as #16211, but that ticket uses a slightly different example to trigger the error upon recompile. The relevant instances are all available in `monad-classes`. {{{ class Monad m => MonadReaderN (n :: Peano) r m instance Monad m => MonadReaderN Zero r (StateT r m) type family CanDo (m :: (* -> *)) (eff :: k) :: Bool type instance CanDo (StateT s m) eff = StateCanDo s eff type family StateCanDo s eff where StateCanDo s (EffState s) = True StateCanDo s (EffReader s) = True type family FindTrue (bs :: [Bool]) :: Peano where FindTrue (True ': t) = Zero FindTrue (False ': t) = Succ (FindTrue t) data EffReader (e :: *) }}} I can reproduce with the attached package using GHC-8.6.3 and cabal: {{{ $ cabal sandbox init $ cabal install --only-dependencies $ cabal build }}} I was unable to minimize the attached example any further. Here's two ways to make the error go away: 1. `import Control.Monad.Primitive ()`. It's unclear why this would help because according to the maintainer of monad-classes, it has no transitive dependencies on `primitive`. 2. Change the signature of `main` to `main :: (MonadReader () (StateT () IO) => IO ()` -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16234 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler