{-# LANGUAGE CPP #-}

{-|
Module:      Data.Functor.Invariant.TH
Copyright:   (C) 2012-2017 Nicolas Frisby, (C) 2015-2017 Ryan Scott
License:     BSD-style (see the file LICENSE)
Maintainer:  Ryan Scott
Portability: Template Haskell

Functions to mechanically derive 'Invariant' or 'Invariant2' instances,
or to splice 'invmap' or 'invmap2' into Haskell source code. You need to enable
the @TemplateHaskell@ language extension in order to use this module.
-}
module Data.Functor.Invariant.TH (
      -- * @deriveInvariant(2)@
      -- $deriveInvariant
      deriveInvariant
    , deriveInvariantOptions
      -- $deriveInvariant2
    , deriveInvariant2
    , deriveInvariant2Options
      -- * @makeInvmap(2)@
      -- $make
    , makeInvmap
    , makeInvmapOptions
    , makeInvmap2
    , makeInvmap2Options
      -- * 'Options'
    , Options(..)
    , defaultOptions
    ) where

import           Control.Monad (unless, when)

import           Data.Functor.Invariant.TH.Internal
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.Functor.Invariant.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 }

{- $deriveInvariant

'deriveInvariant' automatically generates an 'Invariant' instance declaration for a
data type, newtype, or data family instance that has at least one type variable.
This emulates what would (hypothetically) happen if you could attach a @deriving
'Invariant'@ clause to the end of a data declaration. Examples:

@
{-# LANGUAGE TemplateHaskell #-}
import Data.Functor.Invariant.TH

data Pair a = Pair a a
$('deriveInvariant' ''Pair) -- instance Invariant Pair where ...

newtype Alt f a = Alt (f a)
$('deriveInvariant' ''Alt) -- instance Invariant f => Invariant (Alt f) where ...
@

If you are using @template-haskell-2.7.0.0@ or later (i.e., GHC 7.4 or later),
'deriveInvariant' can also be used to derive 'Invariant' instances for data family
instances (which requires the @-XTypeFamilies@ extension). To do so, pass the name of
a data or newtype instance constructor to 'deriveInvariant'.  Note that the generated
code may require the @-XFlexibleInstances@ extension. Some examples:

@
{-# LANGUAGE FlexibleInstances, TemplateHaskell, TypeFamilies #-}
import Data.Functor.Invariant.TH

class AssocClass a b where
    data AssocData a b
instance AssocClass Int b where
    data AssocData Int b = AssocDataInt1 Int | AssocDataInt2 b Int
$('deriveInvariant' 'AssocDataInt1) -- instance Invariant (AssocData Int) where ...
-- Alternatively, one could use $(deriveInvariant 'AssocDataInt2)

data family DataFam a b
newtype instance DataFam () b = DataFamB b
$('deriveInvariant' 'DataFamB) -- instance Invariant (DataFam ())
@

Note that there are some limitations:

* The 'Name' argument to 'deriveInvariant' must not be a type synonym.

* With 'deriveInvariant', the argument's last type variable must be of kind @*@.
  For other ones, type variables of kind @* -> *@ are assumed to require an 'Invariant'
  context. For more complicated scenarios, use 'makeInvmap'.

* If using the @-XDatatypeContexts@, @-XExistentialQuantification@, or @-XGADTs@
  extensions, a constraint cannot mention the last type variable. For example,
  @data Illegal a where I :: Ord a => a -> Illegal a@ cannot have a derived
  'Invariant' instance.

* If the last type variable is used within a data field of a constructor, it must only
  be used in the last argument of the data type constructor. For example, @data Legal a
  = Legal (Either Int a)@ can have a derived 'Invariant' instance, but @data Illegal a =
  Illegal (Either a a)@ cannot.

* Data family instances must be able to eta-reduce the last type variable. In other
  words, if you have a instance of the form:

  @
  data family Family a1 ... an t
  data instance Family e1 ... e2 v = ...
  @

  Then the following conditions must hold:

  1. @v@ must be a type variable.
  2. @v@ must not be mentioned in any of @e1@, ..., @e2@.

