{-# LANGUAGE CPP                    #-}
{-# LANGUAGE UndecidableInstances   #-}

-- {-# OPTIONS -fwarn-unused-binds #-}

{-| The translation of abstract syntax to concrete syntax has two purposes.
    First it allows us to pretty print abstract syntax values without having to
    write a dedicated pretty printer, and second it serves as a sanity check
    for the concrete to abstract translation: translating from concrete to
    abstract and then back again should be (more or less) the identity.
-}
module Agda.Syntax.Translation.AbstractToConcrete
    ( ToConcrete(..)
    , toConcreteCtx
    , abstractToConcrete_
    , abstractToConcreteScope
    , abstractToConcreteHiding
    , runAbsToCon
    , RangeAndPragma(..)
    , abstractToConcreteCtx
    , withScope
    , preserveInteractionIds
    , MonadAbsToCon, AbsToCon, Env
    , noTakenNames
    ) where

import Prelude hiding (null)

import Control.Arrow (first)
import Control.Monad.Reader
import Control.Monad.State

import qualified Control.Monad.Fail as Fail

import qualified Data.Map as Map
import Data.Maybe
import Data.Monoid
import Data.Set (Set)
import qualified Data.Set as Set
import Data.Map (Map)
import qualified Data.Foldable as Fold
import Data.Traversable (traverse)
import Data.Void
import Data.List (sortBy)
import Data.List.NonEmpty (NonEmpty(..))
import qualified Data.List.NonEmpty as NonEmpty

import Agda.Syntax.Common
import Agda.Syntax.Position
import Agda.Syntax.Literal
import Agda.Syntax.Info as A
import Agda.Syntax.Internal (MetaId(..))
import qualified Agda.Syntax.Internal as I
import Agda.Syntax.Fixity
import Agda.Syntax.Concrete as C
import Agda.Syntax.Concrete.Pattern as C
import Agda.Syntax.Abstract as A
import Agda.Syntax.Abstract.Views as A
import Agda.Syntax.Abstract.Pattern as A
import Agda.Syntax.Abstract.PatternSynonyms
import Agda.Syntax.Scope.Base
import Agda.Syntax.Scope.Monad ( tryResolveName )

import Agda.TypeChecking.Monad.State (getScope, getAllPatternSyns)
import Agda.TypeChecking.Monad.Base
import Agda.TypeChecking.Monad.Debug
import Agda.TypeChecking.Monad.Builtin
import Agda.Interaction.Options

import qualified Agda.Utils.AssocList as AssocList
import Agda.Utils.Either
import Agda.Utils.Except
import Agda.Utils.Function
import Agda.Utils.Functor
import Agda.Utils.Lens
import Agda.Utils.Maybe
import Agda.Utils.Monad
import Agda.Utils.Null
import Agda.Utils.Singleton
import Agda.Utils.Pretty

import Agda.Utils.Impossible

-- Environment ------------------------------------------------------------

data Env = Env { takenVarNames :: Set A.Name
                  -- ^ Abstract names currently in scope. Unlike the
                  --   ScopeInfo, this includes names for hidden
                  --   arguments inserted by the system.
               , takenDefNames :: Set C.Name
                  -- ^ Concrete names of all definitions in scope
               , currentScope :: ScopeInfo
               , builtins     :: Map String A.QName
                  -- ^ Certain builtins (like `fromNat`) have special printing
               , preserveIIds :: Bool
                  -- ^ Preserve interaction point ids
               , foldPatternSynonyms :: Bool
               }

makeEnv :: MonadAbsToCon m => ScopeInfo -> m Env
makeEnv scope = do
      -- zero and suc doesn't have to be in scope for natural number literals to work
  let noScopeCheck b = b `elem` [builtinZero, builtinSuc]
      name (I.Def q _)   = Just q
      name (I.Con q _ _) = Just (I.conName q)
      name _             = Nothing
      builtin b = getBuiltin' b >>= \ case
        Just v | Just q <- name v,
                 noScopeCheck b || isNameInScope q scope -> return [(b, q)]
        _                                                -> return []
  ctxVars <- map (fst . I.unDom) <$> asksTC envContext
  letVars <- Map.keys <$> asksTC envLetBindings
  let vars = ctxVars ++ letVars

  -- pick concrete names for in-scope names now so we don't
  -- accidentally shadow them
  forM_ (scope ^. scopeLocals) $ \(y , x) -> do
    pickConcreteName (localVar x) y

  builtinList <- concat <$> mapM builtin [ builtinFromNat, builtinFromString, builtinFromNeg, builtinZero, builtinSuc ]
  foldPatSyns <- optPrintPatternSynonyms <$> pragmaOptions
  return $
    Env { takenVarNames = Set.fromList vars
        , takenDefNames = defs
        , currentScope = scope
        , builtins     = Map.fromList builtinList
        , preserveIIds = False
        , foldPatternSynonyms = foldPatSyns
        }
  where
    -- Jesper, 2018-12-10: It's fine to shadow generalizable names as
    -- they will never show up directly in printed terms.
    notGeneralizeName AbsName{ anameKind = k }  =
      not (k == GeneralizeName || k == DisallowedGeneralizeName)

    defs = Map.keysSet $
           Map.filter (all notGeneralizeName) $
           nsNames $ everythingInScope scope

currentPrecedence :: AbsToCon PrecedenceStack
currentPrecedence = asks $ (^. scopePrecedence) . currentScope

preserveInteractionIds :: AbsToCon a -> AbsToCon a
preserveInteractionIds = local $ \ e -> e { preserveIIds = True }

withPrecedence' :: PrecedenceStack -> AbsToCon a -> AbsToCon a
withPrecedence' ps = local $ \e ->
  e { currentScope = set scopePrecedence ps (currentScope e) }

withPrecedence :: Precedence -> AbsToCon a -> AbsToCon a
withPrecedence p ret = do
  ps <- currentPrecedence
  withPrecedence' (pushPrecedence p ps) ret

withScope :: ScopeInfo -> AbsToCon a -> AbsToCon a
withScope scope = local $ \e -> e { currentScope = scope }

noTakenNames :: AbsToCon a -> AbsToCon a
noTakenNames = local $ \e -> e { takenVarNames = Set.empty }

dontFoldPatternSynonyms :: AbsToCon a -> AbsToCon a
dontFoldPatternSynonyms = local $ \ e -> e { foldPatternSynonyms = False }

-- | Bind a concrete name to an abstract in the translation environment.
addBinding :: C.Name -> A.Name -> Env -> Env
addBinding y x e =
  e { takenVarNames = Set.insert x $ takenVarNames e
    , currentScope = (`updateScopeLocals` currentScope e) $
        AssocList.insert y (LocalVar x __IMPOSSIBLE__ [])
    }

-- | Get a function to check if a name refers to a particular builtin function.
isBuiltinFun :: AbsToCon (A.QName -> String -> Bool)
isBuiltinFun = asks $ is . builtins
  where is m q b = Just q == Map.lookup b m

-- | Resolve a concrete name. If illegally ambiguous fail with the ambiguous names.
resolveName :: KindsOfNames -> Maybe (Set A.Name) -> C.QName -> AbsToCon (Either (NonEmpty A.QName) ResolvedName)
resolveName kinds candidates q = runExceptT $ tryResolveName kinds candidates q

-- | Treat illegally ambiguous names as UnknownNames.
resolveName_ :: C.QName -> [A.Name] -> AbsToCon ResolvedName
resolveName_ q cands = either (const UnknownName) id <$> resolveName allKindsOfNames (Just $ Set.fromList cands) q

-- The Monad --------------------------------------------------------------

-- | We need:
--   - Read access to the AbsToCon environment
--   - Read access to the TC environment
--   - Read access to the TC state
--   - Read and write access to the stConcreteNames part of the TC state
--   - Read access to the options
--   - Permission to print debug messages
type MonadAbsToCon m =
  ( MonadTCEnv m
  , ReadTCState m
  , MonadStConcreteNames m
  , HasOptions m
  , HasBuiltins m
  , MonadDebug m
  )

newtype AbsToCon a = AbsToCon
  { unAbsToCon :: forall m.
      ( MonadReader Env m
      , MonadAbsToCon m
      ) => m a
  }

-- TODO: Is there some way to automatically derive these boilerplate
-- instances?  GeneralizedNewtypeDeriving fails us here.
instance Functor AbsToCon where
  fmap f x = AbsToCon $ fmap f $ unAbsToCon x

instance Applicative AbsToCon where
  pure x = AbsToCon $ pure x
  f <*> m = AbsToCon $ unAbsToCon f <*> unAbsToCon m

instance Monad AbsToCon where
  m >>= f = AbsToCon $ unAbsToCon m >>= unAbsToCon . f
#if __GLASGOW_HASKELL__ < 808
  fail = Fail.fail
#endif

instance Fail.MonadFail AbsToCon where
  fail = error

instance MonadReader Env AbsToCon where
  ask = AbsToCon ask
  local f m = AbsToCon $ local f $ unAbsToCon m

instance MonadTCEnv AbsToCon where
  askTC = AbsToCon askTC
  localTC f m = AbsToCon $ localTC f $ unAbsToCon m

instance ReadTCState AbsToCon where
  getTCState = AbsToCon getTCState
  locallyTCState l f m = AbsToCon $ locallyTCState l f $ unAbsToCon m

instance MonadStConcreteNames AbsToCon where
  runStConcreteNames m = AbsToCon $ runStConcreteNames $ StateT $ unAbsToCon . runStateT m

instance HasOptions AbsToCon where
  pragmaOptions = AbsToCon pragmaOptions
  commandLineOptions = AbsToCon commandLineOptions

instance MonadDebug AbsToCon where
  displayDebugMessage k n s = AbsToCon $ displayDebugMessage k n s
  formatDebugMessage k n s = AbsToCon $ formatDebugMessage k n s
  verboseBracket k n s m = AbsToCon $ verboseBracket k n s $ unAbsToCon m

