path: root/src/HSVIS/Typecheck.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE EmptyDataDeriving #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE GADTs #-}
module HSVIS.Typecheck (
  StageTyped,
  typecheck,
  -- * Typed AST synonyms
  TProgram, TDataDef, TDataField, TFunDef, TFunEq, TKind, TType, TPattern, TRHS, TExpr,
) where

import Control.Monad
import Data.Bifunctor (first, second, bimap)
import Data.Foldable (toList)
import Data.List (inits)
import Data.Map.Strict (Map)
import Data.Maybe (fromMaybe)
import qualified Data.Map.Strict as Map
import Data.Tuple (swap)
import Data.Semigroup (First(..))
import Data.Set (Set)
import qualified Data.Set as Set

import Debug.Trace

import Data.Bag
import Data.Map.Monoidal (MMap(..))
import qualified Data.Map.Monoidal as MMap
import HSVIS.AST
import HSVIS.Parser
import HSVIS.Diagnostic
import HSVIS.Pretty
import HSVIS.Typecheck.Solve


data StageTC

type instance X DataDef StageTC = CKind
type instance X DataField StageTC = Range
type instance X FunDef  StageTC = Range
type instance X TypeSig StageTC = Maybe Range
type instance X FunEq   StageTC = ()
type instance X Kind    StageTC = ()
type instance X Type    StageTC = CKind
type instance X Pattern StageTC = (Range, CType)
type instance X RHS     StageTC = CType
type instance X Expr    StageTC = (Range, CType)

data instance E Type StageTC = TUniVar Int deriving (Show, Eq, Ord)
data instance E Kind StageTC = KUniVar Int deriving (Show, Eq, Ord)
data instance E TypeSig StageTC deriving (Show)

type CProgram = Program StageTC
type CDataDef = DataDef StageTC
type CDataField = DataField StageTC
type CFunDef  = FunDef  StageTC
type CFunEq   = FunEq   StageTC
type CKind    = Kind    StageTC
type CType    = Type    StageTC
type CPattern = Pattern StageTC
type CRHS     = RHS     StageTC
type CExpr    = Expr    StageTC

data StageTyped

type instance X DataDef StageTyped = TKind
type instance X DataField StageTyped = Range
type instance X FunDef  StageTyped = ()
type instance X TypeSig StageTyped = ()
type instance X FunEq   StageTyped = ()
type instance X Kind    StageTyped = ()
type instance X Type    StageTyped = TKind
type instance X Pattern StageTyped = TType
type instance X RHS     StageTyped = TType
type instance X Expr    StageTyped = TType

data instance E Type StageTyped deriving (Show)
data instance E Kind StageTyped deriving (Show)
data instance E TypeSig StageTyped deriving (Show)

type TProgram = Program StageTyped
type TDataDef = DataDef StageTyped
type TDataField = DataField StageTyped
type TFunDef  = FunDef  StageTyped
type TFunEq   = FunEq   StageTyped
type TKind    = Kind    StageTyped
type TType    = Type    StageTyped
type TPattern = Pattern StageTyped
type TRHS     = RHS     StageTyped
type TExpr    = Expr    StageTyped

instance Pretty (E Type StageTC) where
  prettysPrec _ (TUniVar n) = showString ("?t" ++ show n)

instance Pretty (E Kind StageTC) where
  prettysPrec _ (KUniVar n) = showString ("?k" ++ show n)


typecheck :: FilePath -> String -> PProgram -> ([Diagnostic], TProgram)
typecheck fp source prog =
  let (ds, cs, _, _, resprog) =
        runTCM (tcTop prog) (fp, source) 1 (Env mempty mempty mempty)
  in trace ("[tc] resprog = " ++ show resprog) $
     if not (null cs)
       then error $ "Constraints left after typechecker completion: " ++ show cs
       else (toList ds, resprog)

data Constr
  -- Equality constraints: "left" must be equal to "right" because of the thing
  -- at the given range. "left" is the expected thing; "right" is the observed
  -- thing.
  = CEq CType CType Range
  | CEqK CKind CKind Range
  deriving (Show)

data Env = Env (Map Name CKind)   -- ^ types in scope (including variables)
               (Map Name CType)   -- ^ values in scope (constructors and variables)
               (Map Name InvCon)  -- ^ patterns in scope (inverse constructors)
  deriving (Show)

data InvCon = InvCon (Map Name CKind)  -- ^ universally quantified type variables
                     CType    -- ^ input type of the inverse constructor (result of the constructor)
                     [CType]  -- ^ field types
  deriving (Show)

newtype TCM a = TCM {
  runTCM :: (FilePath, String)  -- ^ reader context: file and file contents
         -> Int  -- ^ state: next id to generate
         -> Env  -- ^ state: type and value environment
         -> (Bag Diagnostic  -- ^ writer: diagnostics
            ,Bag Constr      -- ^ writer: constraints
            ,Int, Env, a)
  }

instance Functor TCM where
  fmap f (TCM g) = TCM $ \ctx i env ->
    let (ds, cs, i', env', x) = g ctx i env
    in (ds, cs, i', env', f x)

