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rowtype-yoga/purescript-native/src/PureScript/TypeChecker/Types.hs
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1-----------------------------------------------------------------------------
2--
3-- Module : PureScript.TypeChecker.Types
4-- Copyright : (c) Phil Freeman 2013
5-- License : MIT
6--
7-- Maintainer : Phil Freeman <paf31@cantab.net>
8-- Stability : experimental
9-- Portability :
10--
11-- |
12--
13-----------------------------------------------------------------------------
15module PureScript.TypeChecker.Types (
16 TypeConstraint(..),
17 TypeSolution(..),
18 typeOf
19) where
21import Data.List
22import Data.Function
24import PureScript.Values
25import PureScript.Types
26import PureScript.TypeChecker.Monad
28import Control.Monad.State
29import Control.Monad.Error
31import Control.Applicative
32import Control.Arrow (Kleisli(..), (***))
33import qualified Control.Category as C
35import qualified Data.Map as M
37data TypeConstraint
38 = TypeConstraint Int Type
39 | RowConstraint Int Row deriving Show
41newtype TypeSolution = TypeSolution { runTypeSolution :: (Int -> Type, Int -> Row) }
43emptyTypeSolution :: TypeSolution
44emptyTypeSolution = TypeSolution (TUnknown, RUnknown)
46typeOf :: Value -> Check Type
47typeOf val = do
48 (cs, n, _) <- typeConstraints 0 M.empty val
49 solution <- solveTypeConstraints cs emptyTypeSolution
50 return $ varIfUnknown $ fst (runTypeSolution solution) n
52varIfUnknown :: Type -> Type
53varIfUnknown = flip evalState M.empty . varIfUnknown'
54 where
55 varIfUnknown' :: Type -> State (M.Map Int String) Type
56 varIfUnknown' (TUnknown n) = do
57 m <- get
58 case M.lookup n m of
59 Nothing -> do
60 let name = "t" ++ show (M.size m)
61 put $ M.insert n name m
62 return $ TypeVar name
63 Just name -> return $ TypeVar name
64 varIfUnknown' (Array t) = Array <$> varIfUnknown' t
65 varIfUnknown' (Object row) = Object <$> varIfUnknown'' row
66 varIfUnknown' (Function args ret) = Function <$> mapM varIfUnknown' args <*> varIfUnknown' ret
67 varIfUnknown' (TypeApp t1 t2) = TypeApp <$> varIfUnknown' t1 <*> varIfUnknown' t2
68 varIfUnknown' t = return t
69 varIfUnknown'' :: Row -> State (M.Map Int String) Row
70 varIfUnknown'' (RUnknown n) = do
71 m <- get
72 case M.lookup n m of
73 Nothing -> do
74 let name = "r" ++ show (M.size m)
75 put $ M.insert n name m
76 return $ RowVar name
77 Just name -> return $ RowVar name
78 varIfUnknown'' (RCons name ty row) = RCons name ty <$> varIfUnknown'' row
79 varIfUnknown'' r = return r
81replaceVarsWithUnknowns :: Int -> Type -> (Type, Int)
82replaceVarsWithUnknowns n = (id *** fst) . flip runState (n, M.empty) . replaceVarsWithUnknowns'
83 where
84 replaceVarsWithUnknowns' :: Type -> State (Int, M.Map String Int) Type
85 replaceVarsWithUnknowns' (Array t) = Array <$> replaceVarsWithUnknowns' t
86 replaceVarsWithUnknowns' (Object row) = Object <$> replaceVarsWithUnknowns'' row
87 replaceVarsWithUnknowns' (Function args ret) = Function <$> mapM replaceVarsWithUnknowns' args <*> replaceVarsWithUnknowns' ret
88 replaceVarsWithUnknowns' (TypeApp t1 t2) = TypeApp <$> replaceVarsWithUnknowns' t1 <*> replaceVarsWithUnknowns' t2
89 replaceVarsWithUnknowns' (TypeVar var) = do
90 (n, m) <- get
91 case M.lookup var m of
92 Nothing -> do
93 put (n + 1, M.insert var n m)
94 return $ TUnknown n
95 Just u -> return $ TUnknown u
96 replaceVarsWithUnknowns' t = return t
97 replaceVarsWithUnknowns'' :: Row -> State (Int, M.Map String Int) Row
98 replaceVarsWithUnknowns'' (RowVar var) = do
99 (n, m) <- get
100 case M.lookup var m of
101 Nothing -> do
102 put (n + 1, M.insert var n m)
103 return $ RUnknown n
104 Just u -> return $ RUnknown u