runAbsToCon :: MonadAbsToCon m => AbsToCon c -> m c
runAbsToCon m = do
  scope <- getScope
  verboseBracket "toConcrete" 50 "runAbsToCon" $ do
    reportSLn "toConcrete" 50 $ render $ hsep $
      [ "entering AbsToCon with scope:"
      , prettyList_ (map (text . C.nameToRawName . fst) $ scope ^. scopeLocals)
      ]
    x <- runReaderT (unAbsToCon m) =<< makeEnv scope
    reportSLn "toConcrete" 50 $ "leaving AbsToCon"
    return x

abstractToConcreteScope :: (ToConcrete a c, MonadAbsToCon m)
                        => ScopeInfo -> a -> m c
abstractToConcreteScope scope a = runReaderT (unAbsToCon $ toConcrete a) =<< makeEnv scope

abstractToConcreteCtx :: (ToConcrete a c, MonadAbsToCon m)
                      => Precedence -> a -> m c
abstractToConcreteCtx ctx x = runAbsToCon $ withPrecedence ctx (toConcrete x)

abstractToConcrete_ :: (ToConcrete a c, MonadAbsToCon m)
                    => a -> m c
abstractToConcrete_ = runAbsToCon . toConcrete

abstractToConcreteHiding :: (LensHiding i, ToConcrete a c, MonadAbsToCon m)
                         => i -> a -> m c
abstractToConcreteHiding i = runAbsToCon . toConcreteHiding i

-- Dealing with names -----------------------------------------------------

-- | Names in abstract syntax are fully qualified, but the concrete syntax
--   requires non-qualified names in places. In theory (if all scopes are
--   correct), we should get a non-qualified name when translating back to a
--   concrete name, but I suspect the scope isn't always perfect. In these
--   cases we just throw away the qualified part. It's just for pretty printing
--   anyway...
unsafeQNameToName :: C.QName -> C.Name
unsafeQNameToName = C.unqualify

lookupQName :: AllowAmbiguousNames -> A.QName -> AbsToCon C.QName
lookupQName ambCon x | Just s <- getGeneralizedFieldName x =
  return (C.QName $ C.Name noRange C.InScope $ C.stringNameParts s)
lookupQName ambCon x = do
  ys <- inverseScopeLookupName' ambCon x <$> asks currentScope
  reportSLn "scope.inverse" 100 $
    "inverse looking up abstract name " ++ prettyShow x ++ " yields " ++ prettyShow ys
  loop ys

  where
    -- Found concrete name: check that it is not shadowed by a local
    loop (qy@Qual{}      : _ ) = return qy -- local names cannot be qualified
    loop (qy@(C.QName y) : ys) = lookupNameInScope y >>= \case
      Just x' | x' /= qnameName x -> loop ys
      _ -> return qy
    -- Found no concrete name: make up a new one
    loop [] = case qnameToConcrete x of
      qy@Qual{}    -> return $ setNotInScope qy
      qy@C.QName{} -> C.QName <$> chooseName (qnameName x)

lookupModule :: A.ModuleName -> AbsToCon C.QName
lookupModule (A.MName []) = return $ C.QName $ C.Name noRange InScope [Id "-1"]
  -- Andreas, 2016-10-10 it can happen that we have an empty module name
  -- for instance when we query the current module inside the
  -- frontmatter or module telescope of the top level module.
  -- In this case, we print it as an invalid module name.
  -- (Should only affect debug printing.)
lookupModule x =
    do  scope <- asks currentScope
        case inverseScopeLookupModule x scope of
            (y : _) -> return y
            []      -> return $ mnameToConcrete x
                -- this is what happens for names that are not in scope (private names)

-- | Is this concrete name currently in use by a particular abstract
--   name in the current scope?
lookupNameInScope :: C.Name -> AbsToCon (Maybe A.Name)
lookupNameInScope y =
  fmap localVar . lookup y <$> asks ((^. scopeLocals) . currentScope)

-- | Have we already committed to a specific concrete name for this
--   abstract name? If yes, return the concrete name(s).
hasConcreteNames :: (MonadStConcreteNames m) => A.Name -> m [C.Name]
hasConcreteNames x = Map.findWithDefault [] x <$> useConcreteNames

-- | Commit to a specific concrete name for printing the given
--   abstract name. If the abstract name already has associated
---  concrete name(s), the new name is only used when all previous
---  names are shadowed. Precondition: the abstract name should be in
--   scope.
pickConcreteName :: (MonadStConcreteNames m) => A.Name -> C.Name -> m ()
pickConcreteName x y = modifyConcreteNames $ flip Map.alter x $ \case
    Nothing   -> Just $ [y]
    (Just ys) -> Just $ ys ++ [y]

-- | For the given abstract name, return the names that could shadow it.
shadowingNames :: (ReadTCState m, MonadStConcreteNames m)
               => A.Name -> m (Set RawName)
shadowingNames x = Set.fromList . Map.findWithDefault [] x <$> useR stShadowingNames

toConcreteName :: A.Name -> AbsToCon C.Name
toConcreteName x | y <- nameConcrete x , isNoName y = return y
toConcreteName x = (Map.findWithDefault [] x <$> useConcreteNames) >>= loop
  where
    -- case: we already have picked some name(s) for x
    loop (y:ys) = ifM (isGoodName x y) (return y) (loop ys)

    -- case: we haven't picked a concrete name yet, or all previously
    -- picked names are shadowed, so we pick a new name now
    loop [] = do
      y <- chooseName x
      pickConcreteName x y
      return y

    -- Is 'y' a good concrete name for abstract name 'x'?
    isGoodName :: A.Name -> C.Name -> AbsToCon Bool
    isGoodName x y = do
      zs <- Set.toList <$> asks takenVarNames
      allM zs $ \z -> if x == z then return True else do
        czs <- hasConcreteNames z
        return $ all (/= y) czs


-- | Choose a new unshadowed name for the given abstract name
chooseName :: A.Name -> AbsToCon C.Name
chooseName x = lookupNameInScope (nameConcrete x) >>= \case
  -- If the name is currently in scope, we do not rename it
  Just x' | x == x' -> do
    reportSLn "toConcrete.bindName" 80 $
      "name " ++ C.nameToRawName (nameConcrete x) ++ " already in scope, so not renaming"
    return $ nameConcrete x
  -- Otherwise we pick a name that does not shadow other names
  _ -> do
    taken   <- takenNames
    toAvoid <- shadowingNames x
    let shouldAvoid = (`Set.member` (taken `Set.union` toAvoid)) . C.nameToRawName
        y = firstNonTakenName shouldAvoid $ nameConcrete x
    reportSLn "toConcrete.bindName" 80 $ render $ vcat
      [ "picking concrete name for:" <+> text (C.nameToRawName $ nameConcrete x)
      , "names already taken:      " <+> prettyList_ (Set.toList taken)
      , "names to avoid:           " <+> prettyList_ (Set.toList toAvoid)
      , "concrete name chosen:     " <+> text (C.nameToRawName y)
      ]
    return y

  where
    takenNames :: AbsToCon (Set RawName)
    takenNames = do
      xs <- asks takenDefNames
      ys0 <- asks takenVarNames
      reportSLn "toConcrete.bindName" 90 $ render $ "abstract names of local vars: " <+> prettyList_ (map (C.nameToRawName . nameConcrete) $ Set.toList ys0)
      ys <- Set.fromList . concat <$> mapM hasConcreteNames (Set.toList ys0)
      return $ Set.map C.nameToRawName $ xs `Set.union` ys


-- | Add a abstract name to the scope and produce an available concrete version of it.
bindName :: A.Name -> (C.Name -> AbsToCon a) -> AbsToCon a
bindName x ret = do
  y <- toConcreteName x
  reportSLn "toConcrete.bindName" 30 $ "adding " ++ (C.nameToRawName $ nameConcrete x) ++ " to the scope under concrete name " ++ C.nameToRawName y
  local (addBinding y x) $ ret y

-- | Like 'bindName', but do not care whether name is already taken.
bindName' :: A.Name -> AbsToCon a -> AbsToCon a
bindName' x ret = do
  reportSLn "toConcrete.bindName" 30 $ "adding " ++ (C.nameToRawName $ nameConcrete x) ++ " to the scope with forced name"
  pickConcreteName x y
  applyUnless (isNoName y) (local $ addBinding y x) ret
  where y = nameConcrete x

-- Dealing with precedences -----------------------------------------------

-- | General bracketing function.
bracket' ::    (e -> e)             -- ^ the bracketing function
            -> (PrecedenceStack -> Bool) -- ^ Should we bracket things
                                    --   which have the given
                                    --   precedence?
            -> e -> AbsToCon e
bracket' paren needParen e =
    do  p <- currentPrecedence
        return $ if needParen p then paren e else e

-- | Expression bracketing
bracket :: (PrecedenceStack -> Bool) -> AbsToCon C.Expr -> AbsToCon C.Expr
bracket par m =
    do  e <- m
        bracket' (Paren (getRange e)) par e

-- | Pattern bracketing
bracketP_ :: (PrecedenceStack -> Bool) -> AbsToCon C.Pattern -> AbsToCon C.Pattern
bracketP_ par m =
    do  e <- m
        bracket' (ParenP (getRange e)) par e

{- UNUSED
-- | Pattern bracketing
bracketP :: (PrecedenceStack -> Bool) -> (C.Pattern -> AbsToCon a)
                                 -> ((C.Pattern -> AbsToCon a) -> AbsToCon a)
                                 -> AbsToCon a
bracketP par ret m = m $ \p -> do
    p <- bracket' (ParenP $ getRange p) par p
    ret p
-}

-- | Applications where the argument is a lambda without parentheses need
--   parens more often than other applications.
isLambda :: NamedArg A.Expr -> Bool
isLambda e | notVisible e = False
isLambda e =
  case unScope $ namedArg e of
    A.Lam{}         -> True
    A.AbsurdLam{}   -> True
    A.ExtendedLam{} -> True
    _               -> False

