TcSimplify.lhs 58.4 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 import TcType import TcSMonad  dimitris committed Nov 16, 2011 23 import TcInteract  simonpj@microsoft.com committed Sep 13, 2010 24 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  dimitris committed Nov 16, 2011 43 44 import TrieMap  partain committed Jan 08, 1996 45 46 47 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 48 49 50 51 52 ********************************************************************************* * * * External interface * * * *********************************************************************************  simonpj committed Jan 25, 2001 53   simonpj@microsoft.com committed Sep 13, 2010 54 55 56 \begin{code} simplifyTop :: WantedConstraints -> TcM (Bag EvBind) -- Simplify top-level constraints  simonpj@microsoft.com committed Dec 13, 2010 57 58 59 -- 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 60 simplifyTop wanteds  simonpj committed Apr 19, 2011 61  = simplifyCheck (SimplCheck (ptext (sLit "top level"))) wanteds  simonpj@microsoft.com committed Sep 13, 2010 62   Simon Peyton Jones committed Aug 16, 2011 63 64 65 66 ------------------ simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind) simplifyAmbiguityCheck name wanteds = simplifyCheck (SimplCheck (ptext (sLit "ambiguity check for") <+> ppr name)) wanteds  dimitris committed Nov 16, 2011 67   simonpj@microsoft.com committed Sep 13, 2010 68 69 70 71 72 73 74 75 76 ------------------ 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 77  = do { wanted <- newFlatWanteds DefaultOrigin theta  simonpj committed Apr 19, 2011 78 79  ; _ignored_ev_binds <- simplifyCheck (SimplCheck (ptext (sLit "defaults"))) (mkFlatWC wanted)  simonpj@microsoft.com committed Sep 13, 2010 80 81  ; return () } \end{code}  simonpj committed Jan 25, 2001 82   simonpj committed May 03, 2001 83   dimitris committed Nov 16, 2011 84 ***********************************************************************************  simonpj@microsoft.com committed Sep 13, 2010 85 * *  dimitris committed Nov 16, 2011 86 * Deriving *  simonpj@microsoft.com committed Sep 13, 2010 87 88 * * ***********************************************************************************  simonpj committed May 03, 2001 89   simonpj@microsoft.com committed Sep 13, 2010 90 91 \begin{code} simplifyDeriv :: CtOrigin  simonpj committed Apr 19, 2011 92 93 94 95  -> PredType -> [TyVar] -> ThetaType -- Wanted -> TcM ThetaType -- Needed  simonpj@microsoft.com committed Sep 13, 2010 96 97 -- Given instance (wanted) => C inst_ty -- Simplify 'wanted' as much as possibles  simonpj@microsoft.com committed Jan 12, 2011 98 -- Fail if not possible  simonpj committed Apr 19, 2011 99 simplifyDeriv orig pred tvs theta  simonpj@microsoft.com committed Feb 17, 2011 100 101 102 103 104  = 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 105 106  ; let skol_subst = zipTopTvSubst tvs $map mkTyVarTy tvs_skols  simonpj@microsoft.com committed Jan 12, 2011 107  subst_skol = zipTopTvSubst tvs_skols$ map mkTyVarTy tvs  Simon Peyton Jones committed Jul 23, 2011 108  skol_set = mkVarSet tvs_skols  simonpj committed Apr 19, 2011 109  doc = parens $ptext (sLit "deriving") <+> parens (ppr pred)  simonpj@microsoft.com committed Jan 12, 2011 110 111 112 113 114  ; wanted <- newFlatWanteds orig (substTheta skol_subst theta) ; traceTc "simplifyDeriv" (ppr tvs $$ppr theta$$ ppr wanted) ; (residual_wanted, _binds)  dimitris committed Nov 16, 2011 115 116  <- solveWanteds (SimplInfer doc) NoUntouchables$ mkFlatWC wanted  simonpj@microsoft.com committed Sep 13, 2010 117   simonpj@microsoft.com committed Jan 12, 2011 118 119  ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted) -- See Note [Exotic derived instance contexts]  dimitris committed Nov 16, 2011 120 121 122  get_good :: Ct -> Either PredType Ct get_good ct | validDerivPred skol_set p = Left p | otherwise = Right ct  dimitris committed Nov 29, 2011 123  where p = ctPred ct  simonpj@microsoft.com committed Sep 13, 2010 124   