TH.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE OverloadedRecordDot #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -Wno-name-shadowing #-}
{-# OPTIONS_GHC -Wno-unused-local-binds #-}

module Test.MockCat.TH
  ( showExp,
    expectByExpr,
    makeMock,
    makeMockWithOptions,
    MockOptions (..),
    options,
    makePartialMock,
    makePartialMockWithOptions,
  )
where

import Control.Monad (guard, unless)
import Control.Monad.State (get, modify)
import Control.Monad.Trans.Class (lift)
import Data.Data (Proxy (..))
import Data.Function ((&))
import Data.List (elemIndex, find, nub)
import Data.Maybe (fromMaybe, isJust)
import Data.Text (pack, splitOn, unpack)
import GHC.IO (unsafePerformIO)
import GHC.TypeLits (KnownSymbol, symbolVal)
import Language.Haskell.TH
  ( Cxt,
    Dec (..),
    Exp (..),
    Extension (..),
    Info (..),
    Lit (..),
    Name,
    Pat (..),
    Pred,
    Q,
    Quote (newName),
    TyVarBndr (..),
    Type (..),
    isExtEnabled,
    mkName,
    pprint,
    reify,
    Inline (NoInline),
    RuleMatch (FunLike),
    Phases (AllPhases),
  )
import Language.Haskell.TH.Lib
import Language.Haskell.TH.PprLib (Doc, hcat, parens, text)
import Language.Haskell.TH.Syntax (nameBase)
import Test.MockCat.Cons
import Test.MockCat.Mock
import Test.MockCat.MockT
import Test.MockCat.Param
import Unsafe.Coerce (unsafeCoerce)
import Prelude as P

showExp :: Q Exp -> Q String
showExp qexp = show . pprintExp <$> qexp

pprintExp :: Exp -> Doc
pprintExp (VarE name) = text (nameBase name)
pprintExp (ConE name) = text (nameBase name)
pprintExp (LitE lit) = pprintLit lit
pprintExp (AppE e1 e2) = parens $ hcat [pprintExp e1, text " ", pprintExp e2]
pprintExp (InfixE e1 e2 e3) = pprintInfixE e1 e2 e3
pprintExp (LamE pats body) = parens $ hcat [text "\\", pprintPats pats, text " -> ", pprintExp body]
pprintExp (TupE exps) = parens $ hcat (map (maybe (text "") pprintExp) exps)
pprintExp (ListE exps) = parens $ hcat (map pprintExp exps)
pprintExp (SigE e _) = pprintExp e
pprintExp x = text (pprint x)

pprintInfixE :: Maybe Exp -> Exp -> Maybe Exp -> Doc
pprintInfixE e1 e2 e3 =
  parens $
    hcat
      [ maybe (text "") pprintExp e1,
        maybe (text "") (const (text " ")) e1,
        pprintExp e2,
        text " ",
        maybe (text "") pprintExp e3
      ]

pprintPats :: [Pat] -> Doc
pprintPats = hcat . map pprintPat

pprintPat :: Pat -> Doc
pprintPat (VarP name) = text (nameBase name)
pprintPat p = text (pprint p)

pprintLit :: Lit -> Doc
pprintLit (IntegerL n) = text (show n)
pprintLit (RationalL r) = text (show r)
pprintLit (StringL s) = text (show s)
pprintLit (CharL c) = text (show c)
pprintLit l = text (pprint l)

-- | Create a conditional parameter based on @Q Exp@.
--
--  In applying a mock function, if the argument does not satisfy this condition, an error is raised.
--
--  The conditional expression is displayed in the error message.
expectByExpr :: Q Exp -> Q Exp
expectByExpr qf = do
  str <- showExp qf
  [|ExpectCondition $qf str|]

data MockType = Total | Partial deriving (Eq)

-- | Options for generating mocks.
--
--  - prefix: Stub function prefix
--  - suffix: stub function suffix
--  - implicitMonadicReturn: If True, the return value of the stub function is wrapped in a monad automatically.
--                           If Else, the return value of stub function is not wrapped in a monad,  so required explicitly return monadic values.
data MockOptions = MockOptions {prefix :: String, suffix :: String, implicitMonadicReturn :: Bool}

-- | Default Options.
--
--  Stub function names are prefixed with “_”.
options :: MockOptions
options = MockOptions {prefix = "_", suffix = "", implicitMonadicReturn = True}