-- Dealing with infix declarations ----------------------------------------

-- | If a name is defined with a fixity that differs from the default, we have
--   to generate a fixity declaration for that name.
withInfixDecl :: DefInfo -> C.Name -> AbsToCon [C.Declaration] -> AbsToCon [C.Declaration]
withInfixDecl i x m = do
  ds <- m
  return $ fixDecl ++ synDecl ++ ds
 where fixDecl = [C.Infix (theFixity $ defFixity i) [x] | theFixity (defFixity i) /= noFixity]
       synDecl = [C.Syntax x (theNotation (defFixity i))]

{- UNUSED
withInfixDecls :: [(DefInfo, C.Name)] -> AbsToCon [C.Declaration] -> AbsToCon [C.Declaration]
withInfixDecls = foldr (.) id . map (uncurry withInfixDecl)
-}

-- Dealing with private definitions ---------------------------------------

-- | Add @abstract@, @private@, @instance@ modifiers.
withAbstractPrivate :: DefInfo -> AbsToCon [C.Declaration] -> AbsToCon [C.Declaration]
withAbstractPrivate i m =
    priv (defAccess i)
      . abst (A.defAbstract i)
      . addInstanceB (case A.defInstance i of InstanceDef r -> Just r; NotInstanceDef -> Nothing)
      <$> m
    where
        priv (PrivateAccess UserWritten)
                         ds = [ C.Private  (getRange ds) UserWritten ds ]
        priv _           ds = ds
        abst AbstractDef ds = [ C.Abstract (getRange ds) ds ]
        abst ConcreteDef ds = ds

addInstanceB :: Maybe Range -> [C.Declaration] -> [C.Declaration]
addInstanceB (Just r) ds = [ C.InstanceB r ds ]
addInstanceB Nothing  ds = ds

-- The To Concrete Class --------------------------------------------------

class ToConcrete a c | a -> c where
    toConcrete :: a -> AbsToCon c
    bindToConcrete :: a -> (c -> AbsToCon b) -> AbsToCon b

    -- Christian Sattler, 2017-08-05:
    -- These default implementations are not valid semantically (at least
    -- the second one). Perhaps they (it) should be removed.
    toConcrete     x     = bindToConcrete x return
    bindToConcrete x ret = ret =<< toConcrete x

-- | Translate something in a context of the given precedence.
toConcreteCtx :: ToConcrete a c => Precedence -> a -> AbsToCon c
toConcreteCtx p x = withPrecedence p $ toConcrete x

-- | Translate something in a context of the given precedence.
bindToConcreteCtx :: ToConcrete a c => Precedence -> a -> (c -> AbsToCon b) -> AbsToCon b
bindToConcreteCtx p x ret = withPrecedence p $ bindToConcrete x ret

-- | Translate something in the top context.
toConcreteTop :: ToConcrete a c => a -> AbsToCon c
toConcreteTop = toConcreteCtx TopCtx

-- | Translate something in the top context.
bindToConcreteTop :: ToConcrete a c => a -> (c -> AbsToCon b) -> AbsToCon b
bindToConcreteTop = bindToConcreteCtx TopCtx

-- | Translate something in a context indicated by 'Hiding' info.
toConcreteHiding :: (LensHiding h, ToConcrete a c) => h -> a -> AbsToCon c
toConcreteHiding h =
  case getHiding h of
    NotHidden  -> toConcrete
    Hidden     -> toConcreteTop
    Instance{} -> toConcreteTop

-- | Translate something in a context indicated by 'Hiding' info.
bindToConcreteHiding :: (LensHiding h, ToConcrete a c) => h -> a -> (c -> AbsToCon b) -> AbsToCon b
bindToConcreteHiding h =
  case getHiding h of
    NotHidden  -> bindToConcrete
    Hidden     -> bindToConcreteTop
    Instance{} -> bindToConcreteTop

-- General instances ------------------------------------------------------

instance ToConcrete () () where
  toConcrete = pure

instance ToConcrete Bool Bool where
  toConcrete = pure

instance ToConcrete a c => ToConcrete [a] [c] where
    toConcrete     = mapM toConcrete
    -- Andreas, 2017-04-11, Issue #2543
    -- The naive `thread'ing does not work as we have to undo
    -- changes to the Precedence.
    -- bindToConcrete = thread bindToConcrete
    bindToConcrete []     ret = ret []
    bindToConcrete (a:as) ret = do
      p <- currentPrecedence  -- save precedence
      bindToConcrete a $ \ c ->
        withPrecedence' p $ -- reset precedence
          bindToConcrete as $ \ cs ->
            ret (c : cs)

instance (ToConcrete a1 c1, ToConcrete a2 c2) => ToConcrete (Either a1 a2) (Either c1 c2) where
    toConcrete = traverseEither toConcrete toConcrete
    bindToConcrete (Left x) ret =
        bindToConcrete x $ \x ->
        ret (Left x)
    bindToConcrete (Right y) ret =
        bindToConcrete y $ \y ->
        ret (Right y)

instance (ToConcrete a1 c1, ToConcrete a2 c2) => ToConcrete (a1,a2) (c1,c2) where
    toConcrete (x,y) = liftM2 (,) (toConcrete x) (toConcrete y)
    bindToConcrete (x,y) ret =
        bindToConcrete x $ \x ->
        bindToConcrete y $ \y ->
        ret (x,y)

instance (ToConcrete a1 c1, ToConcrete a2 c2, ToConcrete a3 c3) =>
         ToConcrete (a1,a2,a3) (c1,c2,c3) where
    toConcrete (x,y,z) = reorder <$> toConcrete (x,(y,z))
        where
            reorder (x,(y,z)) = (x,y,z)

    bindToConcrete (x,y,z) ret = bindToConcrete (x,(y,z)) $ ret . reorder
        where
            reorder (x,(y,z)) = (x,y,z)

instance ToConcrete a c => ToConcrete (Arg a) (Arg c) where
    toConcrete (Arg i a) = Arg i <$> toConcreteHiding i a

    bindToConcrete (Arg info x) ret =
      bindToConcreteHiding info x $ ret . Arg info

instance ToConcrete a c => ToConcrete (WithHiding a) (WithHiding c) where
  toConcrete     (WithHiding h a) = WithHiding h <$> toConcreteHiding h a
  bindToConcrete (WithHiding h a) ret = bindToConcreteHiding h a $ \ a ->
    ret $ WithHiding h a

instance ToConcrete a c => ToConcrete (Named name a) (Named name c) where
    toConcrete (Named n x) = Named n <$> toConcrete x
    bindToConcrete (Named n x) ret = bindToConcrete x $ ret . Named n

-- Names ------------------------------------------------------------------

instance ToConcrete A.Name C.Name where
  toConcrete       = toConcreteName
  bindToConcrete x = bindName x

instance ToConcrete BindName C.BoundName where
  toConcrete       = fmap C.mkBoundName_ . toConcreteName . unBind
  bindToConcrete x = bindName (unBind x) . (. C.mkBoundName_)

instance ToConcrete A.QName C.QName where
  toConcrete = lookupQName AmbiguousConProjs

instance ToConcrete A.ModuleName C.QName where
  toConcrete = lookupModule

instance ToConcrete AbstractName C.QName where
  toConcrete = toConcrete . anameName

-- | Assumes name is not 'UnknownName'.
instance ToConcrete ResolvedName C.QName where
  toConcrete = \case
    VarName x _          -> C.QName <$> toConcrete x
    DefinedName _ x      -> toConcrete x
    FieldName xs         -> toConcrete (NonEmpty.head xs)
    ConstructorName xs   -> toConcrete (NonEmpty.head xs)
    PatternSynResName xs -> toConcrete (NonEmpty.head xs)
    UnknownName          -> __IMPOSSIBLE__

-- Expression instance ----------------------------------------------------

instance ToConcrete A.Expr C.Expr where
    toConcrete (Var x)             = Ident . C.QName <$> toConcrete x
    toConcrete (Def x)             = Ident <$> toConcrete x
    toConcrete (Proj ProjPrefix p) = Ident <$> toConcrete (headAmbQ p)
    toConcrete (Proj _          p) = C.Dot noRange . Ident <$> toConcrete (headAmbQ p)
    toConcrete (A.Macro x)         = Ident <$> toConcrete x
    toConcrete e@(Con c)           = tryToRecoverPatternSyn e $ Ident <$> toConcrete (headAmbQ c)
        -- for names we have to use the name from the info, since the abstract
        -- name has been resolved to a fully qualified name (except for
        -- variables)
    toConcrete e@(A.Lit (LitQName r x)) = tryToRecoverPatternSyn e $ do
      x <- lookupQName AmbiguousNothing x
      bracket appBrackets $ return $
        C.App r (C.Quote r) (defaultNamedArg $ C.Ident x)
    toConcrete e@(A.Lit l) = tryToRecoverPatternSyn e $ return $ C.Lit l

