{-# LANGUAGE CPP #-} {-# LANGUAGE PatternGuards #-} {-# LANGUAGE BangPatterns #-} #if __GLASGOW_HASKELL__ >= 704 {-# LANGUAGE Unsafe #-} #endif #ifndef MIN_VERSION_template_haskell #define MIN_VERSION_template_haskell(x,y,z) 1 #endif ----------------------------------------------------------------------------- -- | -- Copyright : (C) 2008-2016 Edward Kmett, (C) 2015-2016 Ryan Scott -- License : BSD-style (see the file LICENSE) -- -- Maintainer : Edward Kmett -- Stability : provisional -- Portability : portable -- -- Functions to mechanically derive 'Bifunctor', 'Bifoldable', -- or 'Bitraversable' instances, or to splice their functions directly into -- source code. You need to enable the @TemplateHaskell@ language extension -- in order to use this module. ---------------------------------------------------------------------------- module Data.Bifunctor.TH ( -- * @derive@- functions -- $derive -- * @make@- functions -- $make -- * 'Bifunctor' deriveBifunctor , deriveBifunctorOptions , makeBimap , makeBimapOptions -- * 'Bifoldable' , deriveBifoldable , deriveBifoldableOptions , makeBifold , makeBifoldOptions , makeBifoldMap , makeBifoldMapOptions , makeBifoldr , makeBifoldrOptions , makeBifoldl , makeBifoldlOptions -- * 'Bitraversable' , deriveBitraversable , deriveBitraversableOptions , makeBitraverse , makeBitraverseOptions , makeBisequenceA , makeBisequenceAOptions , makeBimapM , makeBimapMOptions , makeBisequence , makeBisequenceOptions -- * 'Options' , Options(..) , defaultOptions ) where import Control.Monad (guard, unless, when, zipWithM) import Data.Bifunctor.TH.Internal import Data.Either (rights) import Data.List import qualified Data.Map as Map (fromList, keys, lookup, size) import Data.Maybe import Language.Haskell.TH.Datatype import Language.Haskell.TH.Lib import Language.Haskell.TH.Ppr import Language.Haskell.TH.Syntax ------------------------------------------------------------------------------- -- User-facing API ------------------------------------------------------------------------------- -- | Options that further configure how the functions in "Data.Bifunctor.TH" -- should behave. newtype Options = Options { emptyCaseBehavior :: Bool -- ^ If 'True', derived instances for empty data types (i.e., ones with -- no data constructors) will use the @EmptyCase@ language extension. -- If 'False', derived instances will simply use 'seq' instead. -- (This has no effect on GHCs before 7.8, since @EmptyCase@ is only -- available in 7.8 or later.) } deriving (Eq, Ord, Read, Show) -- | Conservative 'Options' that doesn't attempt to use @EmptyCase@ (to -- prevent users from having to enable that extension at use sites.) defaultOptions :: Options defaultOptions = Options { emptyCaseBehavior = False } {- $derive 'deriveBifunctor', 'deriveBifoldable', and 'deriveBitraversable' automatically generate their respective class instances for a given data type, newtype, or data family instance that has at least two type variable. Examples: @ {-# LANGUAGE TemplateHaskell #-} import Data.Bifunctor.TH data Pair a b = Pair a b $('deriveBifunctor' ''Pair) -- instance Bifunctor Pair where ... data WrapLeftPair f g a b = WrapLeftPair (f a) (g a b) $('deriveBifoldable' ''WrapLeftPair) -- instance (Foldable f, Bifoldable g) => Bifoldable (WrapLeftPair f g) where ... @ If you are using @template-haskell-2.7.0.0@ or later (i.e., GHC 7.4 or later), the @derive@ functions can be used data family instances (which requires the @-XTypeFamilies@ extension). To do so, pass the name of a data or newtype instance constructor (NOT a data family name!) to a @derive@ function. Note that the generated code may require the @-XFlexibleInstances@ extension. Example: @ {-# LANGUAGE FlexibleInstances, TemplateHaskell, TypeFamilies #-} import Data.Bifunctor.TH class AssocClass a b c where data AssocData a b c instance AssocClass Int b c where data AssocData Int b c = AssocDataInt1 Int | AssocDataInt2 b c $('deriveBitraversable' 'AssocDataInt1) -- instance Bitraversable (AssocData Int) where ... -- Alternatively, one could use $(deriveBitraversable 'AssocDataInt2) @ Note that there are some limitations: * The 'Name' argument to a @derive@ function must not be a type synonym. * With a @derive@ function, the last two type variables must both be of kind @*@. Other type variables of kind @* -> *@ are assumed to require a 'Functor', 'Foldable', or 'Traversable' constraint (depending on which @derive@ function is used), and other type variables of kind @* -> * -> *@ are assumed to require an 'Bifunctor', 'Bifoldable', or 'Bitraversable' constraint. If your data type doesn't meet these assumptions, use a @make@ function. * If using the @-XDatatypeContexts@, @-XExistentialQuantification@, or @-XGADTs@ extensions, a constraint cannot mention either of the last two type variables. For example, @data Illegal2 a b where I2 :: Ord a => a -> b -> Illegal2 a b@ cannot have a derived 'Bifunctor' instance. * If either of the last two type variables is used within a constructor argument's type, it must only be used in the last two type arguments. For example, @data Legal a b = Legal (Int, Int, a, b)@ can have a derived 'Bifunctor' instance, but @data Illegal a b = Illegal (a, b, a, b)@ cannot. * Data family instances must be able to eta-reduce the last two type variables. In other words, if you have a instance of the form: @ data family Family a1 ... an t1 t2 data instance Family e1 ... e2 v1 v2 = ... @ Then the following conditions must hold: 1. @v1@ and @v2@ must be distinct type variables. 2. Neither @v1@ not @v2@ must be mentioned in any of @e1@, ..., @e2@. -} {- $make There may be scenarios in which you want to, say, 'bimap' over an arbitrary data type or data family instance without having to make the type an instance of 'Bifunctor'. For these cases, this module provides several functions (all prefixed with @make@-) that splice the appropriate lambda expression into your source code. This is particularly useful for creating instances for sophisticated data types. For example, 'deriveBifunctor' cannot infer the correct type context for @newtype HigherKinded f a b c = HigherKinded (f a b c)@, since @f@ is of kind @* -> * -> * -> *@. However, it is still possible to create a 'Bifunctor' instance for @HigherKinded@ without too much trouble using 'makeBimap': @ {-# LANGUAGE FlexibleContexts, TemplateHaskell #-} import Data.Bifunctor import Data.Bifunctor.TH newtype HigherKinded f a b c = HigherKinded (f a b c) instance Bifunctor (f a) => Bifunctor (HigherKinded f a) where bimap = $(makeBimap ''HigherKinded) @ -} -- | Generates a 'Bifunctor' instance declaration for the given data type or data -- family instance. deriveBifunctor :: Name -> Q [Dec] deriveBifunctor = deriveBifunctorOptions defaultOptions -- | Like 'deriveBifunctor', but takes an 'Options' argument. deriveBifunctorOptions :: Options -> Name -> Q [Dec] deriveBifunctorOptions = deriveBiClass Bifunctor -- | Generates a lambda expression which behaves like 'bimap' (without requiring a -- 'Bifunctor' instance). makeBimap :: Name -> Q Exp makeBimap = makeBimapOptions defaultOptions -- | Like 'makeBimap', but takes an 'Options' argument. makeBimapOptions :: Options -> Name -> Q Exp makeBimapOptions = makeBiFun Bimap -- | Generates a 'Bifoldable' instance declaration for the given data type or data -- family instance. deriveBifoldable :: Name -> Q [Dec] deriveBifoldable = deriveBifoldableOptions defaultOptions -- | Like 'deriveBifoldable', but takes an 'Options' argument. deriveBifoldableOptions :: Options -> Name -> Q [Dec] deriveBifoldableOptions = deriveBiClass Bifoldable --- | Generates a lambda expression which behaves like 'bifold' (without requiring a -- 'Bifoldable' instance). makeBifold :: Name -> Q Exp makeBifold = makeBifoldOptions defaultOptions -- | Like 'makeBifold', but takes an 'Options' argument. makeBifoldOptions :: Options -> Name -> Q Exp makeBifoldOptions opts name = appsE [ makeBifoldMapOptions opts name , varE idValName , varE idValName ] -- | Generates a lambda expression which behaves like 'bifoldMap' (without requiring -- a 'Bifoldable' instance). makeBifoldMap :: Name -> Q Exp makeBifoldMap = makeBifoldMapOptions defaultOptions -- | Like 'makeBifoldMap', but takes an 'Options' argument. makeBifoldMapOptions :: Options -> Name -> Q Exp makeBifoldMapOptions = makeBiFun BifoldMap -- | Generates a lambda expression which behaves like 'bifoldr' (without requiring a -- 'Bifoldable' instance). makeBifoldr :: Name -> Q Exp makeBifoldr = makeBifoldrOptions defaultOptions -- | Like 'makeBifoldr', but takes an 'Options' argument. makeBifoldrOptions :: Options -> Name -> Q Exp makeBifoldrOptions = makeBiFun Bifoldr -- | Generates a lambda expression which behaves like 'bifoldl' (without requiring a -- 'Bifoldable' instance). makeBifoldl :: Name -> Q Exp makeBifoldl = makeBifoldlOptions defaultOptions -- | Like 'makeBifoldl', but takes an 'Options' argument. makeBifoldlOptions :: Options -> Name -> Q Exp makeBifoldlOptions opts name = do f <- newName "f" g <- newName "g" z <- newName "z" t <- newName "t" lamE [varP f, varP g, varP z, varP t] $ appsE [ varE appEndoValName , appsE [ varE getDualValName , appsE [ makeBifoldMapOptions opts name , foldFun f , foldFun g , varE t] ] , varE z ] where foldFun :: Name -> Q Exp foldFun n = infixApp (conE dualDataName) (varE composeValName) (infixApp (conE endoDataName) (varE composeValName) (varE flipValName `appE` varE n) ) -- | Generates a 'Bitraversable' instance declaration for the given data type or data -- family instance. deriveBitraversable :: Name -> Q [Dec] deriveBitraversable = deriveBitraversableOptions defaultOptions -- | Like 'deriveBitraversable', but takes an 'Options' argument. deriveBitraversableOptions :: Options -> Name -> Q [Dec] deriveBitraversableOptions = deriveBiClass Bitraversable -- | Generates a lambda expression which behaves like 'bitraverse' (without -- requiring a 'Bitraversable' instance). makeBitraverse :: Name -> Q Exp makeBitraverse = makeBitraverseOptions defaultOptions -- | Like 'makeBitraverse', but takes an 'Options' argument. makeBitraverseOptions :: Options -> Name -> Q Exp makeBitraverseOptions = makeBiFun Bitraverse -- | Generates a lambda expression which behaves like 'bisequenceA' (without -- requiring a 'Bitraversable' instance). makeBisequenceA :: Name -> Q Exp makeBisequenceA = makeBisequenceAOptions defaultOptions -- | Like 'makeBitraverseA', but takes an 'Options' argument. makeBisequenceAOptions :: Options -> Name -> Q Exp makeBisequenceAOptions opts name = appsE [ makeBitraverseOptions opts name , varE idValName , varE idValName ] -- | Generates a lambda expression which behaves like 'bimapM' (without -- requiring a 'Bitraversable' instance). makeBimapM :: Name -> Q Exp makeBimapM = makeBimapMOptions defaultOptions -- | Like 'makeBimapM', but takes an 'Options' argument. makeBimapMOptions :: Options -> Name -> Q Exp makeBimapMOptions opts name = do f <- newName "f" g <- newName "g" lamE [varP f, varP g] . infixApp (varE unwrapMonadValName) (varE composeValName) $ appsE [ makeBitraverseOptions opts name , wrapMonadExp f , wrapMonadExp g ] where wrapMonadExp :: Name -> Q Exp wrapMonadExp n = infixApp (conE wrapMonadDataName) (varE composeValName) (varE n) -- | Generates a lambda expression which behaves like 'bisequence' (without -- requiring a 'Bitraversable' instance). makeBisequence :: Name -> Q Exp makeBisequence = makeBisequenceOptions defaultOptions -- | Like 'makeBisequence', but takes an 'Options' argument. makeBisequenceOptions :: Options -> Name -> Q Exp makeBisequenceOptions opts name = appsE [ makeBimapMOptions opts name , varE idValName , varE idValName ] ------------------------------------------------------------------------------- -- Code generation ------------------------------------------------------------------------------- -- | Derive a class instance declaration (depending on the BiClass argument's value). deriveBiClass :: BiClass -> Options -> Name -> Q [Dec] deriveBiClass biClass opts name = do info <- reifyDatatype name case info of DatatypeInfo { datatypeContext = ctxt , datatypeName = parentName , datatypeVars = vars , datatypeVariant = variant , datatypeCons = cons } -> do (instanceCxt, instanceType) <- buildTypeInstance biClass parentName ctxt vars variant (:[]) `fmap` instanceD (return instanceCxt) (return instanceType) (biFunDecs biClass opts parentName vars cons) -- | Generates a declaration defining the primary function(s) corresponding to a -- particular class (bimap for Bifunctor, bifoldr and bifoldMap for Bifoldable, and -- bitraverse for Bitraversable). -- -- For why both bifoldr and bifoldMap are derived for Bifoldable, see Trac #7436. biFunDecs :: BiClass -> Options -> Name -> [Type] -> [ConstructorInfo] -> [Q Dec] biFunDecs biClass opts parentName vars cons = map makeFunD $ biClassToFuns biClass where makeFunD :: BiFun -> Q Dec makeFunD biFun = funD (biFunName biFun) [ clause [] (normalB $ makeBiFunForCons biFun opts parentName vars cons) [] ] -- | Generates a lambda expression which behaves like the BiFun argument. makeBiFun :: BiFun -> Options -> Name -> Q Exp makeBiFun biFun opts name = do info <- reifyDatatype name case info of DatatypeInfo { datatypeContext = ctxt , datatypeName = parentName , datatypeVars = vars , datatypeVariant = variant , datatypeCons = cons } -> -- We force buildTypeInstance here since it performs some checks for whether -- or not the provided datatype can actually have bimap/bifoldr/bitraverse/etc. -- implemented for it, and produces errors if it can't. buildTypeInstance (biFunToClass biFun) parentName ctxt vars variant >> makeBiFunForCons biFun opts parentName vars cons -- | Generates a lambda expression for the given constructors. -- All constructors must be from the same type. makeBiFunForCons :: BiFun -> Options -> Name -> [Type] -> [ConstructorInfo] -> Q Exp makeBiFunForCons biFun opts _parentName vars cons = do argNames <- mapM newName $ catMaybes [ Just "f" , Just "g" , guard (biFun == Bifoldr) >> Just "z" , Just "value" ] let ([map1, map2], others) = splitAt 2 argNames z = head others -- If we're deriving bifoldr, this