{-# LANGUAGE CPP #-}
{-# LANGUAGE PatternGuards #-}
#ifdef TRUSTWORTHY
{-# LANGUAGE Trustworthy #-}
#endif

#ifndef MIN_VERSION_template_haskell
#define MIN_VERSION_template_haskell(x,y,z) (defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 706)
#endif
-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Lens.TH
-- Copyright   :  (C) 2012-14 Edward Kmett, Michael Sloan
-- License     :  BSD-style (see the file LICENSE)
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  experimental
-- Portability :  non-portable
--
-----------------------------------------------------------------------------
module Control.Lens.TH
  (
  -- * Constructing Lenses Automatically
    makeLenses, makeLensesFor
  , makeClassy, makeClassyFor, makeClassy_
  , makePrisms
  , makeClassyPrisms
  , makeWrapped
  , makeFields
  , makeFieldsWith
  -- * Constructing Lenses Given a Declaration Quote
  , declareLenses, declareLensesFor
  , declareClassy, declareClassyFor
  , declarePrisms
  , declareWrapped
  , declareFields
  -- * Configuring Lenses
  , makeLensesWith
  , declareLensesWith
  , defaultFieldRules
  , camelCaseFields
  , underscoreFields
  , abbreviatedFields
  , LensRules
  , DefName(..)
  , lensRules
  , lensRulesFor
  , classyRules
  , classyRules_
  , lensField
  , lensClass
  , simpleLenses
  , createClass
  , generateSignatures
  ) where

import Control.Applicative
#if !(MIN_VERSION_template_haskell(2,7,0))
import Control.Monad (ap)
#endif
import qualified Control.Monad.Trans as Trans
import Control.Monad.Trans.Writer
import Control.Lens.Fold
import Control.Lens.Getter
import Control.Lens.Lens
import Control.Lens.Setter
import Control.Lens.Tuple
import Control.Lens.Traversal
import Control.Lens.Internal.TH
import Control.Lens.Internal.FieldTH
import Control.Lens.Internal.PrismTH
import Data.Char (toLower, isUpper)
import Data.Foldable hiding (concat, any)
import Data.List as List
import Data.Map as Map hiding (toList,map,filter)
import Data.Maybe (maybeToList)
import Data.Monoid
import Data.Set as Set hiding (toList,map,filter)
import Data.Set.Lens
import Data.Traversable hiding (mapM)
import Language.Haskell.TH
import Language.Haskell.TH.Lens

#ifdef HLINT
{-# ANN module "HLint: ignore Eta reduce" #-}
{-# ANN module "HLint: ignore Use fewer imports" #-}
{-# ANN module "HLint: ignore Use foldl" #-}
#endif

-- | Generate "simple" optics even when type-changing optics are possible.
-- (e.g. 'Lens'' instead of 'Lens')
simpleLenses :: Lens' LensRules Bool
simpleLenses f r = fmap (\x -> r { _simpleLenses = x}) (f (_simpleLenses r))

-- | Indicate whether or not to supply the signatures for the generated
-- lenses.
--
-- Disabling this can be useful if you want to provide a more restricted type
-- signature or if you want to supply hand-written haddocks.
generateSignatures :: Lens' LensRules Bool
generateSignatures f r =
  fmap (\x -> r { _generateSigs = x}) (f (_generateSigs r))

-- | Create the class if the constructor is 'Control.Lens.Type.Simple' and the
-- 'lensClass' rule matches.
createClass :: Lens' LensRules Bool
createClass f r =
  fmap (\x -> r { _generateClasses = x}) (f (_generateClasses r))

