deriving-compat-0.2: src/Data/Functor/Deriving/Internal.hs
{-# LANGUAGE CPP #-}
{-|
Module: Data.Functor.Deriving.Internal
Copyright: (C) 2015-2016 Ryan Scott
License: BSD-style (see the file LICENSE)
Maintainer: Ryan Scott
Portability: Template Haskell
The machinery needed to derive 'Foldable', 'Functor', and 'Traversable' instances.
For more info on how deriving @Functor@ works, see
<https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/DeriveFunctor this GHC wiki page>.
-}
module Data.Functor.Deriving.Internal (
-- * 'Foldable'
deriveFoldable
, makeFoldMap
, makeFoldr
, makeFold
, makeFoldl
-- * 'Functor'
, deriveFunctor
, makeFmap
-- * 'Traversable'
, deriveTraversable
, makeTraverse
, makeSequenceA
, makeMapM
, makeSequence
) where
import Control.Monad (guard, unless, when, zipWithM)
import Data.Deriving.Internal
import Data.Either (rights)
#if MIN_VERSION_template_haskell(2,8,0) && !(MIN_VERSION_template_haskell(2,10,0))
import Data.Foldable (foldr')
#endif
import Data.List
import qualified Data.Map as Map (fromList, keys, lookup, size)
import Data.Maybe
import Language.Haskell.TH.Lib
import Language.Haskell.TH.Ppr
import Language.Haskell.TH.Syntax
-- | Generates a 'Foldable' instance declaration for the given data type or data
-- family instance.
deriveFoldable :: Name -> Q [Dec]
deriveFoldable = deriveFunctorClass Foldable
-- | Generates a lambda expression which behaves like 'foldMap' (without requiring a
-- 'Foldable' instance).
makeFoldMap :: Name -> Q Exp
makeFoldMap = makeFunctorFun FoldMap
-- | Generates a lambda expression which behaves like 'foldr' (without requiring a
-- 'Foldable' instance).
makeFoldr :: Name -> Q Exp
makeFoldr = makeFunctorFun Foldr
-- | Generates a lambda expression which behaves like 'fold' (without requiring a
-- 'Foldable' instance).
makeFold :: Name -> Q Exp
makeFold name = makeFoldMap name `appE` varE idValName
-- | Generates a lambda expression which behaves like 'foldl' (without requiring a
-- 'Foldable' instance).
makeFoldl :: Name -> Q Exp
makeFoldl name = do
f <- newName "f"
z <- newName "z"
t <- newName "t"
lamE [varP f, varP z, varP t] $
appsE [ varE appEndoValName
, appsE [ varE getDualValName
, appsE [ makeFoldMap name, foldFun f, 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 'Functor' instance declaration for the given data type or data
-- family instance.
deriveFunctor :: Name -> Q [Dec]
deriveFunctor = deriveFunctorClass Functor
-- | Generates a lambda expression which behaves like 'fmap' (without requiring a
-- 'Functor' instance).
makeFmap :: Name -> Q Exp
makeFmap = makeFunctorFun Fmap
-- | Generates a 'Traversable' instance declaration for the given data type or data
-- family instance.
deriveTraversable :: Name -> Q [Dec]
deriveTraversable = deriveFunctorClass Traversable
-- | Generates a lambda expression which behaves like 'traverse' (without requiring a
-- 'Traversable' instance).
makeTraverse :: Name -> Q Exp
makeTraverse = makeFunctorFun Traverse
-- | Generates a lambda expression which behaves like 'sequenceA' (without requiring a
-- 'Traversable' instance).
makeSequenceA :: Name -> Q Exp
makeSequenceA name = makeTraverse name `appE` varE idValName
-- | Generates a lambda expression which behaves like 'mapM' (without requiring a
-- 'Traversable' instance).
makeMapM :: Name -> Q Exp
makeMapM name = do
f <- newName "f"
lam1E (varP f) . infixApp (varE unwrapMonadValName) (varE composeValName) $
makeTraverse name `appE` wrapMonadExp f
where
wrapMonadExp :: Name -> Q Exp
wrapMonadExp n = infixApp (conE wrapMonadDataName) (varE composeValName) (varE n)
-- | Generates a lambda expression which behaves like 'sequence' (without requiring a
-- 'Traversable' instance).