105 replaceVarsWithUnknowns'' (RCons name ty row) = RCons name <$> replaceVarsWithUnknowns' ty <*> replaceVarsWithUnknowns'' row
106 replaceVarsWithUnknowns'' r = return r
108typeConstraints :: Int -> M.Map String Int -> Value -> Check ([TypeConstraint], Int, Int)
109typeConstraints n _ (NumericLiteral _) = return ([TypeConstraint n Number], n, n)
110typeConstraints n _ (StringLiteral _) = return ([TypeConstraint n String], n, n)
111typeConstraints n _ (BooleanLiteral _) = return ([TypeConstraint n Boolean], n, n)
112typeConstraints n m (ArrayLiteral vals) = do
113 (cs, ns, max1) <- typeConstraintsAll n m vals
114 let me = max1 + 1
115 return (cs ++ map (TypeConstraint me . Array . TUnknown) ns, me, me)
116typeConstraints n m (Unary op val) = do
117 (cs, n1, max1) <- typeConstraints n m val
118 let me = max1 + 1
119 return (cs ++ unaryOperatorConstraints op n1 me, me, me)
120typeConstraints n m (Binary op left right) = do
121 (cs1, n1, max1) <- typeConstraints n m left
122 (cs2, n2, max2) <- typeConstraints (max1 + 1) m right
123 let me = max1 + 1
124 return (cs1 ++ cs2 ++ binaryOperatorConstraints op n1 n2 me, me, me)
125typeConstraints n m (ObjectLiteral ps) = do
126 (cs, ns, max1) <- typeConstraintsAll n m (map snd ps)
127 let me = max1 + 1
128 let tys = zipWith (\(name, _) u -> (name, TUnknown u)) ps ns
129 return ((TypeConstraint me (Object (typesToRow tys))) : cs, me, me)
130 where
131 typesToRow [] = REmpty
132 typesToRow ((name, ty):tys) = RCons name ty (typesToRow tys)
133typeConstraints n m (Accessor prop val) = do
134 (cs, n1, max1) <- typeConstraints n m val
135 let me = max1 + 1
136 let rest = max1 + 2
137 return ((TypeConstraint n1 (Object (RCons prop (TUnknown me) (RUnknown rest)))) : cs, me, rest)
138typeConstraints n m (Abs args ret) = do
139 let ns = [n..n + length args - 1]
140 let next = n + length args
141 let m' = m `M.union` M.fromList (zipWith (,) args ns)
142 (cs, n', max1) <- typeConstraints next m' ret
143 let me = max1 + 1
144 return ((TypeConstraint me (Function (map TUnknown ns) (TUnknown n'))) : cs, me, me)
145typeConstraints n m (App f xs) = do
146 (cs1, n1, max1) <- typeConstraints n m f
147 (cs2, ns, max2) <- typeConstraintsAll (max1 + 1) m xs
148 let me = max2 + 1
149 return ((TypeConstraint n1 (Function (map TUnknown ns) (TUnknown me))) : cs1 ++ cs2, me, me)
150typeConstraints n m (Var var) =
151 case M.lookup var m of
152 Nothing -> do
153 env <- get
154 case M.lookup var (names env) of
155 Nothing -> throwError $ var ++ " is undefined"
156 Just ty -> let (replaced, max1) = replaceVarsWithUnknowns (n + 1) ty
157 in return ([TypeConstraint n replaced], n, max1)
158 Just u -> return ([TypeConstraint u (TUnknown n)], n, n)
159typeConstraints n m (Block ss) = do
160 let ret = n + 1
161 (cs, allCodePathsReturn, max1, _) <- typeConstraintsForBlock (ret + 1) m M.empty ret ss
162 guardWith "Block is missing a return statement" allCodePathsReturn
163 return (cs, ret, max1)
164typeConstraints n m (Constructor c) = do
165 env <- get
166 case M.lookup c (names env) of
167 Nothing -> throwError $ c ++ " is undefined"
168 Just ty -> let (replaced, max1) = replaceVarsWithUnknowns (n + 1) ty
169 in return ([TypeConstraint n replaced], n, max1)
170typeConstraints n m (Case val binders) = do
171 (cs1, n1, max1) <- typeConstraints n m val
172 let ret = max1 + 1
173 (cs2, max2) <- typeConstraintsForBinders (ret + 1) m n1 ret binders
174 return (cs1 ++ cs2, ret, max2)
175typeConstraints n m (TypedValue val ty) = do
176 (cs, n1, max1) <- typeConstraints n m val
177 return ([TypeConstraint n1 ty], n1, max1)
178 -- TODO: Kind-check the type
180unaryOperatorConstraints :: UnaryOperator -> Int -> Int -> [TypeConstraint]
181unaryOperatorConstraints Negate val result = [TypeConstraint val Number, TypeConstraint result Number]