-- | 'Lens'' to access the convention for naming fields in our 'LensRules'.
--
-- Defaults to stripping the _ off of the field name, lowercasing the name, and
-- skipping the field if it doesn't start with an '_'. The field naming rule
-- provides the names of all fields in the type as well as the current field.
-- This extra generality enables field naming conventions that depend on the
-- full set of names in a type.
--
-- The field naming rule has access to the type name, the names of all the field
-- of that type (including the field being named), and the name of the field
-- being named.
--
-- TypeName -> FieldNames -> FieldName -> DefinitionNames
lensField :: Lens' LensRules (Name -> [Name] -> Name -> [DefName])
lensField f r = fmap (\x -> r { _fieldToDef = x}) (f (_fieldToDef r))

-- | Retrieve options such as the name of the class and method to put in it to
-- build a class around monomorphic data types. "Classy" lenses are generated
-- when this naming convention is provided.
-- TypeName -> Maybe (ClassName, MainMethodName)
lensClass :: Lens' LensRules (Name -> Maybe (Name, Name))
lensClass f r = fmap (\x -> r { _classyLenses = x }) (f (_classyLenses r))

-- | Rules for making fairly simple partial lenses, ignoring the special cases
-- for isomorphisms and traversals, and not making any classes.
lensRules :: LensRules
lensRules = LensRules
  { _simpleLenses    = False
  , _generateSigs    = True
  , _generateClasses = False
  , _allowIsos       = True
  , _classyLenses    = const Nothing
  , _fieldToDef      = \_ _ n ->
       case nameBase n of
         '_':x:xs -> [TopName (mkName (toLower x:xs))]
         _        -> []
  }

-- | Construct a 'LensRules' value for generating top-level definitions
-- using the given map from field names to definition names.
lensRulesFor ::
  [(String, String)] {- ^ [(Field Name, Definition Name)] -} ->
  LensRules
lensRulesFor fields = lensRules & lensField .~ mkNameLookup fields

mkNameLookup :: [(String,String)] -> Name -> [Name] -> Name -> [DefName]
mkNameLookup kvs _ _ field =
  [ TopName (mkName v) | (k,v) <- kvs, k == nameBase field]

-- | Rules for making lenses and traversals that precompose another 'Lens'.
classyRules :: LensRules
classyRules = LensRules
  { _simpleLenses    = True
  , _generateSigs    = True
  , _generateClasses = True
  , _allowIsos       = False -- generating Isos would hinder "subtyping"
  , _classyLenses    = \n ->
        case nameBase n of
          x:xs -> Just (mkName ("Has" ++ x:xs), mkName (toLower x:xs))
          []   -> Nothing
  , _fieldToDef      = \_ _ n ->
        case nameBase n of
          '_':x:xs -> [TopName (mkName (toLower x:xs))]
          _        -> []
  }

-- | Rules for making lenses and traversals that precompose another 'Lens'
-- using a custom function for naming the class, main class method, and a
-- mapping from field names to definition names.
classyRulesFor
  :: (String -> Maybe (String, String)) {- ^ Type Name -> Maybe (Class Name, Method Name) -} ->
  [(String, String)] {- ^ [(Field Name, Method Name)] -} ->
  LensRules
classyRulesFor classFun fields = classyRules
  & lensClass .~ (over (mapped . both) mkName . classFun . nameBase)
  & lensField .~ mkNameLookup fields

classyRules_ :: LensRules
classyRules_
  = classyRules & lensField .~ \_ _ n -> [TopName (mkName ('_':nameBase n))]

-- | Build lenses (and traversals) with a sensible default configuration.
--
-- /e.g./
--
-- @
-- data FooBar
--   = Foo { _x, _y :: 'Int' }
--   | Bar { _x :: 'Int' }
-- 'makeLenses' ''FooBar
-- @
--
-- will create
--
-- @
-- x :: 'Lens'' FooBar 'Int'
-- x f (Foo a b) = (\\a\' -> Foo a\' b) \<$\> f a
-- x f (Bar a)   = Bar \<$\> f a
-- y :: 'Traversal'' FooBar 'Int'
-- y f (Foo a b) = (\\b\' -> Foo a  b\') \<$\> f b
-- y _ c\@(Bar _) = pure c
-- @
--
-- @
-- 'makeLenses' = 'makeLensesWith' 'lensRules'
-- @
makeLenses :: Name -> DecsQ
makeLenses = makeFieldOptics lensRules