-- | Create a mock of the typeclasses that returns a monad according to the `MockOptions`.
--
--  Given a monad type class, generate the following.
--
--  - MockT instance of the given typeclass
--  - A stub function corresponding to a function of the original class type.
-- The name of stub function is the name of the original function with a “_” appended.
--
--  @
--  class (Monad m) => FileOperation m where
--    writeFile :: FilePath -\> Text -\> m ()
--    readFile :: FilePath -\> m Text
--
--  makeMockWithOptions [t|FileOperation|] options { prefix = "stub_" }
--
--  it "test runMockT" do
--    result \<- runMockT do
--      stub_readFile $ "input.txt" |\> pack "content"
--      stub_writeFile $ "output.text" |\> pack "content" |\> ()
--      somethingProgram
--
--    result `shouldBe` ()
--  @
makeMockWithOptions :: Q Type -> MockOptions -> Q [Dec]
makeMockWithOptions = flip doMakeMock Total

-- | Create a mock of a typeclasses that returns a monad.
--
--  Given a monad type class, generate the following.
--
--  - MockT instance of the given typeclass
--  - A stub function corresponding to a function of the original class type.
-- The name of stub function is the name of the original function with a “_” appended.
--
--  The prefix can be changed.
--  In that case, use `makeMockWithOptions`.
--
--  @
--  class (Monad m) => FileOperation m where
--    writeFile :: FilePath -\> Text -\> m ()
--    readFile :: FilePath -\> m Text
--
--  makeMock [t|FileOperation|]
--
--  spec :: Spec
--  spec = do
--    it "test runMockT" do
--      result \<- runMockT do
--        _readFile $ "input.txt" |\> pack "content"
--        _writeFile $ "output.text" |\> pack "content" |\> ()
--        somethingProgram
--
--      result `shouldBe` ()
--  @
makeMock :: Q Type -> Q [Dec]
makeMock t = doMakeMock t Total options

-- | Create a partial mock of a typeclasses that returns a monad.
--
--  Given a monad type class, generate the following.
--
--  - MockT instance of the given typeclass
--  - A stub function corresponding to a function of the original class type.
-- The name of stub function is the name of the original function with a “_” appended.
--
--  For functions that are not stubbed in the test, the real function is used as appropriate for the context.
--
--  The prefix can be changed.
--  In that case, use `makePartialMockWithOptions`.
--
--  @
--  class Monad m => Finder a b m | a -> b, b -> a where
--    findIds :: m [a]
--    findById :: a -> m b
--
--  instance Finder Int String IO where
--    findIds = pure [1, 2, 3]
--    findById id = pure $ "{id: " <> show id <> "}"
--
--  findValue :: Finder a b m => m [b]
--  findValue = do
--    ids <- findIds
--    mapM findById ids
--
--  makePartialMock [t|Finder|]
--
--  spec :: Spec
--  spec = do
--    it "Use all real functions." do
--      values <- runMockT findValue
--      values `shouldBe` ["{id: 1}", "{id: 2}", "{id: 3}"]
--
--    it "Only findIds should be stubbed." do
--      values <- runMockT do
--        _findIds [1 :: Int, 2]
--        findValue
--      values `shouldBe` ["{id: 1}", "{id: 2}"]
--  @
makePartialMock :: Q Type -> Q [Dec]
makePartialMock t = doMakeMock t Partial options

-- | `makePartialMock` with options
makePartialMockWithOptions :: Q Type -> MockOptions -> Q [Dec]
makePartialMockWithOptions = flip doMakeMock Partial

doMakeMock :: Q Type -> MockType -> MockOptions -> Q [Dec]
doMakeMock t mockType options = do
  verifyExtension DataKinds
  verifyExtension FlexibleInstances
  verifyExtension FlexibleContexts

  ty <- t
  className <- getClassName <$> t

  reify className >>= \case
    ClassI (ClassD _ _ [] _ _) _ ->
      fail $ "A type parameter is required for class " <> show className
    ClassI (ClassD cxt _ typeVars _ decs) _ -> do
      monadVarNames <- getMonadVarNames cxt typeVars
      case nub monadVarNames of
        [] -> fail "Monad parameter not found."
        (monadVarName : rest)
          | length rest > 1 -> fail "Monad parameter must be unique."
          | otherwise -> makeMockDecs ty mockType className monadVarName cxt typeVars decs options
    t -> error $ "unsupported type: " <> show t

makeMockDecs :: Type -> MockType -> Name -> Name -> Cxt -> [TyVarBndr a] -> [Dec] -> MockOptions -> Q [Dec]
makeMockDecs ty mockType className monadVarName cxt typeVars decs options = do
  let classParamNames = filter (className /=) (getClassNames ty)
      newTypeVars = drop (length classParamNames) typeVars
      varAppliedTypes = zipWith (\t i -> VarAppliedType t (safeIndex classParamNames i)) (getTypeVarNames typeVars) [0 ..]