makeSequence :: Name -> Q Exp
makeSequence name = makeMapM name `appE` varE idValName
-------------------------------------------------------------------------------
-- Code generation
-------------------------------------------------------------------------------
-- | Derive a class instance declaration (depending on the FunctorClass argument's value).
deriveFunctorClass :: FunctorClass -> Name -> Q [Dec]
deriveFunctorClass fc name = withType name fromCons where
fromCons :: Name -> Cxt -> [TyVarBndr] -> [Con] -> Maybe [Type] -> Q [Dec]
fromCons name' ctxt tvbs cons mbTys = (:[]) `fmap` do
(instanceCxt, instanceType)
<- buildTypeInstance fc name' ctxt tvbs mbTys
instanceD (return instanceCxt)
(return instanceType)
(functorFunDecs fc cons)
-- | Generates a declaration defining the primary function(s) corresponding to a
-- particular class (fmap for Functor, foldr and foldMap for Foldable, and
-- traverse for Traversable).
--
-- For why both foldr and foldMap are derived for Foldable, see Trac #7436.
functorFunDecs :: FunctorClass -> [Con] -> [Q Dec]
functorFunDecs fc cons = map makeFunD $ functorClassToFuns fc where
makeFunD :: FunctorFun -> Q Dec
makeFunD ff =
funD (functorFunName ff)
[ clause []
(normalB $ makeFunctorFunForCons ff cons)
[]
]
-- | Generates a lambda expression which behaves like the FunctorFun argument.
makeFunctorFun :: FunctorFun -> Name -> Q Exp
makeFunctorFun ff name = withType name fromCons where
fromCons :: Name -> Cxt -> [TyVarBndr] -> [Con] -> Maybe [Type] -> Q Exp
fromCons name' ctxt tvbs cons mbTys =
-- We force buildTypeInstance here since it performs some checks for whether
-- or not the provided datatype can actually have fmap/foldr/traverse/etc.
-- implemented for it, and produces errors if it can't.
buildTypeInstance (functorFunToClass ff) name' ctxt tvbs mbTys
`seq` makeFunctorFunForCons ff cons
-- | Generates a lambda expression for the given constructors.
-- All constructors must be from the same type.
makeFunctorFunForCons :: FunctorFun -> [Con] -> Q Exp
makeFunctorFunForCons ff cons = do
argNames <- mapM newName $ catMaybes [ Just "f"
, guard (ff == Foldr) >> Just "z"
, Just "value"
]
let mapFun:others = argNames
z = head others -- If we're deriving foldr, this will be well defined
-- and useful. Otherwise, it'll be ignored.
value = last others
lamE (map varP argNames)
. appsE
$ [ varE $ functorFunConstName ff
, if null cons
then appE (varE errorValName)
(stringE $ "Void " ++ nameBase (functorFunName ff))
else caseE (varE value)
(map (makeFunctorFunForCon ff z mapFun) cons)
] ++ map varE argNames
-- | Generates a lambda expression for a single constructor.
makeFunctorFunForCon :: FunctorFun -> Name -> Name -> Con -> Q Match
makeFunctorFunForCon ff z mapFun con = do
let conName = constructorName con
(ts, tvMap) <- reifyConTys ff conName mapFun
argNames <- newNameList "_arg" $ length ts
makeFunctorFunForArgs ff z tvMap conName ts argNames
-- | Generates a lambda expression for a single constructor's arguments.
makeFunctorFunForArgs :: FunctorFun
-> Name
-> TyVarMap
-> Name
-> [Type]
-> [Name]
-> Q Match
makeFunctorFunForArgs ff z tvMap conName tys args =
match (conP conName $ map varP args)
(normalB $ functorFunCombine ff conName z args mappedArgs)
[]
where
mappedArgs :: Q [Either Exp Exp]
mappedArgs = zipWithM (makeFunctorFunForArg ff tvMap conName) tys args
-- | Generates a lambda expression for a single argument of a constructor.