-- | Make lenses and traversals for a type, and create a class when the
-- type has no arguments.
--
-- /e.g./
--
-- @
-- data Foo = Foo { _fooX, _fooY :: 'Int' }
-- 'makeClassy' ''Foo
-- @
--
-- will create
--
-- @
-- class HasFoo t where
--   foo :: 'Lens'' t Foo
--   fooX :: 'Lens'' t 'Int'
--   fooX = foo . go where go f (Foo x y) = (\\x\' -> Foo x' y) \<$\> f x
--   fooY :: 'Lens'' t 'Int'
--   fooY = foo . go where go f (Foo x y) = (\\y\' -> Foo x y') \<$\> f y
-- instance HasFoo Foo where
--   foo = id
-- @
--
-- @
-- 'makeClassy' = 'makeLensesWith' 'classyRules'
-- @
makeClassy :: Name -> DecsQ
makeClassy = makeFieldOptics classyRules

-- | Make lenses and traversals for a type, and create a class when the type
-- has no arguments.  Works the same as 'makeClassy' except that (a) it
-- expects that record field names do not begin with an underscore, (b) all
-- record fields are made into lenses, and (c) the resulting lens is prefixed
-- with an underscore.
makeClassy_ :: Name -> DecsQ
makeClassy_ = makeFieldOptics classyRules_

-- | Derive lenses and traversals, specifying explicit pairings
-- of @(fieldName, lensName)@.
--
-- If you map multiple names to the same label, and it is present in the same
-- constructor then this will generate a 'Traversal'.
--
-- /e.g./
--
-- @
-- 'makeLensesFor' [(\"_foo\", \"fooLens\"), (\"baz\", \"lbaz\")] ''Foo
-- 'makeLensesFor' [(\"_barX\", \"bar\"), (\"_barY\", \"bar\")] ''Bar
-- @
makeLensesFor :: [(String, String)] -> Name -> DecsQ
makeLensesFor fields = makeFieldOptics (lensRulesFor fields)

-- | Derive lenses and traversals, using a named wrapper class, and
-- specifying explicit pairings of @(fieldName, traversalName)@.
--
-- Example usage:
--
-- @
-- 'makeClassyFor' \"HasFoo\" \"foo\" [(\"_foo\", \"fooLens\"), (\"bar\", \"lbar\")] ''Foo
-- @
makeClassyFor :: String -> String -> [(String, String)] -> Name -> DecsQ
makeClassyFor clsName funName fields = makeFieldOptics $
  classyRulesFor (const (Just (clsName, funName))) fields

-- | Build lenses with a custom configuration.
makeLensesWith :: LensRules -> Name -> DecsQ
makeLensesWith = makeFieldOptics



-- | Make lenses for all records in the given declaration quote. All record
-- syntax in the input will be stripped off.
--
-- /e.g./
--
-- @
-- declareLenses [d|
--   data Foo = Foo { fooX, fooY :: 'Int' }
--     deriving 'Show'
--   |]
-- @
--
-- will create
--
-- @
-- data Foo = Foo 'Int' 'Int' deriving 'Show'
-- fooX, fooY :: 'Lens'' Foo Int
-- @
declareLenses :: DecsQ -> DecsQ
declareLenses
  = declareLensesWith
  $ lensRules
  & lensField .~ \_ _ n -> [TopName n]

-- | Similar to 'makeLensesFor', but takes a declaration quote.
declareLensesFor :: [(String, String)] -> DecsQ -> DecsQ
declareLensesFor fields
  = declareLensesWith
  $ lensRulesFor fields
  & lensField .~ \_ _ n -> [TopName n]