simonpj@microsoft.com committed Jan 12, 2011 125  ; reportUnsolved (residual_wanted { wc_flat = bad })  simonpj@microsoft.com committed Sep 13, 2010 126   simonpj@microsoft.com committed Jan 12, 2011 127 128  ; let min_theta = mkMinimalBySCs (bagToList good) ; return (substTheta subst_skol min_theta) }  simonpj@microsoft.com committed Sep 13, 2010 129 \end{code}  simonpj committed May 03, 2001 130   simonpj@microsoft.com committed Feb 17, 2011 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 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 156 157 158 159 160 161 162 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 163   simonpj@microsoft.com committed Sep 13, 2010 164 165 166 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 167   simonpj@microsoft.com committed Sep 13, 2010 168 169 Notice that this instance (just) satisfies the Paterson termination conditions. Then we *could* derive an instance decl like this:  simonpj committed Feb 10, 2004 170   simonpj@microsoft.com committed Sep 13, 2010 171 172 173 174  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 175   simonpj@microsoft.com committed Sep 13, 2010 176 177 178 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 179   simonpj@microsoft.com committed Sep 13, 2010 180 181 So for now we simply require that the derived instance context should have only type-variable constraints.  simonpj committed Feb 10, 2004 182   simonpj@microsoft.com committed Sep 13, 2010 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 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 212   dreixel committed Nov 11, 2011 213 214 215 216 217 218 219 220 221 222 223 224 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 225 \begin{code}  Simon Peyton Jones committed Aug 16, 2011 226 simplifyInfer :: Bool  simonpj@microsoft.com committed Jan 12, 2011 227 228 229  -> Bool -- Apply monomorphism restriction -> [(Name, TcTauType)] -- Variables to be generalised, -- and their tau-types  simonpj@microsoft.com committed Sep 13, 2010 230 231 232  -> WantedConstraints -> TcM ([TcTyVar], -- Quantify over these type variables [EvVar], -- ... and these constraints  Simon Peyton Jones committed Aug 16, 2011 233 234 235  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 236  TcEvBinds) -- ... binding these evidence variables  Simon Peyton Jones committed Aug 16, 2011 237 simplifyInfer _top_lvl apply_mr name_taus wanteds  simonpj@microsoft.com committed Jan 12, 2011 238 239 240  | isEmptyWC wanteds = do { gbl_tvs <- tcGetGlobalTyVars -- Already zonked ; zonked_taus <- zonkTcTypes (map snd name_taus)  dreixel committed Nov 11, 2011 241 242 243 244  ; 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 245  ; return (qtvs, [], False, emptyTcEvBinds) }  simonpj committed Feb 10, 2004 246   simonpj@microsoft.com committed Sep 13, 2010 247  | otherwise  simonpj@microsoft.com committed Jan 12, 2011 248 249 250 251  = do { zonked_wanteds <- zonkWC wanteds ; zonked_taus <- zonkTcTypes (map snd name_taus) ; gbl_tvs <- tcGetGlobalTyVars  simonpj@microsoft.com committed Sep 13, 2010 252  ; traceTc "simplifyInfer {" $vcat  Simon Peyton Jones committed Aug 16, 2011 253 254 255 256 257  [ ptext (sLit "names =") <+> ppr (map fst name_taus) , ptext (sLit "taus (zonked) =") <+> ppr zonked_taus , ptext (sLit "gbl_tvs =") <+> ppr gbl_tvs , ptext (sLit "closed =") <+> ppr _top_lvl , ptext (sLit "apply_mr =") <+> ppr apply_mr  simonpj@microsoft.com committed Jan 12, 2011 258  , ptext (sLit "wanted =") <+> ppr zonked_wanteds  simonpj@microsoft.com committed Sep 13, 2010 259 260  ]  simonpj@microsoft.com committed Jan 12, 2011 261 262  -- Step 1 -- Make a guess at the quantified type variables  simonpj@microsoft.com committed Oct 08, 2010 263 264 265  -- Then split the constraints on the baisis of those tyvars -- to avoid unnecessarily simplifying a class constraint -- See Note [Avoid unecessary constraint simplification]  dreixel committed Nov 11, 2011 266  ; let zonked_tau_tvs = tyVarsOfTypes zonked_taus  simonpj@microsoft.com committed