@@ -0,0 +1,953 @@
1+-----------------------------------------------------------------------------
2+--
3+-- Module : Language.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+-- This module implements the type checker
13+--
14+-----------------------------------------------------------------------------
15+
16+{-# OPTIONS_GHC -fno-warn-orphans #-}
17+{-# LANGUAGE FlexibleContexts #-}
18+{-# LANGUAGE MultiParamTypeClasses #-}
19+
20+module Language.PureScript.TypeChecker.Types (
21+ typesOf
22+) where
23+
24+{-
25+ The following functions represent the corresponding type checking judgements:
26+
27+ infer
28+ Synthesize a type for a value
29+
30+ check
31+ Check a value has a given type
32+
33+ checkProperties
34+ Check an object with a given type contains specified properties
35+
36+ checkFunctionApplication
37+ Check a function of a given type returns a value of another type when applied to its arguments
38+
39+ subsumes
40+ Check a type subsumes another type
41+-}
42+
43+import Data.List
44+import Data.Maybe (maybeToList, isNothing, isJust, fromMaybe)
45+import qualified Data.Data as D
46+import Data.Generics
47+ (everythingWithContext, mkM, everywhereM, everything, mkT,
48+ something, everywhere, mkQ)
49+import Data.Generics.Extras
50+
51+import Language.PureScript.Values
52+import Language.PureScript.Types
53+import Language.PureScript.Kinds
54+import Language.PureScript.Names
55+import Language.PureScript.TypeChecker.Monad
56+import Language.PureScript.TypeChecker.Kinds
57+import Language.PureScript.TypeChecker.Synonyms
58+import Language.PureScript.Pretty
59+
60+import Control.Monad.State
61+import Control.Monad.Error
62+import Control.Monad.Unify
63+
64+import Control.Applicative
65+import Control.Arrow (Arrow(..))
66+
67+import qualified Data.Map as M
68+import Data.Function (on)
69+import Data.Ord (comparing)
70+
71+instance Partial Type where
72+ unknown = TUnknown
73+ isUnknown (TUnknown u) = Just u
74+ isUnknown _ = Nothing
75+
76+instance Unifiable Check Type where
77+ (=?=) = unifyTypes
78+
79+-- |
80+-- Unify two types, updating the current substitution
81+--
82+unifyTypes :: Type -> Type -> UnifyT Type Check ()
83+unifyTypes t1 t2 = rethrow (\e -> "Error unifying type " ++ prettyPrintType t1 ++ " with type " ++ prettyPrintType t2 ++ ":\n" ++ e) $
84+ unifyTypes' t1 t2
85+ where
86+ unifyTypes' (TUnknown u1) (TUnknown u2) | u1 == u2 = return ()
87+ unifyTypes' (TUnknown u) t = u =:= t
88+ unifyTypes' t (TUnknown u) = u =:= t
89+ unifyTypes' (SaturatedTypeSynonym name args) ty = do
90+ ty1 <- introduceSkolemScope <=< expandTypeSynonym name $ args
91+ ty1 `unifyTypes` ty
92+ unifyTypes' ty s@(SaturatedTypeSynonym _ _) = s `unifyTypes` ty
93+ unifyTypes' (ForAll ident1 ty1 sc1) (ForAll ident2 ty2 sc2) =
94+ case (sc1, sc2) of
95+ (Just sc1', Just sc2') -> do
96+ sko <- newSkolemConstant
97+ let sk1 = skolemize ident1 sko sc1' ty1
98+ let sk2 = skolemize ident2 sko sc2' ty2
99+ sk1 `unifyTypes` sk2
100+ _ -> throwError (prettyPrintType ty1)
101+ unifyTypes' (ForAll ident ty1 (Just sc)) ty2 = do
102+ sko <- newSkolemConstant
103+ let sk = skolemize ident sko sc ty1
104+ sk `unifyTypes` ty2
105+ unifyTypes' ForAll{} _ = throwError "Skolem variable scope is unspecified"
106+ unifyTypes' ty f@ForAll{} = f `unifyTypes` ty
107+ unifyTypes' (Object row1) (Object row2) = row1 =?= row2
108+ unifyTypes' (TypeVar v1) (TypeVar v2) | v1 == v2 = return ()
109+ unifyTypes' (TypeConstructor c1) (TypeConstructor c2) = do
110+ env <- getEnv
111+ Just moduleName <- checkCurrentModule <$> get
112+ guardWith ("Cannot unify " ++ show c1 ++ " with " ++ show c2 ++ ".") (c1 == c2)
113+ unifyTypes' (TypeApp t3 t4) (TypeApp t5 t6) = do
114+ t3 `unifyTypes` t5
115+ t4 `unifyTypes` t6
116+ unifyTypes' (Skolem s1 _) (Skolem s2 _) | s1 == s2 = return ()
117+ unifyTypes' r1@RCons{} r2 = unifyRows r1 r2
118+ unifyTypes' r1 r2@RCons{} = unifyRows r1 r2
119+ unifyTypes' r1@REmpty r2 = unifyRows r1 r2
120+ unifyTypes' r1 r2@REmpty = unifyRows r1 r2
121+ unifyTypes' t3 t4 = throwError $ "Cannot unify " ++ prettyPrintType t3 ++ " with " ++ prettyPrintType t4 ++ "."
122+
123+-- |
124+-- Unify two rows, updating the current substitution
125+--
126+-- Common labels are first identified, and unified. Remaining labels and types are unified with a
127+-- trailing row unification variable, if appropriate, otherwise leftover labels result in a unification
128+-- error.
