TcSimplify.lhs 56.4 KB
 simonm committed Jan 08, 1998 1 \begin{code}  Ian Lynagh committed Nov 04, 2011 2 3 4 5 6 7 8 {-# OPTIONS -fno-warn-tabs #-} -- The above warning supression flag is a temporary kludge. -- While working on this module you are encouraged to remove it and -- detab the module (please do the detabbing in a separate patch). See -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces -- for details  simonpj@microsoft.com committed Sep 13, 2010 9 module TcSimplify(  Simon Peyton Jones committed Aug 16, 2011 10  simplifyInfer, simplifyAmbiguityCheck,  11  simplifyDefault, simplifyDeriv,  simonpj@microsoft.com committed Sep 13, 2010 12 13  simplifyRule, simplifyTop, simplifyInteractive ) where  partain committed Jan 08, 1996 14   simonm committed Jan 08, 1998 15 #include "HsVersions.h"  partain committed Mar 19, 1996 16   simonpj@microsoft.com committed Sep 13, 2010 17 import HsSyn  simonpj committed Sep 13, 2002 18 import TcRnMonad  simonpj@microsoft.com committed Sep 13, 2010 19 import TcErrors  simonpj@microsoft.com committed Jan 02, 2007 20 import TcMType  simonpj@microsoft.com committed Sep 13, 2010 21 22 23 24 import TcType import TcSMonad import TcInteract import Inst  25 import Unify ( niFixTvSubst, niSubstTvSet )  Simon Marlow committed Oct 11, 2006 26 import Var  simonm committed Dec 02, 1998 27 import VarSet  simonpj@microsoft.com committed Nov 12, 2010 28 import VarEnv  29 import Coercion  simonpj@microsoft.com committed Nov 12, 2010 30 import TypeRep  simonpj@microsoft.com committed Sep 13, 2010 31 32 import Name import NameEnv ( emptyNameEnv )  simonmar committed Dec 10, 2003 33 import Bag  Simon Marlow committed Oct 11, 2006 34 35 import ListSetOps import Util  simonpj@microsoft.com committed Sep 13, 2010 36 37 38 import PrelInfo import PrelNames import Class ( classKey )  Simon Peyton Jones committed Aug 16, 2011 39 import BasicTypes ( RuleName )  simonpj@microsoft.com committed Jan 12, 2011 40 import Control.Monad ( when )  simonpj@microsoft.com committed Sep 13, 2010 41 import Outputable  Ian Lynagh committed Mar 29, 2008 42 import FastString  partain committed Jan 08, 1996 43 44 45 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 46 47 48 49 50 ********************************************************************************* * * * External interface * * * *********************************************************************************  simonpj committed Jan 25, 2001 51   simonpj@microsoft.com committed Sep 13, 2010 52 53 54 \begin{code} simplifyTop :: WantedConstraints -> TcM (Bag EvBind) -- Simplify top-level constraints  simonpj@microsoft.com committed Dec 13, 2010 55 56 57 -- Usually these will be implications, -- but when there is nothing to quantify we don't wrap -- in a degenerate implication, so we do that here instead  simonpj@microsoft.com committed Sep 13, 2010 58 simplifyTop wanteds  simonpj committed Apr 19, 2011 59  = simplifyCheck (SimplCheck (ptext (sLit "top level"))) wanteds  simonpj@microsoft.com committed Sep 13, 2010 60   Simon Peyton Jones committed Aug 16, 2011 61 62 63 64 65 ------------------ simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind) simplifyAmbiguityCheck name wanteds = simplifyCheck (SimplCheck (ptext (sLit "ambiguity check for") <+> ppr name)) wanteds  simonpj@microsoft.com committed Sep 13, 2010 66 67 68 69 70 71 72 73 74 ------------------ simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind) simplifyInteractive wanteds = simplifyCheck SimplInteractive wanteds ------------------ simplifyDefault :: ThetaType -- Wanted; has no type variables in it -> TcM () -- Succeeds iff the constraint is soluble simplifyDefault theta  simonpj@microsoft.com committed Jan 12, 2011 75  = do { wanted <- newFlatWanteds DefaultOrigin theta  simonpj committed Apr 19, 2011 76 77  ; _ignored_ev_binds <- simplifyCheck (SimplCheck (ptext (sLit "defaults"))) (mkFlatWC wanted)  simonpj@microsoft.com committed Sep 13, 2010 78 79  ; return () } \end{code}  simonpj committed Jan 25, 2001 80   simonpj committed May 03, 2001 81   qrczak committed Jul 17, 2001 82   simonpj@microsoft.com committed Sep 13, 2010 83 84 85 86 87 ********************************************************************************* * * * Deriving * * ***********************************************************************************  simonpj committed May 03, 2001 88   simonpj@microsoft.com committed Sep 13, 2010 89 90 \begin{code} simplifyDeriv :: CtOrigin  simonpj committed Apr 19, 2011 91 92 93 94  -> PredType -> [TyVar] -> ThetaType -- Wanted -> TcM ThetaType -- Needed  simonpj@microsoft.com committed Sep 13, 2010 95 96 -- Given instance (wanted) => C inst_ty -- Simplify 'wanted' as much as possibles  simonpj@microsoft.com committed Jan 12, 2011 97 -- Fail if not possible  simonpj committed Apr 19, 2011 98 simplifyDeriv orig pred tvs theta  simonpj@microsoft.com committed Feb 17, 2011 99 100 101 102 103  = do { tvs_skols <- tcInstSkolTyVars tvs -- Skolemize -- The constraint solving machinery -- expects *TcTyVars* not TyVars. -- We use *non-overlappable* (vanilla) skolems -- See Note [Overlap and deriving]  simonpj@microsoft.com committed Sep 13, 2010 104 105  ; let skol_subst = zipTopTvSubst tvs $map mkTyVarTy tvs_skols  simonpj@microsoft.com committed Jan 12, 2011 106  subst_skol = zipTopTvSubst tvs_skols$ map mkTyVarTy tvs  Simon Peyton Jones committed Jul 23, 2011 107  skol_set = mkVarSet tvs_skols  simonpj committed Apr 19, 2011 108  doc = parens $ptext (sLit "deriving") <+> parens (ppr pred)  simonpj@microsoft.com committed Jan 12, 2011 109 110 111 112 113  ; wanted <- newFlatWanteds orig (substTheta skol_subst theta) ; traceTc "simplifyDeriv" (ppr tvs $$ppr theta$$ ppr wanted) ; (residual_wanted, _binds)  simonpj committed Apr 19, 2011 114  <- runTcS (SimplInfer doc) NoUntouchables$  simonpj@microsoft.com committed Jan 12, 2011 115  solveWanteds emptyInert (mkFlatWC wanted)  simonpj@microsoft.com committed Sep 13, 2010 116   simonpj@microsoft.com committed Jan 12, 2011 117 118 119  ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted) -- See Note [Exotic derived instance contexts] get_good :: WantedEvVar -> Either PredType WantedEvVar  Simon Peyton Jones committed Jul 23, 2011 120 121  get_good wev | validDerivPred skol_set p = Left p | otherwise = Right wev  simonpj@microsoft.com committed Jan 12, 2011 122  where p = evVarOfPred wev  simonpj@microsoft.com committed Sep 13, 2010 123   simonpj@microsoft.com committed Jan 12, 2011 124  ; reportUnsolved (residual_wanted { wc_flat = bad })  simonpj@microsoft.com committed Sep 13, 2010 125   simonpj@microsoft.com committed Jan 12, 2011 126 127  ; let min_theta = mkMinimalBySCs (bagToList good) ; return (substTheta subst_skol min_theta) }  simonpj@microsoft.com committed Sep 13, 2010 128 \end{code}  simonpj committed May 03, 2001 129   simonpj@microsoft.com committed Feb 17, 2011 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 Note [Overlap and deriving] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider some overlapping instances: data Show a => Show [a] where .. data Show [Char] where ... Now a data type with deriving: data T a = MkT [a] deriving( Show ) We want to get the derived instance instance Show [a] => Show (T a) where... and NOT instance Show a => Show (T a) where... so that the (Show (T Char)) instance does the Right Thing It's very like the situation when we're inferring the type of a function