-}

-- | Generates an 'Invariant' instance declaration for the given data type or data
-- family instance.
deriveInvariant :: Name -> Q [Dec]
deriveInvariant = deriveInvariantOptions defaultOptions

-- | Like 'deriveInvariant', but takes an 'Options' argument.
deriveInvariantOptions :: Options -> Name -> Q [Dec]
deriveInvariantOptions = deriveInvariantClass Invariant

{- $deriveInvariant2

'deriveInvariant2' automatically generates an 'Invariant2' instance declaration for
a data type, newtype, or data family instance that has at least two type variables.
This emulates what would (hypothetically) happen if you could attach a @deriving
'Invariant2'@ clause to the end of a data declaration. Examples:

@
{-# LANGUAGE TemplateHaskell #-}
import Data.Functor.Invariant.TH

data OneOrNone a b = OneL a | OneR b | None
$('deriveInvariant2' ''OneOrNone) -- instance Invariant2 OneOrNone where ...

newtype Alt2 f a b = Alt2 (f a b)
$('deriveInvariant2' ''Alt2) -- instance Invariant2 f => Invariant2 (Alt2 f) where ...
@

The same restrictions that apply to 'deriveInvariant' also apply to 'deriveInvariant2',
with some caveats:

* With 'deriveInvariant2', the last type variables must both be of kind @*@. For other
  ones, type variables of kind @* -> *@ are assumed to require an 'Invariant'
  constraint, and type variables of kind @* -> * -> *@ are assumed to require an
  'Invariant2' constraint. For more complicated scenarios, use 'makeInvmap2'.

* 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 'Invariant2' instance.

* If either of the last two type variables is used within a data field of a constructor,
  it must only be used in the last two arguments of the data type constructor. For
  example, @data Legal a b = Legal (Int, Int, a, b)@ can have a derived 'Invariant2'
  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@.

-}

-- | Generates an 'Invariant2' instance declaration for the given data type or data
-- family instance.
deriveInvariant2 :: Name -> Q [Dec]
deriveInvariant2 = deriveInvariant2Options defaultOptions

-- | Like 'deriveInvariant2', but takes an 'Options' argument.
deriveInvariant2Options :: Options -> Name -> Q [Dec]
deriveInvariant2Options = deriveInvariantClass Invariant2

{- $make

There may be scenarios in which you want to @invmap@ over an arbitrary data type or
data family instance without having to make the type an instance of 'Invariant'. For
these cases, this module provides several functions (all prefixed with @make-@) that
splice the appropriate lambda expression into your source code. Example:

This is particularly useful for creating instances for sophisticated data types. For
example, 'deriveInvariant' 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 an 'Invariant' instance
for @HigherKinded@ without too much trouble using 'makeInvmap':

@
{-# LANGUAGE FlexibleContexts, TemplateHaskell #-}
import Data.Functor.Invariant
import Data.Functor.Invariant.TH

newtype HigherKinded f a b c = HigherKinded (f a b c)

instance Invariant (f a b) => Invariant (HigherKinded f a b) where
    invmap = $(makeInvmap ''HigherKinded)
@

-}

-- | Generates a lambda expression which behaves like 'invmap' (without requiring an
-- 'Invariant' instance).
makeInvmap :: Name -> Q Exp
makeInvmap = makeInvmapOptions defaultOptions

-- | Like 'makeInvmap', but takes an 'Options' argument.
makeInvmapOptions :: Options -> Name -> Q Exp
makeInvmapOptions = makeInvmapClass Invariant

-- | Generates a lambda expression which behaves like 'invmap2' (without requiring an
-- 'Invariant2' instance).
makeInvmap2 :: Name -> Q Exp
makeInvmap2 = makeInvmap2Options defaultOptions

-- | Like 'makeInvmap2', but takes an 'Options' argument.
makeInvmap2Options :: Options -> Name -> Q Exp
makeInvmap2Options = makeInvmapClass Invariant2

-------------------------------------------------------------------------------
-- Code generation
-------------------------------------------------------------------------------

-- | Derive an Invariant(2) instance declaration (depending on the InvariantClass
-- argument's value).
deriveInvariantClass :: InvariantClass -> Options -> Name -> Q [Dec]
deriveInvariantClass iClass opts name = do
  info <- reifyDatatype name
  case info of
    DatatypeInfo { datatypeContext = ctxt
                 , datatypeName    = parentName
                 , datatypeVars    = vars
                 , datatypeVariant = variant
                 , datatypeCons    = cons
                 } -> do
      (instanceCxt, instanceType)
        <- buildTypeInstance iClass parentName ctxt vars variant
      (:[]) `fmap` instanceD (return instanceCxt)
                             (return instanceType)
                             (invmapDecs iClass opts parentName vars cons)