-- | For each record in the declaration quote, make lenses and traversals for
-- it, and create a class when the type has no arguments. All record syntax
-- in the input will be stripped off.
--
-- /e.g./
--
-- @
-- declareClassy [d|
--   data Foo = Foo { fooX, fooY :: 'Int' }
--     deriving 'Show'
--   |]
-- @
--
-- will create
--
-- @
-- data Foo = Foo 'Int' 'Int' deriving 'Show'
-- class HasFoo t where
--   foo :: 'Lens'' t Foo
-- instance HasFoo Foo where foo = 'id'
-- fooX, fooY :: HasFoo t => 'Lens'' t 'Int'
-- @
declareClassy :: DecsQ -> DecsQ
declareClassy
  = declareLensesWith
  $ classyRules
  & lensField .~ \_ _ n -> [TopName n]

-- | Similar to 'makeClassyFor', but takes a declaration quote.
declareClassyFor ::
  [(String, (String, String))] -> [(String, String)] -> DecsQ -> DecsQ
declareClassyFor classes fields
  = declareLensesWith
  $ classyRulesFor (`Prelude.lookup`classes) fields
  & lensField .~ \_ _ n -> [TopName n]

-- | Generate a 'Prism' for each constructor of each data type.
--
-- /e.g./
--
-- @
-- declarePrisms [d|
--   data Exp = Lit Int | Var String | Lambda{ bound::String, body::Exp }
--   |]
-- @
--
-- will create
--
-- @
-- data Exp = Lit Int | Var String | Lambda { bound::String, body::Exp }
-- _Lit :: 'Prism'' Exp Int
-- _Var :: 'Prism'' Exp String
-- _Lambda :: 'Prism'' Exp (String, Exp)
-- @
declarePrisms :: DecsQ -> DecsQ
declarePrisms = declareWith $ \dec -> do
  emit =<< Trans.lift (makeDecPrisms True dec)
  return dec

-- | Build 'Wrapped' instance for each newtype.
declareWrapped :: DecsQ -> DecsQ
declareWrapped = declareWith $ \dec -> do
  maybeDecs <- Trans.lift (makeWrappedForDec dec)
  forM_ maybeDecs emit
  return dec

-- | @ declareFields = 'declareLensesWith' 'defaultFieldRules' @
declareFields :: DecsQ -> DecsQ
declareFields = declareLensesWith defaultFieldRules

-- | Declare lenses for each records in the given declarations, using the
-- specified 'LensRules'. Any record syntax in the input will be stripped
-- off.
declareLensesWith :: LensRules -> DecsQ -> DecsQ
declareLensesWith rules = declareWith $ \dec -> do
  emit =<< Trans.lift (makeFieldOpticsForDec rules dec)
  return $ stripFields dec

-----------------------------------------------------------------------------
-- Internal TH Implementation
-----------------------------------------------------------------------------

-- | Transform @NewtypeD@s declarations to @DataD@s and @NewtypeInstD@s to
-- @DataInstD@s.
deNewtype :: Dec -> Dec
deNewtype (NewtypeD ctx tyName args c d) = DataD ctx tyName args [c] d
deNewtype (NewtypeInstD ctx tyName args c d) = DataInstD ctx tyName args [c] d
deNewtype d = d


-- | Given a set of names, build a map from those names to a set of fresh names
-- based on them.
freshMap :: Set Name -> Q (Map Name Name)
freshMap ns = Map.fromList <$> for (toList ns) (\ n -> (,) n <$> newName (nameBase n))


apps :: Type -> [Type] -> Type
apps = Prelude.foldl AppT


makeDataDecl :: Dec -> Maybe DataDecl
makeDataDecl dec = case deNewtype dec of
  DataD ctx tyName args cons _ -> Just DataDecl
    { dataContext = ctx
    , tyConName = Just tyName
    , dataParameters = args
    , fullType = apps $ ConT tyName
    , constructors = cons
    }
  DataInstD ctx familyName args cons _ -> Just DataDecl
    { dataContext = ctx
    , tyConName = Nothing
    , dataParameters = map PlainTV vars
    , fullType = \tys -> apps (ConT familyName) $
        substType (Map.fromList $ zip vars tys) args
    , constructors = cons
    }
    where
      -- The list of "type parameters" to a data family instance is not
      -- explicitly specified in the source. Here we define it to be
      -- the set of distinct type variables that appear in the LHS. e.g.
      --
      -- data instance F a Int (Maybe (a, b)) = G
      --
      -- has 2 type parameters: a and b.
      vars = toList $ setOf typeVars args
  _ -> Nothing