f x = show [x] and we want to infer f :: Show [a] => a -> String BOTTOM LINE: use vanilla, non-overlappable skolems when inferring the context for the derived instance. Hence tcInstSkolTyVars not tcInstSuperSkolTyVars  simonpj@microsoft.com committed Sep 13, 2010 155 156 157 158 159 160 161 Note [Exotic derived instance contexts] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In a 'derived' instance declaration, we *infer* the context. It's a bit unclear what rules we should apply for this; the Haskell report is silent. Obviously, constraints like (Eq a) are fine, but what about data T f a = MkT (f a) deriving( Eq ) where we'd get an Eq (f a) constraint. That's probably fine too.  simonpj committed May 03, 2001 162   simonpj@microsoft.com committed Sep 13, 2010 163 164 165 One could go further: consider data T a b c = MkT (Foo a b c) deriving( Eq ) instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)  simonpj committed Feb 10, 2004 166   simonpj@microsoft.com committed Sep 13, 2010 167 168 Notice that this instance (just) satisfies the Paterson termination conditions. Then we *could* derive an instance decl like this:  simonpj committed Feb 10, 2004 169   simonpj@microsoft.com committed Sep 13, 2010 170 171 172 173  instance (C Int a, Eq b, Eq c) => Eq (T a b c) even though there is no instance for (C Int a), because there just *might* be an instance for, say, (C Int Bool) at a site where we need the equality instance for T's.  simonpj committed Feb 10, 2004 174   simonpj@microsoft.com committed Sep 13, 2010 175 176 177 However, this seems pretty exotic, and it's quite tricky to allow this, and yet give sensible error messages in the (much more common) case where we really want that instance decl for C.  simonpj committed Feb 10, 2004 178   simonpj@microsoft.com committed Sep 13, 2010 179 180 So for now we simply require that the derived instance context should have only type-variable constraints.  simonpj committed Feb 10, 2004 181   simonpj@microsoft.com committed Sep 13, 2010 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 Here is another example: data Fix f = In (f (Fix f)) deriving( Eq ) Here, if we are prepared to allow -XUndecidableInstances we could derive the instance instance Eq (f (Fix f)) => Eq (Fix f) but this is so delicate that I don't think it should happen inside 'deriving'. If you want this, write it yourself! NB: if you want to lift this condition, make sure you still meet the termination conditions! If not, the deriving mechanism generates larger and larger constraints. Example: data Succ a = S a data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show Note the lack of a Show instance for Succ. First we'll generate instance (Show (Succ a), Show a) => Show (Seq a) and then instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a) and so on. Instead we want to complain of no instance for (Show (Succ a)). The bottom line ~~~~~~~~~~~~~~~ Allow constraints which consist only of type variables, with no repeats. ********************************************************************************* * * * Inference * * ***********************************************************************************  simonpj committed Feb 10, 2004 211   dreixel committed Nov 11, 2011 212 213 214 215 216 217 218 219 220 221 222 223 Note [Which variables to quantify] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose the inferred type of a function is T kappa (alpha:kappa) -> Int where alpha is a type unification variable and kappa is a kind unification variable Then we want to quantify over *both* alpha and kappa. But notice that kappa appears "at top level" of the type, as well as inside the kind of alpha. So it should be fine to just look for the "top level" kind/type variables of the type, without looking transitively into the kinds of those type variables.  simonpj@microsoft.com committed Sep 13, 2010 224 \begin{code}  Simon Peyton Jones committed Aug 16, 2011 225 simplifyInfer :: Bool  simonpj@microsoft.com committed Jan 12, 2011 226 227 228  -> Bool -- Apply monomorphism restriction -> [(Name, TcTauType)] -- Variables to be generalised, -- and their tau-types  simonpj@microsoft.com committed Sep 13, 2010 229 230 231  -> WantedConstraints -> TcM ([TcTyVar], -- Quantify over these type variables [EvVar], -- ... and these constraints  Simon Peyton Jones committed Aug 16, 2011 232 233 234  Bool, -- The monomorphism restriction did something -- so the results type is not as general as -- it could be  simonpj@microsoft.com committed Sep 13, 2010 235  TcEvBinds) -- ... binding these evidence variables  Simon Peyton Jones committed Aug 16, 2011 236 simplifyInfer _top_lvl apply_mr name_taus wanteds  simonpj@microsoft.com committed Jan 12, 2011 237 238 239  | isEmptyWC wanteds = do { gbl_tvs <- tcGetGlobalTyVars -- Already zonked ; zonked_taus <- zonkTcTypes (map snd name_taus)  dreixel committed Nov 11, 2011 240 241 242 243  ; let tvs_to_quantify = tyVarsOfTypes zonked_taus minusVarSet gbl_tvs -- tvs_to_quantify can contain both kind and type vars -- See Note [Which variables to quantify] ; qtvs <- zonkQuantifiedTyVars tvs_to_quantify  Simon Peyton Jones committed Aug 16, 2011 244  ; return (qtvs, [], False, emptyTcEvBinds) }  simonpj committed Feb 10, 2004 245   simonpj@microsoft.com committed Sep 13, 2010 246  | otherwise  simonpj@microsoft.com committed Jan 12, 2011 247 248 249 250  = do { zonked_wanteds <- zonkWC wanteds ; zonked_taus <- zonkTcTypes (map snd name_taus) ; gbl_tvs <- tcGetGlobalTyVars  simonpj@microsoft.com committed Sep 13, 2010 251  ; traceTc "simplifyInfer {" $vcat  Simon Peyton Jones committed Aug 16, 2011 252 253 254 255 256  [ ptext (sLit "names =") <+> ppr (map fst name_taus) , ptext (sLit "taus (zonked) =") <+> ppr zonked_taus , ptext (sLit "gbl_tvs =") <+> ppr gbl_tvs , ptext (sLit "closed =") <+> ppr _top_lvl , ptext (sLit "apply_mr =") <+> ppr apply_mr  simonpj@microsoft.com committed Jan 12, 2011 257  , ptext (sLit "wanted =") <+> ppr zonked_wanteds  simonpj@microsoft.com committed Sep 13, 2010 258 259  ]  simonpj@microsoft.com committed Jan 12, 2011 260 261  -- Step 1 -- Make a guess at the quantified type variables  simonpj@microsoft.com committed Oct 08, 2010 262 263 264  -- Then split the constraints on the baisis of those tyvars -- to avoid unnecessarily simplifying a class constraint -- See Note [Avoid unecessary constraint simplification]  dreixel committed Nov 11, 2011 265  ; let zonked_tau_tvs = tyVarsOfTypes zonked_taus  simonpj@microsoft.com committed Jan 12, 2011 266  proto_qtvs = growWanteds gbl_tvs zonked_wanteds$  simonpj@microsoft.com committed Oct 08, 2010 267  zonked_tau_tvs minusVarSet gbl_tvs  simonpj@microsoft.com committed Jan 12, 2011 268 269 270 271 272 273 274 275 276 277  (perhaps_bound, surely_free) = partitionBag (quantifyMe proto_qtvs) (wc_flat zonked_wanteds) ; traceTc "simplifyInfer proto" $vcat [ ptext (sLit "zonked_tau_tvs =") <+> ppr zonked_tau_tvs , ptext (sLit "proto_qtvs =") <+> ppr proto_qtvs , ptext (sLit "surely_fref =") <+> ppr surely_free ] ; emitFlats surely_free  278 279 280 281  ; traceTc "sinf"$ vcat [ ptext (sLit "perhaps_bound =") <+> ppr perhaps_bound , ptext (sLit "surely_free =") <+> ppr surely_free ]  simonpj@microsoft.com committed Oct 08, 2010 282   simonpj@microsoft.com committed Jan 12, 2011 283 284 285  -- Step 2 -- Now simplify the possibly-bound constraints ; (simpl_results, tc_binds0)  simonpj committed