  instanceDec <-
    instanceD
      (createCxt cxt mockType className monadVarName newTypeVars varAppliedTypes)
      (createInstanceType ty monadVarName newTypeVars)
      (map (createInstanceFnDec mockType options) decs)

  mockFnDecs <- concat <$> mapM (createMockFnDec monadVarName varAppliedTypes options) decs

  pure $ instanceDec : mockFnDecs

getMonadVarNames :: Cxt -> [TyVarBndr a] -> Q [Name]
getMonadVarNames cxt typeVars = do
  let parentClassInfos = toClassInfos cxt

      typeVarNames = getTypeVarNames typeVars
      -- VarInfos (class names is empty)
      emptyClassVarInfos = map (`VarName2ClassNames` []) typeVarNames

  varInfos <- collectVarInfos parentClassInfos emptyClassVarInfos

  pure $ (\(VarName2ClassNames n _) -> n) <$> filterMonadicVarInfos varInfos

getTypeVarNames :: [TyVarBndr a] -> [Name]
getTypeVarNames = map getTypeVarName

getTypeVarName :: TyVarBndr a -> Name
getTypeVarName (PlainTV name _) = name
getTypeVarName (KindedTV name _ _) = name

toClassInfos :: Cxt -> [ClassName2VarNames]
toClassInfos = map toClassInfo

toClassInfo :: Pred -> ClassName2VarNames
toClassInfo (AppT t1 t2) = do
  let (ClassName2VarNames name vars) = toClassInfo t1
  ClassName2VarNames name (vars ++ getTypeNames t2)
toClassInfo (ConT name) = ClassName2VarNames name []
toClassInfo _ = error "Unsupported Type structure"

getTypeNames :: Pred -> [Name]
getTypeNames (VarT name) = [name]
getTypeNames (ConT name) = [name]
getTypeNames _ = []

collectVarInfos :: [ClassName2VarNames] -> [VarName2ClassNames] -> Q [VarName2ClassNames]
collectVarInfos classInfos = mapM (collectVarInfo classInfos)

collectVarInfo :: [ClassName2VarNames] -> VarName2ClassNames -> Q VarName2ClassNames
collectVarInfo classInfos (VarName2ClassNames vName classNames) = do
  varClassNames <- collectVarClassNames vName classInfos
  pure $ VarName2ClassNames vName (classNames ++ varClassNames)

collectVarClassNames :: Name -> [ClassName2VarNames] -> Q [Name]
collectVarClassNames varName classInfos = do
  let targetClassInfos = filterClassInfo varName classInfos
  concat <$> mapM (collectVarClassNames_ varName) targetClassInfos

collectVarClassNames_ :: Name -> ClassName2VarNames -> Q [Name]
collectVarClassNames_ name (ClassName2VarNames cName vNames) = do
  case elemIndex name vNames of
    Nothing -> pure []
    Just i -> do
      ClassI (ClassD cxt _ typeVars _ _) _ <- reify cName
      let -- type variable names
          typeVarNames = getTypeVarNames typeVars
          -- type variable name of same position
          typeVarName = typeVarNames !! i
          -- parent class information
          parentClassInfos = toClassInfos cxt

      case parentClassInfos of
        [] -> pure [cName]
        _ -> do
          result <- concat <$> mapM (collectVarClassNames_ typeVarName) parentClassInfos
          pure $ cName : result

filterClassInfo :: Name -> [ClassName2VarNames] -> [ClassName2VarNames]
filterClassInfo name = filter (hasVarName name)
  where
    hasVarName :: Name -> ClassName2VarNames -> Bool
    hasVarName name (ClassName2VarNames _ varNames) = name `elem` varNames

filterMonadicVarInfos :: [VarName2ClassNames] -> [VarName2ClassNames]
filterMonadicVarInfos = filter hasMonadInVarInfo

hasMonadInVarInfo :: VarName2ClassNames -> Bool
hasMonadInVarInfo (VarName2ClassNames _ classNames) = ''Monad `elem` classNames

createCxt :: [Pred] -> MockType -> Name -> Name -> [TyVarBndr a] -> [VarAppliedType] -> Q [Pred]
createCxt cxt mockType className monadVarName tyVars varAppliedTypes = do
  newCxt <- mapM (createPred monadVarName) cxt