will be well defined -- and useful. Otherwise, it'll be ignored. value = last others lastTyVars = map varTToName $ drop (length vars - 2) vars tvMap = Map.fromList $ zip lastTyVars [map1, map2] lamE (map varP argNames) . appsE $ [ varE $ biFunConstName biFun , makeFun z value tvMap ] ++ map varE argNames where makeFun :: Name -> Name -> TyVarMap -> Q Exp makeFun z value tvMap = do #if MIN_VERSION_template_haskell(2,9,0) roles <- reifyRoles _parentName #endif case () of _ #if MIN_VERSION_template_haskell(2,9,0) | Just (rs, PhantomR) <- unsnoc roles , Just (_, PhantomR) <- unsnoc rs -> biFunPhantom z value #endif | null cons && emptyCaseBehavior opts && ghc7'8OrLater -> biFunEmptyCase biFun z value | null cons -> biFunNoCons biFun z value | otherwise -> caseE (varE value) (map (makeBiFunForCon biFun z tvMap) cons) ghc7'8OrLater :: Bool #if __GLASGOW_HASKELL__ >= 708 ghc7'8OrLater = True #else ghc7'8OrLater = False #endif #if MIN_VERSION_template_haskell(2,9,0) biFunPhantom :: Name -> Name -> Q Exp biFunPhantom z value = biFunTrivial coerce (varE pureValName `appE` coerce) biFun z where coerce :: Q Exp coerce = varE coerceValName `appE` varE value #endif -- | Generates a lambda expression for a single constructor. makeBiFunForCon :: BiFun -> Name -> TyVarMap -> ConstructorInfo -> Q Match makeBiFunForCon biFun z tvMap (ConstructorInfo { constructorName = conName , constructorContext = ctxt , constructorFields = ts }) = do ts' <- mapM resolveTypeSynonyms ts argNames <- newNameList "_arg" $ length ts' if (any (`predMentionsName` Map.keys tvMap) ctxt || Map.size tvMap < 2) && not (allowExQuant (biFunToClass biFun)) then existentialContextError conName else makeBiFunForArgs biFun z tvMap conName ts' argNames -- | Generates a lambda expression for a single constructor's arguments. makeBiFunForArgs :: BiFun -> Name -> TyVarMap -> Name -> [Type] -> [Name] -> Q Match makeBiFunForArgs biFun z tvMap conName tys args = match (conP conName $ map varP args) (normalB $ biFunCombine biFun conName z args mappedArgs) [] where mappedArgs :: Q [Either Exp Exp] mappedArgs = zipWithM (makeBiFunForArg biFun tvMap conName) tys args -- | Generates a lambda expression for a single argument of a constructor. -- The returned value is 'Right' if its type mentions one of the last two type -- parameters. Otherwise, it is 'Left'. makeBiFunForArg :: BiFun -> TyVarMap -> Name -> Type -> Name -> Q (Either Exp Exp) makeBiFunForArg biFun tvMap conName ty tyExpName = makeBiFunForType biFun tvMap conName True ty `appEitherE` varE tyExpName -- | Generates a lambda expression for a specific type. The returned value is -- 'Right' if its type mentions one of the last two type parameters. Otherwise, -- it is 'Left'. makeBiFunForType :: BiFun -> TyVarMap -> Name -> Bool -> Type -> Q (Either Exp Exp) makeBiFunForType biFun tvMap conName covariant (VarT tyName) = case Map.lookup tyName tvMap of Just mapName -> fmap Right . varE $ if covariant then mapName else contravarianceError conName Nothing -> fmap Left $ biFunTriv biFun makeBiFunForType biFun tvMap conName covariant (SigT ty _) = makeBiFunForType biFun tvMap conName covariant ty makeBiFunForType biFun tvMap conName covariant (ForallT _ _ ty) = makeBiFunForType biFun tvMap conName covariant ty makeBiFunForType biFun tvMap conName covariant ty = let tyCon :: Type tyArgs :: [Type] tyCon:tyArgs = unapplyTy ty numLastArgs :: Int numLastArgs = min 2 $ length tyArgs lhsArgs, rhsArgs :: [Type] (lhsArgs, rhsArgs) = splitAt (length tyArgs - numLastArgs) tyArgs tyVarNames :: [Name] tyVarNames = Map.keys tvMap mentionsTyArgs :: Bool mentionsTyArgs = any (`mentionsName` tyVarNames) tyArgs makeBiFunTuple :: ([Q Pat] -> Q Pat) -> (Int -> Name) -> Int -> Q (Either Exp Exp) makeBiFunTuple mkTupP mkTupleDataName n = do args <- mapM newName $ catMaybes [ Just "x" , guard (biFun == Bifoldr) >> Just "z" ] xs <- newNameList "_tup" n let x = head args z = last args fmap Right $ lamE (map varP args) $ caseE (varE x) [ match (mkTupP $ map varP xs) (normalB $ biFunCombine biFun (mkTupleDataName n) z xs (zipWithM makeBiFunTupleField tyArgs xs) ) [] ] makeBiFunTupleField :: Type -> Name -> Q (Either Exp Exp) makeBiFunTupleField fieldTy fieldName = makeBiFunForType biFun tvMap conName covariant fieldTy `appEitherE` varE fieldName in case tyCon of ArrowT | not (allowFunTys (biFunToClass biFun)) -> noFunctionsError conName | mentionsTyArgs, [argTy, resTy] <- tyArgs -> do x <- newName "x" b <- newName "b" fmap Right . lamE [varP x, varP b] $ covBiFun covariant resTy `appE` (varE x `appE` (covBiFun (not covariant) argTy `appE` varE b)) where covBiFun :: Bool -> Type -> Q Exp covBiFun cov = fmap fromEither . makeBiFunForType biFun tvMap conName cov #if