instance Applicative TCM where
  pure x = TCM $ \_ i env -> (mempty, mempty, i, env, x)
  (<*>) = ap

instance Monad TCM where
  TCM f >>= g = TCM $ \ctx i1 env1 ->
    let (ds2, cs2, i2, env2, x) = f ctx i1 env1
        (ds3, cs3, i3, env3, y) = runTCM (g x) ctx i2 env2
    in (ds2 <> ds3, cs2 <> cs3, i3, env3, y)

class Monad m => MonadRaise m where
  raise :: Severity -> Range -> String -> m ()

instance MonadRaise TCM where
  raise sev rng@(Range (Pos y _) _) msg = TCM $ \(fp, source) i env ->
    (pure (Diagnostic sev fp rng [] (lines source !! y) msg), mempty, i, env, ())

emit :: Constr -> TCM ()
emit c = TCM $ \_ i env -> (mempty, pure c, i, env, ())

collectConstraints :: (Constr -> Maybe b) -> TCM a -> TCM (Bag b, a)
collectConstraints predicate (TCM f) = TCM $ \ctx i env ->
  let (ds, cs, i', env', x) = f ctx i env
      (yes, no) = bagPartition predicate cs
  in (ds, no, i', env', (yes, x))

getFullEnv :: TCM Env
getFullEnv = TCM $ \_ i env -> (mempty, mempty, i, env, env)

putFullEnv :: Env -> TCM ()
putFullEnv env = TCM $ \_ i _ -> (mempty, mempty, i, env, ())

genId :: TCM Int
genId = TCM $ \_ i env -> (mempty, mempty, i + 1, env, i)

getKind :: Name -> TCM (Maybe CKind)
getKind name = do
  Env tenv _ _ <- getFullEnv
  return (Map.lookup name tenv)

getType :: Name -> TCM (Maybe CType)
getType name = do
  Env _ venv _ <- getFullEnv
  return (Map.lookup name venv)

getInvCon :: Name -> TCM (Maybe InvCon)
getInvCon name = do
  Env _ _ icenv <- getFullEnv
  return (Map.lookup name icenv)

modifyTEnv :: (Map Name CKind -> Map Name CKind) -> TCM ()
modifyTEnv f = do
  Env tenv venv icenv <- getFullEnv
  putFullEnv (Env (f tenv) venv icenv)

modifyVEnv :: (Map Name CType -> Map Name CType) -> TCM ()
modifyVEnv f = do
  Env tenv venv icenv <- getFullEnv
  putFullEnv (Env tenv (f venv) icenv)

modifyICEnv :: (Map Name InvCon -> Map Name InvCon) -> TCM ()
modifyICEnv f = do
  Env tenv venv icenv <- getFullEnv
  putFullEnv (Env tenv venv (f icenv))

scopeTEnv :: TCM a -> TCM a
scopeTEnv m = do
  Env origtenv _ _ <- getFullEnv
  res <- m
  modifyTEnv (\_ -> origtenv)
  return res

scopeVEnv :: TCM a -> TCM a
scopeVEnv m = do
  Env _ origvenv _ <- getFullEnv
  res <- m
  modifyVEnv (\_ -> origvenv)
  return res

genKUniVar :: TCM CKind
genKUniVar = KExt () . KUniVar <$> genId

genUniVar :: CKind -> TCM CType
genUniVar k = TExt k . TUniVar <$> genId

getKind' :: Range -> Name -> TCM CKind
getKind' rng name = getKind name >>= \case
  Nothing -> do
    raise SError rng $ "Type not in scope: " ++ pretty name
    genKUniVar
  Just k -> return k

getType' :: Range -> Name -> TCM CType
getType' rng name = getType name >>= \case
  Nothing -> do
    raise SError rng $ "Variable not in scope: " ++ pretty name
    genUniVar (KType ())
  Just k -> return k

tcTop :: PProgram -> TCM TProgram
tcTop prog = do
  (cs, prog') <- collectConstraints Just (tcProgram prog)
  (subK, subT) <- solveConstrs cs
  finaliseProg (substProg subK subT prog')

tcProgram :: PProgram -> TCM CProgram
tcProgram (Program ddefs1 fdefs1) = do
  -- kind-check data definitions and collect ensuing kind constraints
  (kconstrs, ddefs2) <- collectConstraints isCEqK $ do
    ks <- mapM prepareDataDef ddefs1
    zipWithM kcDataDef ks ddefs1

  -- solve the kind constraints and finalise data types
  kinduvars <- solveKindVars kconstrs
  let ddefs3 = map (substDdef kinduvars mempty) ddefs2
  modifyTEnv (Map.map (substKind kinduvars))

  -- generate inverse constructors for all data types
  forM_ ddefs3 $ \ddef ->
    modifyICEnv (Map.fromList (generateInvCons ddef) <>)

  traceM (unlines (map pretty ddefs3))

  -- check the function definitions
  fdefs2 <- tcFunDefBlock fdefs1

  return (Program ddefs3 fdefs2)

-- Bring data type name in scope with a kind of the specified arity
prepareDataDef :: PDataDef -> TCM (CKind, [CKind])
prepareDataDef (DataDef _ name params _) = do
  parkinds <- mapM (\_ -> genKUniVar) params
  let k = foldr (KFun ()) (KType ()) parkinds
  modifyTEnv (Map.insert name k)
  return (k, parkinds)