129+--
130+unifyRows :: Type -> Type -> UnifyT Type Check ()
131+unifyRows r1 r2 =
132+ let
133+ (s1, r1') = rowToList r1
134+ (s2, r2') = rowToList r2
135+ int = [ (t1, t2) | (name, t1) <- s1, (name', t2) <- s2, name == name' ]
136+ sd1 = [ (name, t1) | (name, t1) <- s1, name `notElem` map fst s2 ]
137+ sd2 = [ (name, t2) | (name, t2) <- s2, name `notElem` map fst s1 ]
138+ in do
139+ forM_ int (uncurry (=?=))
140+ unifyRows' sd1 r1' sd2 r2'
141+ where
142+ unifyRows' :: [(String, Type)] -> Type -> [(String, Type)] -> Type -> UnifyT Type Check ()
143+ unifyRows' [] (TUnknown u) sd r = u =:= rowFromList (sd, r)
144+ unifyRows' sd r [] (TUnknown u) = u =:= rowFromList (sd, r)
145+ unifyRows' ((name, ty):row) r others u@(TUnknown un) = do
146+ occursCheck un ty
147+ forM_ row $ \(_, t) -> occursCheck un t
148+ u' <- fresh
149+ u =?= RCons name ty u'
150+ unifyRows' row r others u'
151+ unifyRows' [] REmpty [] REmpty = return ()
152+ unifyRows' [] (TypeVar v1) [] (TypeVar v2) | v1 == v2 = return ()
153+ unifyRows' [] (Skolem s1 _) [] (Skolem s2 _) | s1 == s2 = return ()
154+ unifyRows' sd3 r3 sd4 r4 = throwError $ "Cannot unify (" ++ prettyPrintRow (rowFromList (sd3, r3)) ++ ") with (" ++ prettyPrintRow (rowFromList (sd4, r4)) ++ ")."
155+
156+-- |
157+-- Infer the types of multiple mutually-recursive values, and return elaborated values including
158+-- type class dictionaries and type annotations.
159+--
160+typesOf :: Maybe ModuleName -> ModuleName -> [(Ident, Value)] -> Check [(Ident, (Value, Type))]
161+typesOf mainModuleName moduleName vals = do
162+ tys <- fmap tidyUp . liftUnify $ do
163+ let
164+ -- Map each declaration to a name/value pair, with an optional type, if the declaration is typed
165+ es = map isTyped vals
166+ -- Filter the typed and untyped declarations
167+ typed = filter (isJust . snd . snd) es
168+ untyped = filter (isNothing . snd . snd) es
169+ -- Make a map of names to typed declarations
170+ typedDict = map (\(ident, (_, Just (ty, _))) -> (ident, ty)) typed
171+ -- Create fresh unification variables for the types of untyped declarations
172+ untypedNames <- replicateM (length untyped) fresh
173+ let
174+ -- Make a map of names to the unification variables of untyped declarations
175+ untypedDict = zip (map fst untyped) untypedNames
176+ -- Create the dictionary of all name/type pairs, which will be added to the environment during type checking
177+ dict = M.fromList (map (\(ident, ty) -> ((moduleName, ident), (ty, LocalVariable))) $ typedDict ++ untypedDict)
178+ forM es $ \e@(_, (val, _)) -> do
179+ -- If the declaration is a function, it has access to other values in the binding group.
180+ -- If not, the generated code might fail at runtime since those values might be undefined.
181+ let dict' = if isFunction val then dict else M.empty
182+ triple@(_, (val', ty)) <- case e of
183+ -- Typed declarations
184+ (ident, (val', Just (ty, checkType))) -> do
185+ -- Kind check
186+ kind <- liftCheck $ kindOf moduleName ty
187+ guardWith ("Expected type of kind *, was " ++ prettyPrintKind kind) $ kind == Star
188+ -- Check the type with the new names in scope
189+ ty' <- introduceSkolemScope <=< replaceAllTypeSynonyms $ ty
190+ val'' <- bindNames dict' $ if checkType
191+ then TypedValue True <$> check val' ty' <*> pure ty'
192+ else return (TypedValue False val' ty')
193+ return (ident, (val'', ty'))
194+ -- Untyped declarations
195+ (ident, (val', Nothing)) -> do
196+ -- Infer the type with the new names in scope
197+ TypedValue _ val'' ty <- bindNames dict' $ infer val'
198+ ty =?= fromMaybe (error "name not found in dictionary") (lookup ident untypedDict)
199+ return (ident, (TypedValue True val'' ty, ty))
200+ -- If --main is enabled, need to check that `main` has type Eff eff a for some eff, a
201+ when (Just moduleName == mainModuleName && fst e == Ident "main") $ do
202+ [eff, a] <- replicateM 2 fresh
203+ ty =?= TypeApp (TypeApp (TypeConstructor (Qualified (Just (ModuleName [ProperName "Control", ProperName "Monad", ProperName "Eff"])) (ProperName "Eff"))) eff) a
204+ -- Make sure unification variables do not escape
205+ escapeCheck val' ty
206+ return triple
207+ forM tys $ \(ident, (val, ty)) -> do
208+ -- Replace type class dictionary placeholders with actual dictionaries
209+ val' <- replaceTypeClassDictionaries moduleName val
210+ -- Check skolem variables did not escape their scope
211+ skolemEscapeCheck val'
212+ -- Remove type synonyms placeholders, remove duplicate row fields, and replace
213+ -- top-level unification variables with named type variables.
214+ let val'' = overTypes (desaturateAllTypeSynonyms . setifyAll) val'
215+ ty' = varIfUnknown . desaturateAllTypeSynonyms . setifyAll $ ty
216+ return (ident, (val'', ty'))
217+ where
218+ -- Apply the substitution that was returned from runUnify to both types and (type-annotated) values
219+ tidyUp (ts, sub) = map (\(i, (val, ty)) -> (i, (overTypes (sub $?) val, sub $? ty))) ts
220+
221+-- |
222+-- Check if a value introduces a function
223+--
224+isFunction :: Value -> Bool
225+isFunction (Abs _ _) = True
226+isFunction (TypedValue _ val _) = isFunction val
227+isFunction _ = False
228+
229+-- |
230+-- Check if a value contains a type annotation
231+--
232+isTyped :: (Ident, Value) -> (Ident, (Value, Maybe (Type, Bool)))
233+isTyped (name, TypedValue checkType value ty) = (name, (value, Just (ty, checkType)))
234+isTyped (name, value) = (name, (value, Nothing))
235+
236+-- |
237+-- Map a function over type annotations appearing inside a value
238+--
239+overTypes :: (Type -> Type) -> Value -> Value
240+overTypes f = everywhere (mkT f)
241+
242+-- |
243+-- Replace type class dictionary placeholders with inferred type class dictionaries
244+--
245+replaceTypeClassDictionaries :: ModuleName -> Value -> Check Value
246+replaceTypeClassDictionaries mn = everywhereM' (mkM go)
247+ where
248+ go (TypeClassDictionary constraint dicts) = entails mn dicts constraint
249+ go other = return other
250+
251+-- |
252+-- Check that the current set of type class dictionaries entail the specified type class goal, and, if so,
253+-- return a type class dictionary reference.