MIN_VERSION_template_haskell(2,6,0) UnboxedTupleT n | n > 0 && mentionsTyArgs -> makeBiFunTuple unboxedTupP unboxedTupleDataName n #endif TupleT n | n > 0 && mentionsTyArgs -> makeBiFunTuple tupP tupleDataName n _ -> do itf <- isTyFamily tyCon if any (`mentionsName` tyVarNames) lhsArgs || (itf && mentionsTyArgs) then outOfPlaceTyVarError conName else if any (`mentionsName` tyVarNames) rhsArgs then fmap Right . biFunApp biFun . appsE $ ( varE (fromJust $ biFunArity biFun numLastArgs) : map (fmap fromEither . makeBiFunForType biFun tvMap conName covariant) rhsArgs ) else fmap Left $ biFunTriv biFun ------------------------------------------------------------------------------- -- Template Haskell reifying and AST manipulation ------------------------------------------------------------------------------- -- For the given Types, generate an instance context and head. Coming up with -- the instance type isn't as simple as dropping the last types, as you need to -- be wary of kinds being instantiated with *. -- See Note [Type inference in derived instances] buildTypeInstance :: BiClass -- ^ Bifunctor, Bifoldable, or Bitraversable -> Name -- ^ The type constructor or data family name -> Cxt -- ^ The datatype context -> [Type] -- ^ The types to instantiate the instance with -> DatatypeVariant -- ^ Are we dealing with a data family instance or not -> Q (Cxt, Type) buildTypeInstance biClass tyConName dataCxt varTysOrig variant = do -- Make sure to expand through type/kind synonyms! Otherwise, the -- eta-reduction check might get tripped up over type variables in a -- synonym that are actually dropped. -- (See GHC Trac #11416 for a scenario where this actually happened.) varTysExp <- mapM resolveTypeSynonyms varTysOrig let remainingLength :: Int remainingLength = length varTysOrig - 2 droppedTysExp :: [Type] droppedTysExp = drop remainingLength varTysExp droppedStarKindStati :: [StarKindStatus] droppedStarKindStati = map canRealizeKindStar droppedTysExp -- Check there are enough types to drop and that all of them are either of -- kind * or kind k (for some kind variable k). If not, throw an error. when (remainingLength < 0 || any (== NotKindStar) droppedStarKindStati) $ derivingKindError biClass tyConName let droppedKindVarNames :: [Name] droppedKindVarNames = catKindVarNames droppedStarKindStati -- Substitute kind * for any dropped kind variables varTysExpSubst :: [Type] varTysExpSubst = map (substNamesWithKindStar droppedKindVarNames) varTysExp remainingTysExpSubst, droppedTysExpSubst :: [Type] (remainingTysExpSubst, droppedTysExpSubst) = splitAt remainingLength varTysExpSubst -- All of the type variables mentioned in the dropped types -- (post-synonym expansion) droppedTyVarNames :: [Name] droppedTyVarNames = freeVariables droppedTysExpSubst -- If any of the dropped types were polykinded, ensure that they are of kind * -- after substituting * for the dropped kind variables. If not, throw an error. unless (all hasKindStar droppedTysExpSubst) $ derivingKindError biClass tyConName let preds :: [Maybe Pred] kvNames :: [[Name]] kvNames' :: [Name] -- Derive instance constraints (and any kind variables which are specialized -- to * in those constraints) (preds, kvNames) = unzip $ map (deriveConstraint biClass) remainingTysExpSubst kvNames' = concat kvNames -- Substitute the kind variables specialized in the constraints with * remainingTysExpSubst' :: [Type] remainingTysExpSubst' = map (substNamesWithKindStar kvNames') remainingTysExpSubst -- We now substitute all of the specialized-to-* kind variable names with -- *, but in the original types, not the synonym-expanded types. The reason -- we do this is a superficial one: we want the derived instance to resemble -- the datatype written in source code as closely as possible. For example, -- for the following data family instance: -- -- data family Fam a -- newtype instance Fam String = Fam String -- -- We'd want to generate the instance: -- -- instance C (Fam String) -- -- Not: -- -- instance C (Fam [Char]) remainingTysOrigSubst :: [Type] remainingTysOrigSubst = map (substNamesWithKindStar (union droppedKindVarNames kvNames')) $ take remainingLength varTysOrig isDataFamily :: Bool isDataFamily = case variant of Datatype -> False Newtype -> False DataInstance -> True NewtypeInstance -> True remainingTysOrigSubst' :: [Type] -- See Note [Kind signatures in derived instances] for an explanation -- of the isDataFamily check. remainingTysOrigSubst' = if isDataFamily then remainingTysOrigSubst else map unSigT remainingTysOrigSubst instanceCxt :: Cxt instanceCxt = catMaybes preds instanceType :: Type instanceType = AppT (ConT $ biClassName biClass) $ applyTyCon tyConName remainingTysOrigSubst' -- If the datatype context