  monadAppT <- appT (conT ''Monad) (varT monadVarName)

  let hasMonad = P.any (\(ClassName2VarNames c _) -> c == ''Monad) $ toClassInfos newCxt

  pure $ case mockType of
    Total -> newCxt ++ ([monadAppT | not hasMonad])
    Partial -> do
      let classAppT = constructClassAppT className $ toVarTs tyVars
          varAppliedClassAppT = updateType classAppT varAppliedTypes
      newCxt ++ ([monadAppT | not hasMonad]) ++ [varAppliedClassAppT]

toVarTs :: [TyVarBndr a] -> [Type]
toVarTs tyVars = VarT <$> getTypeVarNames tyVars

constructClassAppT :: Name -> [Type] -> Type
constructClassAppT className = foldl AppT (ConT className)

createPred :: Name -> Pred -> Q Pred
createPred monadVarName a@(AppT t@(ConT ty) b@(VarT varName))
  | monadVarName == varName && ty == ''Monad = pure a
  | monadVarName == varName && ty /= ''Monad = appT (pure t) (appT (conT ''MockT) (varT varName))
  | otherwise = appT (createPred monadVarName t) (pure b)
createPred monadVarName (AppT ty a@(VarT varName))
  | monadVarName == varName = appT (pure ty) (appT (conT ''MockT) (varT varName))
  | otherwise = appT (createPred monadVarName ty) (pure a)
createPred monadVarName (AppT ty1 ty2) = appT (createPred monadVarName ty1) (createPred monadVarName ty2)
createPred _ ty = pure ty

createInstanceType :: Type -> Name -> [TyVarBndr a] -> Q Type
createInstanceType className monadName tvbs = do
  types <- mapM (tyVarBndrToType monadName) tvbs
  pure $ foldl AppT className types

tyVarBndrToType :: Name -> TyVarBndr a -> Q Type
tyVarBndrToType monadName (PlainTV name _)
  | monadName == name = appT (conT ''MockT) (varT monadName)
  | otherwise = varT name
tyVarBndrToType monadName (KindedTV name _ _)
  | monadName == name = appT (conT ''MockT) (varT monadName)
  | otherwise = varT name

createInstanceFnDec :: MockType -> MockOptions -> Dec -> Q Dec
createInstanceFnDec mockType options (SigD fnName funType) = do
  names <- sequence $ typeToNames funType
  let r = mkName "result"
      params = varP <$> names
      args = varE <$> names
      fnNameStr = createFnName fnName options

      fnBody = case mockType of
        Total -> generateInstanceMockFnBody fnNameStr args r options
        Partial -> generateInstanceRealFnBody fnName fnNameStr args r options

      fnClause = clause params (normalB fnBody) []
  funD fnName [fnClause]
createInstanceFnDec _ _ dec = fail $ "unsuported dec: " <> pprint dec

generateInstanceMockFnBody :: String -> [Q Exp] -> Name -> MockOptions -> Q Exp
generateInstanceMockFnBody fnNameStr args r options = do
  returnExp <- if options.implicitMonadicReturn
    then [| pure $(varE r) |]
    else [| lift $(varE r) |]
  [|
    MockT $ do
      defs <- get
      let mock =
            defs
              & findParam (Proxy :: Proxy $(litT (strTyLit fnNameStr)))
              & fromMaybe (error $ "no answer found stub function `" ++ fnNameStr ++ "`.")
          $(bangP $ varP r) = $(generateStubFn args [|mock|])
      $(pure returnExp)
    |]

generateInstanceRealFnBody :: Name -> String -> [Q Exp] -> Name -> MockOptions -> Q Exp
generateInstanceRealFnBody fnName fnNameStr args r options = do
  returnExp <- if options.implicitMonadicReturn
    then [| pure $(varE r) |]
    else [| lift $(varE r) |]
  [|
    MockT $ do
      defs <- get
      case findParam (Proxy :: Proxy $(litT (strTyLit fnNameStr))) defs of
        Just mock -> do
          let $(bangP $ varP r) = $(generateStubFn args [|mock|])
          $(pure returnExp)
        Nothing -> lift $ $(foldl appE (varE fnName) args)
    |]

generateStubFn :: [Q Exp] -> Q Exp -> Q Exp
generateStubFn [] = $([|generateConstantStubFn|])
generateStubFn args = $([|generateNotConstantsStubFn args|])