-- The returned value is 'Right' if its type mentions the last type
-- parameter. Otherwise, it is 'Left'.
makeFunctorFunForArg :: FunctorFun
-> TyVarMap
-> Name
-> Type
-> Name
-> Q (Either Exp Exp)
makeFunctorFunForArg ff tvMap conName ty tyExpName =
makeFunctorFunForType ff tvMap conName True ty `appEitherE` varE tyExpName
-- | Generates a lambda expression for a specific type. The returned value is
-- 'Right' if its type mentions the last type parameter. Otherwise,
-- it is 'Left'.
makeFunctorFunForType :: FunctorFun
-> TyVarMap
-> Name
-> Bool
-> Type
-> Q (Either Exp Exp)
makeFunctorFunForType ff tvMap conName covariant (VarT tyName) =
case Map.lookup tyName tvMap of
Just mapName -> fmap Right $
if covariant
then varE mapName
else contravarianceError conName
-- Invariant: this should only happen when deriving fmap
Nothing -> fmap Left $ functorFunTriv ff
makeFunctorFunForType ff tvMap conName covariant (SigT ty _) =
makeFunctorFunForType ff tvMap conName covariant ty
makeFunctorFunForType ff tvMap conName covariant (ForallT _ _ ty) =
makeFunctorFunForType ff tvMap conName covariant ty
makeFunctorFunForType ff tvMap conName covariant ty =
let tyCon :: Type
tyArgs :: [Type]
tyCon:tyArgs = unapplyTy ty
numLastArgs :: Int
numLastArgs = min 1 $ 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
makeFunctorFunTuple :: Type -> Name -> Q (Either Exp Exp)
makeFunctorFunTuple fieldTy fieldName =
makeFunctorFunForType ff tvMap conName covariant fieldTy
`appEitherE` varE fieldName
in case tyCon of
ArrowT
| not (allowFunTys (functorFunToClass ff)) -> noFunctionsError conName
| mentionsTyArgs, [argTy, resTy] <- tyArgs ->
do x <- newName "x"
b <- newName "b"
fmap Right . lamE [varP x, varP b] $
covFunctorFun covariant resTy `appE` (varE x `appE`
(covFunctorFun (not covariant) argTy `appE` varE b))
where
covFunctorFun :: Bool -> Type -> Q Exp
covFunctorFun cov = fmap fromEither . makeFunctorFunForType ff tvMap conName cov
TupleT n
| n > 0 && mentionsTyArgs -> do
args <- mapM newName $ catMaybes [ Just "x"
, guard (ff == Foldr) >> Just "z"
]
xs <- newNameList "_tup" n
let x = head args
z = last args
fmap Right $ lamE (map varP args) $ caseE (varE x)
[ match (tupP $ map varP xs)
(normalB $ functorFunCombine ff
(tupleDataName n)
z
xs
(zipWithM makeFunctorFunTuple tyArgs xs)
)
[]
]
_ -> do
itf <- isTyFamily tyCon
if any (`mentionsName` tyVarNames) lhsArgs || (itf && mentionsTyArgs)
then outOfPlaceTyVarError conName
else if any (`mentionsName` tyVarNames) rhsArgs
then fmap Right . functorFunApp ff . appsE $
( varE (functorFunName ff)
: map (fmap fromEither . makeFunctorFunForType ff tvMap conName covariant)
rhsArgs
)
else fmap Left $ functorFunTriv ff
-------------------------------------------------------------------------------
-- Template Haskell reifying and AST manipulation
-------------------------------------------------------------------------------
-- | Boilerplate for top level splices.
--
-- The given Name must meet one of two criteria:
--
-- 1. It must be the name of a type constructor of a plain data type or newtype.
-- 2. It must be the name of a data family instance or newtype instance constructor.