254+--
255+entails :: ModuleName -> [TypeClassDictionaryInScope] -> (Qualified ProperName, [Type]) -> Check Value
256+entails moduleName context goal@(className, tys) = do
257+ env <- getEnv
258+ case go env goal of
259+ [] -> throwError $ "No " ++ show className ++ " instance found for " ++ intercalate ", " (map prettyPrintType tys)
260+ (dict : _) -> return dict
261+ where
262+ go env (className', tys') =
263+ [ mkDictionary (canonicalizeDictionary tcd) args
264+ | tcd <- context
265+ -- Choose type class dictionaries in scope in the current module
266+ , filterModule tcd
267+ -- Make sure the type class name matches the one we are trying to satisfy
268+ , className' == tcdClassName tcd
269+ -- Make sure the type unifies with the type in the type instance definition
270+ , subst <- maybeToList . (>>= verifySubstitution) . fmap concat $ zipWithM (typeHeadsAreEqual moduleName env) tys' (tcdInstanceTypes tcd)
271+ -- Solve any necessary subgoals
272+ , args <- solveSubgoals env subst (tcdDependencies tcd) ]
273+ -- Create dictionaries for subgoals which still need to be solved by calling go recursively
274+ -- E.g. the goal (Show a, Show b) => Show (Either a b) can be satisfied if the current type
275+ -- unifies with Either a b, and we can satisfy the subgoals Show a and Show b recursively.
276+ solveSubgoals :: Environment -> [(String, Type)] -> Maybe [(Qualified ProperName, [Type])] -> [Maybe [Value]]
277+ solveSubgoals _ _ Nothing = return Nothing
278+ solveSubgoals env subst (Just subgoals) = do
279+ dict <- mapM (go env . second (map (replaceAllTypeVars subst))) subgoals
280+ return $ Just dict
281+ -- Make a dictionary from subgoal dictionaries by applying the correct function
282+ mkDictionary :: Qualified Ident -> Maybe [Value] -> Value
283+ mkDictionary fnName Nothing = Var fnName
284+ mkDictionary fnName (Just []) = App (Var fnName) (ObjectLiteral [])
285+ mkDictionary fnName (Just dicts) = foldl App (Var fnName) dicts
286+ -- Filter out type dictionaries which are in scope in the current module
287+ filterModule :: TypeClassDictionaryInScope -> Bool
288+ filterModule (TypeClassDictionaryInScope { tcdName = Qualified (Just mn) _ }) | mn == moduleName = True
289+ filterModule (TypeClassDictionaryInScope { tcdName = Qualified Nothing _ }) = True
290+ filterModule _ = False
291+ -- Resolve a type class dictionary in scope to an actual value by following any (TCDAlias) pointers
292+ -- which originated from module imports
293+ canonicalizeDictionary :: TypeClassDictionaryInScope -> Qualified Ident
294+ canonicalizeDictionary (TypeClassDictionaryInScope { tcdType = TCDRegular, tcdName = nm }) = nm
295+ canonicalizeDictionary (TypeClassDictionaryInScope { tcdType = TCDAlias nm }) = nm
296+ -- Ensure that a substitution is valid
297+ verifySubstitution :: [(String, Type)] -> Maybe [(String, Type)]
298+ verifySubstitution subst = do
299+ let grps = groupBy ((==) `on` fst) subst
300+ guard (all ((==) 1 . length . nubBy ((==) `on` snd)) grps)
301+ return $ map head grps
302+
303+-- |
304+-- Check whether the type heads of two types are equal (for the purposes of type class dictionary lookup),
305+-- and return a substitution from type variables to types which makes the type heads unify.
306+--
307+typeHeadsAreEqual :: ModuleName -> Environment -> Type -> Type -> Maybe [(String, Type)]
308+typeHeadsAreEqual _ _ (Skolem s1 _) (Skolem s2 _) | s1 == s2 = Just []
309+typeHeadsAreEqual _ _ (TypeVar v) t = Just [(v, t)]
310+typeHeadsAreEqual _ _ t (TypeVar v) = Just [(v, t)]
311+typeHeadsAreEqual m e (TypeConstructor c1) (TypeConstructor c2) | c1 == c2 = Just []
312+typeHeadsAreEqual m e (TypeApp h1 (TypeVar v)) (TypeApp h2 arg) = (:) (v, arg) <$> typeHeadsAreEqual m e h1 h2
313+typeHeadsAreEqual m e t1@(TypeApp _ _) t2@(TypeApp _ (TypeVar _)) = typeHeadsAreEqual m e t2 t1
314+typeHeadsAreEqual m e (SaturatedTypeSynonym name args) t2 = case expandTypeSynonym' e m name args of
315+ Left _ -> Nothing
316+ Right t1 -> typeHeadsAreEqual m e t1 t2
317+typeHeadsAreEqual _ _ _ _ = Nothing
318+
319+-- |
320+-- Ensure unsolved unification variables do not escape
321+--
322+escapeCheck :: Value -> Type -> UnifyT Type Check ()
323+escapeCheck value ty = do
324+ subst <- unifyCurrentSubstitution <$> UnifyT get
325+ let visibleUnknowns = nub $ unknowns $ subst $? ty
326+ let allUnknowns = findAllTypes value
327+ forM_ allUnknowns $ \t -> do
328+ let unsolvedUnknowns = nub . unknowns $ subst $? t
329+ guardWith "Escape check fails" $ null $ unsolvedUnknowns \\ visibleUnknowns
330+
331+-- |
332+-- Find all type annotations occuring inside a value
333+--
334+findAllTypes :: Value -> [Type]
335+findAllTypes = everything (++) (mkQ [] go)
336+ where
337+ go (TypedValue _ _ ty) = [ty]
338+ go _ = []
339+
340+-- |
341+-- Ensure skolem variables do not escape their scope
342+--
343+skolemEscapeCheck :: Value -> Check ()
344+skolemEscapeCheck (TypedValue False _ _) = return ()
345+skolemEscapeCheck root@TypedValue{} =
346+ -- Every skolem variable is created when a ForAll type is skolemized.
347+ -- This determines the scope of that skolem variable, which is copied from the SkolemScope
348+ -- field of the ForAll constructor.
349+ -- We traverse the tree top-down, and collect any SkolemScopes introduced by ForAlls.
350+ -- If a Skolem is encountered whose SkolemScope is not in the current list, we have found
351+ -- an escaped skolem variable.