    -- Andreas, 2014-05-17  We print question marks with their
    -- interaction id, in case @metaNumber /= Nothing@
    -- Ulf, 2017-09-20  ... or @preserveIIds == True@.
    toConcrete (A.QuestionMark i ii) = do
      preserve <- asks preserveIIds
      return $ C.QuestionMark (getRange i) $
                 interactionId ii <$ guard (preserve || isJust (metaNumber i))

    toConcrete (A.Underscore i)     = return $
      C.Underscore (getRange i) $
        prettyShow . NamedMeta (metaNameSuggestion i) . MetaId . metaId <$> metaNumber i

    toConcrete (A.Dot i e) =
      C.Dot (getRange i) <$> toConcrete e

    toConcrete e@(A.App i e1 e2) = do
      is <- isBuiltinFun
      -- Special printing of desugared overloaded literals:
      --  fromNat 4        --> 4
      --  fromNeg 4        --> -4
      --  fromString "foo" --> "foo"
      -- Only when the corresponding conversion function is in scope and was
      -- inserted by the system.
      case (getHead e1, namedArg e2) of
        (Just (HdDef q), l@A.Lit{})
          | any (is q) [builtinFromNat, builtinFromString], visible e2,
            getOrigin i == Inserted -> toConcrete l
        (Just (HdDef q), A.Lit (LitNat r n))
          | q `is` builtinFromNeg, visible e2,
            getOrigin i == Inserted -> toConcrete (A.Lit (LitNat r (-n)))
        _ ->
          tryToRecoverPatternSyn e
          $ tryToRecoverOpApp e
          $ tryToRecoverNatural e
          -- or fallback to App
          $ bracket (appBrackets' $ preferParenless (appParens i) && isLambda e2)
          $ do e1' <- toConcreteCtx FunctionCtx e1
               e2' <- toConcreteCtx (ArgumentCtx $ appParens i) e2
               return $ C.App (getRange i) e1' e2'

    toConcrete (A.WithApp i e es) =
      bracket withAppBrackets $ do
        e <- toConcreteCtx WithFunCtx e
        es <- mapM (toConcreteCtx WithArgCtx) es
        return $ C.WithApp (getRange i) e es

    toConcrete (A.AbsurdLam i h) =
      bracket lamBrackets $ return $ C.AbsurdLam (getRange i) h
    toConcrete e@(A.Lam i _ _)      =
        tryToRecoverOpApp e   -- recover sections
        $ case lamView e of
            ([], e) -> toConcrete e
            (bs, e) -> bracket lamBrackets $
                bindToConcrete (map makeDomainFree bs) $ \ bs -> do
                  e  <- toConcreteTop e
                  return $ C.Lam (getRange i) bs e
        where
          -- #3238 GA: We drop the hidden lambda abstractions which have
          -- been inserted by the machine rather than the user. This means
          -- that the result of lamView may actually be an empty list of
          -- binders.
          lamView :: A.Expr -> ([A.LamBinding], A.Expr)
          lamView (A.Lam _ b@(A.DomainFree _ x) e)
            | isInsertedHidden x = lamView e
            | otherwise = case lamView e of
              (bs@(A.DomainFree{} : _), e) -> (b:bs, e)
              _                            -> ([b] , e)
          lamView (A.Lam _ b@(A.DomainFull A.TLet{}) e) = case lamView e of
            (bs@(A.DomainFull _ : _), e) -> (b:bs, e)
            _                            -> ([b], e)
          lamView (A.Lam _ (A.DomainFull (A.TBind r t xs ty)) e) =
            case filter (not . isInsertedHidden) xs of
              []  -> lamView e
              xs' -> let b = A.DomainFull (A.TBind r t xs' ty) in
                case lamView e of
                  (bs@(A.DomainFull _ : _), e) -> (b:bs, e)
                  _                            -> ([b], e)
          lamView e = ([], e)
    toConcrete (A.ExtendedLam i di qname cs) =
        bracket lamBrackets $ do
          decls <- concat <$> toConcrete cs
          let namedPat np = case getHiding np of
                 NotHidden  -> namedArg np
                 Hidden     -> C.HiddenP noRange (unArg np)
                 Instance{} -> C.InstanceP noRange (unArg np)
              -- we know all lhs are of the form `.extlam p1 p2 ... pn`,
              -- with the name .extlam leftmost. It is our mission to remove it.
          let removeApp (C.RawAppP r (_:es)) = return $ C.RawAppP r es
              removeApp (C.AppP (C.IdentP _) np) = return $ namedPat np
              removeApp (C.AppP p np) = do
                p <- removeApp p
                return $ C.AppP p np

              -- Andreas, 2018-06-18, issue #3136
              -- Empty pattern list also allowed in extended lambda,
              -- thus, we might face the unapplied .extendedlambda identifier.
              removeApp x@C.IdentP{} = return $ C.RawAppP (getRange x) []

              removeApp p = do
                reportSLn "extendedlambda" 50 $ "abstractToConcrete removeApp p = " ++ show p
                return p -- __IMPOSSIBLE__ -- Andreas, this is actually not impossible, my strictification exposed this sleeping bug
          let decl2clause (C.FunClause lhs rhs wh ca) = do
                let p = lhsOriginalPattern lhs
                reportSLn "extendedlambda" 50 $ "abstractToConcrete extended lambda pattern p = " ++ show p
                p' <- removeApp p
                reportSLn "extendedlambda" 50 $ "abstractToConcrete extended lambda pattern p' = " ++ show p'
                return $ LamClause lhs{ lhsOriginalPattern = p' } rhs wh ca
              decl2clause _ = __IMPOSSIBLE__
          C.ExtendedLam (getRange i) <$> mapM decl2clause decls
    toConcrete (A.Pi _ [] e) = toConcrete e
    toConcrete t@(A.Pi i _ _)  = case piTel t of
      (tel, e) ->
        bracket piBrackets
        $ bindToConcrete tel $ \ tel' -> do
             e' <- toConcreteTop e
             return $ C.Pi tel' e'
      where
        piTel (A.Pi _ tel e) = first (tel ++) $ piTel e
        piTel e              = ([], e)

    toConcrete (A.Generalized _ e) = C.Generalized <$> toConcrete e

    toConcrete (A.Fun i a b) =
        bracket piBrackets
        $ do a' <- toConcreteCtx ctx a
             b' <- toConcreteTop b
             let dom = setQuantity (getQuantity a') $ defaultArg $ addRel a' $ mkArg a'
             return $ C.Fun (getRange i) dom b'
             -- Andreas, 2018-06-14, issue #2513
             -- TODO: print attributes
        where
            ctx = if isRelevant a then FunctionSpaceDomainCtx else DotPatternCtx
            addRel a e = case getRelevance a of
                           Irrelevant -> C.Dot (getRange a) e
                           NonStrict  -> C.DoubleDot (getRange a) e
                           _          -> e
            mkArg (Arg info e) = case getHiding info of
                                          Hidden     -> HiddenArg   (getRange e) (unnamed e)
                                          Instance{} -> InstanceArg (getRange e) (unnamed e)
                                          NotHidden  -> e

    toConcrete (A.Set i 0)  = return $ C.Set (getRange i)
    toConcrete (A.Set i n)  = return $ C.SetN (getRange i) n
    toConcrete (A.Prop i 0) = return $ C.Prop (getRange i)
    toConcrete (A.Prop i n) = return $ C.PropN (getRange i) n

    toConcrete (A.Let i ds e) =
        bracket lamBrackets
        $ bindToConcrete ds $ \ds' -> do
             e'  <- toConcreteTop e
             return $ C.Let (getRange i) (concat ds') (Just e')

    toConcrete (A.Rec i fs) =
      bracket appBrackets $ do
        C.Rec (getRange i) . map (fmap (\x -> ModuleAssignment x [] defaultImportDir)) <$> toConcreteTop fs

    toConcrete (A.RecUpdate i e fs) =
      bracket appBrackets $ do
        C.RecUpdate (getRange i) <$> toConcrete e <*> toConcreteTop fs

    toConcrete (A.ETel tel) = C.ETel <$> toConcrete tel

    toConcrete (A.ScopedExpr _ e) = toConcrete e
    toConcrete (A.Quote i) = return $ C.Quote (getRange i)
    toConcrete (A.QuoteTerm i) = return $ C.QuoteTerm (getRange i)
    toConcrete (A.Unquote i) = return $ C.Unquote (getRange i)
    toConcrete (A.Tactic i e xs) = do
      e' <- toConcrete e
      xs' <- toConcrete xs
      let r      = getRange i
          rawtac = foldl (C.App r) e' xs'
      return $ C.Tactic (getRange i) rawtac

    -- Andreas, 2012-04-02: TODO!  print DontCare as irrAxiom
    -- Andreas, 2010-10-05 print irrelevant things as ordinary things
    toConcrete (A.DontCare e) = C.Dot r . C.Paren r  <$> toConcrete e
       where r = getRange e
    toConcrete (A.PatternSyn n) = C.Ident <$> toConcrete (headAmbQ n)

makeDomainFree :: A.LamBinding -> A.LamBinding
makeDomainFree b@(A.DomainFull (A.TBind _ tac [x] t)) =
  case unScope t of
    A.Underscore A.MetaInfo{metaNumber = Nothing} ->
      A.DomainFree tac x
    _ -> b
makeDomainFree b = b

-- Christian Sattler, 2017-08-05, fixing #2669
-- Both methods of ToConcrete (FieldAssignment' a) (FieldAssignment' c) need
-- to be implemented, each in terms of the corresponding one of ToConcrete a c.
-- This mirrors the instance ToConcrete (Arg a) (Arg c).
-- The default implementations of ToConcrete are not valid semantically.
instance ToConcrete a c => ToConcrete (FieldAssignment' a) (FieldAssignment' c) where
    toConcrete = traverse toConcrete

    bindToConcrete (FieldAssignment name a) ret =
      bindToConcrete a $ ret . FieldAssignment name