Apr 19, 2011 286  <- runTcS (SimplInfer (ppr (map fst name_taus))) NoUntouchables $ simonpj@microsoft.com committed Jan 12, 2011 287 288 289 290 291 292 293 294 295 296 297  simplifyWithApprox (zonked_wanteds { wc_flat = perhaps_bound }) ; when (insolubleWC simpl_results) -- Fail fast if there is an insoluble constraint (do { reportUnsolved simpl_results; failM }) -- Step 3 -- Split again simplified_perhaps_bound, because some unifications -- may have happened, and emit the free constraints. ; gbl_tvs <- tcGetGlobalTyVars ; zonked_tau_tvs <- zonkTcTyVarsAndFV zonked_tau_tvs ; zonked_simples <- zonkWantedEvVars (wc_flat simpl_results)  simonpj@microsoft.com committed Oct 08, 2010 298  ; let init_tvs = zonked_tau_tvs minusVarSet gbl_tvs  Simon Peyton Jones committed Aug 16, 2011 299 300 301 302  poly_qtvs = growWantedEVs gbl_tvs zonked_simples init_tvs (pbound, pfree) = partitionBag (quantifyMe poly_qtvs) zonked_simples -- Monomorphism restriction  simonpj@microsoft.com committed Oct 08, 2010 303  mr_qtvs = init_tvs minusVarSet constrained_tvs  simonpj@microsoft.com committed Jan 12, 2011 304  constrained_tvs = tyVarsOfEvVarXs zonked_simples  Simon Peyton Jones committed Aug 16, 2011 305 306 307 308 309  mr_bites = apply_mr && not (isEmptyBag pbound) (qtvs, (bound, free)) | mr_bites = (mr_qtvs, (emptyBag, zonked_simples)) | otherwise = (poly_qtvs, (pbound, pfree))  simonpj@microsoft.com committed Jan 12, 2011 310 311  ; emitFlats free  Simon Peyton Jones committed Aug 16, 2011 312  ; if isEmptyVarSet qtvs && isEmptyBag bound  simonpj@microsoft.com committed Jan 12, 2011 313 314 315  then ASSERT( isEmptyBag (wc_insol simpl_results) ) do { traceTc "} simplifyInfer/no quantification" empty ; emitImplications (wc_impl simpl_results)  Simon Peyton Jones committed Aug 16, 2011 316  ; return ([], [], mr_bites, EvBinds tc_binds0) }  simonpj@microsoft.com committed Jan 12, 2011 317 318 319 320  else do -- Step 4, zonk quantified variables { let minimal_flat_preds = mkMinimalBySCs$ map evVarOfPred $bagToList bound  Simon Peyton Jones committed Aug 16, 2011 321 322  skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty) | (name, ty) <- name_taus ]  simonpj@microsoft.com committed Jan 12, 2011 323 324 325 326  -- Don't add the quantified variables here, because -- they are also bound in ic_skols and we want them to be -- tidied uniformly  dreixel committed Nov 11, 2011 327  ; qtvs_to_return <- zonkQuantifiedTyVars qtvs  simonpj@microsoft.com committed Jan 12, 2011 328 329 330 331 332 333 334  -- Step 5 -- Minimize bound' and emit an implication ; minimal_bound_ev_vars <- mapM TcMType.newEvVar minimal_flat_preds ; ev_binds_var <- newTcEvBinds ; mapBagM_ (\(EvBind evar etrm) -> addTcEvBind ev_binds_var evar etrm) tc_binds0 ; lcl_env <- getLclTypeEnv  dreixel committed Nov 11, 2011 335  ; gloc <- getCtLoc skol_info  simonpj@microsoft.com committed Jan 12, 2011 336 337 338 339 340 341 342 343 344 345 346 347  ; let implic = Implic { ic_untch = NoUntouchables , ic_env = lcl_env , ic_skols = mkVarSet qtvs_to_return , ic_given = minimal_bound_ev_vars , ic_wanted = simpl_results { wc_flat = bound } , ic_insol = False , ic_binds = ev_binds_var , ic_loc = gloc } ; emitImplication implic ; traceTc "} simplifyInfer/produced residual implication for quantification"$ vcat [ ptext (sLit "implic =") <+> ppr implic -- ic_skols, ic_given give rest of result  Simon Peyton Jones committed Aug 16, 2011 348  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return  simonpj@microsoft.com committed Jan 12, 2011 349 350 351 352 353  , ptext (sLit "spb =") <+> ppr zonked_simples , ptext (sLit "bound =") <+> ppr bound ]  Simon Peyton Jones committed Aug 16, 2011 354 355  ; return ( qtvs_to_return, minimal_bound_ev_vars , mr_bites, TcEvBinds ev_binds_var) } }  simonpj@microsoft.com committed Jan 12, 2011 356 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 357 358   simonpj@microsoft.com committed Jan 12, 2011 359 360 Note [Minimize by Superclasses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Sep 13, 2010 361   simonpj@microsoft.com committed Jan 12, 2011 362 363 364 365 366 367 368 When we quantify over a constraint, in simplifyInfer we need to quantify over a constraint that is minimal in some sense: For instance, if the final wanted constraint is (Eq alpha, Ord alpha), we'd like to quantify over Ord alpha, because we can just get Eq alpha from superclass selection from Ord alpha. This minimization is what mkMinimalBySCs does. Then, simplifyInfer uses the minimal constraint to check the original wanted.  simonpj@microsoft.com committed Sep 13, 2010 369   simonpj@microsoft.com committed Jan 12, 2011 370 371 372 373 \begin{code} simplifyWithApprox :: WantedConstraints -> TcS WantedConstraints simplifyWithApprox wanted = do { traceTcS "simplifyApproxLoop" (ppr wanted)  simonpj committed Feb 10, 2004 374   simonpj@microsoft.com committed Jan 12, 2011 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391  ; results <- solveWanteds emptyInert wanted ; let (residual_implics, floats) = approximateImplications (wc_impl results) -- If no new work was produced then we are done with simplifyApproxLoop ; if insolubleWC results || isEmptyBag floats then return results else solveWanteds emptyInert (WC { wc_flat = floats unionBags wc_flat results , wc_impl = residual_implics , wc_insol = emptyBag }) } approximateImplications :: Bag Implication -> (Bag Implication, Bag WantedEvVar) -- Extracts any nested constraints that don't mention the skolems approximateImplications impls = do_bag (float_implic emptyVarSet) impls  simonpj@microsoft.com committed Sep 13, 2010 392  where  simonpj@microsoft.com committed Jan 12, 2011 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413  do_bag :: forall a b c. (a -> (Bag b, Bag c)) -> Bag a -> (Bag b, Bag c) do_bag f = foldrBag (plus . f) (emptyBag, emptyBag) plus :: forall b c. (Bag b, Bag c) -> (Bag b, Bag c) -> (Bag b, Bag c) plus (a1,b1) (a2,b2) = (a1 unionBags a2, b1 unionBags b2) float_implic :: TyVarSet -> Implication -> (Bag Implication, Bag WantedEvVar) float_implic skols imp = (unitBag (imp { ic_wanted = wanted' }), floats) where (wanted', floats) = float_wc (skols unionVarSet ic_skols imp) (ic_wanted imp) float_wc skols wc@(WC { wc_flat = flat, wc_impl = implic }) = (wc { wc_flat = flat', wc_impl = implic' }, floats1 unionBags floats2) where (flat', floats1) = do_bag (float_flat skols) flat (implic', floats2) = do_bag (float_implic skols) implic float_flat :: TcTyVarSet -> WantedEvVar -> (Bag WantedEvVar, Bag WantedEvVar) float_flat skols wev | tyVarsOfEvVarX wev disjointVarSet skols = (emptyBag, unitBag wev) | otherwise = (unitBag wev, emptyBag)  simonpj@microsoft.com committed Sep 13, 2010 414 \end{code}  simonpj committed Feb 10, 2004 415   simonpj@microsoft.com committed Sep 13, 2010 416 \begin{code}  simonpj@microsoft.com committed Oct 08, 2010 417 418 -- (growX gbls wanted tvs) grows a seed 'tvs' against the -- X-constraint 'wanted', nuking the 'gbls' at each stage  simonpj@microsoft.com committed Jan 12, 2011 419 420 -- It's conservative in that if the seed could *possibly* -- grow to include a type variable, then it does  simonpj@microsoft.com committed Oct 19, 2010 421   simonpj@microsoft.com committed Jan 12, 2011 422 423 424 425 426 427 428 growWanteds :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet growWanteds gbl_tvs wc = fixVarSet (growWC gbl_tvs wc) growWantedEVs :: TyVarSet -> Bag WantedEvVar -> TyVarSet -> TyVarSet growWantedEVs gbl_tvs ws tvs | isEmptyBag ws = tvs | otherwise = fixVarSet (growPreds gbl_tvs evVarOfPred ws) tvs  simonpj@microsoft.com committed Oct 19, 2010 429   simonpj@microsoft.com committed Jan 12, 2011 430 431 432 433 434 -------- Helper functions, do not do fixpoint ------------------------ growWC :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet growWC gbl_tvs wc = growImplics gbl_tvs (wc_impl wc) . growPreds gbl_tvs evVarOfPred (wc_flat wc) . growPreds gbl_tvs evVarOfPred (wc_insol wc)  simonpj@microsoft.com committed Sep 13, 2010 435   simonpj@microsoft.com committed Jan 12, 2011 436 437 438 439 440 441 442 443 444 445 446 447 448 449 growImplics :: TyVarSet -> Bag Implication -> TyVarSet -> TyVarSet growImplics gbl_tvs implics tvs = foldrBag grow_implic tvs implics where grow_implic implic tvs = grow tvs minusVarSet ic_skols implic where grow = growWC gbl_tvs (ic_wanted implic) . growPreds gbl_tvs evVarPred (listToBag (ic_given implic)) -- We must grow from givens too; see test IPRun growPreds :: TyVarSet -> (a -> PredType) -> Bag a -> TyVarSet -> TyVarSet growPreds gbl_tvs get_pred items tvs = foldrBag extend tvs items  simonpj@microsoft.com committed Oct 19, 2010 450  where  simonpj@microsoft.com committed Jan 12, 2011 451 452  extend item tvs = tvs unionVarSet (growPredTyVars (get_pred item) tvs minusVarSet gbl_tvs)  simonpj@microsoft.com committed Sep 13, 2010 453 454 455 456 457 458 459  -------------------- quantifyMe :: TyVarSet -- Quantifying over these -> WantedEvVar -> Bool -- True <=> quantify over this wanted quantifyMe qtvs wev | isIPPred pred = True -- Note [Inheriting implicit parameters]  batterseapower committed Sep 06, 2011 460  | otherwise = tyVarsOfType pred intersectsVarSet qtvs  simonpj@microsoft.com committed Sep 13, 2010 461  where  simonpj@microsoft.com committed Jan 12, 2011 462  pred = evVarOfPred wev  simonpj@microsoft.com committed Sep 13, 2010 463 \end{code}  simonpj committed Jan 25, 2001 464   simonpj@microsoft.com committed Oct 08, 2010 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 Note [Avoid unecessary constraint simplification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When inferring the type of a let-binding, with simplifyInfer, try to avoid unnecessariliy simplifying class constraints. Doing so aids sharing, but it also helps with delicate situations like instance C t => C [t] where .. f :: C [t] => .... f x = let g y = ...(constraint C [t])... in ... When inferring a type for 'g', we don't want to apply the instance decl, because then we can't satisfy (C t). So we just notice that g isn't quantified over 't' and partition the contraints before simplifying. This only half-works, but then let-generalisation only half-works.  simonpj@microsoft.com committed Nov 24, 2006 483 484 Note [Inheriting implicit parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj committed May 03, 2001 485 486 487 Consider this: f x = (x::Int) + ?y  simonpj committed Jan 25, 2001 488   simonpj committed May 03, 2001 489 490 491 where f is *not* a top-level binding. From the RHS of f we'll get the constraint (?y::Int). There are two types we might infer for f:  simonpj committed Jan 25, 2001 492   simonpj committed May 03, 2001 493 494 495  f :: Int -> Int (so we get ?y from the context of f's definition), or  simonpj committed Jan 25, 2001 496 497 498  f :: (?y::Int) => Int -> Int  simonpj committed May 03, 2001 499 500 501 502 503 504 At first you might think the first was better, becuase then ?y behaves like a free variable of the definition, rather than having to be passed at each call site. But of course, the WHOLE IDEA is that ?y should be passed at each call site (that's what dynamic binding means) so we'd better infer the second.  simonpj committed Oct 25, 2001 505 506 BOTTOM LINE: when *inferring types* you *must* quantify over implicit parameters. See the predicate isFreeWhenInferring.  simonpj committed Jun 25, 2001 507   simonpj committed Oct 25, 2001 508   simonpj@microsoft.com committed Sep 13, 2010 509 510 511 512 513 ********************************************************************************* * * * RULES * * * ***********************************************************************************  simonpj committed May 12, 2004 514   simonpj@microsoft.com committed Sep 13, 2010 515 516 Note [Simplifying RULE lhs constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Dec 13, 2010 517 On the LHS of transformation rules we only simplify only equalities,  simonpj@microsoft.com committed Sep 13, 2010 518 519 520 521 but not dictionaries. We want to keep dictionaries unsimplified, to serve as the available stuff for the RHS of the rule. We *do* want to simplify equalities, however, to detect ill-typed rules that cannot be applied.  simonpj committed May 12, 2004 522   simonpj@microsoft.com committed Sep 13, 2010 523 524 525 Implementation: the TcSFlags carried by the TcSMonad controls the amount of simplification, so simplifyRuleLhs just sets the flag appropriately.  simonpj committed May 18, 1999 526   simonpj@microsoft.com committed May 19, 2006 527 528 529 530 531 532 533 534 535 536 Example. Consider the following left-hand side of a rule f (x == y) (y > z) = ... If we typecheck this expression we get constraints d1 :: Ord a, d2 :: Eq a We do NOT want to "simplify" to the LHS forall x::a, y::a, z::a, d1::Ord a. f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ... Instead we want forall x::a, y::a, z::a, d1::Ord a, d2::Eq a. f ((==) d2 x y) ((>) d1 y z) = ...  simonpj committed Jun 28, 1999 537   simonpj@microsoft.com committed May 19, 2006 538 Here is another example:  simonpj committed Feb 28, 2001 539 540  fromIntegral :: (Integral a, Num b) => a -> b {-# RULES "foo" fromIntegral = id :: Int -> Int #-}  simonpj@microsoft.com committed May 19, 2006 541 542 In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont* want to get  simonpj committed Feb 28, 2001 543  forall dIntegralInt.  simonpj@microsoft.com committed May 19, 2006 544  fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int  simonpj committed Mar 09, 2005 545 because the scsel will mess up RULE matching. Instead we want  simonpj committed Feb 28, 2001 546  forall dIntegralInt, dNumInt.  simonpj@microsoft.com committed May 19, 2006 547  fromIntegral Int Int dIntegralInt dNumInt = id Int  simonpj committed Feb 28, 2001 548   simonpj@microsoft.com committed May 19, 2006 549 550 551 552 553 554 555 Even if we have g (x == y) (y == z) = .. where the two dictionaries are *identical*, we do NOT WANT forall x::a, y::a, z::a, d1::Eq a f ((==) d1 x y) ((>) d1 y z) = ... because that will only match if the dict args are (visibly) equal. Instead we want to quantify over the dictionaries separately.  simonpj committed May 18, 1999 556   simonpj@microsoft.com committed Sep 13, 2010 557 558 In short, simplifyRuleLhs must *only* squash equalities, leaving all dicts unchanged, with absolutely no sharing.  lewie committed Mar 02, 2000 559   simonpj@microsoft.com committed Sep 13, 2010 560 561 562 563 564 565 566 567 568 HOWEVER, under a nested implication things are different Consider f :: (forall a. Eq a => a->a) -> Bool -> ... {-# RULES "foo" forall (v::forall b. Eq b => b->b). f b True = ... #=} Here we *must* solve the wanted (Eq a) from the given (Eq a) resulting from skolemising the agument type of g. So we revert to SimplCheck when going under an implication.  