352+ case everythingWithContext [] (++) (mkQ ((,) []) go) root of
353+ [] -> return ()
354+ ((binding, val) : _) -> throwError $ "Rigid/skolem type variable bound by " ++ maybe "<unknown>" prettyPrintValue binding ++ " has escaped at " ++ prettyPrintValue val
355+ where
356+ go :: Value -> [(SkolemScope, Value)] -> ([(Maybe Value, Value)], [(SkolemScope, Value)])
357+ go val@(TypedValue _ _ (ForAll _ _ (Just sco))) scos = ([], (sco, val) : scos)
358+ go val@(TypedValue _ _ ty) scos = case collectSkolems ty \\ map fst scos of
359+ (sco : _) -> ([(findBindingScope sco, val)], scos)
360+ _ -> ([], scos)
361+ where
362+ collectSkolems :: Type -> [SkolemScope]
363+ collectSkolems = nub . everything (++) (mkQ [] collect)
364+ where
365+ collect (Skolem _ scope) = [scope]
366+ collect _ = []
367+ go _ scos = ([], scos)
368+ findBindingScope :: SkolemScope -> Maybe Value
369+ findBindingScope sco = something (mkQ Nothing go') root
370+ where
371+ go' val@(TypedValue _ _ (ForAll _ _ (Just sco'))) | sco == sco' = Just val
372+ go' _ = Nothing
373+skolemEscapeCheck val = throwError $ "Untyped value passed to skolemEscapeCheck: " ++ prettyPrintValue val
374+
375+-- |
376+-- Ensure a row contains no duplicate labels
377+--
378+setify :: Type -> Type
379+setify = rowFromList . first (M.toList . M.fromList) . rowToList
380+
381+-- |
382+-- \"Setify\" all rows occuring inside a value
383+--
384+setifyAll :: (D.Data d) => d -> d
385+setifyAll = everywhere (mkT setify)
386+
387+-- |
388+-- Replace outermost unsolved unification variables with named type variables
389+--
390+varIfUnknown :: Type -> Type
391+varIfUnknown ty =
392+ let unks = nub $ unknowns ty
393+ toName = (:) 't' . show
394+ ty' = everywhere (mkT typeToVar) ty
395+ typeToVar :: Type -> Type
396+ typeToVar (TUnknown (Unknown u)) = TypeVar (toName u)
397+ typeToVar t = t
398+ in mkForAll (sort . map (toName . runUnknown) $ unks) ty'
399+
400+-- |
401+-- Remove any ForAlls and ConstrainedType constructors in a type by introducing new unknowns
402+-- or TypeClassDictionary values.
403+--
404+-- This is necessary during type checking to avoid unifying a polymorphic type with a
405+-- unification variable.
406+--
407+instantiatePolyTypeWithUnknowns :: Value -> Type -> UnifyT Type Check (Value, Type)
408+instantiatePolyTypeWithUnknowns val (ForAll ident ty _) = do
409+ ty' <- replaceVarWithUnknown ident ty
410+ instantiatePolyTypeWithUnknowns val ty'
411+instantiatePolyTypeWithUnknowns val (ConstrainedType constraints ty) = do
412+ env <- getEnv
413+ Just moduleName <- checkCurrentModule <$> get
414+ dicts <- getTypeClassDictionaries
415+ (_, ty') <- instantiatePolyTypeWithUnknowns (error "Types under a constraint cannot themselves be constrained") ty
416+ return (foldl App val (map (flip TypeClassDictionary dicts) constraints), ty')
417+instantiatePolyTypeWithUnknowns val ty = return (val, ty)
418+
419+-- |
420+-- Replace a single type variable with a new unification variable
421+--
422+replaceVarWithUnknown :: String -> Type -> UnifyT Type Check Type
423+replaceVarWithUnknown ident ty = do
424+ tu <- fresh
425+ return $ replaceTypeVars ident tu ty
426+
427+-- |
428+-- Replace fully applied type synonyms with the @SaturatedTypeSynonym@ data constructor, which helps generate
429+-- better error messages during unification.
430+--
431+replaceAllTypeSynonyms' :: (D.Data d) => Environment -> ModuleName -> d -> Either String d
432+replaceAllTypeSynonyms' env moduleName d =
433+ let
434+ syns = map (\(name, (args, _)) -> (name, length args)) . M.toList $ typeSynonyms env
435+ in
436+ saturateAllTypeSynonyms env moduleName syns d
437+
438+replaceAllTypeSynonyms :: (Functor m, Monad m, MonadState CheckState m, MonadError String m) => (D.Data d) => d -> m d
439+replaceAllTypeSynonyms d = do
440+ env <- getEnv
441+ Just moduleName <- checkCurrentModule <$> get
442+ either throwError return $ replaceAllTypeSynonyms' env moduleName d
443+
444+-- |
445+-- \"Desaturate\" @SaturatedTypeSynonym@s
446+--
447+desaturateAllTypeSynonyms :: (D.Data d) => d -> d
448+desaturateAllTypeSynonyms = everywhere (mkT replaceSaturatedTypeSynonym)
449+ where
450+ replaceSaturatedTypeSynonym (SaturatedTypeSynonym name args) = foldl TypeApp (TypeConstructor name) args
451+ replaceSaturatedTypeSynonym t = t
452+
453+-- |
454+-- Replace a type synonym and its arguments with the aliased type
455+--
456+expandTypeSynonym' :: Environment -> ModuleName -> Qualified ProperName -> [Type] -> Either String Type
457+expandTypeSynonym' env moduleName name args =
458+ case M.lookup name (typeSynonyms env) of
459+ Just (synArgs, body) -> do
460+ let repl = replaceAllTypeVars (zip synArgs args) body
461+ replaceAllTypeSynonyms' env moduleName repl
462+ Nothing -> error "Type synonym was not defined"
463+
464+expandTypeSynonym :: (Functor m, Monad m, MonadState CheckState m, MonadError String m) => Qualified ProperName -> [Type] -> m Type
465+expandTypeSynonym name args = do
466+ env <- getEnv
467+ Just moduleName <- checkCurrentModule <$> get