182unaryOperatorConstraints Not val result = [TypeConstraint val Boolean, TypeConstraint result Boolean]
183unaryOperatorConstraints BitwiseNot val result = [TypeConstraint val Number, TypeConstraint result Number]
185binaryOperatorConstraints :: BinaryOperator -> Int -> Int -> Int -> [TypeConstraint]
186binaryOperatorConstraints Add = symBinOpConstraints Number
187binaryOperatorConstraints Subtract = symBinOpConstraints Number
188binaryOperatorConstraints Multiply = symBinOpConstraints Number
189binaryOperatorConstraints Divide = symBinOpConstraints Number
190binaryOperatorConstraints Modulus = symBinOpConstraints Number
191binaryOperatorConstraints LessThan = symBinOpConstraints Number
192binaryOperatorConstraints LessThanOrEqualTo = symBinOpConstraints Number
193binaryOperatorConstraints GreaterThan = symBinOpConstraints Number
194binaryOperatorConstraints GreaterThanOrEqualTo = symBinOpConstraints Number
195binaryOperatorConstraints BitwiseAnd = symBinOpConstraints Number
196binaryOperatorConstraints BitwiseOr = symBinOpConstraints Number
197binaryOperatorConstraints BitwiseXor = symBinOpConstraints Number
198binaryOperatorConstraints ShiftLeft = symBinOpConstraints Number
199binaryOperatorConstraints ShiftRight = symBinOpConstraints Number
200binaryOperatorConstraints ZeroFillShiftRight = symBinOpConstraints Number
201binaryOperatorConstraints EqualTo = equalityBinOpConstraints
202binaryOperatorConstraints NotEqualTo = equalityBinOpConstraints
203binaryOperatorConstraints And = symBinOpConstraints Boolean
204binaryOperatorConstraints Or = symBinOpConstraints Boolean
205binaryOperatorConstraints Concat = symBinOpConstraints String
207equalityBinOpConstraints :: Int -> Int -> Int -> [TypeConstraint]
208equalityBinOpConstraints left right result = [TypeConstraint left (TUnknown right), TypeConstraint result Boolean]
210symBinOpConstraints :: Type -> Int -> Int -> Int -> [TypeConstraint]
211symBinOpConstraints ty left right result = [TypeConstraint left ty, TypeConstraint right ty, TypeConstraint result ty]
213typeConstraintsForBinder :: Int -> M.Map String Int -> Int -> Binder -> Check ([TypeConstraint], Int, M.Map String Int)
214typeConstraintsForBinder n m val (VarBinder name) =
215 return ([TypeConstraint n (TUnknown val)], n, M.insert name n m)
216typeConstraintsForBinder n m val (ConstructorBinder ctor binder) = do
217 env <- get
218 case M.lookup ctor (names env) of
219 Just f@(Function [ty] ret) -> do
220 let obj = n
221 let (Function [ty'] ret', max1) = replaceVarsWithUnknowns n f
222 (cs, max2, m1) <- typeConstraintsForBinder (max1 + 1) m obj binder
223 return ((TypeConstraint val ret') : (TypeConstraint obj ty') : cs, max2, m1)
224 Just _ -> throwError $ ctor ++ " is not a constructor"
225 _ -> throwError $ "Constructor " ++ ctor ++ " is not defined"
226typeConstraintsForBinder n m val (ObjectBinder props) = do undefined
228typeConstraintsForBinders :: Int -> M.Map String Int -> Int -> Int -> [(Binder, Value)] -> Check ([TypeConstraint], Int)
229typeConstraintsForBinders n _ _ _ [] = return ([], n)
230typeConstraintsForBinders n m nval ret ((binder, val):bs) = do
231 (cs1, max1, m1) <- typeConstraintsForBinder n m nval binder
232 (cs2, n2, max2) <- typeConstraints (max1 + 1) m1 val
233 (cs3, max2) <- typeConstraintsForBinders (max2 + 1) m nval ret bs
234 return ((TypeConstraint n2 (TUnknown ret)) : cs1 ++ cs2 ++ cs3, max2)
236typeConstraintsForStatement :: Int -> M.Map String Int -> M.Map String Int -> Int -> Statement -> Check ([TypeConstraint], Bool, Int, M.Map String Int)
237typeConstraintsForStatement n m mass ret (VariableIntroduction name val) = do
238 case M.lookup name (m `M.union` mass) of
239 Nothing -> do
240 (cs1, n1, max1) <- typeConstraints n m val
241 return $ (cs1, False, max1, M.insert name n1 mass)