--
-- Any other value will result in an exception.
withType :: Name
-> (Name -> Cxt -> [TyVarBndr] -> [Con] -> Maybe [Type] -> Q a)
-> Q a
withType name f = do
info <- reify name
case info of
TyConI dec ->
case dec of
DataD ctxt _ tvbs
#if MIN_VERSION_template_haskell(2,11,0)
_
#endif
cons _ -> f name ctxt tvbs cons Nothing
NewtypeD ctxt _ tvbs
#if MIN_VERSION_template_haskell(2,11,0)
_
#endif
con _ -> f name ctxt tvbs [con] Nothing
_ -> fail $ ns ++ "Unsupported type: " ++ show dec
#if MIN_VERSION_template_haskell(2,7,0)
# if MIN_VERSION_template_haskell(2,11,0)
DataConI _ _ parentName -> do
# else
DataConI _ _ parentName _ -> do
# endif
parentInfo <- reify parentName
case parentInfo of
# if MIN_VERSION_template_haskell(2,11,0)
FamilyI (DataFamilyD _ tvbs _) decs ->
# else
FamilyI (FamilyD DataFam _ tvbs _) decs ->
# endif
let instDec = flip find decs $ \dec -> case dec of
DataInstD _ _ _
# if MIN_VERSION_template_haskell(2,11,0)
_
# endif
cons _ -> any ((name ==) . constructorName) cons
NewtypeInstD _ _ _
# if MIN_VERSION_template_haskell(2,11,0)
_
# endif
con _ -> name == constructorName con
_ -> error $ ns ++ "Must be a data or newtype instance."
in case instDec of
Just (DataInstD ctxt _ instTys
# if MIN_VERSION_template_haskell(2,11,0)
_
# endif
cons _)
-> f parentName ctxt tvbs cons $ Just instTys
Just (NewtypeInstD ctxt _ instTys
# if MIN_VERSION_template_haskell(2,11,0)
_
# endif
con _)
-> f parentName ctxt tvbs [con] $ Just instTys
_ -> fail $ ns ++
"Could not find data or newtype instance constructor."
_ -> fail $ ns ++ "Data constructor " ++ show name ++
" is not from a data family instance constructor."
# if MIN_VERSION_template_haskell(2,11,0)
FamilyI DataFamilyD{} _ ->
# else
FamilyI (FamilyD DataFam _ _ _) _ ->
# endif
fail $ ns ++
"Cannot use a data family name. Use a data family instance constructor instead."
_ -> fail $ ns ++ "The name must be of a plain data type constructor, "
++ "or a data family instance constructor."
#else
DataConI{} -> dataConIError
_ -> fail $ ns ++ "The name must be of a plain type constructor."
#endif
where
ns :: String
ns = "Data.Functor.Deriving.Internal.withType: "
-- | Deduces the instance context and head for an instance.
buildTypeInstance :: FunctorClass
-- ^ Functor, Foldable, or Traversable
-> Name
-- ^ The type constructor or data family name
-> Cxt
-- ^ The datatype context
-> [TyVarBndr]
-- ^ The type variables from the data type/data family declaration
-> Maybe [Type]
-- ^ 'Just' the types used to instantiate a data family instance,
-- or 'Nothing' if it's a plain data type
-> Q (Cxt, Type)
-- Plain data type/newtype case
buildTypeInstance fc tyConName dataCxt tvbs Nothing =
let varTys :: [Type]
varTys = map tvbToType tvbs
in buildTypeInstanceFromTys fc tyConName dataCxt varTys False
-- Data family instance case
--
-- The CPP is present to work around a couple of annoying old GHC bugs.
-- See Note [Polykinded data families in Template Haskell]
buildTypeInstance fc parentName dataCxt tvbs (Just instTysAndKinds) = do
#if !(MIN_VERSION_template_haskell(2,8,0)) || MIN_VERSION_template_haskell(2,10,0)
let instTys :: [Type]
instTys = zipWith stealKindForType tvbs instTysAndKinds
#else
let kindVarNames :: [Name]
kindVarNames = nub $ concatMap (tyVarNamesOfType . tvbKind) tvbs
numKindVars :: Int
numKindVars = length kindVarNames
givenKinds, givenKinds' :: [Kind]
givenTys :: [Type]
(givenKinds, givenTys) = splitAt numKindVars instTysAndKinds
givenKinds' = map sanitizeStars givenKinds
-- A GHC 7.6-specific bug requires us to replace all occurrences of
-- (ConT GHC.Prim.*) with StarT, or else Template Haskell will reject it.