-- Assumes that the kind of the name itself has already been registered with
-- the correct arity (this is done by prepareDataDef).
kcDataDef :: (CKind, [CKind]) -> PDataDef -> TCM CDataDef
kcDataDef (kd, parkinds) (DataDef _ name params cons) = do
  -- ensure unicity of type param names
  params' <-
    let prenames = Set.fromList (map snd params)
        namegen = filter (`Set.notMember` prenames) [Name ('t' : show i) | i <- [1::Int ..]]
    in forM (zip3 params (inits (map snd params)) namegen) $ \((rng, pname), previous, replname) ->
         if pname `elem` previous
           then do raise SError rng $ "Duplicate type parameter: " ++ pretty pname
                   return replname
           else return pname

  -- kind-check the constructors
  cons' <- scopeTEnv $ do
    modifyTEnv (Map.fromList (zip params' parkinds) <>)
    forM cons $ \(cname, fields) -> do
      fields' <- forM fields $ \(DataField () t) ->
        DataField (extOf t) <$> kcType KCTMNormal (Just (KType ())) t
      return (cname, fields')

  return (DataDef kd name (zip parkinds params') cons')

generateInvCons :: CDataDef -> [(Name, InvCon)]
generateInvCons (DataDef k tname params cons) =
  let tyvars = Map.fromList (map swap params)
      resty = TApp (KType ()) (TCon k tname) (map (uncurry TVar) params)
  in [(cname, InvCon tyvars resty (map dataFieldType fields)) | (cname, fields) <- cons]

promoteDownK :: Maybe CKind -> TCM CKind
promoteDownK Nothing = genKUniVar
promoteDownK (Just k) = return k

downEqK :: Range -> Maybe CKind -> CKind -> TCM ()
downEqK _ Nothing _ = return ()
downEqK rng (Just k1) k2 = emit $ CEqK k1 k2 rng

data KCTypeMode ext ret where
  -- | Kind-check a normal type: out-of-scope type variables are reported as errors.
  KCTMNormal :: KCTypeMode () CType

  -- | Kind-check an open type: out-of-scope type variables are returned. This
  -- is used to check function type signatures, which may have an implicit
  -- forall telescope at the head.
  KCTMOpen :: KCTypeMode (MMap Name (First CKind)) (CType, Map Name CKind)

-- | Given (maybe) the expected kind of this type, and a type, check it for
-- kind-correctness.
kcType :: forall ext ret. KCTypeMode ext ret -> Maybe CKind -> PType -> TCM ret
kcType KCTMNormal mdown t = snd <$> kcType' KCTMNormal mdown t
kcType KCTMOpen mdown t = second (\(MMap m) -> Map.map getFirst m) . swap <$> kcType' KCTMOpen mdown t

-- | Given (maybe) the expected kind of this type, and a type, check it for
-- kind-correctness.
kcType' :: forall ext ret. Monoid ext => KCTypeMode ext ret -> Maybe CKind -> PType -> TCM (ext, CType)
kcType' mode mdown = \case
  TApp rng t ts -> do
    (ext1, t') <- kcType' mode Nothing t
    (ext2, ts') <- sequence <$> mapM (kcType' mode Nothing) ts
    retk <- promoteDownK mdown
    let expected = foldr (KFun ()) retk (map extOf ts')
    emit $ CEqK (extOf t') expected rng
    return (ext1 <> ext2, TApp retk t' ts')

  TTup rng ts -> do
    (ext, ts') <- sequence <$> mapM (kcType' mode (Just (KType ()))) ts
    forM_ (zip (map extOf ts) ts') $ \(trng, ct) ->
      emit $ CEqK (extOf ct) (KType ()) trng
    downEqK rng mdown (KType ())
    return (ext, TTup (KType ()) ts')

  TList rng t -> do
    (ext, t') <- kcType' mode (Just (KType ())) t
    emit $ CEqK (extOf t') (KType ()) (extOf t)
    downEqK rng mdown (KType ())
    return (ext, TList (KType ()) t')

  TFun rng t1 t2 -> do
    (ext1, t1') <- kcType' mode (Just (KType ())) t1
    (ext2, t2') <- kcType' mode (Just (KType ())) t2
    emit $ CEqK (extOf t1') (KType ()) (extOf t1)
    emit $ CEqK (extOf t2') (KType ()) (extOf t2)
    downEqK rng mdown (KType ())
    return (ext1 <> ext2, TFun (KType ()) t1' t2')

  TCon rng n -> do
    k <- getKind' rng n
    downEqK rng mdown k
    return (mempty, TCon k n)

  TVar rng n -> do
    k <- getKind' rng n
    downEqK rng mdown k
    return (case mode of KCTMNormal -> ()
                         KCTMOpen -> MMap.singleton n (pure k)
           ,TVar k n)