simonpj committed Sep 13, 2002 569 570  \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 571 572 573 574 575 576 577 578 579 simplifyRule :: RuleName -> [TcTyVar] -- Explicit skolems -> WantedConstraints -- Constraints from LHS -> WantedConstraints -- Constraints from RHS -> TcM ([EvVar], -- LHS dicts TcEvBinds, -- Evidence for LHS TcEvBinds) -- Evidence for RHS -- See Note [Simplifying RULE lhs constraints] simplifyRule name tv_bndrs lhs_wanted rhs_wanted  simonpj@microsoft.com committed Jan 12, 2011 580 581 582 583 584 585 586  = do { loc <- getCtLoc (RuleSkol name) ; zonked_lhs <- zonkWC lhs_wanted ; let untch = NoUntouchables -- We allow ourselves to unify environment -- variables; hence *no untouchables* ; (lhs_results, lhs_binds)  simonpj committed Apr 19, 2011 587  <- runTcS (SimplRuleLhs name) untch $ 588  solveWanteds emptyInert zonked_lhs  simonpj@microsoft.com committed Jan 12, 2011 589 590 591 592 593 594 595  ; traceTc "simplifyRule"$ vcat [ text "zonked_lhs" <+> ppr zonked_lhs , text "lhs_results" <+> ppr lhs_results , text "lhs_binds" <+> ppr lhs_binds , text "rhs_wanted" <+> ppr rhs_wanted ]  simonpj@microsoft.com committed Sep 13, 2010 596 597  -- Don't quantify over equalities (judgement call here)  simonpj@microsoft.com committed Jan 12, 2011 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612  ; let (eqs, dicts) = partitionBag (isEqPred . evVarOfPred) (wc_flat lhs_results) lhs_dicts = map evVarOf (bagToList dicts) -- Dicts and implicit parameters -- Fail if we have not got down to unsolved flats ; ev_binds_var <- newTcEvBinds ; emitImplication $Implic { ic_untch = untch , ic_env = emptyNameEnv , ic_skols = mkVarSet tv_bndrs , ic_given = lhs_dicts , ic_wanted = lhs_results { wc_flat = eqs } , ic_insol = insolubleWC lhs_results , ic_binds = ev_binds_var , ic_loc = loc }  simonpj@microsoft.com committed Sep 13, 2010 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628  -- Notice that we simplify the RHS with only the explicitly -- introduced skolems, allowing the RHS to constrain any -- unification variables. -- Then, and only then, we call zonkQuantifiedTypeVariables -- Example foo :: Ord a => a -> a -- foo_spec :: Int -> Int -- {-# RULE "foo" foo = foo_spec #-} -- Here, it's the RHS that fixes the type variable -- So we don't want to make untouchable the type -- variables in the envt of the RHS, because they include -- the template variables of the RULE -- Hence the rather painful ad-hoc treatement here ; rhs_binds_var@(EvBindsVar evb_ref _) <- newTcEvBinds  simonpj committed Apr 19, 2011 629 630  ; let doc = ptext (sLit "rhs of rule") <+> doubleQuotes (ftext name) ; rhs_binds1 <- simplifyCheck (SimplCheck doc)$  simonpj@microsoft.com committed Jan 12, 2011 631 632 633 634 635 636 637 638 639 640 641  WC { wc_flat = emptyBag , wc_insol = emptyBag , wc_impl = unitBag $Implic { ic_untch = NoUntouchables , ic_env = emptyNameEnv , ic_skols = mkVarSet tv_bndrs , ic_given = lhs_dicts , ic_wanted = rhs_wanted , ic_insol = insolubleWC rhs_wanted , ic_binds = rhs_binds_var , ic_loc = loc } }  simonpj@microsoft.com committed Sep 13, 2010 642 643 644 645 646  ; rhs_binds2 <- readTcRef evb_ref ; return ( lhs_dicts , EvBinds lhs_binds , EvBinds (rhs_binds1 unionBags evBindMapBinds rhs_binds2)) }  simonpj committed Sep 13, 2002 647 648 649 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 650 651 652 653 654 ********************************************************************************* * * * Main Simplifier * * * ***********************************************************************************  lewie committed Mar 02, 2000 655 656  \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 simplifyCheck :: SimplContext -> WantedConstraints -- Wanted -> TcM (Bag EvBind) -- Solve a single, top-level implication constraint -- e.g. typically one created from a top-level type signature -- f :: forall a. [a] -> [a] -- f x = rhs -- We do this even if the function has no polymorphism: -- g :: Int -> Int -- g y = rhs -- (whereas for *nested* bindings we would not create -- an implication constraint for g at all.) -- -- Fails if can't solve something in the input wanteds simplifyCheck ctxt wanteds  simonpj@microsoft.com committed Jan 12, 2011 673  = do { wanteds <- zonkWC wanteds  simonpj@microsoft.com committed Sep 13, 2010 674 675 676 677  ; traceTc "simplifyCheck {" (vcat [ ptext (sLit "wanted =") <+> ppr wanteds ])  simonpj@microsoft.com committed Jan 12, 2011 678 679  ; (unsolved, ev_binds) <- runTcS ctxt NoUntouchables$ solveWanteds emptyInert wanteds  simonpj@microsoft.com committed Sep 13, 2010 680 681  ; traceTc "simplifyCheck }" $ simonpj@microsoft.com committed Jan 12, 2011 682  ptext (sLit "unsolved =") <+> ppr unsolved  simonpj@microsoft.com committed Sep 13, 2010 683   simonpj@microsoft.com committed Jan 12, 2011 684  ; reportUnsolved unsolved  simonpj@microsoft.com committed Sep 13, 2010 685 686 687 688  ; return ev_binds } ----------------  simonpj@microsoft.com committed Jan 12, 2011 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 solveWanteds :: InertSet -- Given -> WantedConstraints -> TcS WantedConstraints solveWanteds inert wanted = do { (unsolved_flats, unsolved_implics, insols) <- solve_wanteds inert wanted ; return (WC { wc_flat = keepWanted unsolved_flats -- Discard Derived , wc_impl = unsolved_implics , wc_insol = insols }) } solve_wanteds :: InertSet -- Given -> WantedConstraints -> TcS (Bag FlavoredEvVar, Bag Implication, Bag FlavoredEvVar) -- solve_wanteds iterates when it is able to float equalities -- out of one or more of the implications solve_wanteds inert wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols }) = do { traceTcS "solveWanteds {" (ppr wanted) -- Try the flat bit  simonpj@microsoft.com committed Jan 13, 2011 708 709 710 711 712  -- Discard from insols all the derived/given constraints -- because they will show up again when we try to solve -- everything else. Solving them a second time is a bit -- of a waste, but the code is simple, and the program is -- wrong anyway!  simonpj@microsoft.com committed Jan 12, 2011 713 714 715 716 717  ; let all_flats = flats unionBags keepWanted insols ; inert1 <- solveInteractWanted inert (bagToList all_flats) ; (unsolved_flats, unsolved_implics) <- simpl_loop 1 inert1 implics  simonpj@microsoft.com committed Nov 12, 2010 718 719  ; bb <- getTcEvBindsBag ; tb <- getTcSTyBindsMap  simonpj@microsoft.com committed Sep 13, 2010 720  ; traceTcS "solveWanteds }"$  simonpj@microsoft.com committed Nov 12, 2010 721  vcat [ text "unsolved_flats =" <+> ppr unsolved_flats  simonpj@microsoft.com committed Jan 12, 2011 722  , text "unsolved_implics =" <+> ppr unsolved_implics  simonpj@microsoft.com committed Nov 12, 2010 723 724 725 726  , text "current evbinds =" <+> vcat (map ppr (varEnvElts bb)) , text "current tybinds =" <+> vcat (map ppr (varEnvElts tb)) ]  simonpj@microsoft.com committed Jan 12, 2011 727  ; (subst, remaining_flats) <- solveCTyFunEqs unsolved_flats  simonpj@microsoft.com committed Nov 12, 2010 728  -- See Note [Solving Family Equations]  simonpj@microsoft.com committed Jan 12, 2011 729 730 731 732 733 734 735 736  -- NB: remaining_flats has already had subst applied ; let (insoluble_flats, unsolved_flats) = partitionBag isCFrozenErr remaining_flats ; return ( mapBag (substFlavoredEvVar subst . deCanonicalise) unsolved_flats , mapBag (substImplication