242 Just ty -> throwError $ "Variable with name " ++ name ++ " already exists."
243typeConstraintsForStatement n m mass ret (Assignment name val) = do
244 case M.lookup name (m `M.union` mass) of
245 Nothing -> throwError $ "No local variable with name " ++ name
246 Just ty -> do
247 (cs1, n1, max1) <- typeConstraints n m val
248 return ((TypeConstraint n1 (TUnknown ty)) : cs1, False, max1, mass)
249typeConstraintsForStatement n m mass ret (While val inner) = do
250 (cs1, n1, max1) <- typeConstraints n m val
251 (cs2, allCodePathsReturn, max2, _) <- typeConstraintsForBlock (max1 + 1) m mass ret inner
252 return $ ((TypeConstraint n1 Boolean) : cs1 ++ cs2, allCodePathsReturn, max2, mass)
253typeConstraintsForStatement n m mass ret (IfThenElse val thens Nothing) = do
254 (cs1, n1, max1) <- typeConstraints n m val
255 (cs2, allCodePathsReturn, max2, _) <- typeConstraintsForBlock (max1 + 1) m mass ret thens
256 return $ ((TypeConstraint n1 Boolean) : cs1 ++ cs2, allCodePathsReturn, max2, mass)
257typeConstraintsForStatement n m mass ret (IfThenElse val thens (Just elses)) = do
258 (cs1, n1, max1) <- typeConstraints n m val
259 (cs2, allCodePathsReturn1, max2, _) <- typeConstraintsForBlock (max1 + 1) m mass ret thens
260 (cs3, allCodePathsReturn2, max3, _) <- typeConstraintsForBlock (max2 + 1) m mass ret elses
261 return $ ((TypeConstraint n1 Boolean) : cs1 ++ cs2 ++ cs3, allCodePathsReturn1 && allCodePathsReturn2, max3, mass)
262typeConstraintsForStatement n m mass ret (For (init, cond, cont) inner) = do
263 (cs1, b1, max1, mass1) <- typeConstraintsForStatement n m mass ret init
264 guardWith "Cannot return from inside for" $ not b1
265 (cs2, n1, max2) <- typeConstraints (max1 + 1) (m `M.union` mass1) cond
266 (cs3, b2, max3, mass2) <- typeConstraintsForStatement (max2 + 1) (m `M.union` mass1) mass1 ret cont
267 guardWith "Cannot return from inside for" $ not b2
268 (cs4, allCodePathsReturn, max4, _) <- typeConstraintsForBlock (max3 + 1) (m `M.union` mass2) mass2 ret inner
269 return $ ((TypeConstraint n1 Boolean) : cs1 ++ cs2 ++ cs3 ++ cs4, allCodePathsReturn, max4, mass)
270typeConstraintsForStatement n m mass ret (Return val) = do
271 (cs1, n1, max1) <- typeConstraints n m val
272 return ((TypeConstraint n1 (TUnknown ret)) : cs1, True, max1, mass)
274typeConstraintsForBlock :: Int -> M.Map String Int -> M.Map String Int -> Int -> [Statement] -> Check ([TypeConstraint], Bool, Int, M.Map String Int)
275typeConstraintsForBlock n _ mass _ [] = return ([], False, n, mass)
276typeConstraintsForBlock n m mass ret (s:ss) = do
277 (cs1, b1, max1, mass1) <- typeConstraintsForStatement n (m `M.union` mass) mass ret s
278 case (b1, ss) of
279 (True, []) -> return (cs1, True, max1, mass1)
280 (True, _) -> throwError "Unreachable code"
281 (False, ss) -> do
282 (cs2, b2, max2, mass2) <- typeConstraintsForBlock (max1 + 1) m mass1 ret ss
283 return (cs1 ++ cs2, b2, max2, mass2)
285typeConstraintsAll :: Int -> M.Map String Int -> [Value] -> Check ([TypeConstraint], [Int], Int)
286typeConstraintsAll n _ [] = return ([], [], n)
287typeConstraintsAll n m (t:ts) = do