  TForall rng n t -> do  -- implicit forall
    k1 <- genKUniVar
    k2 <- genKUniVar
    downEqK rng mdown k2
    (ext, t') <- scopeTEnv $ do
      modifyTEnv (Map.insert n k1)
      kcType' mode (Just k2) t
    return (ext, TForall k2 n t')  -- not 'k1 -> k2' because the forall is implicit

tcFunDefBlock :: [PFunDef] -> TCM [CFunDef]
tcFunDefBlock fdefs = do
  -- generate preliminary unification variables for the functions' types
  bound <- mapM (\(FunDef _ n _ _) -> (n,) <$> genUniVar (KType ())) fdefs
  defs' <- forM fdefs $ \def@(FunDef _ name _ _) ->
    scopeVEnv $ do
      modifyVEnv (Map.fromList [(n, t) | (n, t) <- bound, n /= name] <>)
      tcFunDef def

  -- take the actual found types for typechecking the body (and link them
  -- to the variables generated above)
  let bound2 = map (\(FunDef _ n (TypeSig _ ty) _) -> (n, ty)) defs'
  forM_ (zip3 fdefs bound bound2) $ \(fdef, (_, tvar), (_, ty)) ->
    emit $ CEq ty tvar (extOf fdef)  -- which is expected/observed? which range? /shrug/

  return defs'

tcFunDef :: PFunDef -> TCM CFunDef
tcFunDef (FunDef rng name msig eqs) = do
  when (not $ allEq (fmap (length . funeqPats) eqs)) $
    raise SError rng "Function equations have differing numbers of arguments"

  (typ, msigrng) <- case msig of
    TypeSig _ sig -> do
      (typ, freetvars) <- kcType KCTMOpen (Just (KType ())) sig
      TODO -- We need to check that these free type variables do not escape.
           -- Perhaps with levels on unification variables? Associate a level
           -- to a generated uvar, and increment the global level counter when
           -- passing below a forall.
           -- But how do we deal with functions without a type signature
           -- anyway? We should be able to infer a polymorphic type for them.
      return (foldr (\(n, k) -> TForall k n) typ (Map.assocs freetvars)
             ,Just (extOf sig))
    TypeSigExt _ NoTypeSig -> (,Nothing) <$> genUniVar (KType ())

  eqs' <- scopeVEnv $ do
    modifyVEnv (Map.insert name typ)  -- allow function to be recursive
    mapM (tcFunEq typ) eqs

  return (FunDef rng name (TypeSig msigrng typ) eqs')

tcFunEq :: CType -> PFunEq -> TCM CFunEq
tcFunEq down (FunEq rng name pats rhs) = do
  -- getFullEnv >>= \env -> traceM $ "[tcFunEq] Env = " ++ show env
  (argtys, rhsty) <- unfoldFunTy rng (length pats) down
  scopeVEnv $ do
    pats' <- zipWithM tcPattern argtys pats
    rhs' <- tcRHS rhsty rhs
    return (FunEq () name pats' rhs')

-- | Brings the bound variables in scope
tcPattern :: CType -> PPattern -> TCM CPattern
tcPattern down = \case
  PWildcard rng -> return $ PWildcard (rng, down)

  PVar rng n -> modifyVEnv (Map.insert n down) >> return (PVar (rng, down) n)

  PAs rng n p -> do
    modifyVEnv (Map.insert n down)
    p' <- tcPattern down p
    return $ PAs (rng, snd (extOf p')) n p'

  PCon rng n ps ->
    getInvCon n >>= \case
      Just (InvCon tyvars match fields) -> do
        unisub <- mapM genUniVar tyvars  -- substitution for the universally quantified variables
        let match' = substType mempty mempty unisub match
            fields' = map (substType mempty mempty unisub) fields
        emit $ CEq down match' rng
        PCon (rng, match') n <$> zipWithM tcPattern fields' ps
      Nothing -> do
        raise SError rng $ "Constructor not in scope: " ++ pretty n
        return (PWildcard (rng, down))

  POp rng p1 op p2 ->
    case op of
      OCons -> do
        eltty <- genUniVar (KType ())
        let listty = TList (KType ()) eltty
        emit $ CEq down listty rng
        p1' <- tcPattern eltty p1
        p2' <- tcPattern listty p2
        return (POp (rng, listty) p1' OCons p2')
      _ -> do
        raise SError rng $ "Operator is not a constructor: " ++ pretty op
        return (PWildcard (rng, down))

  PList rng ps -> do
    eltty <- genUniVar (KType ())
    let listty = TList (KType ()) eltty
    emit $ CEq down listty rng
    PList (rng, listty) <$> mapM (tcPattern eltty) ps

  PTup rng ps -> do
    ts <- mapM (\_ -> genUniVar (KType ())) ps
    emit $ CEq down (TTup (KType ()) ts) rng
    PTup (rng, TTup (KType ()) ts) <$> zipWithM tcPattern ts ps

tcRHS :: CType -> PRHS -> TCM CRHS
tcRHS _ (Guarded _ _) = error "typecheck: Guards not yet supported"
tcRHS down (Plain _ e) = do
  e' <- tcExpr down e
  return $ Plain (snd (extOf e')) e'

tcExpr :: CType -> PExpr -> TCM CExpr
tcExpr down = \case
  ELit rng lit -> do
    let ty = case lit of
               LInt{} -> TCon (KType ()) (Name "Int")
               LFloat{} -> TCon (KType ()) (Name "Double")
               LChar{} -> TCon (KType ()) (Name "Char")
               LString{} -> TList (KType ()) (TCon (KType ()) (Name "Char"))
    emit $ CEq down ty rng
    return $ ELit (rng, ty) lit