468+ either throwError return $ expandTypeSynonym' env moduleName name args
469+
470+-- |
471+-- Ensure a set of property names and value does not contain duplicate labels
472+--
473+ensureNoDuplicateProperties :: (MonadError String m) => [(String, Value)] -> m ()
474+ensureNoDuplicateProperties ps = guardWith "Duplicate property names" $ length (nub . map fst $ ps) == length ps
475+
476+-- |
477+-- Infer a type for a value, rethrowing any error to provide a more useful error message
478+--
479+infer :: Value -> UnifyT Type Check Value
480+infer val = rethrow (\e -> "Error inferring type of term " ++ prettyPrintValue val ++ ":\n" ++ e) $ infer' val
481+
482+-- |
483+-- Infer a type for a value
484+--
485+infer' :: Value -> UnifyT Type Check Value
486+infer' v@(NumericLiteral _) = return $ TypedValue True v tyNumber
487+infer' v@(StringLiteral _) = return $ TypedValue True v tyString
488+infer' v@(BooleanLiteral _) = return $ TypedValue True v tyBoolean
489+infer' (ArrayLiteral vals) = do
490+ ts <- mapM infer vals
491+ els <- fresh
492+ forM_ ts $ \(TypedValue _ _ t) -> els =?= TypeApp tyArray t
493+ return $ TypedValue True (ArrayLiteral ts) els
494+infer' (ObjectLiteral ps) = do
495+ ensureNoDuplicateProperties ps
496+ ts <- mapM (infer . snd) ps
497+ let fields = zipWith (\name (TypedValue _ _ t) -> (name, t)) (map fst ps) ts
498+ ty = Object $ rowFromList (fields, REmpty)
499+ return $ TypedValue True (ObjectLiteral (zip (map fst ps) ts)) ty
500+infer' (ObjectUpdate o ps) = do
501+ ensureNoDuplicateProperties ps
502+ row <- fresh
503+ newVals <- zipWith (\(name, _) t -> (name, t)) ps <$> mapM (infer . snd) ps
504+ let newTys = map (\(name, TypedValue _ _ ty) -> (name, ty)) newVals
505+ oldTys <- zip (map fst ps) <$> replicateM (length ps) fresh
506+ let oldTy = Object $ rowFromList (oldTys, row)
507+ o' <- TypedValue True <$> check o oldTy <*> pure oldTy
508+ return $ TypedValue True (ObjectUpdate o' newVals) $ Object $ rowFromList (newTys, row)
509+infer' (Accessor prop val) = do
510+ typed@(TypedValue _ _ objTy) <- infer val
511+ propTy <- inferProperty objTy prop
512+ case propTy of
513+ Nothing -> do
514+ field <- fresh
515+ rest <- fresh
516+ _ <- subsumes Nothing objTy (Object (RCons prop field rest))
517+ return $ TypedValue True (Accessor prop typed) field
518+ Just ty -> return $ TypedValue True (Accessor prop typed) ty
519+infer' (Abs (Left arg) ret) = do
520+ ty <- fresh
521+ Just moduleName <- checkCurrentModule <$> get
522+ bindLocalVariables moduleName [(arg, ty)] $ do
523+ body@(TypedValue _ _ bodyTy) <- infer' ret
524+ return $ TypedValue True (Abs (Left arg) body) $ function ty bodyTy
525+infer' (Abs (Right _) _) = error "Binder was not desugared"
526+infer' (App f arg) = do
527+ f'@(TypedValue _ _ ft) <- infer f
528+ (ret, app) <- checkFunctionApplication f' ft arg
529+ return $ TypedValue True app ret
530+infer' (Var var) = do
531+ Just moduleName <- checkCurrentModule <$> get
532+ ty <- introduceSkolemScope <=< replaceAllTypeSynonyms <=< lookupVariable moduleName $ var
533+ case ty of
534+ ConstrainedType constraints ty' -> do
535+ env <- getEnv
536+ dicts <- getTypeClassDictionaries
537+ return $ TypedValue True (foldl App (Var var) (map (flip TypeClassDictionary dicts) constraints)) ty'
538+ _ -> return $ TypedValue True (Var var) ty
539+infer' v@(Constructor c) = do
540+ env <- getEnv
541+ Just moduleName <- checkCurrentModule <$> get
542+ case M.lookup c (dataConstructors env) of
543+ Nothing -> throwError $ "Constructor " ++ show c ++ " is undefined"
544+ Just ty -> do ty' <- introduceSkolemScope <=< replaceAllTypeSynonyms $ ty
545+ return $ TypedValue True v ty'
546+infer' (Case vals binders) = do
547+ ts <- mapM infer vals
548+ ret <- fresh
549+ binders' <- checkBinders (map (\(TypedValue _ _ t) -> t) ts) ret binders
550+ return $ TypedValue True (Case ts binders') ret
551+infer' (IfThenElse cond th el) = do
552+ cond' <- check cond tyBoolean
553+ v2@(TypedValue _ _ t2) <- infer th
554+ v3@(TypedValue _ _ t3) <- infer el
555+ t2 =?= t3
556+ return $ TypedValue True (IfThenElse cond' v2 v3) t2
557+infer' (TypedValue checkType val ty) = do
558+ Just moduleName <- checkCurrentModule <$> get
559+ kind <- liftCheck $ kindOf moduleName ty
560+ guardWith ("Expected type of kind *, was " ++ prettyPrintKind kind) $ kind == Star
561+ ty' <- introduceSkolemScope <=< replaceAllTypeSynonyms $ ty
562+ val' <- if checkType then check val ty' else return val
563+ return $ TypedValue True val' ty'
564+infer' _ = error "Invalid argument to infer"
565+
566+-- |
567+-- Infer the type of a property inside a record with a given type
568+--
569+inferProperty :: Type -> String -> UnifyT Type Check (Maybe Type)
570+inferProperty (Object row) prop = do
571+ let (props, _) = rowToList row
572+ return $ lookup prop props
573+inferProperty (SaturatedTypeSynonym name args) prop = do
574+ replaced <- introduceSkolemScope <=< expandTypeSynonym name $ args
575+ inferProperty replaced prop
576+inferProperty (ForAll ident ty _) prop = do