-- | Generates a declaration defining the primary function corresponding to a
-- particular class (invmap for Invariant and invmap2 for Invariant2).
invmapDecs :: InvariantClass -> Options -> Name -> [Type] -> [ConstructorInfo]
           -> [Q Dec]
invmapDecs iClass opts parentName vars cons =
    [ funD (invmapName iClass)
           [ clause []
                    (normalB $ makeInvmapForCons iClass opts parentName vars cons)
                    []
           ]
    ]

-- | Generates a lambda expression which behaves like invmap (for Invariant),
-- or invmap2 (for Invariant2).
makeInvmapClass :: InvariantClass -> Options -> Name -> Q Exp
makeInvmapClass iClass 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 invmap/invmap2
      -- implemented for it, and produces errors if it can't.
      buildTypeInstance iClass parentName ctxt vars variant
        >> makeInvmapForCons iClass opts parentName vars cons

-- | Generates a lambda expression for invmap(2) for the given constructors.
-- All constructors must be from the same type.
makeInvmapForCons :: InvariantClass -> Options -> Name -> [Type] -> [ConstructorInfo]
                  -> Q Exp
makeInvmapForCons iClass opts _parentName vars cons = do
    value      <- newName "value"
    covMaps    <- newNameList "covMap" numNbs
    contraMaps <- newNameList "contraMap" numNbs

    let mapFuns    = zip covMaps contraMaps
        lastTyVars = map varTToName $ drop (length vars - fromEnum iClass) vars
        tvMap      = Map.fromList $ zip lastTyVars mapFuns
        argNames   = concat (transpose [covMaps, contraMaps]) ++ [value]
    lamE (map varP argNames)
        . appsE
        $ [ varE $ invmapConstName iClass
          , makeFun value tvMap
          ] ++ map varE argNames
  where
    numNbs :: Int
    numNbs = fromEnum iClass

    makeFun :: Name -> TyVarMap -> Q Exp
    makeFun value tvMap = do
#if MIN_VERSION_template_haskell(2,9,0)
      roles <- reifyRoles _parentName
      let rroles = roles
#endif
      case () of
        _

#if MIN_VERSION_template_haskell(2,9,0)
          | (length rroles >= numNbs) &&
            (all (== PhantomR) (take numNbs rroles))
         -> varE coerceValName `appE` varE value
#endif

          | null cons && emptyCaseBehavior opts && ghc7'8OrLater
         -> caseE (varE value) []

          | null cons
         -> appE (varE seqValName) (varE value) `appE`
            appE (varE errorValName)
                 (stringE $ "Void " ++ nameBase (invmapName iClass))

          | otherwise
         -> caseE (varE value)
                  (map (makeInvmapForCon iClass tvMap) cons)

    ghc7'8OrLater :: Bool
#if __GLASGOW_HASKELL__ >= 708
    ghc7'8OrLater = True
#else
    ghc7'8OrLater = False
#endif

-- | Generates a lambda expression for invmap(2) for a single constructor.
makeInvmapForCon :: InvariantClass -> TyVarMap -> ConstructorInfo -> Q Match
makeInvmapForCon iClass 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 < fromEnum iClass
       then existentialContextError conName
       else makeInvmapForArgs iClass tvMap conName ts' argNames

makeInvmapForArgs :: InvariantClass
                  -> TyVarMap
                  -> Name
                  -> [Type]
                  -> [Name]
                  -> Q Match
makeInvmapForArgs iClass tvMap conName tys args =
    let mappedArgs :: [Q Exp]
        mappedArgs = zipWith (makeInvmapForArg iClass conName tvMap) tys args
     in match (conP conName $ map varP args)
              (normalB . appsE $ conE conName:mappedArgs)
              []

-- | Generates a lambda expression for invmap(2) for an argument of a constructor.
makeInvmapForArg :: InvariantClass
                 -> Name
                 -> TyVarMap
                 -> Type
                 -> Name
                 -> Q Exp
makeInvmapForArg iClass conName tvis ty tyExpName =
    appE (makeInvmapForType iClass conName tvis True ty) (varE tyExpName)