  EVar rng n -> do
    ty <- getType' rng n
    emit $ CEq down ty rng
    return $ EVar (rng, ty) n

  ECon rng n -> do
    ty <- getType' rng n
    emit $ CEq down ty rng
    return $ EVar (rng, ty) n

  EList rng es -> do
    eltty <- genUniVar (KType ())
    let listty = TList (KType ()) eltty
    emit $ CEq down listty rng
    EList (rng, listty) <$> mapM (tcExpr listty) es

  ETup rng es -> do
    ts <- mapM (\_ -> genUniVar (KType ())) es
    emit $ CEq down (TTup (KType ()) ts) rng
    ETup (rng, TTup (KType ()) ts) <$> zipWithM tcExpr ts es

  EApp rng e1 es -> do
    argtys <- mapM (\_ -> genUniVar (KType ())) es
    let funty = foldr (TFun (KType ())) down argtys
    EApp (rng, funty) <$> tcExpr funty e1
                      <*> zipWithM tcExpr argtys es

  -- TODO: these types are way too monomorphic and in any case these
  -- ~operators~ functions should not be built-in
  EOp rng e1 op e2 -> do
    let int = TCon (KType ()) (Name "Int")
        bool = TCon (KType ()) (Name "Bool")
    (rty, aty1, aty2) <- case op of
      OAdd -> return (int, int, int)
      OSub -> return (int, int, int)
      OMul -> return (int, int, int)
      ODiv -> return (int, int, int)
      OMod -> return (int, int, int)
      OEqu -> return (bool, int, int)
      OPow -> return (int, int, int)
      OCons -> do
        eltty <- genUniVar (KType ())
        let listty = TList (KType ()) eltty
        return (listty, eltty, listty)
    emit $ CEq down rty rng
    e1' <- tcExpr aty1 e1
    e2' <- tcExpr aty2 e2
    return (EOp (rng, rty) e1' op e2')

  EIf rng e1 e2 e3 -> do
    e1' <- tcExpr (TCon (KType ()) (Name "Bool")) e1
    e2' <- tcExpr down e2
    e3' <- tcExpr down e3
    return (EIf (rng, down) e1' e2' e3')

  ECase rng e1 alts -> do
    ty <- genUniVar (KType ())
    e1' <- tcExpr ty e1
    alts' <- forM alts $ \(pat, rhs) ->
      scopeVEnv $
        (,) <$> tcPattern ty pat <*> tcRHS down rhs
    return $ ECase (rng, down) e1' alts'

  ELet rng defs body -> do
    defs' <- tcFunDefBlock defs
    let bound2 = map (\(FunDef _ n (TypeSig _ ty) _) -> (n, ty)) defs'
    scopeVEnv $ do
      modifyVEnv (Map.fromList bound2 <>)
      body' <- tcExpr down body
      return $ ELet (rng, down) defs' body'

  EError rng -> return $ EError (rng, down)

unfoldFunTy :: Range -> Int -> CType -> TCM ([CType], CType)
unfoldFunTy _ n t | n <= 0 = return ([], t)
unfoldFunTy rng n (TFun _ t1 t2) = do
  (params, core) <- unfoldFunTy rng (n - 1) t2
  return (t1 : params, core)
unfoldFunTy rng n t = do
  vars <- replicateM n (genUniVar (KType ()))
  core <- genUniVar (KType ())
  let expected = foldr (TFun (KType ())) core vars
  emit $ CEq expected t rng
  return (vars, core)

solveConstrs :: MonadRaise m => Bag Constr -> m (Map Int CKind, Map Int CType)
solveConstrs constrs = do
  let (tcs, kcs) = partitionConstrs constrs
  subK <- solveKindVars kcs
  subT <- solveTypeVars tcs
  let subT' = Map.map (substType subK mempty mempty) subT
  return (subK, subT')

solveKindVars :: MonadRaise m => Bag (CKind, CKind, Range) -> m (Map Int CKind)
solveKindVars cs = do
  let (asg, errs) =
        solveConstraints
          reduce
          (foldMap pure . kindUniVars)
          substKind
          (\case KExt () (KUniVar v) -> Just v
                 _ -> Nothing)
          kindSize
          (toList cs)

  forM_ errs $ \case
    UEUnequal k1 k2 rng ->
      raise SError rng $
        "Kind mismatch:\n\
        \- " ++ pretty k1 ++ "\n\
        \- " ++ pretty k2
    UERecursive uvar k rng ->
      raise SError rng $
        "Kind cannot be recursive: " ++ pretty (KExt () (KUniVar uvar)) ++ " = " ++ pretty k

