The @FamInst@ type: family instance heads \begin{code}

    module FamInst (
            checkFamInstConsistency, tcExtendLocalFamInstEnv, tcGetFamInstEnvs
        ) where

    import HscTypes
    import FamInstEnv
    import TcMType
    import TcRnMonad
    import TyCon
    import Name
    import Module
    import SrcLoc
    import Outputable
    import UniqFM
    import FastString

    import Maybes
    import Control.Monad
    import Data.Map (Map)
    import qualified Data.Map as Map

\end{code} %************************************************************************ %* * Optimised overlap checking for family instances %* * %************************************************************************ For any two family instance modules that we import directly or indirectly, we check whether the instances in the two modules are consistent, *unless* we can be certain that the instances of the two modules have already been checked for consistency during the compilation of modules that we import. Why do we need to check? Consider module X1 where module X2 where data T1 data T2 type instance F T1 b = Int type instance F a T2 = Char f1 :: F T1 a -> Int f2 :: Char -> F a T2 f1 x = x f2 x = x Now if we import both X1 and X2 we could make (f2 . f1) :: Int -> Char. Notice that neither instance is an orphan. How do we know which pairs of modules have already been checked? Any pair of modules where both modules occur in the `HscTypes.dep_finsts' set (of the `HscTypes.Dependencies') of one of our directly imported modules must have already been checked. Everything else, we check now. (So that we can be certain that the modules in our `HscTypes.dep_finsts' are consistent.) \begin{code}

    -- The optimisation of overlap tests is based on determining pairs of modules
    -- whose family instances need to be checked for consistency.
    --
    data ModulePair = ModulePair Module Module

    -- canonical order of the components of a module pair
    --
    canon :: ModulePair -> (Module, Module)
    canon (ModulePair m1 m2) | m1 < m2   = (m1, m2)
    			 | otherwise = (m2, m1)

    instance Eq ModulePair where
      mp1 == mp2 = canon mp1 == canon mp2

    instance Ord ModulePair where
      mp1 `compare` mp2 = canon mp1 `compare` canon mp2

    -- Sets of module pairs
    --
    type ModulePairSet = Map ModulePair ()

    listToSet :: [ModulePair] -> ModulePairSet
    listToSet l = Map.fromList (zip l (repeat ()))

    checkFamInstConsistency :: [Module] -> [Module] -> TcM ()
    checkFamInstConsistency famInstMods directlyImpMods
      = do { dflags     <- getDOpts
           ; (eps, hpt) <- getEpsAndHpt

           ; let { -- Fetch the iface of a given module.  Must succeed as
     	       -- all imported modules must already have been loaded.
    	       modIface mod =
    	         case lookupIfaceByModule dflags hpt (eps_PIT eps) mod of
                       Nothing    -> panic "FamInst.checkFamInstConsistency"
                       Just iface -> iface

                 ; hmiModule     = mi_module . hm_iface
    	     ; hmiFamInstEnv = mkFamInstEnv . md_fam_insts . hm_details
    	     ; mkFamInstEnv  = extendFamInstEnvList emptyFamInstEnv
                 ; hptModInsts   = [ (hmiModule hmi, hmiFamInstEnv hmi)
    			       | hmi <- eltsUFM hpt]
                 ; modInstsEnv   = eps_mod_fam_inst_env eps	-- external modules
    			       `extendModuleEnvList`	-- plus
    			       hptModInsts		-- home package modules
    	     ; groups        = map (dep_finsts . mi_deps . modIface)
    				   directlyImpMods
    	     ; okPairs       = listToSet $ concatMap allPairs groups
    	         -- instances of okPairs are consistent
    	     ; criticalPairs = listToSet $ allPairs famInstMods
    	         -- all pairs that we need to consider
                 ; toCheckPairs  = Map.keys $ criticalPairs `Map.difference` okPairs
    	         -- the difference gives us the pairs we need to check now
    	     }