mentions any of the dropped type variables, -- we can't derive an instance, so throw an error. when (any (`predMentionsName` droppedTyVarNames) dataCxt) $ datatypeContextError tyConName instanceType -- Also ensure the dropped types can be safely eta-reduced. Otherwise, -- throw an error. unless (canEtaReduce remainingTysExpSubst' droppedTysExpSubst) $ etaReductionError instanceType return (instanceCxt, instanceType) -- | Attempt to derive a constraint on a Type. If successful, return -- Just the constraint and any kind variable names constrained to *. -- Otherwise, return Nothing and the empty list. -- -- See Note [Type inference in derived instances] for the heuristics used to -- come up with constraints. deriveConstraint :: BiClass -> Type -> (Maybe Pred, [Name]) deriveConstraint biClass t | not (isTyVar t) = (Nothing, []) | otherwise = case hasKindVarChain 1 t of Just ns -> ((`applyClass` tName) `fmap` biClassConstraint biClass 1, ns) _ -> case hasKindVarChain 2 t of Just ns -> ((`applyClass` tName) `fmap` biClassConstraint biClass 2, ns) _ -> (Nothing, []) where tName :: Name tName = varTToName t {- Note [Kind signatures in derived instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It is possible to put explicit kind signatures into the derived instances, e.g., instance C a => C (Data (f :: * -> *)) where ... But it is preferable to avoid this if possible. If we come up with an incorrect kind signature (which is entirely possible, since our type inferencer is pretty unsophisticated - see Note [Type inference in derived instances]), then GHC will flat-out reject the instance, which is quite unfortunate. Plain old datatypes have the advantage that you can avoid using any kind signatures at all in their instances. This is because a datatype declaration uses all type variables, so the types that we use in a derived instance uniquely determine their kinds. As long as we plug in the right types, the kind inferencer can do the rest of the work. For this reason, we use unSigT to remove all kind signatures before splicing in the instance context and head. Data family instances are trickier, since a data family can have two instances that are distinguished by kind alone, e.g., data family Fam (a :: k) data instance Fam (a :: * -> *) data instance Fam (a :: *) If we dropped the kind signatures for C (Fam a), then GHC will have no way of knowing which instance we are talking about. To avoid this scenario, we always include explicit kind signatures in data family instances. There is a chance that the inferred kind signatures will be incorrect, but if so, we can always fall back on the make- functions. Note [Type inference in derived instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Type inference is can be tricky to get right, and we want to avoid recreating the entirety of GHC's type inferencer in Template Haskell. For this reason, we will probably never come up with derived instance contexts that are as accurate as GHC's. But that doesn't mean we can't do anything! There are a couple of simple things we can do to make instance contexts that work for 80% of use cases: 1. If one of the last type parameters is polykinded, then its kind will be specialized to * in the derived instance. We note what kind variable the type parameter had and substitute it with * in the other types as well. For example, imagine you had data Data (a :: k) (b :: k) (c :: k) Then you'd want to derived instance to be: instance C (Data (a :: *)) Not: instance C (Data (a :: k)) 2. We naïvely come up with instance constraints using the following criteria: (i) If there's a type parameter n of kind k1 -> k2 (where k1/k2 are * or kind variables), then generate a Functor n constraint, and if k1/k2 are kind variables, then substitute k1/k2 with * elsewhere in the types. We must consider the case where they are kind variables because you might have a scenario like this: newtype Compose (f :: k3 -> *) (g :: k1 -> k2 -> k3) (a :: k1) (b :: k2) = Compose (f (g a b)) Which would have a derived Bifunctor instance of: instance (Functor f, Bifunctor g) => Bifunctor (Compose f g) where ... (ii) If there's a type parameter n of kind k1 -> k2 -> k3 (where k1/k2/k3 are * or kind variables), then generate a Bifunctor n constraint and perform kind substitution as in the other case. -} {- Note [Matching functions with GADT type variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When deriving Bifoldable, there is a tricky corner case to consider: data Both a b where BothCon :: x -> x -> Both x x Which fold functions should be applied to which arguments of BothCon? We have a choice, since both the function of type (a -> m) and of type (b -> m) can be applied to either