-- Binder instances -------------------------------------------------------

-- If there is no label we set it to the bound name, to make renaming the bound
-- name safe.
forceNameIfHidden :: NamedArg A.Binder -> NamedArg A.Binder
forceNameIfHidden x
  | isJust $ getNameOf  x = x
  | visible x             = x
  | otherwise             = setNameOf (Just name) x
  where
    name = WithOrigin Inserted
         $ Ranged (getRange x)
         $ C.nameToRawName $ nameConcrete
         $ unBind $ A.binderName $ namedArg x

instance ToConcrete a b => ToConcrete (A.Binder' a) (C.Binder' b) where
  bindToConcrete (A.Binder p a) ret =
    bindToConcrete a $ \ a ->
    bindToConcrete p $ \ p ->
    ret $ C.Binder p a

instance ToConcrete A.LamBinding C.LamBinding where
    bindToConcrete (A.DomainFree t x) ret = do
      t <- traverse toConcrete t
      let setTac x = x { bnameTactic = t }
      bindToConcrete (forceNameIfHidden x) $
        ret . C.DomainFree . updateNamedArg (fmap setTac)
    bindToConcrete (A.DomainFull b) ret = bindToConcrete b $ ret . C.DomainFull

instance ToConcrete A.TypedBinding C.TypedBinding where
    bindToConcrete (A.TBind r t xs e) ret = do
        t <- traverse toConcrete t
        bindToConcrete (map forceNameIfHidden xs) $ \ xs -> do
          e <- toConcreteTop e
          let setTac x = x { bnameTactic = t }
          ret $ C.TBind r (map (updateNamedArg (fmap setTac)) xs) e
    bindToConcrete (A.TLet r lbs) ret =
        bindToConcrete lbs $ \ ds -> do
        ret $ C.TLet r $ concat ds

instance ToConcrete A.LetBinding [C.Declaration] where
    bindToConcrete (A.LetBind i info x t e) ret =
        bindToConcrete x $ \ x ->
        do (t, (e, [], [], [])) <- toConcrete (t, A.RHS e Nothing)
           ret $ addInstanceB (if isInstance info then Just noRange else Nothing) $
               [ C.TypeSig info Nothing (C.boundName x) t
               , C.FunClause (C.LHS (C.IdentP $ C.QName $ C.boundName x) [] [] NoEllipsis)
                             e C.NoWhere False
               ]
    -- TODO: bind variables
    bindToConcrete (LetPatBind i p e) ret = do
        p <- toConcrete p
        e <- toConcrete e
        ret [ C.FunClause (C.LHS p [] [] NoEllipsis) (C.RHS e) NoWhere False ]
    bindToConcrete (LetApply i x modapp _ _) ret = do
      x' <- unqualify <$> toConcrete x
      modapp <- toConcrete modapp
      let r = getRange modapp
          open = fromMaybe DontOpen $ minfoOpenShort i
          dir  = fromMaybe defaultImportDir{ importDirRange = r } $ minfoDirective i
      -- This is no use since toAbstract LetDefs is in localToAbstract.
      local (openModule' x dir id) $
        ret [ C.ModuleMacro (getRange i) x' modapp open dir ]
    bindToConcrete (LetOpen i x _) ret = do
      x' <- toConcrete x
      let dir = fromMaybe defaultImportDir $ minfoDirective i
      local (openModule' x dir restrictPrivate) $
            ret [ C.Open (getRange i) x' dir ]
    bindToConcrete (LetDeclaredVariable _) ret =
      -- Note that the range of the declaration site is dropped.
      ret []

instance ToConcrete A.WhereDeclarations WhereClause where
  bindToConcrete (A.WhereDecls _ []) ret = ret C.NoWhere
  bindToConcrete (A.WhereDecls (Just am) [A.Section _ _ _ ds]) ret = do
    ds' <- declsToConcrete ds
    cm  <- unqualify <$> lookupModule am
    -- Andreas, 2016-07-08 I put PublicAccess in the following SomeWhere
    -- Should not really matter for printing...
    let wh' = (if isNoName cm then AnyWhere else SomeWhere cm PublicAccess) $ ds'
    local (openModule' am defaultImportDir id) $ ret wh'
  bindToConcrete (A.WhereDecls _ ds) ret =
    ret . AnyWhere =<< declsToConcrete ds

mergeSigAndDef :: [C.Declaration] -> [C.Declaration]
mergeSigAndDef (C.RecordSig _ x bs e : C.RecordDef r y ind eta c _ fs : ds)
  | x == y = C.Record r y ind eta c bs e fs : mergeSigAndDef ds
mergeSigAndDef (C.DataSig _ x bs e : C.DataDef r y _ cs : ds)
  | x == y = C.Data r y bs e cs : mergeSigAndDef ds
mergeSigAndDef (d : ds) = d : mergeSigAndDef ds
mergeSigAndDef [] = []

openModule' :: A.ModuleName -> C.ImportDirective -> (Scope -> Scope) -> Env -> Env
openModule' x dir restrict env = env{currentScope = set scopeModules mods' sInfo}
  where sInfo = currentScope env
        amod  = sInfo ^. scopeCurrent
        mods  = sInfo ^. scopeModules
        news  = setScopeAccess PrivateNS
                $ applyImportDirective dir
                $ maybe emptyScope restrict
                $ Map.lookup x mods
        mods' = Map.update (Just . (`mergeScope` news)) amod mods


-- Declaration instances --------------------------------------------------

declsToConcrete :: [A.Declaration] -> AbsToCon [C.Declaration]
declsToConcrete ds = mergeSigAndDef . concat <$> toConcrete ds

instance ToConcrete A.RHS (C.RHS, [C.RewriteEqn], [WithHiding C.Expr], [C.Declaration]) where
    toConcrete (A.RHS e (Just c)) = return (C.RHS c, [], [], [])
    toConcrete (A.RHS e Nothing) = do
      e <- toConcrete e
      return (C.RHS e, [], [], [])
    toConcrete A.AbsurdRHS = return (C.AbsurdRHS, [], [], [])
    toConcrete (A.WithRHS _ es cs) = do
      es <- toConcrete es
      cs <- noTakenNames $ concat <$> toConcrete cs
      return (C.AbsurdRHS, [], es, cs)
    toConcrete (A.RewriteRHS xeqs _spats rhs wh) = do
      wh <- declsToConcrete (A.whereDecls wh)
      (rhs, eqs', es, whs) <- toConcrete rhs
      unless (null eqs') __IMPOSSIBLE__
      eqs <- toConcrete xeqs
      return (rhs, eqs, es, wh ++ whs)

instance (ToConcrete p q, ToConcrete a b) =>
         ToConcrete (RewriteEqn' qn p a) (RewriteEqn' () q b) where
  toConcrete = \case
    Rewrite es    -> Rewrite <$> mapM (toConcrete . (\ (_, e) -> ((),e))) es
    Invert qn pes -> Invert () <$> mapM toConcrete pes

instance ToConcrete (Maybe A.QName) (Maybe C.Name) where
  toConcrete = mapM (toConcrete . qnameName)

instance ToConcrete (Constr A.Constructor) C.Declaration where
  toConcrete (Constr (A.ScopedDecl scope [d])) =
    withScope scope $ toConcrete (Constr d)
  toConcrete (Constr (A.Axiom _ i info Nothing x t)) = do
    x' <- unsafeQNameToName <$> toConcrete x
    t' <- toConcreteTop t
    return $ C.TypeSig info Nothing x' t'
  toConcrete (Constr (A.Axiom _ _ _ (Just _) _ _)) = __IMPOSSIBLE__
  toConcrete (Constr d) = head <$> toConcrete d

instance ToConcrete a C.LHS => ToConcrete (A.Clause' a) [C.Declaration] where
  toConcrete (A.Clause lhs _ rhs wh catchall) =
      bindToConcrete lhs $ \case
          C.LHS p _ _ ell -> do
            bindToConcrete wh $ \ wh' -> do
                (rhs', eqs, with, wcs) <- toConcreteTop rhs
                return $ FunClause (C.LHS p eqs with ell) rhs' wh' catchall : wcs

instance ToConcrete A.ModuleApplication C.ModuleApplication where
  toConcrete (A.SectionApp tel y es) = do
    y  <- toConcreteCtx FunctionCtx y
    bindToConcrete tel $ \ tel -> do
      es <- toConcreteCtx argumentCtx_ es
      let r = fuseRange y es
      return $ C.SectionApp r tel (foldl (C.App r) (C.Ident y) es)

  toConcrete (A.RecordModuleInstance recm) = do
    recm <- toConcrete recm
    return $ C.RecordModuleInstance (getRange recm) recm

instance ToConcrete A.Declaration [C.Declaration] where
  toConcrete (ScopedDecl scope ds) =
    withScope scope (declsToConcrete ds)

  toConcrete (A.Axiom _ i info mp x t) = do
    x' <- unsafeQNameToName <$> toConcrete x
    withAbstractPrivate i $
      withInfixDecl i x'  $ do
      t' <- toConcreteTop t
      return $
        (case mp of
           Nothing   -> []
           Just occs -> [C.Pragma (PolarityPragma noRange x' occs)]) ++
        [C.Postulate (getRange i) [C.TypeSig info Nothing x' t']]

  toConcrete (A.Generalize s i j x t) = do
    x' <- unsafeQNameToName <$> toConcrete x
    tac <- traverse toConcrete (defTactic i)
    withAbstractPrivate i $
      withInfixDecl i x'  $ do
      t' <- toConcreteTop t
      return [C.Generalize (getRange i) [C.TypeSig j tac x' $ C.Generalized t']]

  toConcrete (A.Field i x t) = do
    x' <- unsafeQNameToName <$> toConcrete x
    tac <- traverse toConcrete (defTactic i)
    withAbstractPrivate i $
      withInfixDecl i x'  $ do
      t' <- toConcreteTop t
      return [C.FieldSig (A.defInstance i) tac x' t']

  toConcrete (A.Primitive i x t) = do
    x' <- unsafeQNameToName <$> toConcrete x
    withAbstractPrivate i $
      withInfixDecl i x'  $ do
      t' <- toConcreteTop t
      return [C.Primitive (getRange i) [C.TypeSig defaultArgInfo Nothing x' t']]
        -- Primitives are always relevant.