generateNotConstantsStubFn :: [Q Exp] -> Q Exp -> Q Exp
generateNotConstantsStubFn args mock = foldl appE [|stubFn $(mock)|] args

generateConstantStubFn :: Q Exp -> Q Exp
generateConstantStubFn mock = [|stubFn $(mock)|]

createMockFnDec :: Name -> [VarAppliedType] -> MockOptions -> Dec -> Q [Dec]
createMockFnDec monadVarName varAppliedTypes options (SigD sigFnName ty) = do
  let fnName = createFnName sigFnName options
      mockFnName = mkName fnName
      params = mkName "p"
      updatedType = updateType ty varAppliedTypes
      fnType = if options.implicitMonadicReturn
        then createMockBuilderFnType monadVarName updatedType
        else updatedType

  fnDecs <- if isNotConstantFunctionType ty then
    doCreateMockFnDecs fnName mockFnName params fnType monadVarName updatedType
  else
    if options.implicitMonadicReturn then
      doCreateConstantMockFnDecs fnName mockFnName fnType monadVarName
    else
      doCreateEmptyVerifyParamMockFnDecs fnName mockFnName params fnType monadVarName updatedType

  pragmaDec <- pragInlD mockFnName NoInline FunLike AllPhases

  pure $ pragmaDec : fnDecs

createMockFnDec _ _ _ dec = fail $ "unsupport dec: " <> pprint dec

doCreateMockFnDecs :: (Quote m) => String -> Name -> Name -> Type -> Name -> Type -> m [Dec]
doCreateMockFnDecs funNameStr mockFunName params funType monadVarName updatedType = do
  newFunSig <- do
    let verifyParams = createMockBuilderVerifyParams updatedType
    sigD
      mockFunName
      [t|
        (MockBuilder $(varT params) ($(pure funType)) ($(pure verifyParams)), Monad $(varT monadVarName)) =>
        $(varT params) ->
        MockT $(varT monadVarName) ()
        |]

  createMockFn <- [|createNamedMock|]

  mockBody <- createMockBody funNameStr createMockFn
  newFun <- funD mockFunName [clause [varP $ mkName "p"] (normalB (pure mockBody)) []]

  pure $ newFunSig : [newFun]

doCreateConstantMockFnDecs :: (Quote m) => String -> Name -> Type -> Name -> m [Dec]
doCreateConstantMockFnDecs funNameStr mockFunName ty monadVarName = do
  newFunSig <- sigD mockFunName [t|(Monad $(varT monadVarName)) => $(pure ty) -> MockT $(varT monadVarName) ()|]
  createMockFn <- [|createNamedConstantMock|]
  mockBody <- createMockBody funNameStr createMockFn
  newFun <- funD mockFunName [clause [varP $ mkName "p"] (normalB (pure mockBody)) []]
  pure $ newFunSig : [newFun]

-- MockBuilder constraints, but verifyParams are empty.
doCreateEmptyVerifyParamMockFnDecs :: (Quote m) => String -> Name -> Name -> Type -> Name -> Type -> m [Dec]
doCreateEmptyVerifyParamMockFnDecs funNameStr mockFunName params funType monadVarName updatedType = do
  newFunSig <- do
    let verifyParams = createMockBuilderVerifyParams updatedType
    sigD
      mockFunName
      [t|
        (MockBuilder $(varT params) ($(pure funType)) (), Monad $(varT monadVarName)) =>
        $(varT params) ->
        MockT $(varT monadVarName) ()
        |]

  createMockFn <- [|createNamedMock|]

  mockBody <- createMockBody funNameStr createMockFn
  newFun <- funD mockFunName [clause [varP $ mkName "p"] (normalB (pure mockBody)) []]

  pure $ newFunSig : [newFun]

createMockBody :: (Quote m) => String -> Exp -> m Exp
createMockBody funNameStr createMockFn =
  [|
    MockT $
      modify
        ( ++
            [ Definition
                (Proxy :: Proxy $(litT (strTyLit funNameStr)))
                (unsafePerformIO $ $(pure createMockFn) $(litE (stringL funNameStr)) p)
                shouldApplyToAnything
            ]
        )
    |]

isNotConstantFunctionType :: Type -> Bool
isNotConstantFunctionType (AppT (AppT ArrowT _) _) = True
isNotConstantFunctionType (AppT t1 t2) = isNotConstantFunctionType t1 || isNotConstantFunctionType t2
isNotConstantFunctionType (TupleT _) = False
isNotConstantFunctionType (ForallT _ _ t) = isNotConstantFunctionType t
isNotConstantFunctionType _ = False