-- Luckily, (ConT GHC.Prim.*) only seems to occur in this one spot.
sanitizeStars :: Kind -> Kind
sanitizeStars = go
where
go :: Kind -> Kind
go (AppT t1 t2) = AppT (go t1) (go t2)
go (SigT t k) = SigT (go t) (go k)
go (ConT n) | n == starKindName = StarT
go t = t
-- If we run this code with GHC 7.8, we might have to generate extra type
-- variables to compensate for any type variables that Template Haskell
-- eta-reduced away.
-- See Note [Polykinded data families in Template Haskell]
xTypeNames <- newNameList "tExtra" (length tvbs - length givenTys)
let xTys :: [Type]
xTys = map VarT xTypeNames
-- ^ Because these type variables were eta-reduced away, we can only
-- determine their kind by using stealKindForType. Therefore, we mark
-- them as VarT to ensure they will be given an explicit kind annotation
-- (and so the kind inference machinery has the right information).
substNamesWithKinds :: [(Name, Kind)] -> Type -> Type
substNamesWithKinds nks t = foldr' (uncurry substNameWithKind) t nks
-- The types from the data family instance might not have explicit kind
-- annotations, which the kind machinery needs to work correctly. To
-- compensate, we use stealKindForType to explicitly annotate any
-- types without kind annotations.
instTys :: [Type]
instTys = map (substNamesWithKinds (zip kindVarNames givenKinds'))
-- ^ Note that due to a GHC 7.8-specific bug
-- (see Note [Polykinded data families in Template Haskell]),
-- there may be more kind variable names than there are kinds
-- to substitute. But this is OK! If a kind is eta-reduced, it
-- means that is was not instantiated to something more specific,
-- so we need not substitute it. Using stealKindForType will
-- grab the correct kind.
$ zipWith stealKindForType tvbs (givenTys ++ xTys)
#endif
buildTypeInstanceFromTys fc parentName dataCxt instTys True
-- For the given Types, generate an instance context and head. Coming up with
-- the instance type isn't as simple as dropping the last type, as you need to
-- be wary of kinds being instantiated with *.
-- See Note [Type inference in derived instances]
buildTypeInstanceFromTys :: FunctorClass
-- ^ Functor, Foldable, or Traversable
-> Name
-- ^ The type constructor or data family name
-> Cxt
-- ^ The datatype context
-> [Type]
-- ^ The types to instantiate the instance with
-> Bool
-- ^ True if it's a data family, False otherwise
-> Q (Cxt, Type)
buildTypeInstanceFromTys fc tyConName dataCxt varTysOrig isDataFamily = 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 expandSyn varTysOrig
let remainingLength :: Int
remainingLength = length varTysOrig - 1
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 fc 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 = concatMap tyVarNamesOfType 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 fc 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 fc) 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
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 $ functorClassName fc)
$ 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 :: FunctorClass -> Type -> (Maybe Pred, [Name])
deriveConstraint fc t
| not (isTyVar t) = (Nothing, [])
| otherwise = case hasKindVarChain 1 t of
Just ns -> (Just (applyClass (functorClassName fc) tName), ns)
Nothing -> (Nothing, [])
where
tName :: Name
tName = varTToName t
{-
Note [Polykinded data families in Template Haskell]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to come up with the correct instance context and head for an instance, e.g.,
instance C a => C (Data a) where ...
We need to know the exact types and kinds used to instantiate the instance. For
plain old datatypes, this is simple: every type must be a type variable, and
Template Haskell reliably tells us the type variables and their kinds.
Doing the same for data families proves to be much harder for three reasons:
1. On any version of Template Haskell, it may not tell you what an instantiated
type's kind is. For instance, in the following data family instance:
data family Fam (f :: * -> *) (a :: *)
data instance Fam f a
Then if we use TH's reify function, it would tell us the TyVarBndrs of the
data family declaration are:
[KindedTV f (AppT (AppT ArrowT StarT) StarT),KindedTV a StarT]
and the instantiated types of the data family instance are:
[VarT f1,VarT a1]
We can't just pass [VarT f1,VarT a1] to buildTypeInstanceFromTys, since we
have no way of knowing their kinds. Luckily, the TyVarBndrs tell us what the
kind is in case an instantiated type isn't a SigT, so we use the stealKindForType
function to ensure all of the instantiated types are SigTs before passing them
to buildTypeInstanceFromTys.