-- | Generates a lambda expression for invmap(2) for a specific type.
-- The generated expression depends on the number of type variables.
makeInvmapForType :: InvariantClass
                  -> Name
                  -> TyVarMap
                  -> Bool
                  -> Type
                  -> Q Exp
makeInvmapForType _ _ tvMap covariant (VarT tyName) =
    case Map.lookup tyName tvMap of
         Just (covMap, contraMap) ->
             varE $ if covariant then covMap else contraMap
         Nothing -> do -- Produce a lambda expression rather than id, addressing Trac #7436
             x <- newName "x"
             lamE [varP x] $ varE x
makeInvmapForType iClass conName tvMap covariant (SigT ty _) =
    makeInvmapForType iClass conName tvMap covariant ty
makeInvmapForType iClass conName tvMap covariant (ForallT _ _ ty)
    = makeInvmapForType iClass conName tvMap covariant ty
makeInvmapForType iClass conName tvMap covariant ty =
    let tyCon  :: Type
        tyArgs :: [Type]
        tyCon:tyArgs = unapplyTy ty

        numLastArgs :: Int
        numLastArgs = min (fromEnum iClass) (length tyArgs)

        lhsArgs, rhsArgs :: [Type]
        (lhsArgs, rhsArgs) = splitAt (length tyArgs - numLastArgs) tyArgs

        tyVarNames :: [Name]
        tyVarNames = Map.keys tvMap

        doubleMap :: (Bool -> Type -> Q Exp) -> [Type] -> [Q Exp]
        doubleMap _ []     = []
        doubleMap f (t:ts) = f covariant t : f (not covariant) t : doubleMap f ts

        mentionsTyArgs :: Bool
        mentionsTyArgs = any (`mentionsName` tyVarNames) tyArgs

        makeInvmapTuple :: ([Q Pat] -> Q Pat) -> ([Q Exp] -> Q Exp) -> Int -> Q Exp
        makeInvmapTuple mkTupP mkTupE n = do
            x  <- newName "x"
            xs <- newNameList "x" n
            lamE [varP x] $ caseE (varE x)
                [ match (mkTupP $ map varP xs)
                        (normalB . mkTupE $ zipWith makeInvmapTupleField tyArgs xs)
                        []
                ]

        makeInvmapTupleField :: Type -> Name -> Q Exp
        makeInvmapTupleField fieldTy fieldName =
            appE (makeInvmapForType iClass conName tvMap covariant fieldTy) $ varE fieldName

     in case tyCon of
          ArrowT | mentionsTyArgs ->
              let [argTy, resTy] = tyArgs
               in do x <- newName "x"
                     b <- newName "b"
                     lamE [varP x, varP b] $
                       makeInvmapForType iClass conName tvMap covariant resTy `appE` (varE x `appE`
                         (makeInvmapForType iClass conName tvMap (not covariant) argTy `appE` varE b))
#if MIN_VERSION_template_haskell(2,6,0)
          UnboxedTupleT n
            | n > 0 && mentionsTyArgs -> makeInvmapTuple unboxedTupP unboxedTupE n
#endif
          TupleT n
            | n > 0 && mentionsTyArgs -> makeInvmapTuple tupP tupE n
          _ -> do
              itf <- isTyFamily tyCon
              if any (`mentionsName` tyVarNames) lhsArgs || (itf && mentionsTyArgs)
                   then outOfPlaceTyVarError conName tyVarNames
                   else if any (`mentionsName` tyVarNames) rhsArgs
                        then appsE $
                             ( varE (invmapName (toEnum numLastArgs))
                             : doubleMap (makeInvmapForType iClass conName tvMap) rhsArgs
                             )
                        else do x <- newName "x"
                                lamE [varP x] $ varE x

-------------------------------------------------------------------------------
-- 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 :: InvariantClass
                  -- ^ Invariant or Invariant2
                  -> 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 iClass 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 - fromEnum iClass

        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 iClass 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 there are of kind *
    -- after substituting * for the dropped kind variables. If not, throw an error.
    unless (all hasKindStar droppedTysExpSubst) $
      derivingKindError iClass 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 iClass) 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 $ invariantClassName iClass)
                     $ 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 :: InvariantClass -> Type -> (Maybe Pred, [Name])
deriveConstraint iClass t
  | not (isTyVar t) = (Nothing, [])
  | otherwise = case hasKindVarChain 1 t of
      Just ns | iClass >= Invariant
              -> (Just (applyClass invariantTypeName tName), ns)
      _ -> case hasKindVarChain 2 t of
                Just ns | iClass == Invariant2
                        -> (Just (applyClass invariant2TypeName tName), 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 an Invariant 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 Invariant2 instance of:

          instance (Invariant f, Invariant2 g) => Invariant2 (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 Invariant2 n constraint and perform
        kind substitution as in the other case.
-}

-------------------------------------------------------------------------------
-- 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 :: InvariantClass -> Name -> a
derivingKindError iClass 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 "
    . showString (pprint . createKindChain $ fromEnum iClass)
    $ ""
  where
    className :: String
    className = nameBase $ invariantClassName iClass

-- | 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