           ; mapM_ (check modInstsEnv) toCheckPairs
           }
      where
        allPairs []     = []
        allPairs (m:ms) = map (ModulePair m) ms ++ allPairs ms

        -- The modules are guaranteed to be in the environment, as they are either
        -- already loaded in the EPS or they are in the HPT.
        --
        check modInstsEnv (ModulePair m1 m2)
          = let { instEnv1 = (expectJust "checkFamInstConsistency") . lookupModuleEnv modInstsEnv $ m1
    	    ; instEnv2 = (expectJust "checkFamInstConsistency") . lookupModuleEnv modInstsEnv $ m2
    	    ; insts1   = famInstEnvElts instEnv1
    	    }
            in
    	mapM_ (checkForConflicts (emptyFamInstEnv, instEnv2)) insts1

\end{code} %************************************************************************ %* * Extending the family instance environment %* * %************************************************************************ \begin{code}

    -- Add new locally-defined family instances
    tcExtendLocalFamInstEnv :: [FamInst] -> TcM a -> TcM a
    tcExtendLocalFamInstEnv fam_insts thing_inside
     = do { env <- getGblEnv
          ; inst_env' <- foldlM addLocalFamInst (tcg_fam_inst_env env) fam_insts
          ; let env' = env { tcg_fam_insts    = fam_insts ++ tcg_fam_insts env,
    			 tcg_fam_inst_env = inst_env' }
          ; setGblEnv env' thing_inside
          }

    -- Check that the proposed new instance is OK,
    -- and then add it to the home inst env
    addLocalFamInst :: FamInstEnv -> FamInst -> TcM FamInstEnv
    addLocalFamInst home_fie famInst
      = do {       -- Load imported instances, so that we report
    	       -- overlaps correctly
           ; eps <- getEps
           ; let inst_envs = (eps_fam_inst_env eps, home_fie)

    	       -- Check for conflicting instance decls
           ; checkForConflicts inst_envs famInst

    	       -- OK, now extend the envt
           ; return (extendFamInstEnv home_fie famInst)
           }

\end{code} %************************************************************************ %* * Checking an instance against conflicts with an instance env %* * %************************************************************************ Check whether a single family instance conflicts with those in two instance environments (one for the EPS and one for the HPT). \begin{code}

    checkForConflicts :: (FamInstEnv, FamInstEnv) -> FamInst -> TcM ()
    checkForConflicts inst_envs famInst
      = do { 	-- To instantiate the family instance type, extend the instance
    		-- envt with completely fresh template variables
    		-- This is important because the template variables must
    		-- not overlap with anything in the things being looked up
    		-- (since we do unification).
    		-- We use tcInstSkolType because we don't want to allocate
    		-- fresh *meta* type variables.

           ; skol_tvs <- tcInstSkolTyVars (tyConTyVars (famInstTyCon famInst))
           ; let conflicts = lookupFamInstEnvConflicts inst_envs famInst skol_tvs
           ; unless (null conflicts) $
    	   conflictInstErr famInst (fst (head conflicts))
           }
      where

    conflictInstErr :: FamInst -> FamInst -> TcRn ()
    conflictInstErr famInst conflictingFamInst
      = addFamInstLoc famInst $
        addErr (hang (ptext (sLit "Conflicting family instance declarations:"))
    	       2 (pprFamInsts [famInst, conflictingFamInst]))

    addFamInstLoc :: FamInst -> TcRn a -> TcRn a
    addFamInstLoc famInst thing_inside
      = setSrcSpan (mkSrcSpan loc loc) thing_inside
      where
        loc = getSrcLoc famInst

\end{code} \begin{code}

    tcGetFamInstEnvs :: TcM (FamInstEnv, FamInstEnv)
    -- Gets both the external-package inst-env
    -- and the home-pkg inst env (includes module being compiled)
    tcGetFamInstEnvs
      = do { eps <- getEps; env <- getGblEnv
           ; return (eps_fam_inst_env eps, tcg_fam_inst_env env)
           }

\end{code}