updateType :: Type -> [VarAppliedType] -> Type
updateType (AppT (VarT v1) (VarT v2)) varAppliedTypes = do
  let x = maybe (VarT v1) ConT (findClass v1 varAppliedTypes)
      y = maybe (VarT v2) ConT (findClass v2 varAppliedTypes)
  AppT x y
updateType ty _ = ty

hasClass :: Name -> [VarAppliedType] -> Bool
hasClass varName = P.any (\(VarAppliedType v c) -> (v == varName) && isJust c)

findClass :: Name -> [VarAppliedType] -> Maybe Name
findClass varName types = do
  guard $ hasClass varName types
  (VarAppliedType _ c) <- find (\(VarAppliedType v _) -> v == varName) types
  c

createFnName :: Name -> MockOptions -> String
createFnName funName options = do
  options.prefix <> nameBase funName <> options.suffix

createMockBuilderFnType :: Name -> Type -> Type
createMockBuilderFnType monadVarName a@(AppT (VarT var) ty)
  | monadVarName == var = ty
  | otherwise = a
createMockBuilderFnType monadVarName (AppT ty ty2) = AppT ty (createMockBuilderFnType monadVarName ty2)
createMockBuilderFnType monadVarName (ForallT _ _ ty) = createMockBuilderFnType monadVarName ty
createMockBuilderFnType _ ty = ty

createMockBuilderVerifyParams :: Type -> Type
createMockBuilderVerifyParams (AppT (AppT ArrowT ty) (AppT (VarT _) _)) = AppT (ConT ''Param) ty
createMockBuilderVerifyParams (AppT (AppT ArrowT ty) ty2) =
  AppT (AppT (ConT ''(:>)) (AppT (ConT ''Param) ty)) (createMockBuilderVerifyParams ty2)
createMockBuilderVerifyParams (AppT (VarT _) (ConT c)) = AppT (ConT ''Param) (ConT c)
createMockBuilderVerifyParams (ForallT _ _ ty) = createMockBuilderVerifyParams ty
createMockBuilderVerifyParams a = a

findParam :: (KnownSymbol sym) => Proxy sym -> [Definition] -> Maybe a
findParam pa definitions = do
  let definition = find (\(Definition s _ _) -> symbolVal s == symbolVal pa) definitions
  fmap (\(Definition _ mock _) -> unsafeCoerce mock) definition

typeToNames :: Type -> [Q Name]
typeToNames (AppT (AppT ArrowT _) t2) = newName "a" : typeToNames t2
typeToNames (ForallT _ _ ty) = typeToNames ty
typeToNames _ = []

getClassName :: Type -> Name
getClassName (ConT name) = name
getClassName (AppT ty _) = getClassName ty
getClassName d = error $ "unsupported class definition: " <> show d

getClassNames :: Type -> [Name]
getClassNames (AppT (ConT name1) (ConT name2)) = [name1, name2]
getClassNames (AppT ty (ConT name)) = getClassNames ty ++ [name]
getClassNames (AppT ty1 ty2) = getClassNames ty1 ++ getClassNames ty2
getClassNames _ = []

data ClassName2VarNames = ClassName2VarNames Name [Name]

instance Show ClassName2VarNames where
  show (ClassName2VarNames cName varNames) = showClassDef cName varNames

data VarName2ClassNames = VarName2ClassNames Name [Name]

instance Show VarName2ClassNames where
  show (VarName2ClassNames varName classNames) = show varName <> " class is " <> unwords (showClassName <$> classNames)

data VarAppliedType = VarAppliedType {name :: Name, appliedClassName :: Maybe Name}
  deriving (Show)

showClassName :: Name -> String
showClassName n = splitLast "." $ show n

showClassDef :: Name -> [Name] -> String
showClassDef className varNames = showClassName className <> " " <> unwords (show <$> varNames)

splitLast :: String -> String -> String
splitLast delimiter = last . split delimiter

split :: String -> String -> [String]
split delimiter str = unpack <$> splitOn (pack delimiter) (pack str)

safeIndex :: [a] -> Int -> Maybe a
safeIndex [] _ = Nothing
safeIndex (x : _) 0 = Just x
safeIndex (_ : xs) n
  | n < 0 = Nothing
  | otherwise = safeIndex xs (n - 1)

verifyExtension :: Extension -> Q ()
verifyExtension e = isExtEnabled e >>= flip unless (fail $ "Language extensions `" ++ show e ++ "` is required.")