subst) unsolved_implics , mapBag (substFlavoredEvVar subst . deCanonicalise) insoluble_flats ) }  simonpj@microsoft.com committed Nov 10, 2006 737  where  simonpj@microsoft.com committed Jan 12, 2011 738 739  simpl_loop :: Int -> InertSet  simonpj@microsoft.com committed Sep 13, 2010 740  -> Bag Implication  simonpj@microsoft.com committed Jan 12, 2011 741 742  -> TcS (CanonicalCts, Bag Implication) -- CanonicalCts are Wanted or Derived simpl_loop n inert implics  simonpj@microsoft.com committed Sep 13, 2010 743  | n>10  dimitris@microsoft.com committed Dec 09, 2010 744  = trace "solveWanteds: loop" $-- Always bleat  simonpj@microsoft.com committed Sep 13, 2010 745  do { traceTcS "solveWanteds: loop" (ppr inert) -- Bleat more informatively  simonpj@microsoft.com committed Jan 12, 2011 746  ; let (_, unsolved_cans) = extractUnsolved inert  dimitris@microsoft.com committed Dec 09, 2010 747  ; return (unsolved_cans, implics) }  simonpj@microsoft.com committed Sep 13, 2010 748 749  | otherwise  simonpj@microsoft.com committed Nov 12, 2010 750 751 752  = do { traceTcS "solveWanteds: simpl_loop start {"$ vcat [ text "n =" <+> ppr n , text "implics =" <+> ppr implics  simonpj@microsoft.com committed Jan 12, 2011 753 754 755  , text "inert =" <+> ppr inert ] ; let (just_given_inert, unsolved_cans) = extractUnsolved inert  simonpj@microsoft.com committed Feb 11, 2011 756  -- unsolved_cans contains either Wanted or Derived!  simonpj@microsoft.com committed Sep 13, 2010 757   758  ; (implic_eqs, unsolved_implics)  simonpj@microsoft.com committed Feb 11, 2011 759  <- solveNestedImplications just_given_inert unsolved_cans implics  simonpj@microsoft.com committed Nov 12, 2010 760 761  -- Apply defaulting rules if and only if there  simonpj@microsoft.com committed Sep 13, 2010 762 763  -- no floated equalities. If there are, they may -- solve the remaining wanteds, so don't do defaulting.  simonpj@microsoft.com committed Jan 12, 2011 764 765 766  ; improve_eqs <- if not (isEmptyBag implic_eqs) then return implic_eqs else applyDefaultingRules just_given_inert unsolved_cans  simonpj@microsoft.com committed Sep 13, 2010 767   simonpj@microsoft.com committed Nov 12, 2010 768  ; traceTcS "solveWanteds: simpl_loop end }" $ simonpj@microsoft.com committed Jan 12, 2011 769 770  vcat [ text "improve_eqs =" <+> ppr improve_eqs , text "unsolved_flats =" <+> ppr unsolved_cans  simonpj@microsoft.com committed Nov 12, 2010 771 772  , text "unsolved_implics =" <+> ppr unsolved_implics ]  simonpj@microsoft.com committed Jan 12, 2011 773 774 775 776  ; (improve_eqs_already_in_inert, inert_with_improvement) <- solveInteract inert improve_eqs ; if improve_eqs_already_in_inert then  dimitris@microsoft.com committed Dec 09, 2010 777  return (unsolved_cans, unsolved_implics)  simonpj@microsoft.com committed Sep 13, 2010 778  else  simonpj@microsoft.com committed Jan 12, 2011 779 780 781  simpl_loop (n+1) inert_with_improvement -- Contain unsolved_cans and the improve_eqs unsolved_implics  782 783  }  dimitris committed May 17, 2011 784 785 786 787 givensFromWanteds :: SimplContext -> CanonicalCts -> Bag FlavoredEvVar -- Extract the Wanted ones from CanonicalCts and conver to -- Givens; not Given/Solved, see Note [Preparing inert set for implications] givensFromWanteds _ctxt = foldrBag getWanted emptyBag  simonpj@microsoft.com committed Feb 11, 2011 788 789 790  where getWanted :: CanonicalCt -> Bag FlavoredEvVar -> Bag FlavoredEvVar getWanted cc givens  dimitris committed May 17, 2011 791 792 793 794 795 796 797 798 799 800 801 802 803  | pushable_wanted cc = let given = mkEvVarX (cc_id cc) (mkGivenFlavor (cc_flavor cc) UnkSkol) in given consBag givens -- and not mkSolvedFlavor, -- see Note [Preparing inert set for implications] | otherwise = givens pushable_wanted :: CanonicalCt -> Bool pushable_wanted cc | not (isCFrozenErr cc) , isWantedCt cc = isEqPred (evVarPred (cc_id cc)) -- see Note [Preparing inert set for implications] | otherwise = False  simonpj@microsoft.com committed Feb 11, 2011 804 805 solveNestedImplications :: InertSet -> CanonicalCts -> Bag Implication  simonpj@microsoft.com committed Jan 12, 2011 806  -> TcS (Bag FlavoredEvVar, Bag Implication)  simonpj@microsoft.com committed Feb 11, 2011 807 solveNestedImplications just_given_inert unsolved_cans implics  808 809 810  | isEmptyBag implics = return (emptyBag, emptyBag) | otherwise  simonpj@microsoft.com committed Feb 11, 2011 811 812  = do { -- See Note [Preparing inert set for implications] -- Push the unsolved wanteds inwards, but as givens  dimitris committed May 17, 2011 813 814 815 816  ; simpl_ctx <- getTcSContext ; let pushed_givens = givensFromWanteds simpl_ctx unsolved_cans  simonpj@microsoft.com committed Feb 11, 2011 817 818 819  tcs_untouchables = filterVarSet isFlexiTcsTv$ tyVarsOfEvVarXs pushed_givens -- See Note [Extra TcsTv untouchables]  simonpj@microsoft.com committed Jan 12, 2011 820   simonpj@microsoft.com committed Feb 17, 2011 821 822  ; traceTcS "solveWanteds: preparing inerts for implications {" (vcat [ppr tcs_untouchables, ppr pushed_givens])  dimitris committed May 17, 2011 823 824  ; (_, inert_for_implics) <- solveInteract just_given_inert pushed_givens  825   simonpj@microsoft.com committed Feb 11, 2011 826  ; traceTcS "solveWanteds: } now doing nested implications {" $ 827 828 829 830 831 832 833 834 835 836 837 838  vcat [ text "inerts_for_implics =" <+> ppr inert_for_implics , text "implics =" <+> ppr implics ] ; (implic_eqs, unsolved_implics) <- flatMapBagPairM (solveImplication tcs_untouchables inert_for_implics) implics ; traceTcS "solveWanteds: done nested implications }"$ vcat [ text "implic_eqs =" <+> ppr implic_eqs , text "unsolved_implics =" <+> ppr unsolved_implics ] ; return (implic_eqs, unsolved_implics) }  simonpj@microsoft.com committed Jan 12, 2011 839 840 841 842 843 solveImplication :: TcTyVarSet -- Untouchable TcS unification variables -> InertSet -- Given -> Implication -- Wanted -> TcS (Bag FlavoredEvVar, -- All wanted or derived unifications: var = type Bag Implication) -- Unsolved rest (always empty or singleton)  simonpj@microsoft.com committed Sep 13, 2010 844 845 846 847 848 849 850 -- Returns: -- 1. A bag of floatable wanted constraints, not mentioning any skolems, -- that are of the form unification var = type -- -- 2. Maybe a unsolved implication, empty if entirely solved! -- -- Precondition: everything is zonked by now  851 solveImplication tcs_untouchables inert  simonpj@microsoft.com committed Oct 06, 2010 852 853 854 855  imp@(Implic { ic_untch = untch , ic_binds = ev_binds , ic_skols = skols , ic_given = givens  simonpj@microsoft.com committed Sep 13, 2010 856  , ic_wanted = wanteds  simonpj@microsoft.com committed Oct 06, 2010 857  , ic_loc = loc })  858  = nestImplicTcS ev_binds (untch, tcs_untouchables) $ simonpj@microsoft.com committed Oct 19, 2010 859 860 861 862  recoverTcS (return (emptyBag, emptyBag))$ -- Recover from nested failures. Even the top level is -- just a bunch of implications, so failing at the first -- one is bad  simonpj@microsoft.com committed Sep 13, 2010 863 864 865  do { traceTcS "solveImplication {" (ppr imp) -- Solve flat givens  simonpj@microsoft.com committed Jan 12, 2011 866  ; given_inert <- solveInteractGiven inert loc givens  simonpj@microsoft.com committed Sep 13, 2010 867 868  -- Simplify the wanteds  simonpj@microsoft.com committed Jan 12, 2011 869 870 871 872 873 874  ; (unsolved_flats, unsolved_implics, insols) <- solve_wanteds given_inert wanteds ; let (res_flat_free, res_flat_bound) = floatEqualities skols givens unsolved_flats final_flat = keepWanted res_flat_bound  simonpj@microsoft.com committed Sep 13, 2010 875   simonpj@microsoft.com committed Jan 12, 2011 876 877 878 879 880 881  ; let res_wanted = WC { wc_flat = final_flat , wc_impl = unsolved_implics , wc_insol = insols } res_implic = unitImplication $imp { ic_wanted = res_wanted , ic_insol = insolubleWC res_wanted }  simonpj@microsoft.com committed Sep 13, 2010 882 883 884  ; traceTcS "solveImplication end }"$ vcat [ text "res_flat_free =" <+> ppr res_flat_free  simonpj@microsoft.com committed Jan 12, 2011 885  , text "res_implic =" <+> ppr res_implic ]  simonpj@microsoft.com committed Sep 13, 2010 886   simonpj@microsoft.com committed Jan 12, 2011 887  ; return (res_flat_free, res_implic) }  simonpj@microsoft.com committed Sep 13, 2010 888 889   simonpj@microsoft.com committed Jan 12, 2011 890 891 892 893 894 895 floatEqualities :: TcTyVarSet -> [EvVar] -> Bag FlavoredEvVar -> (Bag FlavoredEvVar, Bag FlavoredEvVar) -- Post: The returned FlavoredEvVar's are only Wanted or Derived -- and come from the input wanted ev vars or deriveds floatEqualities skols can_given wantders | hasEqualities can_given = (emptyBag, wantders)  simonpj@microsoft.com committed Nov 12, 2010 896  -- Note [Float Equalities out of Implications]  simonpj@microsoft.com committed Jan 12, 2011 897 898 899 900  | otherwise = partitionBag is_floatable wantders where is_floatable :: FlavoredEvVar -> Bool  batterseapower committed Sep 06, 2011 901 902  is_floatable (EvVarX eqv _fl) | isEqPred (evVarPred eqv) = skols disjointVarSet tvs_under_fsks (evVarPred eqv)  simonpj@microsoft.com committed Jan 12, 2011 903  is_floatable _flev = False  simonpj@microsoft.com committed Nov 12, 2010 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920  tvs_under_fsks :: Type -> TyVarSet -- ^ NB: for type synonyms tvs_under_fsks does /not/ expand the synonym tvs_under_fsks (TyVarTy tv) | not (isTcTyVar tv) = unitVarSet tv | FlatSkol ty <- tcTyVarDetails tv = tvs_under_fsks ty | otherwise = unitVarSet tv tvs_under_fsks (TyConApp _ tys) = unionVarSets (map tvs_under_fsks tys) tvs_under_fsks (FunTy arg res) = tvs_under_fsks arg unionVarSet tvs_under_fsks res tvs_under_fsks (AppTy fun arg) = tvs_under_fsks fun unionVarSet tvs_under_fsks arg tvs_under_fsks (ForAllTy tv ty) -- The kind of a coercion binder -- can mention type variables! | isTyVar tv = inner_tvs delVarSet tv | otherwise {- Coercion -} = -- ASSERT( not (tv elemVarSet inner_tvs) ) inner_tvs unionVarSet tvs_under_fsks (tyVarKind tv) where inner_tvs = tvs_under_fsks ty  lewie committed Mar 02, 2000 921 \end{code}  partain committed Jan 08, 1996 922   simonpj@microsoft.com committed Sep 13, 2010 923 924 925 926 Note [Preparing inert set for implications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Before solving the nested implications, we convert any unsolved flat wanteds to givens, and add them to the inert set. Reasons:  927 928  a) In checking mode, suppresses unnecessary errors. We already have  simonpj@microsoft.com committed Sep 13, 2010 929  on unsolved-wanted error; adding it to the givens prevents any  simonpj@microsoft.com committed Feb 11, 2011 930  consequential errors from showing up  931   simonpj@microsoft.com committed Sep 13, 2010 932 933 934 935  b) More importantly, in inference mode, we are going to quantify over this constraint, and we *don't* want to quantify over any constraints that are deducible from it.  simonpj@microsoft.com committed Feb 11, 2011 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958  c) Flattened type-family equalities must be exposed to the nested constraints. Consider F b ~ alpha, (forall c. F b ~ alpha) Obviously this is soluble with [alpha := F b]. But the unification is only done by solveCTyFunEqs, right at the end of solveWanteds, and if we aren't careful we'll end up with an unsolved goal inside the implication. We need to "push" the as-yes-unsolved (F b ~ alpha) inwards, as a *given*, so that it can be used to solve the inner (F b ~ alpha). See Trac #4935. d) There are other cases where interactions between wanteds that can help to solve a constraint. For example class C a b | a -> b (C Int alpha), (forall d. C d blah => C Int a) If we push the (C Int alpha) inwards, as a given, it can produce a fundep (alpha~a) and this can float out again and be used to fix alpha. (In general we can't float class constraints out just in case (C d blah) might help to solve (C Int a).)  simonpj@microsoft.com committed Sep 13, 2010 959 960 961 962 963 964 965 966 The unsolved wanteds are *canonical* but they may not be *inert*, because when made into a given they might interact with other givens. Hence the call to solveInteract. Example: Original inert set = (d :_g D a) /\ (co :_w a ~ [beta]) We were not able to solve (a ~w [beta]) but we can't just assume it as given because the resulting set is not inert. Hence we have to do a  967 968 'solveInteract' step first.  dimitris committed May 17, 2011 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 Finally, note that we convert them to [Given] and NOT [Given/Solved]. The reason is that Given/Solved are weaker than Givens and may be discarded. As an example consider the inference case, where we may have, the following original constraints: [Wanted] F Int ~ Int (F Int ~ a => F Int ~ a) If we convert F Int ~ Int to [Given/Solved] instead of Given, then the next given (F Int ~ a) is going to cause the Given/Solved to be ignored, casting the (F Int ~ a) insoluble. Hence we should really convert the residual wanteds to plain old Given. We need only push in unsolved equalities both in checking mode and inference mode: (1) In checking mode we should not push given dictionaries in because of example LongWayOverlapping.hs, where we might get strange overlap errors between far-away constraints in the program. But even in checking mode, we must still push type family equations. Consider: type instance F True a b = a type instance F False a b = b [w] F c a b ~ gamma (c ~ True) => a ~ gamma (c ~ False) => b ~ gamma Since solveCTyFunEqs happens at the very end of solving, the only way to solve the two implications is temporarily consider (F c a b ~ gamma) as Given (NB: not merely Given/Solved because it has to interact with the top-level instance environment) and push it inside the implications. Now, when we come out again at the end, having solved the implications solveCTyFunEqs will solve this equality. (2) In inference mode, we recheck the final constraint in checking mode and hence we will be able to solve inner implications from top-level quantified constraints nonetheless.  