  toConcrete (A.FunDef i _ _ cs) =
    withAbstractPrivate i $ concat <$> toConcrete cs

  toConcrete (A.DataSig i x bs t) =
    withAbstractPrivate i $
    bindToConcrete (A.generalizeTel bs) $ \ tel' -> do
      x' <- unsafeQNameToName <$> toConcrete x
      t' <- toConcreteTop t
      return [ C.DataSig (getRange i) x' (map C.DomainFull tel') t' ]

  toConcrete (A.DataDef i x uc bs cs) =
    withAbstractPrivate i $
    bindToConcrete (map makeDomainFree $ dataDefParams bs) $ \ tel' -> do
      (x',cs') <- first unsafeQNameToName <$> toConcrete (x, map Constr cs)
      return [ C.DataDef (getRange i) x' tel' cs' ]

  toConcrete (A.RecSig i x bs t) =
    withAbstractPrivate i $
    bindToConcrete (A.generalizeTel bs) $ \ tel' -> do
      x' <- unsafeQNameToName <$> toConcrete x
      t' <- toConcreteTop t
      return [ C.RecordSig (getRange i) x' (map C.DomainFull tel') t' ]

  toConcrete (A.RecDef  i x uc ind eta c bs t cs) =
    withAbstractPrivate i $
    bindToConcrete (map makeDomainFree $ dataDefParams bs) $ \ tel' -> do
      (x',cs') <- first unsafeQNameToName <$> toConcrete (x, map Constr cs)
      return [ C.RecordDef (getRange i) x' ind eta Nothing tel' cs' ]

  toConcrete (A.Mutual i ds) = declsToConcrete ds

  toConcrete (A.Section i x (A.GeneralizeTel _ tel) ds) = do
    x <- toConcrete x
    bindToConcrete tel $ \ tel -> do
      ds <- declsToConcrete ds
      return [ C.Module (getRange i) x tel ds ]

  toConcrete (A.Apply i x modapp _ _) = do
    x  <- unsafeQNameToName <$> toConcrete x
    modapp <- toConcrete modapp
    let r = getRange modapp
        open = fromMaybe DontOpen $ minfoOpenShort i
        dir  = fromMaybe defaultImportDir{ importDirRange = r } $ minfoDirective i
    return [ C.ModuleMacro (getRange i) x modapp open dir ]

  toConcrete (A.Import i x _) = do
    x <- toConcrete x
    let open = fromMaybe DontOpen $ minfoOpenShort i
        dir  = fromMaybe defaultImportDir $ minfoDirective i
    return [ C.Import (getRange i) x Nothing open dir]

  toConcrete (A.Pragma i p)     = do
    p <- toConcrete $ RangeAndPragma (getRange i) p
    return [C.Pragma p]

  toConcrete (A.Open i x _) = do
    x <- toConcrete x
    return [C.Open (getRange i) x defaultImportDir]

  toConcrete (A.PatternSynDef x xs p) = do
    C.QName x <- toConcrete x
    bindToConcrete xs $ \xs -> (:[]) . C.PatternSyn (getRange x) x xs <$> dontFoldPatternSynonyms (toConcrete (vacuous p :: A.Pattern))

  toConcrete (A.UnquoteDecl _ i xs e) = do
    let unqual (C.QName x) = return x
        unqual _           = __IMPOSSIBLE__
    xs <- mapM (unqual <=< toConcrete) xs
    (:[]) . C.UnquoteDecl (getRange i) xs <$> toConcrete e

  toConcrete (A.UnquoteDef i xs e) = do
    let unqual (C.QName x) = return x
        unqual _           = __IMPOSSIBLE__
    xs <- mapM (unqual <=< toConcrete) xs
    (:[]) . C.UnquoteDef (getRange i) xs <$> toConcrete e


data RangeAndPragma = RangeAndPragma Range A.Pragma

instance ToConcrete RangeAndPragma C.Pragma where
  toConcrete (RangeAndPragma r p) = case p of
    A.OptionsPragma xs  -> return $ C.OptionsPragma r xs
    A.BuiltinPragma b x       -> C.BuiltinPragma r b <$> toConcrete x
    A.BuiltinNoDefPragma b x  -> C.BuiltinPragma r b <$> toConcrete x
    A.RewritePragma r' x      -> C.RewritePragma r r' <$> toConcrete x
    A.CompilePragma b x s -> do
      x <- toConcrete x
      return $ C.CompilePragma r b x s
    A.StaticPragma x -> C.StaticPragma r <$> toConcrete x
    A.InjectivePragma x -> C.InjectivePragma r <$> toConcrete x
    A.InlinePragma b x -> C.InlinePragma r b <$> toConcrete x
    A.EtaPragma x    -> C.EtaPragma    r <$> toConcrete x
    A.DisplayPragma f ps rhs ->
      C.DisplayPragma r <$> toConcrete (A.DefP (PatRange noRange) (unambiguous f) ps) <*> toConcrete rhs

-- Left hand sides --------------------------------------------------------

instance ToConcrete A.SpineLHS C.LHS where
  bindToConcrete lhs = bindToConcrete (A.spineToLhs lhs :: A.LHS)

instance ToConcrete A.LHS C.LHS where
    bindToConcrete (A.LHS i lhscore) ret = do
      bindToConcreteCtx TopCtx lhscore $ \ lhs ->
          ret $ C.LHS (reintroduceEllipsis (lhsEllipsis i) lhs) [] [] NoEllipsis

instance ToConcrete A.LHSCore C.Pattern where
  bindToConcrete = bindToConcrete . lhsCoreToPattern

appBracketsArgs :: [arg] -> PrecedenceStack -> Bool
appBracketsArgs []    _   = False
appBracketsArgs (_:_) ctx = appBrackets ctx

-- Auxiliary wrappers for processing the bindings in patterns in the right order.
newtype UserPattern a  = UserPattern a
newtype SplitPattern a = SplitPattern a
newtype BindingPattern = BindingPat A.Pattern
newtype FreshenName = FreshenName BindName

instance ToConcrete FreshenName A.Name where
  bindToConcrete (FreshenName BindName{ unBind = x }) ret = bindToConcrete x $ \ y -> ret x { nameConcrete = y }

-- Pass 1: (Issue #2729)
-- Takes care of binding the originally user-written pattern variables, but doesn't actually
-- translate anything to Concrete.
instance ToConcrete (UserPattern A.Pattern) A.Pattern where
  bindToConcrete (UserPattern p) ret = do
    reportSLn "toConcrete.pat" 100 $ "binding pattern (pass 1)" ++ show p
    case p of
      A.VarP bx -> do
        let x = unBind bx
        case isInScope x of
          InScope            -> bindName' x $ ret $ A.VarP bx
          C.NotInScope       -> bindName x $ \y ->
                                ret $ A.VarP $ mkBindName $ x { nameConcrete = y }
      A.WildP{}              -> ret p
      A.ProjP{}              -> ret p
      A.AbsurdP{}            -> ret p
      A.LitP{}               -> ret p
      A.DotP{}               -> ret p
      A.EqualP{}             -> ret p
      -- Andreas, 2017-09-03, issue #2729:
      -- Do not go into patterns generated by case-split here!
      -- They are treated in a second pass.
      A.ConP i c args
        | conPatOrigin i == ConOSplit -> ret p
        | otherwise          -> bindToConcrete (map UserPattern args) $ ret . A.ConP i c
      A.DefP i f args        -> bindToConcrete (map UserPattern args) $ ret . A.DefP i f
      A.PatternSynP i f args -> bindToConcrete (map UserPattern args) $ ret . A.PatternSynP i f
      A.RecP i args          -> bindToConcrete ((map . fmap) UserPattern args) $ ret . A.RecP i
      A.AsP i x p            -> bindName' (unBind x) $
                                bindToConcrete (UserPattern p) $ \ p ->
                                ret (A.AsP i x p)
      A.WithP i p            -> bindToConcrete (UserPattern p) $ ret . A.WithP i

instance ToConcrete (UserPattern (NamedArg A.Pattern)) (NamedArg A.Pattern) where
  bindToConcrete (UserPattern np) ret =
    case getOrigin np of
      CaseSplit -> ret np
      _         -> bindToConcrete (fmap (fmap UserPattern) np) ret

-- Pass 2a: locate case-split pattern.  Don't bind anything!
instance ToConcrete (SplitPattern A.Pattern) A.Pattern where
  bindToConcrete (SplitPattern p) ret = do
    reportSLn "toConcrete.pat" 100 $ "binding pattern (pass 2a)" ++ show p
    case p of
      A.VarP x               -> ret p
      A.WildP{}              -> ret p
      A.ProjP{}              -> ret p
      A.AbsurdP{}            -> ret p
      A.LitP{}               -> ret p
      A.DotP{}               -> ret p
      A.EqualP{}             -> ret p
      -- Andreas, 2017-09-03, issue #2729:
      -- For patterns generated by case-split here, switch to freshening & binding.
      A.ConP i c args
        | conPatOrigin i == ConOSplit
                             -> bindToConcrete ((map . fmap . fmap) BindingPat args) $ ret . A.ConP i c
        | otherwise          -> bindToConcrete (map SplitPattern args) $ ret . A.ConP i c
      A.DefP i f args        -> bindToConcrete (map SplitPattern args) $ ret . A.DefP i f
      A.PatternSynP i f args -> bindToConcrete (map SplitPattern args) $ ret . A.PatternSynP i f
      A.RecP i args          -> bindToConcrete ((map . fmap) SplitPattern args) $ ret . A.RecP i
      A.AsP i x p            -> bindToConcrete (SplitPattern p)  $ \ p ->
                                ret (A.AsP i x p)
      A.WithP i p            -> bindToConcrete (SplitPattern p) $ ret . A.WithP i

instance ToConcrete (SplitPattern (NamedArg A.Pattern)) (NamedArg A.Pattern) where
  bindToConcrete (SplitPattern np) ret =
    case getOrigin np of
      CaseSplit -> bindToConcrete (fmap (fmap BindingPat  ) np) ret
      _         -> bindToConcrete (fmap (fmap SplitPattern) np) ret