288 (cs, n', max1) <- typeConstraints n m t
289 (cs', ns, max2) <- typeConstraintsAll (max1 + 1) m ts
290 return (cs ++ cs', n':ns, max2)
292solveTypeConstraints :: [TypeConstraint] -> TypeSolution -> Check TypeSolution
293solveTypeConstraints [] s = return s
294solveTypeConstraints (TypeConstraint n t:cs) s = do
295 guardWith "Occurs check failed" $ not $ typeOccursCheck n t
296 let s' = let (f, g) = runTypeSolution s
297 in TypeSolution (replaceTypeInType n t . f, replaceTypeInRow n t . g)
298 cs' <- fmap concat $ mapM (substituteTypeInConstraint n t) cs
299 solveTypeConstraints cs' s'
300solveTypeConstraints (RowConstraint n r:cs) s = do
301 guardWith "Occurs check failed" $ not $ rowOccursCheck n r
302 let s' = let (f, g) = runTypeSolution s
303 in TypeSolution $ (replaceRowInType n r . f, replaceRowInRow n r . g)
304 cs' <- fmap concat $ mapM (substituteRowInConstraint n r) cs
305 solveTypeConstraints cs' s'
307substituteTypeInConstraint :: Int -> Type -> TypeConstraint -> Check [TypeConstraint]
308substituteTypeInConstraint n s (TypeConstraint m t)
309 | n == m = unifyTypes s t
310 | otherwise = return [TypeConstraint m (replaceTypeInType n s t)]
311substituteTypeInConstraint n s (RowConstraint m r)
312 = return [RowConstraint m (replaceTypeInRow n s r)]
314substituteRowInConstraint :: Int -> Row -> TypeConstraint -> Check [TypeConstraint]
315substituteRowInConstraint n r (TypeConstraint m t)
316 = return [TypeConstraint m (replaceRowInType n r t)]
317substituteRowInConstraint n r (RowConstraint m r1)
318 | m == n = unifyRows r r1
319 | otherwise = return [RowConstraint m (replaceRowInRow n r r1)]
321replaceTypeInType :: Int -> Type -> Type -> Type
322replaceTypeInType n t (TUnknown m) | m == n = t
323replaceTypeInType n t (Array t1) = Array $ replaceTypeInType n t t1
324replaceTypeInType n t (Object row) = Object $ replaceTypeInRow n t row
325replaceTypeInType n t (Function args ret) = Function (map (replaceTypeInType n t) args) (replaceTypeInType n t ret)
326replaceTypeInType n t (TypeApp t1 t2) = TypeApp (replaceTypeInType n t t1) (replaceTypeInType n t t2)
327replaceTypeInType _ _ other = other
329replaceTypeInRow :: Int -> Type -> Row -> Row
330replaceTypeInRow n t (RCons name ty row) = RCons name (replaceTypeInType n t ty) (replaceTypeInRow n t row)
331replaceTypeInRow n t other = other
333replaceRowInType :: Int -> Row -> Type -> Type
334replaceRowInType n r (Array t1) = Array $ replaceRowInType n r t1
335replaceRowInType n r (Object row) = Object $ replaceRowInRow n r row
336replaceRowInType n r (Function args ret) = Function (map (replaceRowInType n r) args) (replaceRowInType n r ret)
337replaceRowInType n r (TypeApp t1 t2) = TypeApp (replaceRowInType n r t1) (replaceRowInType n r t2)
338replaceRowInType _ _ other = other
340replaceRowInRow :: Int -> Row -> Row -> Row
341replaceRowInRow n r (RUnknown m) | m == n = r
342replaceRowInRow n r (RCons name ty row) = RCons name (replaceRowInType n r ty) (replaceRowInRow n r row)
343replaceRowInRow _ _ other = other
345unifyTypes :: Type -> Type -> Check [TypeConstraint]