Jan 12, 2011 267  proto_qtvs = growWanteds gbl_tvs zonked_wanteds$  simonpj@microsoft.com committed Oct 08, 2010 268  zonked_tau_tvs minusVarSet gbl_tvs  simonpj@microsoft.com committed Jan 12, 2011 269 270 271 272 273 274 275 276 277  (perhaps_bound, surely_free) = partitionBag (quantifyMe proto_qtvs) (wc_flat zonked_wanteds) ; traceTc "simplifyInfer proto" $vcat [ ptext (sLit "zonked_tau_tvs =") <+> ppr zonked_tau_tvs , ptext (sLit "proto_qtvs =") <+> ppr proto_qtvs , ptext (sLit "surely_fref =") <+> ppr surely_free ]  dimitris committed Nov 16, 2011 278  ; emitWantedCts surely_free  279 280 281 282  ; traceTc "sinf"$ vcat [ ptext (sLit "perhaps_bound =") <+> ppr perhaps_bound , ptext (sLit "surely_free =") <+> ppr surely_free ]  simonpj@microsoft.com committed Oct 08, 2010 283   simonpj@microsoft.com committed Jan 12, 2011 284 285 286  -- Step 2 -- Now simplify the possibly-bound constraints ; (simpl_results, tc_binds0)  dimitris committed Nov 16, 2011 287  <- runTcS (SimplInfer (ppr (map fst name_taus))) NoUntouchables emptyInert emptyWorkList $ simonpj@microsoft.com committed Jan 12, 2011 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  dimitris committed Nov 16, 2011 298  ; zonked_simples <- zonkCts (wc_flat simpl_results)  simonpj@microsoft.com committed Oct 08, 2010 299  ; let init_tvs = zonked_tau_tvs minusVarSet gbl_tvs  Simon Peyton Jones committed Aug 16, 2011 300 301 302 303  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 304  mr_qtvs = init_tvs minusVarSet constrained_tvs  dimitris committed Nov 16, 2011 305  constrained_tvs = tyVarsOfCts zonked_simples  Simon Peyton Jones committed Aug 16, 2011 306 307 308 309 310  mr_bites = apply_mr && not (isEmptyBag pbound) (qtvs, (bound, free)) | mr_bites = (mr_qtvs, (emptyBag, zonked_simples)) | otherwise = (poly_qtvs, (pbound, pfree))  dimitris committed Nov 16, 2011 311  ; emitWantedCts free  simonpj@microsoft.com committed Jan 12, 2011 312   Simon Peyton Jones committed Aug 16, 2011 313  ; if isEmptyVarSet qtvs && isEmptyBag bound  simonpj@microsoft.com committed Jan 12, 2011 314 315 316  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 317  ; return ([], [], mr_bites, EvBinds tc_binds0) }  simonpj@microsoft.com committed Jan 12, 2011 318 319 320  else do -- Step 4, zonk quantified variables  dimitris committed Nov 16, 2011 321  { let minimal_flat_preds = mkMinimalBySCs$  dimitris committed Nov 29, 2011 322  map ctPred $bagToList bound  Simon Peyton Jones committed Aug 16, 2011 323 324  skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty) | (name, ty) <- name_taus ]  simonpj@microsoft.com committed Jan 12, 2011 325 326 327 328  -- 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 329  ; qtvs_to_return <- zonkQuantifiedTyVars qtvs  simonpj@microsoft.com committed Jan 12, 2011 330 331 332 333 334 335 336  -- 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 337  ; gloc <- getCtLoc skol_info  simonpj@microsoft.com committed Jan 12, 2011 338 339 340 341 342 343 344 345 346 347 348 349  ; 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 350  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return  simonpj@microsoft.com committed Jan 12, 2011 351 352 353 354 355  , ptext (sLit "spb =") <+> ppr zonked_simples , ptext (sLit "bound =") <+> ppr bound ]  Simon Peyton Jones committed Aug 16, 2011 356 357  ; return ( qtvs_to_return, minimal_bound_ev_vars , mr_bites, TcEvBinds ev_binds_var) } }  simonpj@microsoft.com committed Jan 12, 2011 358 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 359 360   simonpj@microsoft.com committed Jan 12, 2011 361 362 Note [Minimize by Superclasses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Sep 13, 2010 363   simonpj@microsoft.com committed Jan 12, 2011 364 365 366 367 368 369 370 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 371   simonpj@microsoft.com committed Jan 12, 2011 372 \begin{code}  dimitris committed Nov 16, 2011 373   simonpj@microsoft.com committed Jan 12, 2011 374 simplifyWithApprox :: WantedConstraints -> TcS