1005 1006 Note [Extra TcsTv untouchables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Feb 11, 2011 1007 1008 1009 1010 1011 Furthemore, we record the inert set simplifier-generated unification variables of the TcsTv kind (such as variables from instance that have been applied, or unification flattens). These variables must be passed to the implications as extra untouchable variables. Otherwise we have the danger of double unifications. Example (from trac ticket #4494):  1012 1013 1014  (F Int ~ uf) /\ (forall a. C a => F Int ~ beta)  simonpj@microsoft.com committed Feb 11, 2011 1015 1016 1017 In this example, beta is touchable inside the implication. The first solveInteract step leaves 'uf' ununified. Then we move inside the implication where a new constraint  1018  uf ~ beta  simonpj@microsoft.com committed Feb 11, 2011 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 emerges. We may spontaneously solve it to get uf := beta, so the whole implication disappears but when we pop out again we are left with (F Int ~ uf) which will be unified by our final solveCTyFunEqs stage and uf will get unified *once more* to (F Int). The solution is to record the TcsTvs (i.e. the simplifier-generated unification variables) that are generated when solving the flats, and make them untouchables for the nested implication. In the example above uf would become untouchable, so beta would be forced to be unified as beta := uf. NB: A consequence is that every simplifier-generated TcsTv variable that gets floated out of an implication becomes now untouchable next time we go inside that implication to solve any residual constraints. In effect, by floating an equality out of the implication we are committing to have it solved in the outside.  1035   simonpj@microsoft.com committed Feb 14, 2011 1036 1037 1038 1039 Note [Float Equalities out of Implications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We want to float equalities out of vanilla existentials, but *not* out of GADT pattern matches.  1040   simonpj@microsoft.com committed Sep 13, 2010 1041   simonpj@microsoft.com committed Nov 12, 2010 1042 1043 \begin{code}  simonpj@microsoft.com committed Jan 12, 2011 1044 solveCTyFunEqs :: CanonicalCts -> TcS (TvSubst, CanonicalCts)  simonpj@microsoft.com committed Nov 12, 2010 1045 1046 1047 1048 -- Default equalities (F xi ~ alpha) by setting (alpha := F xi), whenever possible -- See Note [Solving Family Equations] -- Returns: a bunch of unsolved constraints from the original CanonicalCts and implications -- where the newly generated equalities (alpha := F xi) have been substituted through.  simonpj@microsoft.com committed Jan 12, 2011 1049 solveCTyFunEqs cts  simonpj@microsoft.com committed Nov 12, 2010 1050  = do { untch <- getUntouchables  simonpj@microsoft.com committed Jan 12, 2011 1051 1052  ; let (unsolved_can_cts, (ni_subst, cv_binds)) = getSolvableCTyFunEqs untch cts  simonpj@microsoft.com committed Nov 12, 2010 1053  ; traceTcS "defaultCTyFunEqs" (vcat [text "Trying to default family equations:"  simonpj@microsoft.com committed Jan 12, 2011 1054  , ppr ni_subst, ppr cv_binds  simonpj@microsoft.com committed Nov 12, 2010 1055  ])  simonpj@microsoft.com committed Jan 12, 2011 1056 1057 1058 1059  ; mapM_ solve_one cv_binds ; return (niFixTvSubst ni_subst, unsolved_can_cts) } where  1060  solve_one (cv,tv,ty) = do { setWantedTyBind tv ty  batterseapower committed Sep 06, 2011 1061  ; setEqBind cv (mkReflCo ty) }  simonpj@microsoft.com committed Nov 12, 2010 1062   simonpj@microsoft.com committed Jan 12, 2011 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 ------------ type FunEqBinds = (TvSubstEnv, [(CoVar, TcTyVar, TcType)]) -- The TvSubstEnv is not idempotent, but is loop-free -- See Note [Non-idempotent substitution] in Unify emptyFunEqBinds :: FunEqBinds emptyFunEqBinds = (emptyVarEnv, []) extendFunEqBinds :: FunEqBinds -> CoVar -> TcTyVar -> TcType -> FunEqBinds extendFunEqBinds (tv_subst, cv_binds) cv tv ty = (extendVarEnv tv_subst tv ty, (cv, tv, ty):cv_binds) ------------  1075 getSolvableCTyFunEqs :: TcsUntouchables  simonpj@microsoft.com committed Jan 12, 2011 1076  -> CanonicalCts -- Precondition: all Wanteds or Derived!  simonpj@microsoft.com committed Nov 12, 2010 1077 1078  -> (CanonicalCts, FunEqBinds) -- Postcondition: returns the unsolvables getSolvableCTyFunEqs untch cts  simonpj@microsoft.com committed Jan 12, 2011 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103  = Bag.foldlBag dflt_funeq (emptyCCan, emptyFunEqBinds) cts where dflt_funeq :: (CanonicalCts, FunEqBinds) -> CanonicalCt -> (CanonicalCts, FunEqBinds) dflt_funeq (cts_in, feb@(tv_subst, _)) (CFunEqCan { cc_id = cv , cc_flavor = fl , cc_fun = tc , cc_tyargs = xis , cc_rhs = xi }) | Just tv <- tcGetTyVar_maybe xi -- RHS is a type variable , isTouchableMetaTyVar_InRange untch tv -- And it's a *touchable* unification variable , typeKind xi isSubKind tyVarKind tv -- Must do a small kind check since TcCanonical invariants -- on family equations only impose compatibility, not subkinding , not (tv elemVarEnv tv_subst) -- Check not in extra_binds -- See Note [Solving Family Equations], Point 1 , not (tv elemVarSet niSubstTvSet tv_subst (tyVarsOfTypes xis)) -- Occurs check: see Note [Solving Family Equations], Point 2  dimitris committed May 17, 2011 1104  = ASSERT ( not (isGivenOrSolved fl) )  simonpj@microsoft.com committed Jan 12, 2011 1105 1106 1107 1108  (cts_in, extendFunEqBinds feb cv tv (mkTyConApp tc xis)) dflt_funeq (cts_in, fun_eq_binds) ct = (cts_in extendCCans ct, fun_eq_binds)  simonpj@microsoft.com committed Nov 12, 2010 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 \end{code} Note [Solving Family Equations] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ After we are done with simplification we may be left with constraints of the form: [Wanted] F xis ~ beta If 'beta' is a touchable unification variable not already bound in the TyBinds then we'd like to create a binding for it, effectively "defaulting" it to be 'F xis'. When is it ok to do so? 1) 'beta' must not already be defaulted to something. Example: [Wanted] F Int ~ beta <~ Will default [beta := F Int] [Wanted] F Char ~ beta <~ Already defaulted, can't default again. We have to report this as unsolved. 2) However, we must still do an occurs check when defaulting (F xis ~ beta), to set [beta := F xis] only if beta is not among the free variables of xis. 3) Notice that 'beta' can't be bound in ty binds already because we rewrite RHS of type family equations. See Inert Set invariants in TcInteract.  simonpj@microsoft.com committed Sep 13, 2010 1132 1133 1134 1135 1136 1137 1138 ********************************************************************************* * * * Defaulting and disamgiguation * * * ********************************************************************************* Basic plan behind applyDefaulting rules:  1139   simonpj@microsoft.com committed Sep 13, 2010 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165  Step 1: Split wanteds into defaultable groups, groups' and the rest rest_wanted' For each defaultable group, do: For each possible substitution for [alpha |-> tau] where alpha' is the group's variable, do: 1) Make up new TcEvBinds 2) Extend TcS with (groupVariable 3) given_inert <- solveOne inert (given : a ~ tau) 4) (final_inert,unsolved) <- solveWanted (given_inert) (group_constraints) 5) if unsolved == empty then sneakyUnify a |-> tau write the evidence bins return (final_inert ++ group_constraints,[]) -- will contain the info (alpha |-> tau)!! goto next defaultable group if unsolved <> empty then throw away evidence binds try next substitution If you've run out of substitutions for this group, too bad, you failed return (inert,group) goto next defaultable group Step 2: Collect all the (canonical-cts, wanteds) gathered this way. - Do a solveGiven over the canonical-cts to make sure they are inert ------------------------------------------------------------------------------------------  chak@cse.unsw.edu.au. committed Oct 01, 2008 1166   simonm committed Jan 08, 1998 1167