-- | A data, newtype, data instance or newtype instance declaration.
data DataDecl = DataDecl
  { dataContext :: Cxt -- ^ Datatype context.
  , tyConName :: Maybe Name
    -- ^ Type constructor name, or Nothing for a data family instance.
  , dataParameters :: [TyVarBndr] -- ^ List of type parameters
  , fullType :: [Type] -> Type
    -- ^ Create a concrete record type given a substitution to
    -- 'detaParameters'.
  , constructors :: [Con] -- ^ Constructors
  -- , derivings :: [Name] -- currently not needed
  }



-- | Build 'Wrapped' instance for a given newtype
makeWrapped :: Name -> DecsQ
makeWrapped nm = do
  inf <- reify nm
  case inf of
    TyConI decl -> do
      maybeDecs <- makeWrappedForDec decl
      maybe (fail "makeWrapped: Unsupported data type") return maybeDecs
    _ -> fail "makeWrapped: Expected the name of a newtype or datatype"

makeWrappedForDec :: Dec -> Q (Maybe [Dec])
makeWrappedForDec decl = case makeDataDecl decl of
  Just dataDecl | [con]   <- constructors dataDecl
                , [field] <- toListOf (conFields._2) con
    -> do wrapped   <- makeWrappedInstance dataDecl con field
          rewrapped <- makeRewrappedInstance dataDecl
          return (Just [rewrapped, wrapped])
  _ -> return Nothing

makeRewrappedInstance :: DataDecl -> DecQ
makeRewrappedInstance dataDecl = do

   t <- varT <$> newName "t"

   let typeArgs = map (view name) (dataParameters dataDecl)

   typeArgs' <- do
     m <- freshMap (Set.fromList typeArgs)
     return (substTypeVars m typeArgs)

       -- Con a b c...
   let appliedType  = return (fullType dataDecl (map VarT typeArgs))

       -- Con a' b' c'...
       appliedType' = return (fullType dataDecl (map VarT typeArgs'))

       -- Con a' b' c'... ~ t
#if MIN_VERSION_template_haskell(2,10,0)
       eq = AppT. AppT EqualityT <$> appliedType' <*> t
#else
       eq = equalP appliedType' t
#endif

       -- Rewrapped (Con a b c...) t
       klass = conT rewrappedTypeName `appsT` [appliedType, t]

   -- instance (Con a' b' c'... ~ t) => Rewrapped (Con a b c...) t
   instanceD (cxt [eq]) klass []

makeWrappedInstance :: DataDecl-> Con -> Type -> DecQ
makeWrappedInstance dataDecl con fieldType = do

  let conName = view name con
  let typeArgs = toListOf typeVars (dataParameters dataDecl)

  -- Con a b c...
  let appliedType  = fullType dataDecl (map VarT typeArgs)

  -- type Unwrapped (Con a b c...) = $fieldType
  let unwrappedATF = tySynInstD' unwrappedTypeName [return appliedType] (return fieldType)

  -- Wrapped (Con a b c...)
  let klass        = conT wrappedTypeName `appT` return appliedType

  -- _Wrapped' = iso (\(Con x) -> x) Con
  let wrapFun      = conE conName
  let unwrapFun    = newName "x" >>= \x -> lam1E (conP conName [varP x]) (varE x)
  let body         = appsE [varE isoValName, unwrapFun, wrapFun]
  let isoMethod    = funD _wrapped'ValName [clause [] (normalB body) []]