argument. In such a scenario, the second fold function takes precedence over the first fold function, so the derived Bifoldable instance would be: instance Bifoldable Both where bifoldMap _ g (BothCon x1 x2) = g x1 <> g x2 This is not an arbitrary choice, as this definition ensures that bifoldMap id = Foldable.foldMap for a derived Bifoldable instance for Both. -} ------------------------------------------------------------------------------- -- Error messages ------------------------------------------------------------------------------- -- | Either the given data type doesn't have enough type variables, or one of -- the type variables to be eta-reduced cannot realize kind *. derivingKindError :: BiClass -> Name -> a derivingKindError biClass tyConName = error . showString "Cannot derive well-kinded instance of form ‘" . showString className . showChar ' ' . showParen True ( showString (nameBase tyConName) . showString " ..." ) . showString "‘\n\tClass " . showString className . showString " expects an argument of kind * -> * -> *" $ "" where className :: String className = nameBase $ biClassName biClass -- | One of the last two type variables appeard in a contravariant position -- when deriving Bifoldable or Bitraversable. contravarianceError :: Name -> a contravarianceError conName = error . showString "Constructor ‘" . showString (nameBase conName) . showString "‘ must not use the last type variable(s) in a function argument" $ "" -- | A constructor has a function argument in a derived Bifoldable or Bitraversable -- instance. noFunctionsError :: Name -> a noFunctionsError conName = error . showString "Constructor ‘" . showString (nameBase conName) . showString "‘ must not contain function types" $ "" -- | The data type has a DatatypeContext which mentions one of the eta-reduced -- type variables. datatypeContextError :: Name -> Type -> a datatypeContextError dataName instanceType = error . showString "Can't make a derived instance of ‘" . showString (pprint instanceType) . showString "‘:\n\tData type ‘" . showString (nameBase dataName) . showString "‘ must not have a class context involving the last type argument(s)" $ "" -- | The data type has an existential constraint which mentions one of the -- eta-reduced type variables. existentialContextError :: Name -> a existentialContextError conName = error . showString "Constructor ‘" . showString (nameBase conName) . showString "‘ must be truly polymorphic in the last argument(s) of the data type" $ "" -- | The data type mentions one of the n eta-reduced type variables in a place other -- than the last nth positions of a data type in a constructor's field. outOfPlaceTyVarError :: Name -> a outOfPlaceTyVarError conName = error . showString "Constructor ‘" . showString (nameBase conName) . showString "‘ must only use its last two type variable(s) within" . showString " the last two argument(s) of a data type" $ "" -- | One of the last type variables cannot be eta-reduced (see the canEtaReduce -- function for the criteria it would have to meet). etaReductionError :: Type -> a etaReductionError instanceType = error $ "Cannot eta-reduce to an instance of form \n\tinstance (...) => " ++ pprint instanceType ------------------------------------------------------------------------------- -- Class-specific constants ------------------------------------------------------------------------------- -- | A representation of which class is being derived. data BiClass = Bifunctor | Bifoldable | Bitraversable -- | A representation of which function is being generated. data BiFun = Bimap | Bifoldr | BifoldMap | Bitraverse deriving Eq biFunConstName :: BiFun -> Name biFunConstName Bimap = bimapConstValName biFunConstName Bifoldr = bifoldrConstValName biFunConstName BifoldMap = bifoldMapConstValName biFunConstName Bitraverse = bitraverseConstValName biClassName :: BiClass -> Name biClassName Bifunctor = bifunctorTypeName biClassName Bifoldable = bifoldableTypeName biClassName Bitraversable = bitraversableTypeName biFunName :: BiFun -> Name biFunName Bimap = bimapValName biFunName Bifoldr = bifoldrValName biFunName BifoldMap = bifoldMapValName biFunName Bitraverse = bitraverseValName biClassToFuns :: BiClass -> [BiFun] biClassToFuns Bifunctor = [Bimap] biClassToFuns Bifoldable = [Bifoldr, BifoldMap] biClassToFuns Bitraversable = [Bitraverse] biFunToClass :: BiFun -> BiClass biFunToClass Bimap = Bifunctor biFunToClass Bifoldr = Bifoldable biFunToClass BifoldMap = Bifoldable biFunToClass Bitraverse = Bitraversable biClassConstraint :: BiClass -> Int -> Maybe Name biClassConstraint Bifunctor 1 = Just functorTypeName biClassConstraint Bifoldable 1 = Just foldableTypeName biClassConstraint Bitraversable 1 = Just