2. On GHC 7.6 and 7.8, a bug is present in which Template Haskell lists all of
the specified kinds of a data family instance efore any of the instantiated
types. Fortunately, this is easy to deal with: you simply count the number of
distinct kind variables in the data family declaration, take that many elements
from the front of the Types list of the data family instance, substitute the
kind variables with their respective instantiated kinds (which you took earlier),
and proceed as normal.
3. On GHC 7.8, an even uglier bug is present (GHC Trac #9692) in which Template
Haskell might not even list all of the Types of a data family instance, since
they are eta-reduced away! And yes, kinds can be eta-reduced too.
The simplest workaround is to count how many instantiated types are missing from
the list and generate extra type variables to use in their place. Luckily, we
needn't worry much if its kind was eta-reduced away, since using stealKindForType
will get it back.
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 criterion:
(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 :: k2 -> *) (g :: k1 -> k2) (a :: k1)
= Compose (f (g a))
Which would have a derived Functor instance of:
instance (Functor f, Functor g) => Functor (Compose f g) where ...
-}
-- Determines the types of a constructor's arguments as well as the last type
-- parameters (along with their map functions), expanding through any type synonyms.
-- The type parameters are determined on a constructor-by-constructor basis since
-- they may be refined to be particular types in a GADT.
reifyConTys :: FunctorFun
-> Name
-> Name
-> Q ([Type], TyVarMap)
reifyConTys ff conName mapFun = do
info <- reify conName
(ctxt, uncTy) <- case info of
DataConI _ ty _
#if !(MIN_VERSION_template_haskell(2,11,0))
_
#endif
-> fmap uncurryTy (expandSyn ty)
_ -> fail "Must be a data constructor"
let (argTys, [resTy]) = splitAt (length uncTy - 1) uncTy
unapResTy = unapplyTy resTy
-- If one of the last type variables is refined to a particular type
-- (i.e., not truly polymorphic), we mark it with Nothing and filter
-- it out later, since we only apply map functions to arguments of
-- a type that it (1) one of the last type variables, and (2)
-- of a truly polymorphic type.
mbTvNames = map varTToName_maybe $
drop (length unapResTy - 1) unapResTy
tvMap = Map.fromList
. catMaybes -- Drop refined types
$ zipWith (\mbTvName sp ->
fmap (\tvName -> (tvName, sp)) mbTvName)
mbTvNames [mapFun]
if (any (`predMentionsName` Map.keys tvMap) ctxt
|| Map.size tvMap < 1)
&& not (allowExQuant (functorFunToClass ff))
then existentialContextError conName
else return (argTys, tvMap)
-------------------------------------------------------------------------------
-- 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 :: FunctorClass -> Name -> Q a
derivingKindError fc tyConName = fail
. 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 $ functorClassName fc
-- | The last type variable appeared in a contravariant position
-- when deriving Functor.
contravarianceError :: Name -> Q a
contravarianceError conName = fail
. showString "Constructor ‘"
. showString (nameBase conName)
. showString "‘ must not use the last type variable in a function argument"
$ ""
-- | A constructor has a function argument in a derived Foldable or Traversable
-- instance.
noFunctionsError :: Name -> Q a
noFunctionsError conName = fail
. 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 -> Q a
datatypeContextError dataName instanceType = fail
. 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 -> Q a
existentialContextError conName = fail
. 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 -> Q a
outOfPlaceTyVarError conName = fail
. 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 -> Q a
etaReductionError instanceType = fail $
"Cannot eta-reduce to an instance of form \n\tinstance (...) => "
++ pprint instanceType
#if !(MIN_VERSION_template_haskell(2,7,0))
-- | Template Haskell didn't list all of a data family's instances upon reification
-- until template-haskell-2.7.0.0, which is necessary for a derived instance to work.
dataConIError :: Q a
dataConIError = fail
. showString "Cannot use a data constructor."
. showString "\n\t(Note: if you are trying to derive for a data family instance,"
. showString "\n\tuse GHC >= 7.4 instead.)"