  -- default unconstrained kind variables to Type
  let unconstrKUVars = foldMap kindUniVars (Map.elems asg) Set.\\ Map.keysSet asg
      defaults = Map.fromList (map (,KType ()) (toList unconstrKUVars))
      asg' = Map.map (substKind defaults) asg <> defaults

  return asg'
  where
    reduce :: CKind -> CKind -> Range -> (Bag (Int, CKind, Range), Bag (CKind, CKind, Range))
    reduce lhs rhs rng = case (lhs, rhs) of
      -- unification variables produce constraints on a unification variable
      (KExt () (KUniVar i), KExt () (KUniVar j)) | i == j -> mempty
      (KExt () (KUniVar i), k                  ) -> (pure (i, k, rng), mempty)
      (k                  , KExt () (KUniVar i)) -> (pure (i, k, rng), mempty)

      -- if lhs and rhs have equal prefixes, recurse
      (KType ()   , KType ()   ) -> mempty
      (KFun () a b, KFun () c d) -> reduce a c rng <> reduce b d rng

      -- otherwise, this is a kind mismatch
      (k1, k2) -> (mempty, pure (k1, k2, rng))

    kindSize :: CKind -> Int
    kindSize KType{} = 1
    kindSize (KFun () a b) = 1 + kindSize a + kindSize b
    kindSize (KExt () KUniVar{}) = 2

solveTypeVars :: MonadRaise m => Bag (CType, CType, Range) -> m (Map Int CType)
solveTypeVars cs = do
  let (asg, errs) =
        solveConstraints
          reduce
          (foldMap pure . typeUniVars)
          (\m -> substType mempty m mempty)
          (\case TExt _ (TUniVar v) -> Just v
                 _ -> Nothing)
          typeSize
          (toList cs)

  forM_ errs $ \case
    UEUnequal t1 t2 rng ->
      raise SError rng $
        "Type mismatch:\n\
        \- " ++ pretty t1 ++ "\n\
        \- " ++ pretty t2
    UERecursive uvar t rng ->
      raise SError rng $
        "Type cannot be recursive: " ++ pretty (TExt (extOf t) (TUniVar uvar)) ++ " = " ++ pretty t

  return asg
  where
    reduce :: CType -> CType -> Range -> (Bag (Int, CType, Range), Bag (CType, CType, Range))
    reduce lhs rhs rng = case (lhs, rhs) of
      -- unification variables produce constraints on a unification variable
      (TExt _ (TUniVar i), TExt _ (TUniVar j)) | i == j -> mempty
      (TExt _ (TUniVar i), t                 ) -> (pure (i, t, rng), mempty)
      (t                 , TExt _ (TUniVar i)) -> (pure (i, t, rng), mempty)

      -- if lhs and rhs have equal prefixes, recurse
      (TApp _ t ts, TApp _ t' ts') -> reduce t t' rng <> foldMap (\(a, b) -> reduce a b rng) (zip ts ts')
      (TTup _ ts, TTup _ ts') -> foldMap (\(a, b) -> reduce a b rng) (zip ts ts')
      (TList _ t, TList _ t') -> reduce t t' rng
      (TFun _ t1 t2, TFun _ t1' t2') -> reduce t1 t1' rng <> reduce t2 t2' rng
      (TCon _ n1, TCon _ n2) | n1 == n2 -> mempty
      (TVar _ n1, TVar _ n2) | n1 == n2 -> mempty
      (TForall _ n1 t1, TForall k n2 t2) ->
        reduce t1 (substType mempty mempty (Map.singleton n2 (TVar k n1)) t2) rng

      -- otherwise, this is a kind mismatch
      (k1, k2) -> (mempty, pure (k1, k2, rng))

    typeSize :: CType -> Int
    typeSize (TApp _ t ts) = typeSize t + sum (map typeSize ts)
    typeSize (TTup _ ts) = sum (map typeSize ts)
    typeSize (TList _ t) = 1 + typeSize t
    typeSize (TFun _ t1 t2) = typeSize t1 + typeSize t2
    typeSize (TCon _ _) = 1
    typeSize (TVar _ _) = 1
    typeSize (TForall _ _ t) = 1 + typeSize t
    typeSize (TExt _ TUniVar{}) = 2

partitionConstrs :: Foldable t => t Constr -> (Bag (CType, CType, Range), Bag (CKind, CKind, Range))
partitionConstrs = foldMap $ \case CEq t1 t2 r -> (pure (t1, t2, r), mempty)
                                   CEqK k1 k2 r -> (mempty, pure (k1, k2, r))

-------------------- SUBSTITUTION FUNCTIONS --------------------
-- These take some of:
-- - an instantiation map for kind unification variables (Map Int CKind)
-- - an instantiation map for type unification variables (Map Int CType)
-- - an instantiation map for type variables (Map Name CType)

substProg :: Map Int CKind -> Map Int CType -> CProgram -> CProgram
substProg mk mt (Program ds fs) = Program (map (substDdef mk mt) ds) (map (substFdef mk mt) fs)

substDdef :: Map Int CKind -> Map Int CType -> CDataDef -> CDataDef
substDdef mk mt (DataDef k name pars cons) =
  DataDef (substKind mk k) name
          (map (first (substKind mk)) pars)
          (map (second (map (substDataField mk mt))) cons)

substDataField :: Map Int CKind -> Map Int CType -> CDataField -> CDataField
substDataField mk mt (DataField rng t) = DataField rng (substType mk mt mempty t)