577+ replaced <- replaceVarWithUnknown ident ty
578+ inferProperty replaced prop
579+inferProperty _ _ = return Nothing
580+
581+-- |
582+-- Infer the types of variables brought into scope by a binder
583+--
584+inferBinder :: Type -> Binder -> UnifyT Type Check (M.Map Ident Type)
585+inferBinder _ NullBinder = return M.empty
586+inferBinder val (StringBinder _) = val =?= tyString >> return M.empty
587+inferBinder val (NumberBinder _) = val =?= tyNumber >> return M.empty
588+inferBinder val (BooleanBinder _) = val =?= tyBoolean >> return M.empty
589+inferBinder val (VarBinder name) = return $ M.singleton name val
590+inferBinder val (ConstructorBinder ctor binders) = do
591+ env <- getEnv
592+ Just moduleName <- checkCurrentModule <$> get
593+ case M.lookup ctor (dataConstructors env) of
594+ Just ty -> do
595+ (_, fn) <- instantiatePolyTypeWithUnknowns (error "Data constructor types cannot contains constraints") ty
596+ go binders fn
597+ where
598+ go [] ty' = do
599+ subsumes Nothing val ty'
600+ return M.empty
601+ go (binder : binders') (TypeApp (TypeApp t obj) ret) | t == tyFunction =
602+ M.union <$> inferBinder obj binder <*> go binders' ret
603+ go _ _ = throwError $ "Wrong number of arguments to constructor " ++ show ctor
604+ _ -> throwError $ "Constructor " ++ show ctor ++ " is not defined"
605+inferBinder val (ObjectBinder props) = do
606+ row <- fresh
607+ rest <- fresh
608+ m1 <- inferRowProperties row rest props
609+ val =?= Object row
610+ return m1
611+ where
612+ inferRowProperties :: Type -> Type -> [(String, Binder)] -> UnifyT Type Check (M.Map Ident Type)
613+ inferRowProperties nrow row [] = nrow =?= row >> return M.empty
614+ inferRowProperties nrow row ((name, binder):binders) = do
615+ propTy <- fresh
616+ m1 <- inferBinder propTy binder
617+ m2 <- inferRowProperties nrow (RCons name propTy row) binders
618+ return $ m1 `M.union` m2
619+inferBinder val (ArrayBinder binders) = do
620+ el <- fresh
621+ m1 <- M.unions <$> mapM (inferBinder el) binders
622+ val =?= TypeApp tyArray el
623+ return m1
624+inferBinder val (ConsBinder headBinder tailBinder) = do
625+ el <- fresh
626+ m1 <- inferBinder el headBinder
627+ m2 <- inferBinder val tailBinder
628+ val =?= TypeApp tyArray el
629+ return $ m1 `M.union` m2
630+inferBinder val (NamedBinder name binder) = do
631+ m <- inferBinder val binder
632+ return $ M.insert name val m
633+
634+-- |
635+-- Check the types of the return values in a set of binders in a case statement
636+--
637+checkBinders :: [Type] -> Type -> [([Binder], Maybe Guard, Value)] -> UnifyT Type Check [([Binder], Maybe Guard, Value)]
638+checkBinders _ _ [] = return []
639+checkBinders nvals ret ((binders, grd, val):bs) = do
640+ Just moduleName <- checkCurrentModule <$> get
641+ m1 <- M.unions <$> zipWithM inferBinder nvals binders
642+ r <- bindLocalVariables moduleName (M.toList m1) $ do
643+ val' <- TypedValue True <$> check val ret <*> pure ret
644+ case grd of
645+ Nothing -> return (binders, Nothing, val')
646+ Just g -> do
647+ g' <- check g tyBoolean
648+ return (binders, Just g', val')
649+ rs <- checkBinders nvals ret bs
650+ return $ r : rs
651+
652+-- |
653+-- Check that a local variable name is not already used
654+--
655+assignVariable :: Ident -> UnifyT Type Check ()
656+assignVariable name = do
657+ env <- checkEnv <$> get
658+ Just moduleName <- checkCurrentModule <$> get
659+ case M.lookup (moduleName, name) (names env) of
660+ Just _ -> UnifyT . lift . throwError $ "Variable with name " ++ show name ++ " already exists."
661+ _ -> return ()
662+
663+-- |
664+-- Generate a new skolem constant
665+--
666+newSkolemConstant :: UnifyT Type Check Int
667+newSkolemConstant = runUnknown <$> fresh'
668+
669+-- |
670+-- Generate a new skolem scope
671+--
672+newSkolemScope :: UnifyT Type Check SkolemScope
673+newSkolemScope = SkolemScope . runUnknown <$> fresh'
674+
675+-- |
676+-- Skolemize a type variable by replacing its instances with fresh skolem constants
677+--
678+skolemize :: String -> Int -> SkolemScope -> Type -> Type
679+skolemize ident sko scope = replaceTypeVars ident (Skolem sko scope)
680+
681+-- |
682+-- Introduce skolem scope at every occurence of a ForAll
683+--
684+introduceSkolemScope :: Type -> UnifyT Type Check Type
685+introduceSkolemScope = everywhereM (mkM go)
686+ where
687+ go (ForAll ident ty Nothing) = ForAll ident ty <$> (Just <$> newSkolemScope)
688+ go other = return other
689+
690+-- |
691+-- Check the type of a value, rethrowing errors to provide a better error message
692+--
693+check :: Value -> Type -> UnifyT Type Check Value
694+check val ty = rethrow errorMessage $ check' val ty
695+ where
696+ errorMessage msg =
697+ "Error checking type of term " ++
698+ prettyPrintValue val ++
699+ " against type " ++
700+ prettyPrintType ty ++
701+ ":\n" ++
702+ msg
703+
704+-- |
705+-- Check the type of a value
706+--
707+check' :: Value -> Type -> UnifyT Type Check Value
708+check' val (ForAll ident ty _) = do
709+ scope <- newSkolemScope
710+ sko <- newSkolemConstant
711+ let sk = skolemize ident sko scope ty