$ ""
#endif
-------------------------------------------------------------------------------
-- Class-specific constants
-------------------------------------------------------------------------------
-- | A representation of which class is being derived.
data FunctorClass = Functor | Foldable | Traversable
-- | A representation of which function is being generated.
data FunctorFun = Fmap | Foldr | FoldMap | Traverse
deriving Eq
instance Show FunctorFun where
showsPrec _ Fmap = showString "fmap"
showsPrec _ Foldr = showString "foldr"
showsPrec _ FoldMap = showString "foldMap"
showsPrec _ Traverse = showString "traverse"
functorFunConstName :: FunctorFun -> Name
functorFunConstName Fmap = fmapConstValName
functorFunConstName Foldr = foldrConstValName
functorFunConstName FoldMap = foldMapConstValName
functorFunConstName Traverse = traverseConstValName
functorClassName :: FunctorClass -> Name
functorClassName Functor = functorTypeName
functorClassName Foldable = foldableTypeName
functorClassName Traversable = traversableTypeName
functorFunName :: FunctorFun -> Name
functorFunName Fmap = fmapValName
functorFunName Foldr = foldrValName
functorFunName FoldMap = foldMapValName
functorFunName Traverse = traverseValName
functorClassToFuns :: FunctorClass -> [FunctorFun]
functorClassToFuns Functor = [Fmap]
functorClassToFuns Foldable = [Foldr, FoldMap]
functorClassToFuns Traversable = [Traverse]
functorFunToClass :: FunctorFun -> FunctorClass
functorFunToClass Fmap = Functor
functorFunToClass Foldr = Foldable
functorFunToClass FoldMap = Foldable
functorFunToClass Traverse = Traversable
allowFunTys :: FunctorClass -> Bool
allowFunTys Functor = True
allowFunTys _ = False
allowExQuant :: FunctorClass -> Bool
allowExQuant Foldable = True
allowExQuant _ = False
-- See Trac #7436 for why explicit lambdas are used
functorFunTriv :: FunctorFun -> Q Exp
functorFunTriv Fmap = do
x <- newName "x"
lam1E (varP x) $ varE x
-- We filter out trivial expressions from derived foldr, foldMap, and traverse
-- implementations, so if we attempt to call functorFunTriv on one of those
-- methods, we've done something wrong.
functorFunTriv ff = return . error $ "functorFunTriv: " ++ show ff
functorFunApp :: FunctorFun -> Q Exp -> Q Exp
functorFunApp Foldr e = do
x <- newName "x"
z <- newName "z"
lamE [varP x, varP z] $ appsE [e, varE z, varE x]
functorFunApp _ e = e
functorFunCombine :: FunctorFun
-> Name
-> Name
-> [Name]
-> Q [Either Exp Exp]
-> Q Exp
functorFunCombine Fmap = fmapCombine
functorFunCombine Foldr = foldrCombine
functorFunCombine FoldMap = foldMapCombine
functorFunCombine Traverse = traverseCombine
fmapCombine :: Name
-> Name
-> [Name]
-> Q [Either Exp Exp]
-> Q Exp
fmapCombine conName _ _ = fmap (foldl' AppE (ConE conName) . fmap fromEither)
-- foldr, foldMap, and traverse are handled differently from fmap, since
-- they filter out subexpressions whose types do not mention the last
-- type parameter. See
-- https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/DeriveFunctor#AlternativestrategyforderivingFoldableandTraversable
-- for further discussion.
foldrCombine :: Name
-> Name
-> [Name]
-> Q [Either Exp Exp]
-> Q Exp
foldrCombine _ zName _ = fmap (foldr AppE (VarE zName) . rights)
foldMapCombine :: Name
-> Name
-> [Name]
-> Q [Either Exp Exp]
-> Q Exp
foldMapCombine _ _ _ = fmap (go . rights)
where
go :: [Exp] -> Exp
go [] = VarE memptyValName
go es = foldr1 (AppE . AppE (VarE mappendValName)) es
traverseCombine :: Name
-> Name
-> [Name]
-> Q [Either Exp Exp]
-> Q Exp
traverseCombine 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:es) = foldl' (\e1 e2 -> InfixE (Just e1) (VarE apValName) (Just e2))
(VarE fmapValName `AppE` conExp `AppE` e) es
return . go . rights $ ess