346unifyTypes (TUnknown u) t = do
347 guardWith "Occurs check failed" $ not $ typeOccursCheck u t
348 return [TypeConstraint u t]
349unifyTypes t (TUnknown u) = do
350 guardWith "Occurs check failed" $ not $ typeOccursCheck u t
351 return [TypeConstraint u t]
352unifyTypes Number Number = return []
353unifyTypes String String = return []
354unifyTypes Boolean Boolean = return []
355unifyTypes (Array s) (Array t) = unifyTypes s t
356unifyTypes (Object row1) (Object row2) = unifyRows row1 row2
357unifyTypes (Function args1 ret1) (Function args2 ret2) = do
358 cs1 <- fmap concat $ zipWithM unifyTypes args1 args2
359 cs2 <- unifyTypes ret1 ret2
360 return $ cs1 ++ cs2
361unifyTypes (TypeVar v1) (TypeVar v2) | v1 == v2 = return []
362unifyTypes (TypeConstructor c1) (TypeConstructor c2) | c1 == c2 = return []
363unifyTypes (TypeVar v1) (TypeVar v2) = return []
364unifyTypes (TypeApp t1 t2) (TypeApp t3 t4) = do
365 cs1 <- unifyTypes t1 t3
366 cs2 <- unifyTypes t2 t4
367 return $ cs1 ++ cs2
368unifyTypes t1 t2 = throwError $ "Cannot unify " ++ show t1 ++ " with " ++ show t2 ++ "."
370unifyRows :: Row -> Row -> Check [TypeConstraint]
371unifyRows r1 r2 =
372 let
373 (s1, r1') = toList r1
374 (s2, r2') = toList r2
375 int = [ (t1, t2) | (name, t1) <- s1, (name', t2) <- s2, name == name' ]
376 sd1 = [ (name, t1) | (name, t1) <- s1, not (elem name (map fst s2)) ]
377 sd2 = [ (name, t2) | (name, t2) <- s2, not (elem name (map fst s1)) ]
378 in rethrow (const $ "Cannot unify " ++ show r1 ++ " with " ++ show r2) $ do
379 cs1 <- fmap concat $ mapM (uncurry unifyTypes) int
380 cs2 <- unifyRows' (fromList (sd1, r1')) (fromList (sd2, r2'))
381 return $ cs1 ++ cs2
382 where
383 unifyRows' :: Row -> Row -> Check [TypeConstraint]
384 unifyRows' r (RUnknown u) = do
385 guardWith "Occurs check failed" $ not $ rowOccursCheck u r
386 return [RowConstraint u r]
387 unifyRows' (RUnknown u) r = do
388 guardWith "Occurs check failed" $ not $ rowOccursCheck u r
389 return [RowConstraint u r]
390 unifyRows' (RowVar v1) (RowVar v2) | v1 == v2 = return []
391 unifyRows' REmpty REmpty = return []
392 unifyRows' r1 r2 = throwError $ "Cannot unify " ++ show r1 ++ " with " ++ show r2 ++ "."
393 toList :: Row -> ([(String, Type)], Row)
394 toList (RCons name ty row) = let (tys, rest) = toList row
395 in ((name, ty):tys, rest)
396 toList r = ([], r)
397 fromList :: ([(String, Type)], Row) -> Row
398 fromList ([], r) = r
399 fromList ((name, t):ts, r) = RCons name t (fromList (ts, r))
401typeOccursCheck :: Int -> Type -> Bool
402typeOccursCheck u (TUnknown u') | u == u' = True
403typeOccursCheck u (Array t) = typeOccursCheck u t
404typeOccursCheck u (Object row) = rowOccursCheck u row
405typeOccursCheck u (Function args ret) = any (typeOccursCheck u) args || typeOccursCheck u ret
406typeOccursCheck u (TypeApp s t) = typeOccursCheck u s || typeOccursCheck u t
407typeOccursCheck _ _ = False
409rowOccursCheck :: Int -> Row -> Bool
410rowOccursCheck u (RUnknown u') | u == u' = True
411rowOccursCheck u (RCons name ty row) = typeOccursCheck u ty || rowOccursCheck u row
412rowOccursCheck _ _ = False