  -- instance Wrapped (Con a b c...) where
  --   type Unwrapped (Con a b c...) = fieldType
  --   _Wrapped' = iso (\(Con x) -> x) Con
  instanceD (cxt []) klass [unwrappedATF, isoMethod]

#if !(MIN_VERSION_template_haskell(2,7,0))
-- | The orphan instance for old versions is bad, but programming without 'Applicative' is worse.
instance Applicative Q where
  pure = return
  (<*>) = ap
#endif

overHead :: (a -> a) -> [a] -> [a]
overHead _ []     = []
overHead f (x:xs) = f x : xs

-- | Field rules for fields in the form @ _prefix_fieldname @
underscoreFields :: LensRules
underscoreFields = defaultFieldRules & lensField .~ underscoreNamer

underscoreNamer :: Name -> [Name] -> Name -> [DefName]
underscoreNamer _ _ field = maybeToList $ do
  _      <- prefix field'
  method <- niceLens
  cls    <- classNaming
  return (MethodName (mkName cls) (mkName method))
  where
    field' = nameBase field
    prefix ('_':xs) | '_' `List.elem` xs = Just (takeWhile (/= '_') xs)
    prefix _                             = Nothing
    niceLens    = prefix field' <&> \n -> drop (length n + 2) field'
    classNaming = niceLens <&> ("Has_" ++)

-- | Field rules for fields in the form @ prefixFieldname or _prefixFieldname @
-- If you want all fields to be lensed, then there is no reason to use an @_@ before the prefix.
-- If any of the record fields leads with an @_@ then it is assume a field without an @_@ should not have a lens created.
--
-- __Note__: The @prefix@ must be the same as the typename (with the first
-- letter lowercased). This is a change from lens versions before lens 4.5.
-- If you want the old behaviour, use 'makeLensesWith' 'abbreviatedFields'
camelCaseFields :: LensRules
camelCaseFields = defaultFieldRules

camelCaseNamer :: Name -> [Name] -> Name -> [DefName]
camelCaseNamer tyName fields field = maybeToList $ do

  fieldPart <- stripPrefix expectedPrefix (nameBase field)
  method    <- computeMethod fieldPart
  let cls = "Has" ++ fieldPart
  return (MethodName (mkName cls) (mkName method))

  where
  expectedPrefix = optUnderscore ++ overHead toLower (nameBase tyName)

  optUnderscore  = ['_' | any (isPrefixOf "_" . nameBase) fields ]

  computeMethod (x:xs) | isUpper x = Just (toLower x : xs)
  computeMethod _                  = Nothing


-- | Field rules fields in the form @ prefixFieldname or _prefixFieldname @
-- If you want all fields to be lensed, then there is no reason to use an @_@ before the prefix.
-- If any of the record fields leads with an @_@ then it is assume a field without an @_@ should not have a lens created.
--
-- Note that @prefix@ may be any string of characters that are not uppercase
-- letters. (In particular, it may be arbitrary string of lowercase letters
-- and numbers) This is the behavior that 'defaultFieldRules' had in lens
-- 4.4 and earlier.
abbreviatedFields :: LensRules
abbreviatedFields = defaultFieldRules { _fieldToDef = abbreviatedNamer }

abbreviatedNamer :: Name -> [Name] -> Name -> [DefName]
abbreviatedNamer _ fields field = maybeToList $ do

  fieldPart <- stripMaxLc (nameBase field)
  method    <- computeMethod fieldPart
  let cls = "Has" ++ fieldPart
  return (MethodName (mkName cls) (mkName method))

  where
  stripMaxLc f = do x <- stripPrefix optUnderscore f
                    case break isUpper x of
                      (p,s) | List.null p || List.null s -> Nothing
                            | otherwise                  -> Just s
  optUnderscore  = ['_' | any (isPrefixOf "_" . nameBase) fields ]

  computeMethod (x:xs) | isUpper x = Just (toLower x : xs)
  computeMethod _                  = Nothing