substFdef :: Map Int CKind -> Map Int CType -> CFunDef -> CFunDef
substFdef mk mt (FunDef rng n (TypeSig sigrng sig) eqs) =
  FunDef rng n
         (TypeSig sigrng (substType mk mt mempty sig))
         (fmap (substFunEq mk mt) eqs)

substFunEq :: Map Int CKind -> Map Int CType -> CFunEq -> CFunEq
substFunEq mk mt (FunEq () n ps rhs) =
  FunEq () n
        (map (substPattern mk mt) ps)
        (substRHS mk mt rhs)

substRHS :: Map Int CKind -> Map Int CType -> CRHS -> CRHS
substRHS _  _  (Guarded _ _) = error "typecheck: guards unsupported"
substRHS mk mt (Plain t e) = Plain (substType mk mt mempty t) (substExpr mk mt e)

substPattern :: Map Int CKind -> Map Int CType -> CPattern -> CPattern
substPattern mk mt = go
  where
    go (PWildcard e) = PWildcard (goExt e)
    go (PVar e n) = PVar (goExt e) n
    go (PAs e n p) = PAs (goExt e) n (go p)
    go (PCon e n ps) = PCon (goExt e) n (map go ps)
    go (POp e p1 op p2) = POp (goExt e) (go p1) op (go p2)
    go (PList e ps) = PList (goExt e) (map go ps)
    go (PTup e ps) = PTup (goExt e) (map go ps)

    goExt (rng, t) = (rng, substType mk mt mempty t)

substExpr :: Map Int CKind -> Map Int CType -> CExpr -> CExpr
substExpr mk mt = go
  where
    go (ELit e lit) = ELit (goExt e) lit
    go (EVar e n) = EVar (goExt e) n
    go (ECon e n) = ECon (goExt e) n
    go (EList e es) = EList (goExt e) (map go es)
    go (ETup e es) = ETup (goExt e) (map go es)
    go (EApp e e1 es) = EApp (goExt e) (go e1) (map go es)
    go (EOp e e1 op e2) = EOp (goExt e) (go e1) op (go e2)
    go (EIf e e1 e2 e3) = EIf (goExt e) (go e1) (go e2) (go e3)
    go (ECase e e1 alts) = ECase (goExt e) (go e1) (map (bimap (substPattern mk mt) (substRHS mk mt)) alts)
    go (ELet e defs body) = ELet (goExt e) (map (substFdef mk mt) defs) (go body)
    go (EError e) = EError (goExt e)

    goExt (rng, t) = (rng, substType mk mt mempty t)

substType :: Map Int CKind -> Map Int CType -> Map Name CType -> CType -> CType
substType mk mt mtv = go
  where
    go (TApp k t ts) = TApp (goKind k) (go t) (map go ts)
    go (TTup k ts) = TTup (goKind k) (map go ts)
    go (TList k t) = TList (goKind k) (go t)
    go (TFun k t1 t2) = TFun (goKind k) (go t1) (go t2)
    go (TCon k n) = TCon (goKind k) n
    go (TVar k n) = fromMaybe (TVar (goKind k) n) (Map.lookup n mtv)
    go (TForall k n t) = TForall (goKind k) n (go t)
    go (TExt k (TUniVar v)) = fromMaybe (TExt (goKind k) (TUniVar v)) (Map.lookup v mt)

    goKind = substKind mk

substKind :: Map Int CKind -> CKind -> CKind
substKind m = \case
  KType () -> KType ()
  KFun () k1 k2 -> KFun () (substKind m k1) (substKind m k2)
  k@(KExt () (KUniVar v)) -> fromMaybe k (Map.lookup v m)

-------------------- FINALISATION FUNCTIONS --------------------
-- These report free type unification variables.

finaliseProg :: MonadRaise m => CProgram -> m TProgram
finaliseProg (Program ds fs) =
  Program <$> traverse finaliseDdef ds <*> traverse finaliseFdef fs

finaliseDdef :: MonadRaise m => CDataDef -> m TDataDef
finaliseDdef (DataDef k name pars cons) =
  DataDef <$> finaliseKind k <*> pure name
          <*> traverse (firstM finaliseKind) pars
          <*> traverse (secondM (traverse finaliseDataField)) cons

finaliseDataField :: MonadRaise m => CDataField -> m TDataField
finaliseDataField (DataField rng t) = DataField rng <$> finaliseType rng t

finaliseFdef :: MonadRaise m => CFunDef -> m TFunDef
finaliseFdef (FunDef rng n (TypeSig msigrng sig) eqs) =
  FunDef () n
         <$> (TypeSig () <$> finaliseType (fromMaybe rng msigrng) sig)
         <*> traverse finaliseFunEq eqs

finaliseFunEq :: MonadRaise m => CFunEq -> m TFunEq
finaliseFunEq (FunEq () n ps rhs) =
  FunEq () n
        <$> traverse finalisePattern ps
        <*> finaliseRHS rhs

finaliseRHS :: MonadRaise m => CRHS -> m TRHS
finaliseRHS (Guarded _ _) = error "typecheck: guards unsupported"
finaliseRHS (Plain t e) = Plain <$> finaliseType (fst (extOf e)) t <*> finaliseExpr e