712+ val' <- check val sk
713+ return $ TypedValue True val' (ForAll ident ty (Just scope))
714+check' val t@(ConstrainedType constraints ty) = do
715+ env <- getEnv
716+ Just moduleName <- checkCurrentModule <$> get
717+ dictNames <- forM constraints $ \(Qualified _ (ProperName className), _) -> do
718+ n <- liftCheck freshDictionaryName
719+ return $ Ident $ "__dict_" ++ className ++ "_" ++ show n
720+ val' <- withTypeClassDictionaries (zipWith (\name (className, instanceTy) ->
721+ TypeClassDictionaryInScope name className instanceTy Nothing TCDRegular) (map (Qualified Nothing) dictNames)
722+ constraints) $ check val ty
723+ return $ TypedValue True (foldr (Abs . Left) val' dictNames) t
724+check' val (SaturatedTypeSynonym name args) = do
725+ ty <- introduceSkolemScope <=< expandTypeSynonym name $ args
726+ val' <- check val ty
727+ return $ TypedValue True val' ty
728+check' val u@(TUnknown _) = do
729+ val'@(TypedValue _ _ ty) <- infer val
730+ -- Don't unify an unknown with an inferred polytype
731+ (val'', ty') <- instantiatePolyTypeWithUnknowns val' ty
732+ ty' =?= u
733+ return $ TypedValue True val'' ty'
734+check' v@(NumericLiteral _) t | t == tyNumber =
735+ return $ TypedValue True v t
736+check' v@(StringLiteral _) t | t == tyString =
737+ return $ TypedValue True v t
738+check' v@(BooleanLiteral _) t | t == tyBoolean =
739+ return $ TypedValue True v t
740+check' (ArrayLiteral vals) t@(TypeApp a ty) | a == tyArray = do
741+ array <- ArrayLiteral <$> forM vals (`check` ty)
742+ return $ TypedValue True array t
743+check' (Abs (Left arg) ret) ty@(TypeApp (TypeApp t argTy) retTy) | t == tyFunction = do
744+ Just moduleName <- checkCurrentModule <$> get
745+ ret' <- bindLocalVariables moduleName [(arg, argTy)] $ check ret retTy
746+ return $ TypedValue True (Abs (Left arg) ret') ty
747+check' (Abs (Right _) _) _ = error "Binder was not desugared"
748+check' (App f arg) ret = do
749+ f'@(TypedValue _ _ ft) <- infer f
750+ (ret', app) <- checkFunctionApplication f' ft arg
751+ subsumes Nothing ret' ret
752+ return $ TypedValue True app ret
753+check' v@(Var var) ty = do
754+ Just moduleName <- checkCurrentModule <$> get
755+ repl <- introduceSkolemScope <=< replaceAllTypeSynonyms <=< lookupVariable moduleName $ var
756+ ty' <- introduceSkolemScope <=< replaceAllTypeSynonyms $ ty
757+ v' <- subsumes (Just v) repl ty'
758+ case v' of
759+ Nothing -> throwError "Unable to check type subsumption"
760+ Just v'' -> return $ TypedValue True v'' ty'
761+check' (TypedValue checkType val ty1) ty2 = do
762+ Just moduleName <- checkCurrentModule <$> get
763+ kind <- liftCheck $ kindOf moduleName ty1
764+ guardWith ("Expected type of kind *, was " ++ prettyPrintKind kind) $ kind == Star
765+ ty1' <- introduceSkolemScope <=< replaceAllTypeSynonyms $ ty1
766+ val' <- subsumes (Just val) ty1' ty2
767+ case val' of
768+ Nothing -> throwError "Unable to check type subsumption"
769+ Just val'' -> do
770+ val''' <- if checkType then check val'' ty1 else return val''
771+ return $ TypedValue checkType (TypedValue True val''' ty1) ty2
772+check' (Case vals binders) ret = do
773+ vals' <- mapM infer vals
774+ let ts = map (\(TypedValue _ _ t) -> t) vals'
775+ binders' <- checkBinders ts ret binders
776+ return $ TypedValue True (Case vals' binders') ret
777+check' (IfThenElse cond th el) ty = do
778+ cond' <- check cond tyBoolean
779+ th' <- check th ty
780+ el' <- check el ty
781+ return $ TypedValue True (IfThenElse cond' th' el') ty
782+check' (ObjectLiteral ps) t@(Object row) = do
783+ ensureNoDuplicateProperties ps
784+ ps' <- checkProperties ps row False
785+ return $ TypedValue True (ObjectLiteral ps') t
786+check' (ObjectUpdate obj ps) t@(Object row) = do
787+ ensureNoDuplicateProperties ps
788+ us <- zip (map fst ps) <$> replicateM (length ps) fresh
789+ let (propsToCheck, rest) = rowToList row
790+ propsToRemove = map fst ps
791+ remainingProps = filter (\(p, _) -> p `notElem` propsToRemove) propsToCheck
792+ obj' <- check obj (Object (rowFromList (us ++ remainingProps, rest)))
793+ ps' <- checkProperties ps row True
794+ return $ TypedValue True (ObjectUpdate obj' ps') t
795+check' (Accessor prop val) ty = do
796+ rest <- fresh
797+ val' <- check val (Object (RCons prop ty rest))
798+ return $ TypedValue True (Accessor prop val') ty
799+check' (Constructor c) ty = do
800+ env <- getEnv
801+ Just moduleName <- checkCurrentModule <$> get
802+ case M.lookup c (dataConstructors env) of
803+ Nothing -> throwError $ "Constructor " ++ show c ++ " is undefined"
804+ Just ty1 -> do
805+ repl <- introduceSkolemScope <=< replaceAllTypeSynonyms $ ty1
806+ _ <- subsumes Nothing repl ty
807+ return $ TypedValue True (Constructor c) ty
808+check' val ty = throwError $ prettyPrintValue val ++ " does not have type " ++ prettyPrintType ty
809+
810+-- |
811+-- Check the type of a collection of named record fields
812+--
813+-- The @lax@ parameter controls whether or not every record member has to be provided. For object updates, this is not the case.