-- Pass 2b:
-- Takes care of freshening and binding pattern variables introduced by case split.
-- Still does not translate anything to Concrete.
instance ToConcrete BindingPattern A.Pattern where
  bindToConcrete (BindingPat p) ret = do
    reportSLn "toConcrete.pat" 100 $ "binding pattern (pass 2b)" ++ show p
    case p of
      A.VarP x               -> bindToConcrete (FreshenName x) $ ret . A.VarP . mkBindName
      A.WildP{}              -> ret p
      A.ProjP{}              -> ret p
      A.AbsurdP{}            -> ret p
      A.LitP{}               -> ret p
      A.DotP{}               -> ret p
      A.EqualP{}             -> ret p
      A.ConP i c args        -> bindToConcrete (map (updateNamedArg BindingPat) args) $ ret . A.ConP i c
      A.DefP i f args        -> bindToConcrete (map (updateNamedArg BindingPat) args) $ ret . A.DefP i f
      A.PatternSynP i f args -> bindToConcrete (map (updateNamedArg BindingPat) args) $ ret . A.PatternSynP i f
      A.RecP i args          -> bindToConcrete ((map . fmap)        BindingPat args) $ ret . A.RecP i
      A.AsP i x p            -> bindToConcrete (FreshenName x) $ \ x ->
                                bindToConcrete (BindingPat p)  $ \ p ->
                                ret (A.AsP i (mkBindName x) p)
      A.WithP i p            -> bindToConcrete (BindingPat p) $ ret . A.WithP i

instance ToConcrete A.Pattern C.Pattern where
  bindToConcrete p ret = do
    prec <- currentPrecedence
    bindToConcrete (UserPattern p) $ \ p -> do
      bindToConcrete (SplitPattern p) $ \ p -> do
        ret =<< do withPrecedence' prec $ toConcrete p
  toConcrete p =
    case p of
      A.VarP x ->
        C.IdentP . C.QName . C.boundName <$> toConcrete x

      A.WildP i ->
        return $ C.WildP (getRange i)

      A.ConP i c args  -> tryOp (headAmbQ c) (A.ConP i c) args

      A.ProjP i ProjPrefix p -> C.IdentP <$> toConcrete (headAmbQ p)
      A.ProjP i _          p -> C.DotP noRange . C.Ident <$> toConcrete (headAmbQ p)

      A.DefP i x args -> tryOp (headAmbQ x) (A.DefP i x)  args

      A.AsP i x p -> do
        (x, p) <- toConcreteCtx argumentCtx_ (x, p)
        return $ C.AsP (getRange i) (C.boundName x) p

      A.AbsurdP i ->
        return $ C.AbsurdP (getRange i)

      A.LitP (LitQName r x) -> do
        x <- lookupQName AmbiguousNothing x
        bracketP_ appBrackets $ return $ C.AppP (C.QuoteP r) (defaultNamedArg (C.IdentP x))
      A.LitP l ->
        return $ C.LitP l

      -- Andreas, 2018-06-19, issue #3130
      -- Print .p as .(p) if p is a projection
      -- to avoid confusion with projection pattern.
      A.DotP i e@A.Proj{} -> C.DotP r . C.Paren r <$> toConcreteCtx TopCtx e
        where r = getRange i

      -- gallais, 2019-02-12, issue #3491
      -- Print p as .(p) if p is a variable but there is a projection of the
      -- same name in scope.
      A.DotP i e@(A.Var v) -> do
        let r = getRange i
        -- Erase @v@ to a concrete name and resolve it back to check whether
        -- we have a conflicting field name.
        cn <- toConcreteName v
        resolveName (someKindsOfNames [FldName]) Nothing (C.QName cn) >>= \ case
          -- If we do then we print .(v) rather than .v
          Right FieldName{} -> do
            reportSLn "print.dotted" 50 $ "Wrapping ambiguous name " ++ show (nameConcrete v)
            C.DotP r . C.Paren r <$> toConcrete (A.Var v)
          Right _ -> printDotDefault i e
          Left _ -> __IMPOSSIBLE__

      A.DotP i e -> printDotDefault i e

      A.EqualP i es -> do
        C.EqualP (getRange i) <$> toConcrete es

      A.PatternSynP i n args -> tryOp (headAmbQ n) (A.PatternSynP i n) args

      A.RecP i as ->
        C.RecP (getRange i) <$> mapM (traverse toConcrete) as

      A.WithP i p -> C.WithP (getRange i) <$> toConcreteCtx WithArgCtx p

    where

    printDotDefault :: PatInfo -> A.Expr -> AbsToCon C.Pattern
    printDotDefault i e = do
      c <- toConcreteCtx DotPatternCtx e
      let r = getRange i
      case c of
        -- Andreas, 2016-02-04 print ._ pattern as _ pattern,
        -- following the fusing of WildP and ImplicitP.
        C.Underscore{} -> return $ C.WildP r
        _ -> return $ C.DotP r c

    tryOp :: A.QName -> (A.Patterns -> A.Pattern) -> A.Patterns -> AbsToCon C.Pattern
    tryOp x f args = do
      -- Andreas, 2016-02-04, Issue #1792
      -- To prevent failing of tryToRecoverOpAppP for overapplied operators,
      -- we take off the exceeding arguments first
      -- and apply them pointwise with C.AppP later.
      let (args1, args2) = splitAt (numHoles x) args
      let funCtx = applyUnless (null args2) (withPrecedence FunctionCtx)
      tryToRecoverPatternSynP (f args) $ funCtx (tryToRecoverOpAppP $ f args1) >>= \case
        Just c  -> applyTo args2 c
        Nothing -> applyTo args . C.IdentP =<< toConcrete x
    -- Note: applyTo [] c = return c
    applyTo args c = bracketP_ (appBracketsArgs args) $ do
      foldl C.AppP c <$> toConcreteCtx argumentCtx_ args

instance ToConcrete (Maybe A.Pattern) (Maybe C.Pattern) where
  toConcrete = traverse toConcrete

-- Helpers for recovering natural number literals

tryToRecoverNatural :: A.Expr -> AbsToCon C.Expr -> AbsToCon C.Expr
tryToRecoverNatural e def = do
  is <- isBuiltinFun
  caseMaybe (recoverNatural is e) def $ return . C.Lit . LitNat noRange

recoverNatural :: (A.QName -> String -> Bool) -> A.Expr -> Maybe Integer
recoverNatural is e = explore (`is` builtinZero) (`is` builtinSuc) 0 e
  where
    explore :: (A.QName -> Bool) -> (A.QName -> Bool) -> Integer -> A.Expr -> Maybe Integer
    explore isZero isSuc k (A.App _ (A.Con c) t) | Just f <- getUnambiguous c, isSuc f
                                                = (explore isZero isSuc $! k + 1) (namedArg t)
    explore isZero isSuc k (A.Con c) | Just x <- getUnambiguous c, isZero x = Just k
    explore isZero isSuc k (A.Lit (LitNat _ l)) = Just (k + l)
    explore _ _ _ _                             = Nothing

-- Helpers for recovering C.OpApp ------------------------------------------

data Hd = HdVar A.Name | HdCon A.QName | HdDef A.QName | HdSyn A.QName

data MaybeSection a
  = YesSection
  | NoSection a
  deriving (Eq, Show, Functor, Foldable, Traversable)

fromNoSection :: a -> MaybeSection a -> a
fromNoSection fallback = \case
  YesSection  -> fallback
  NoSection x -> x

instance HasRange a => HasRange (MaybeSection a) where
  getRange = \case
    YesSection  -> noRange
    NoSection a -> getRange a

getHead :: A.Expr -> Maybe Hd
getHead (Var x)          = Just (HdVar x)
getHead (Def f)          = Just (HdDef f)
getHead (Proj o f)       = Just (HdDef $ headAmbQ f)
getHead (Con c)          = Just (HdCon $ headAmbQ c)
getHead (A.PatternSyn n) = Just (HdSyn $ headAmbQ n)
getHead _                = Nothing

cOpApp :: Range -> C.QName -> A.Name -> [MaybeSection C.Expr] -> C.Expr
cOpApp r x n es =
  C.OpApp r x (Set.singleton n)
          (map (defaultNamedArg . placeholder) eps)
  where
    x0 = C.unqualify x
    positions | isPrefix  x0 =                [ Middle | _ <- drop 1 es ] ++ [End]
              | isPostfix x0 = [Beginning] ++ [ Middle | _ <- drop 1 es ]
              | isInfix x0   = [Beginning] ++ [ Middle | _ <- drop 2 es ] ++ [End]
              | otherwise    =                [ Middle | _ <- es ]
    eps = zip es positions
    placeholder (YesSection , pos ) = Placeholder pos
    placeholder (NoSection e, _pos) = noPlaceholder (Ordinary e)

tryToRecoverOpApp :: A.Expr -> AbsToCon C.Expr -> AbsToCon C.Expr
tryToRecoverOpApp e def = fromMaybeM def $
  recoverOpApp bracket (isLambda . defaultNamedArg) cOpApp view e
  where
    view :: A.Expr -> Maybe (Hd, [NamedArg (MaybeSection (AppInfo, A.Expr))])
    view e
        -- Do we have a series of inserted lambdas?
      | Just xs@(_:_) <- traverse insertedName bs =
        (,) <$> getHead hd <*> sectionArgs (map (unBind . A.binderName) xs) args
      where
        LamView     bs body = A.lamView e
        Application hd args = A.appView' body

        -- Only inserted domain-free visible lambdas come from sections.
        insertedName (A.DomainFree _ x)
          | getOrigin x == Inserted && visible x = Just $ namedArg x
        insertedName _ = Nothing