finalisePattern :: MonadRaise m => CPattern -> m TPattern
finalisePattern = \case
  PWildcard e -> PWildcard <$> goExt e
  PVar e n -> PVar <$> goExt e <*> pure n
  PAs e n p -> PAs <$> goExt e <*> pure n <*> finalisePattern p
  PCon e n ps -> PCon <$> goExt e <*> pure n <*> traverse finalisePattern ps
  POp e p1 op p2 -> POp <$> goExt e <*> finalisePattern p1 <*> pure op <*> finalisePattern p2
  PList e ps -> PList <$> goExt e <*> traverse finalisePattern ps
  PTup e ps -> PTup <$> goExt e <*> traverse finalisePattern ps
  where
    goExt (rng, t) = finaliseType rng t

finaliseExpr :: MonadRaise m => CExpr -> m TExpr
finaliseExpr = \case
  ELit e lit -> ELit <$> goExt e <*> pure lit
  EVar e n -> EVar <$> goExt e <*> pure n
  ECon e n -> ECon <$> goExt e <*> pure n
  EList e es -> EList <$> goExt e <*> traverse finaliseExpr es
  ETup e es -> ETup <$> goExt e <*> traverse finaliseExpr es
  EApp e e1 es -> EApp <$> goExt e <*> finaliseExpr e1 <*> traverse finaliseExpr es
  EOp e e1 op e2 -> EOp <$> goExt e <*> finaliseExpr e1 <*> pure op <*> finaliseExpr e2
  EIf e e1 e2 e3 -> EIf <$> goExt e <*> finaliseExpr e1 <*> finaliseExpr e2 <*> finaliseExpr e3
  ECase e e1 alts -> ECase <$> goExt e <*> finaliseExpr e1 <*> traverse (bimapM finalisePattern finaliseRHS) alts
  ELet e defs body -> ELet <$> goExt e <*> traverse finaliseFdef defs <*> finaliseExpr body
  EError e -> EError <$> goExt e
  where
    goExt (rng, t) = finaliseType rng t

finaliseType :: MonadRaise m => Range -> CType -> m TType
finaliseType rng toptype = go toptype
  where
    go (TApp k t ts) = TApp <$> finaliseKind k <*> go t <*> traverse go ts
    go (TTup k ts) = TTup <$> finaliseKind k <*> traverse go ts
    go (TList k t) = TList <$> finaliseKind k <*> go t
    go (TFun k t1 t2) = TFun <$> finaliseKind k <*> go t1 <*> go t2
    go (TCon k n) = TCon <$> finaliseKind k <*> pure n
    go (TVar k n) = TVar <$> finaliseKind k <*> pure n
    go (TForall k n t) = TForall <$> finaliseKind k <*> pure n <*> go t
    go t@(TExt k TUniVar{}) = do
      raise SError rng $ "Ambiguous type unification variable " ++ pretty t ++ " in type: " ++ pretty toptype
      TVar <$> finaliseKind k <*> pure (Name "$_error")

finaliseKind :: MonadRaise m => CKind -> m TKind
finaliseKind = \case
  KType () -> pure (KType ())
  KFun () k1 k2 -> KFun () <$> finaliseKind k1 <*> finaliseKind k2
  k@(KExt () KUniVar{}) -> error $ "Unresolved kind variable: " ++ pretty k

-------------------- END OF FINALISATION FUNCTIONS --------------------

typeUniVars :: CType -> Set Int
typeUniVars = \case
  TApp _ t ts -> typeUniVars t <> foldMap typeUniVars ts
  TTup _ ts -> foldMap typeUniVars ts
  TList _ t -> typeUniVars t
  TFun _ t1 t2 -> typeUniVars t1 <> typeUniVars t2
  TCon _ _ -> mempty
  TVar _ _ -> mempty
  TForall _ _ t -> typeUniVars t
  TExt _ (TUniVar v) -> Set.singleton v

kindUniVars :: CKind -> Set Int
kindUniVars = \case
  KType{} -> mempty
  KFun () a b -> kindUniVars a <> kindUniVars b
  KExt () (KUniVar v) -> Set.singleton v

allEq :: (Eq a, Foldable t) => t a -> Bool
allEq l = case toList l of
            [] -> True
            x : xs -> all (== x) xs

funeqPats :: FunEq s -> [Pattern s]
funeqPats (FunEq _ _ pats _) = pats

dataFieldType :: DataField s -> Type s
dataFieldType (DataField _ t) = t

isCEqK :: Constr -> Maybe (CKind, CKind, Range)
isCEqK (CEqK k1 k2 rng) = Just (k1, k2, rng)
isCEqK _ = Nothing

firstM :: Functor f => (a -> f b) -> (a, c) -> f (b, c)
firstM f (x, y) = (,y) <$> f x

secondM :: Functor f => (b -> f c) -> (a, b) -> f (a, c)
secondM f (x, y) = (x,) <$> f y

bimapM :: Applicative f => (a -> f b) -> (c -> f d) -> (a, c) -> f (b, d)
bimapM f g (x, y) = (,) <$> f x <*> g y