814+--
815+checkProperties :: [(String, Value)] -> Type -> Bool -> UnifyT Type Check [(String, Value)]
816+checkProperties ps row lax = let (ts, r') = rowToList row in go ps ts r' where
817+ go [] [] REmpty = return []
818+ go [] [] u@(TUnknown _) = do u =?= REmpty
819+ return []
820+ go [] [] (Skolem _ _) | lax = return []
821+ go [] ((p, _): _) _ | lax = return []
822+ | otherwise = throwError $ prettyPrintValue (ObjectLiteral ps) ++ " does not have property " ++ p
823+ go ((p,_):_) [] REmpty = throwError $ "Property " ++ p ++ " is not present in closed object type " ++ prettyPrintRow row
824+ go ((p,v):ps') [] u@(TUnknown _) = do
825+ v'@(TypedValue _ _ ty) <- infer v
826+ rest <- fresh
827+ u =?= RCons p ty rest
828+ ps'' <- go ps' [] rest
829+ return $ (p, v') : ps''
830+ go ((p,v):ps') ts r =
831+ case lookup p ts of
832+ Nothing -> do
833+ v'@(TypedValue _ _ ty) <- infer v
834+ rest <- fresh
835+ r =?= RCons p ty rest
836+ ps'' <- go ps' ts rest
837+ return $ (p, v') : ps''
838+ Just ty -> do
839+ v' <- check v ty
840+ ps'' <- go ps' (delete (p, ty) ts) r
841+ return $ (p, v') : ps''
842+ go _ _ _ = throwError $ prettyPrintValue (ObjectLiteral ps) ++ " does not have type " ++ prettyPrintType (Object row)
843+
844+-- |
845+-- Check the type of a function application, rethrowing errors to provide a better error message
846+--
847+checkFunctionApplication :: Value -> Type -> Value -> UnifyT Type Check (Type, Value)
848+checkFunctionApplication fn fnTy arg = rethrow errorMessage $ checkFunctionApplication' fn fnTy arg
849+ where
850+ errorMessage msg = "Error applying function of type "
851+ ++ prettyPrintType fnTy
852+ ++ " to argument " ++ prettyPrintValue arg
853+ ++ ":\n" ++ msg
854+
855+-- |
856+-- Check the type of a function application
857+--
858+checkFunctionApplication' :: Value -> Type -> Value -> UnifyT Type Check (Type, Value)
859+checkFunctionApplication' fn (TypeApp (TypeApp tyFunction' argTy) retTy) arg = do
860+ tyFunction' =?= tyFunction
861+ arg' <- check arg argTy
862+ return (retTy, App fn arg')
863+checkFunctionApplication' fn (ForAll ident ty _) arg = do
864+ replaced <- replaceVarWithUnknown ident ty
865+ checkFunctionApplication fn replaced arg
866+checkFunctionApplication' fn u@(TUnknown _) arg = do
867+ arg' <- do
868+ TypedValue _ arg' t <- infer arg
869+ (arg'', t') <- instantiatePolyTypeWithUnknowns arg' t
870+ return $ TypedValue True arg'' t'
871+ let ty = (\(TypedValue _ _ t) -> t) arg'
872+ ret <- fresh
873+ u =?= function ty ret
874+ return (ret, App fn arg')
875+checkFunctionApplication' fn (SaturatedTypeSynonym name tyArgs) arg = do
876+ ty <- introduceSkolemScope <=< expandTypeSynonym name $ tyArgs
877+ checkFunctionApplication fn ty arg
878+checkFunctionApplication' fn (ConstrainedType constraints fnTy) arg = do
879+ env <- getEnv
880+ dicts <- getTypeClassDictionaries
881+ Just moduleName <- checkCurrentModule <$> get
882+ checkFunctionApplication' (foldl App fn (map (flip TypeClassDictionary dicts) constraints)) fnTy arg
883+checkFunctionApplication' _ fnTy arg = throwError $ "Cannot apply a function of type "
884+ ++ prettyPrintType fnTy
885+ ++ " to argument " ++ prettyPrintValue arg
886+
887+-- |
888+-- Check whether one type subsumes another, rethrowing errors to provide a better error message
889+--
890+subsumes :: Maybe Value -> Type -> Type -> UnifyT Type Check (Maybe Value)
891+subsumes val ty1 ty2 = rethrow errorMessage $ subsumes' val ty1 ty2
892+ where
893+ errorMessage msg = "Error checking that type "
894+ ++ prettyPrintType ty1
895+ ++ " subsumes type "
896+ ++ prettyPrintType ty2
897+ ++ ":\n" ++ msg
898+
899+-- |
900+-- Check whether one type subsumes another
901+--
902+subsumes' :: Maybe Value -> Type -> Type -> UnifyT Type Check (Maybe Value)
903+subsumes' val (ForAll ident ty1 _) ty2 = do
904+ replaced <- replaceVarWithUnknown ident ty1
905+ subsumes val replaced ty2
906+subsumes' val ty1 (ForAll ident ty2 sco) =
907+ case sco of
908+ Just sco' -> do
909+ sko <- newSkolemConstant
910+ let sk = skolemize ident sko sco' ty2
911+ subsumes val ty1 sk
912+ Nothing -> throwError "Skolem variable scope is unspecified"
913+subsumes' val (TypeApp (TypeApp f1 arg1) ret1) (TypeApp (TypeApp f2 arg2) ret2) | f1 == tyFunction && f2 == tyFunction = do
914+ subsumes Nothing arg2 arg1
915+ subsumes Nothing ret1 ret2
916+ return val
917+subsumes' val (SaturatedTypeSynonym name tyArgs) ty2 = do
918+ ty1 <- introduceSkolemScope <=< expandTypeSynonym name $ tyArgs
919+ subsumes val ty1 ty2
920+subsumes' val ty1 (SaturatedTypeSynonym name tyArgs) = do
921+ ty2 <- introduceSkolemScope <=< expandTypeSynonym name $ tyArgs
922+ subsumes val ty1 ty2
923+subsumes' (Just val) (ConstrainedType constraints ty1) ty2 = do
924+ env <- getEnv
925+ dicts <- getTypeClassDictionaries
926+ Just moduleName <- checkCurrentModule <$> get
927+ _ <- subsumes' Nothing ty1 ty2
928+ return . Just $ foldl App val (map (flip TypeClassDictionary dicts) constraints)
929+subsumes' val (Object r1) (Object r2) = do
930+ let
931+ (ts1, r1') = rowToList r1
932+ (ts2, r2') = rowToList r2
933+ ts1' = sortBy (comparing fst) ts1
934+ ts2' = sortBy (comparing fst) ts2
935+ go ts1' ts2' r1' r2'
936+ return val
937+ where
938+ go [] ts2 r1' r2' = r1' =?= rowFromList (ts2, r2')
939+ go ts1 [] r1' r2' = r2' =?= rowFromList (ts1, r1')
940+ go ((p1, ty1) : ts1) ((p2, ty2) : ts2) r1' r2'
941+ | p1 == p2 = do subsumes Nothing ty1 ty2
942+ go ts1 ts2 r1' r2'
943+ | p1 < p2 = do rest <- fresh
944+ r2' =?= RCons p1 ty1 rest
945+ go ts1 ((p2, ty2) : ts2) r1' rest
946+ | p1 > p2 = do rest <- fresh
947+ r1' =?= RCons p2 ty2 rest
948+ go ((p1, ty1) : ts1) ts2 rest r2'
949+subsumes' val ty1 ty2@(Object _) = subsumes val ty2 ty1
950+subsumes' val ty1 ty2 = do
951+ ty1 =?= ty2
952+ return val
953+