-- | Generate overloaded field accessors.
--
-- /e.g/
--
-- @
-- data Foo a = Foo { _fooX :: 'Int', _fooY : a }
-- newtype Bar = Bar { _barX :: 'Char' }
-- makeFields ''Foo
-- makeFields ''Bar
-- @
--
-- will create
--
-- @
-- _fooXLens :: Lens' (Foo a) Int
-- _fooYLens :: Lens (Foo a) (Foo b) a b
-- class HasX s a | s -> a where
--   x :: Lens' s a
-- instance HasX (Foo a) Int where
--   x = _fooXLens
-- class HasY s a | s -> a where
--   y :: Lens' s a
-- instance HasY (Foo a) a where
--   y = _fooYLens
-- _barXLens :: Iso' Bar Char
-- instance HasX Bar Char where
--   x = _barXLens
-- @
--
-- For details, see 'camelCaseFields'.
--
-- @
-- makeFields = 'makeLensesWith' 'defaultFieldRules'
-- @
makeFields :: Name -> DecsQ
makeFields = makeFieldOptics camelCaseFields

-- | Deprecated alias for 'makeLensesWith'
makeFieldsWith :: LensRules -> Name -> DecsQ
makeFieldsWith = makeLensesWith
{-# DEPRECATED makeFieldsWith "Use `makeLensesWith`, functionality merged" #-}

defaultFieldRules :: LensRules
defaultFieldRules = LensRules
  { _simpleLenses    = True
  , _generateSigs    = True
  , _generateClasses = True  -- classes will still be skipped if they already exist
  , _allowIsos       = False -- generating Isos would hinder field class reuse
  , _classyLenses    = const Nothing
  , _fieldToDef      = camelCaseNamer
  }


-- Declaration quote stuff

declareWith :: (Dec -> Declare Dec) -> DecsQ -> DecsQ
declareWith fun = (runDeclare . traverseDataAndNewtype fun =<<)

-- | Monad for emitting top-level declarations as a side effect.
type Declare = WriterT (Endo [Dec]) Q

runDeclare :: Declare [Dec] -> DecsQ
runDeclare dec = do
  (out, endo) <- runWriterT dec
  return $ out ++ appEndo endo []

emit :: [Dec] -> Declare ()
emit decs = tell $ Endo (decs++)

-- | Traverse each data, newtype, data instance or newtype instance
-- declaration.
traverseDataAndNewtype :: (Applicative f) => (Dec -> f Dec) -> [Dec] -> f [Dec]
traverseDataAndNewtype f decs = traverse go decs
  where
    go dec = case dec of
      DataD{} -> f dec
      NewtypeD{} -> f dec
      DataInstD{} -> f dec
      NewtypeInstD{} -> f dec

      -- Recurse into instance declarations because they main contain
      -- associated data family instances.
      InstanceD ctx inst body -> InstanceD ctx inst <$> traverse go body

      _ -> pure dec

stripFields :: Dec -> Dec
stripFields dec = case dec of
  DataD ctx tyName tyArgs cons derivings ->
    DataD ctx tyName tyArgs (map deRecord cons) derivings
  NewtypeD ctx tyName tyArgs con derivings ->
    NewtypeD ctx tyName tyArgs (deRecord con) derivings
  DataInstD ctx tyName tyArgs cons derivings ->
    DataInstD ctx tyName tyArgs (map deRecord cons) derivings
  NewtypeInstD ctx tyName tyArgs con derivings ->
    NewtypeInstD ctx tyName tyArgs (deRecord con) derivings
  _ -> dec

deRecord :: Con -> Con
deRecord con@NormalC{} = con
deRecord con@InfixC{} = con
deRecord (ForallC tyVars ctx con) = ForallC tyVars ctx $ deRecord con
deRecord (RecC conName fields) = NormalC conName (map dropFieldName fields)
  where dropFieldName (_, str, typ) = (str, typ)