traversableTypeName biClassConstraint biClass 2 = Just $ biClassName biClass biClassConstraint _ _ = Nothing biFunArity :: BiFun -> Int -> Maybe Name biFunArity Bimap 1 = Just fmapValName biFunArity Bifoldr 1 = Just foldrValName biFunArity BifoldMap 1 = Just foldMapValName biFunArity Bitraverse 1 = Just traverseValName biFunArity biFun 2 = Just $ biFunName biFun biFunArity _ _ = Nothing allowFunTys :: BiClass -> Bool allowFunTys Bifunctor = True allowFunTys _ = False allowExQuant :: BiClass -> Bool allowExQuant Bifoldable = True allowExQuant _ = False -- See Trac #7436 for why explicit lambdas are used biFunTriv :: BiFun -> Q Exp biFunTriv Bimap = do x <- newName "x" lamE [varP x] $ varE x -- The biFunTriv definitions for bifoldr, bifoldMap, and bitraverse might seem -- useless, but they do serve a purpose. -- See Note [biFunTriv for Bifoldable and Bitraversable] biFunTriv Bifoldr = do z <- newName "z" lamE [wildP, varP z] $ varE z biFunTriv BifoldMap = lamE [wildP] $ varE memptyValName biFunTriv Bitraverse = varE pureValName biFunApp :: BiFun -> Q Exp -> Q Exp biFunApp Bifoldr e = do x <- newName "x" z <- newName "z" lamE [varP x, varP z] $ appsE [e, varE z, varE x] biFunApp _ e = e biFunCombine :: BiFun -> Name -> Name -> [Name] -> Q [Either Exp Exp] -> Q Exp biFunCombine Bimap = bimapCombine biFunCombine Bifoldr = bifoldrCombine biFunCombine BifoldMap = bifoldMapCombine biFunCombine Bitraverse = bitraverseCombine bimapCombine :: Name -> Name -> [Name] -> Q [Either Exp Exp] -> Q Exp bimapCombine conName _ _ = fmap (foldl' AppE (ConE conName) . fmap fromEither) -- bifoldr, bifoldMap, and bitraverse are handled differently from bimap, since -- they filter out subexpressions whose types do not mention one of the last two -- type parameters. See -- https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/DeriveFunctor#AlternativestrategyforderivingFoldableandTraversable -- for further discussion. bifoldrCombine :: Name -> Name -> [Name] -> Q [Either Exp Exp] -> Q Exp bifoldrCombine _ zName _ = fmap (foldr AppE (VarE zName) . rights) bifoldMapCombine :: Name -> Name -> [Name] -> Q [Either Exp Exp] -> Q Exp bifoldMapCombine _ _ _ = fmap (go . rights) where go :: [Exp] -> Exp go [] = VarE memptyValName go es = foldr1 (AppE . AppE (VarE mappendValName)) es bitraverseCombine :: Name -> Name -> [Name] -> Q [Either Exp Exp] -> Q Exp bitraverseCombine conName _ args essQ = do ess <- essQ let argTysTyVarInfo :: [Bool] argTysTyVarInfo = map isRight ess argsWithTyVar, argsWithoutTyVar :: [Name] (argsWithTyVar, argsWithoutTyVar) = partitionByList argTysTyVarInfo args conExpQ :: Q Exp conExpQ | null argsWithTyVar = appsE (conE conName:map varE argsWithoutTyVar) | otherwise = do bs <- newNameList "b" $ length args let bs' = filterByList argTysTyVarInfo bs vars = filterByLists argTysTyVarInfo (map varE bs) (map varE args) lamE (map varP bs') (appsE (conE conName:vars)) conExp <- conExpQ let go :: [Exp] -> Exp go [] = VarE pureValName `AppE` conExp go [e] = VarE fmapValName `AppE` conExp `AppE` e go (e1:e2:es) = foldl' (\se1 se2 -> InfixE (Just se1) (VarE apValName) (Just se2)) (VarE liftA2ValName `AppE` conExp `AppE` e1 `AppE` e2) es return . go . rights $ ess biFunEmptyCase :: BiFun -> Name -> Name -> Q Exp biFunEmptyCase biFun z value = biFunTrivial emptyCase (varE pureValName `appE` emptyCase) biFun z where emptyCase :: Q Exp emptyCase = caseE (varE value) [] biFunNoCons :: BiFun -> Name -> Name -> Q Exp biFunNoCons biFun z value = biFunTrivial seqAndError (varE pureValName `appE` seqAndError) biFun z where seqAndError :: Q Exp seqAndError = appE (varE seqValName) (varE value) `appE` appE (varE errorValName) (stringE $ "Void " ++ nameBase (biFunName biFun)) biFunTrivial :: Q Exp -> Q Exp -> BiFun -> Name -> Q Exp biFunTrivial bimapE bitraverseE biFun z = go biFun where go :: BiFun -> Q Exp go Bimap = bimapE go Bifoldr = varE z go BifoldMap = varE memptyValName go Bitraverse = bitraverseE {- Note [biFunTriv for Bifoldable and Bitraversable] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When deriving Bifoldable and Bitraversable, we filter out any subexpressions whose type does not mention one of the last two type parameters. From this, you might think that we don't need to implement biFunTriv for bifoldr, bifoldMap, or bitraverse at all, but in fact we do need to. Imagine the following data type: data T a b = MkT a (T Int b) In a derived Bifoldable T instance, you would generate the following bifoldMap definition: bifoldMap f g (MkT a1 a2) = f a1 <> bifoldMap (\_ -> mempty) g arg2 You need to fill in biFunTriv (\_ -> mempty) as the first argument to the recursive call to bifoldMap, since that is how the algorithm handles polymorphic recursion. -}