        -- Build section arguments. Need to check that:
        -- lambda bound variables appear in the right order and only as
        -- top-level arguments.
        sectionArgs :: [A.Name] -> [NamedArg (AppInfo, A.Expr)] -> Maybe [NamedArg (MaybeSection (AppInfo, A.Expr))]
        sectionArgs xs = go xs
          where
            noXs = getAll . foldExpr (\ case A.Var x -> All (notElem x xs)
                                             _       -> All True) . snd . namedArg
            go [] [] = return []
            go (y : ys) (arg : args)
              | visible arg
              , A.Var y' <- snd $ namedArg arg
              , y == y' = (fmap (YesSection <$) arg :) <$> go ys args
            go ys (arg : args)
              | visible arg, noXs arg = ((fmap . fmap) NoSection arg :) <$> go ys args
            go _ _ = Nothing

    view e = (, (map . fmap . fmap) NoSection args) <$> getHead hd
      where Application hd args = A.appView' e

tryToRecoverOpAppP :: A.Pattern -> AbsToCon (Maybe C.Pattern)
tryToRecoverOpAppP p = do
  res <- recoverOpApp bracketP_ (const False) opApp view p
  reportS "print.op" 90
    [ "tryToRecoverOpApp"
    , "in:  " ++ show p
    , "out: " ++ show res
    ]
  return res
  where
    opApp r x n ps = C.OpAppP r x (Set.singleton n) $
      map (defaultNamedArg . fromNoSection __IMPOSSIBLE__) ps
      -- `view` does not generate any `Nothing`s

    appInfo = defaultAppInfo_

    view :: A.Pattern -> Maybe (Hd, [NamedArg (MaybeSection (AppInfo, A.Pattern))])
    view p = case p of
      ConP _        cs ps -> Just (HdCon (headAmbQ cs), (map . fmap . fmap) (NoSection . (appInfo,)) ps)
      DefP _        fs ps -> Just (HdDef (headAmbQ fs), (map . fmap . fmap) (NoSection . (appInfo,)) ps)
      PatternSynP _ ns ps -> Just (HdSyn (headAmbQ ns), (map . fmap . fmap) (NoSection . (appInfo,)) ps)
      _                   -> Nothing
      -- ProjP _ _ d   -> Just (HdDef (headAmbQ d), [])   -- ? Andreas, 2016-04-21

recoverOpApp :: forall a c . (ToConcrete a c, HasRange c)
  => ((PrecedenceStack -> Bool) -> AbsToCon c -> AbsToCon c)
  -> (a -> Bool)  -- ^ Check for lambdas
  -> (Range -> C.QName -> A.Name -> [MaybeSection c] -> c)
  -> (a -> Maybe (Hd, [NamedArg (MaybeSection (AppInfo, a))]))
  -> a
  -> AbsToCon (Maybe c)
recoverOpApp bracket isLam opApp view e = case view e of
  Nothing -> mDefault
  Just (hd, args)
    | all visible args    -> do
      let  args' = map namedArg args
      case hd of
        HdVar  n
          | isNoName n    -> mDefault
          | otherwise     -> doQNameHelper (Left n) args'
        HdDef qn
          | isExtendedLambdaName qn
                          -> mDefault
          | otherwise     -> doQNameHelper (Right qn) args'
        -- HdDef qn          -> doQNameHelper (Right qn) args'
        HdCon qn          -> doQNameHelper (Right qn) args'
        HdSyn qn          -> doQNameHelper (Right qn) args'
    | otherwise           -> mDefault
  where
  mDefault = return Nothing

  skipParens :: MaybeSection (AppInfo, a) -> Bool
  skipParens = \case
     YesSection       -> False
     NoSection (i, e) -> isLam e && preferParenless (appParens i)

  doQNameHelper :: Either A.Name A.QName -> [MaybeSection (AppInfo, a)] -> AbsToCon (Maybe c)
  doQNameHelper n args = do
    x <- either (C.QName <.> toConcrete) toConcrete n
    let n' = either id A.qnameName n
    -- #1346: The fixity of the abstract name is not necessarily correct, it depends on which
    -- concrete name we choose! Make sure to resolve ambiguities with n'.
    fx <- resolveName_ x [n'] <&> \ case
            VarName y _                -> y ^. lensFixity
            DefinedName _ q            -> q ^. lensFixity
            FieldName (q :| _)         -> q ^. lensFixity
            ConstructorName (q :| _)   -> q ^. lensFixity
            PatternSynResName (q :| _) -> q ^. lensFixity
            UnknownName                -> noFixity
    doQName fx x n' args (C.nameParts $ C.unqualify x)

  doQName :: Fixity -> C.QName -> A.Name -> [MaybeSection (AppInfo, a)] -> [NamePart] -> AbsToCon (Maybe c)

  -- fall-back (wrong number of arguments or no holes)
  doQName _ x _ es xs
    | null es                 = mDefault
    | length es /= numHoles x = mDefault

  -- binary case
  doQName fixity x n as xs
    | Hole <- head xs
    , Hole <- last xs = do
        let a1  = head as
            an  = last as
            as' = case as of
                    as@(_ : _ : _) -> init $ tail as
                    _              -> __IMPOSSIBLE__
        Just <$> do
          bracket (opBrackets' (skipParens an) fixity) $ do
            e1 <- traverse (toConcreteCtx (LeftOperandCtx fixity) . snd) a1
            es <- (mapM . traverse) (toConcreteCtx InsideOperandCtx . snd) as'
            en <- traverse (uncurry $ toConcreteCtx . RightOperandCtx fixity . appParens) an
            return $ opApp (getRange (e1, en)) x n ([e1] ++ es ++ [en])

  -- prefix
  doQName fixity x n as xs
    | Hole <- last xs = do
        let an  = last as
            as' = case as of
                    as@(_ : _) -> init as
                    _          -> __IMPOSSIBLE__
        Just <$> do
          bracket (opBrackets' (skipParens an) fixity) $ do
            es <- (mapM . traverse) (toConcreteCtx InsideOperandCtx . snd) as'
            en <- traverse (\ (i, e) -> toConcreteCtx (RightOperandCtx fixity $ appParens i) e) an
            return $ opApp (getRange (n, en)) x n (es ++ [en])

  -- postfix
  doQName fixity x n as xs
    | Hole <- head xs = do
        let a1  = head as
            as' = tail as
        e1 <- traverse (toConcreteCtx (LeftOperandCtx fixity) . snd) a1
        es <- (mapM . traverse) (toConcreteCtx InsideOperandCtx . snd) as'
        Just <$> do
          bracket (opBrackets fixity) $
            return $ opApp (getRange (e1, n)) x n ([e1] ++ es)

  -- roundfix
  doQName _ x n as xs = do
    es <- (mapM . traverse) (toConcreteCtx InsideOperandCtx . snd) as
    Just <$> do
      bracket roundFixBrackets $
        return $ opApp (getRange x) x n es

-- Recovering pattern synonyms --------------------------------------------

-- | Recover pattern synonyms for expressions.
tryToRecoverPatternSyn :: A.Expr -> AbsToCon C.Expr -> AbsToCon C.Expr
tryToRecoverPatternSyn e fallback
  | userWritten e = fallback
  | litOrCon e    = recoverPatternSyn apply matchPatternSyn e fallback
  | otherwise     = fallback
  where
    userWritten (A.App info _ _) = getOrigin info == UserWritten
    userWritten _                = False  -- this means we always use pattern synonyms for nullary constructors

    -- Only literals or constructors can head pattern synonym definitions
    litOrCon e =
      case A.appView e of
        Application Con{}   _ -> True
        Application A.Lit{} _ -> True
        _                     -> False

    apply c args = A.unAppView $ Application (A.PatternSyn $ unambiguous c) args

-- | Recover pattern synonyms in patterns.
tryToRecoverPatternSynP :: A.Pattern -> AbsToCon C.Pattern -> AbsToCon C.Pattern
tryToRecoverPatternSynP = recoverPatternSyn apply matchPatternSynP
  where apply c args = PatternSynP patNoRange (unambiguous c) args

-- | General pattern synonym recovery parameterised over expression type
recoverPatternSyn :: ToConcrete a c =>
  (A.QName -> [NamedArg a] -> a)         -> -- applySyn
  (PatternSynDefn -> a -> Maybe [Arg a]) -> -- match
  a -> AbsToCon c -> AbsToCon c
recoverPatternSyn applySyn match e fallback = do
  doFold <- asks foldPatternSynonyms
  if not doFold then fallback else do
    psyns  <- getAllPatternSyns
    scope  <- getScope
    reportSLn "toConcrete.patsyn" 100 $ render $ hsep $
      [ "Scope when attempting to recover pattern synonyms:"
      , pretty scope
      ]
    let isConP ConP{} = True    -- #2828: only fold pattern synonyms with
        isConP _      = False   --        constructor rhs
        cands = [ (q, args, score rhs)
                | (q, psyndef@(_, rhs)) <- reverse $ Map.toList psyns
                , isConP rhs
                , Just args <- [match psyndef e]
                -- #3879: only fold pattern synonyms with an unqualified concrete name in scope
                -- Note that we only need to consider the head of the inverse lookup result: they
                -- are already sorted from shortest to longest!
                , C.QName{} <- Fold.toList $ listToMaybe $ inverseScopeLookupName q scope
                ]
        cmp (_, _, x) (_, _, y) = flip compare x y
    reportSLn "toConcrete.patsyn" 50 $ render $ hsep $
      [ "Found pattern synonym candidates:"
      , prettyList_ $ map (\ (q,_,_) -> q) cands
      ]
    case sortBy cmp cands of
      (q, args, _) : _ -> toConcrete $ applySyn q $ (map . fmap) unnamed args
      []               -> fallback
  where
    -- Heuristic to pick the best pattern synonym: the one that folds the most
    -- constructors.
    score :: Pattern' Void -> Int
    score = getSum . foldAPattern con
      where con ConP{} = 1
            con _      = 0

-- Some instances that are related to interaction with users -----------

instance ToConcrete InteractionId C.Expr where
    toConcrete (InteractionId i) = return $ C.QuestionMark noRange (Just i)

instance ToConcrete NamedMeta C.Expr where
    toConcrete i = do
      return $ C.Underscore noRange (Just $ prettyShow i)