WantedConstraints  dimitris committed Nov 17, 2011 375 -- Post: returns only wanteds (no deriveds)  simonpj@microsoft.com committed Jan 12, 2011 376 377 simplifyWithApprox wanted = do { traceTcS "simplifyApproxLoop" (ppr wanted)  simonpj committed Feb 10, 2004 378   dimitris committed Nov 16, 2011 379 380 381 382 383 384 385 386 387  ; let all_flats = wc_flat wanted unionBags keepWanted (wc_insol wanted) ; solveInteractCts $bagToList all_flats ; unsolved_implics <- simpl_loop 1 (wc_impl wanted) ; let (residual_implics,floats) = approximateImplications unsolved_implics -- Solve extra stuff for real: notice that all the extra unsolved constraints will -- be in the inerts of the monad, so we are OK ; traceTcS "simplifyApproxLoop"$ text "Calling solve_wanteds!"  dimitris committed Nov 17, 2011 388 389 390 391 392  ; wants_or_ders <- solve_wanteds (WC { wc_flat = floats -- They are floated so they are not in the evvar cache , wc_impl = residual_implics , wc_insol = emptyBag }) ; return $wants_or_ders { wc_flat = keepWanted (wc_flat wants_or_ders) } }  simonpj@microsoft.com committed Jan 12, 2011 393   dimitris committed Nov 16, 2011 394 395  approximateImplications :: Bag Implication -> (Bag Implication, Cts)  simonpj@microsoft.com committed Jan 12, 2011 396 397 398 -- 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 399  where  simonpj@microsoft.com committed Jan 12, 2011 400 401 402 403 404  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)  dimitris committed Nov 16, 2011 405  float_implic :: TyVarSet -> Implication -> (Bag Implication, Cts)  simonpj@microsoft.com committed Jan 12, 2011 406 407 408 409 410 411 412 413 414 415 416  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  dimitris committed Nov 16, 2011 417 418 419 420  float_flat :: TcTyVarSet -> Ct -> (Cts, Cts) float_flat skols ct | tyVarsOfCt ct disjointVarSet skols = (emptyBag, unitBag ct) | otherwise = (unitBag ct, emptyBag)  simonpj@microsoft.com committed Sep 13, 2010 421 \end{code}  simonpj committed Feb 10, 2004 422   simonpj@microsoft.com committed Sep 13, 2010 423 \begin{code}  simonpj@microsoft.com committed Oct 08, 2010 424 425 -- (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 426 427 -- 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 428   simonpj@microsoft.com committed Jan 12, 2011 429 430 431 growWanteds :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet growWanteds gbl_tvs wc = fixVarSet (growWC gbl_tvs wc)  dimitris committed Nov 16, 2011 432 growWantedEVs :: TyVarSet -> Cts -> TyVarSet -> TyVarSet  simonpj@microsoft.com committed Jan 12, 2011 433 434 growWantedEVs gbl_tvs ws tvs | isEmptyBag ws = tvs  dimitris committed Nov 29, 2011 435  | otherwise = fixVarSet (growPreds gbl_tvs ctPred ws) tvs  simonpj@microsoft.com committed Oct 19, 2010 436   simonpj@microsoft.com committed Jan 12, 2011 437 438 439 -------- Helper functions, do not do fixpoint ------------------------ growWC :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet growWC gbl_tvs wc = growImplics gbl_tvs (wc_impl wc) .  dimitris committed Nov 29, 2011 440 441  growPreds gbl_tvs ctPred (wc_flat wc) . growPreds gbl_tvs ctPred (wc_insol wc)  simonpj@microsoft.com committed Sep 13, 2010 442   simonpj@microsoft.com committed Jan 12, 2011 443 444 445 446 447 448 449 450 451 452 453 454 455 456 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 457  where  simonpj@microsoft.com committed Jan 12, 2011 458 459  extend item tvs = tvs unionVarSet (growPredTyVars (get_pred item) tvs minusVarSet gbl_tvs)  simonpj@microsoft.com committed Sep 13, 2010 460 461 462  -------------------- quantifyMe :: TyVarSet -- Quantifying over these  dimitris committed Nov 16, 2011 463  -> Ct  simonpj@microsoft.com committed Sep 13, 2010 464  -> Bool -- True <=> quantify over this wanted  dimitris committed Nov 16, 2011 465 quantifyMe qtvs ct  simonpj@microsoft.com committed Sep 13, 2010 466  | isIPPred pred = True -- Note [Inheriting implicit parameters]  batterseapower committed Sep 06, 2011 467  | otherwise = tyVarsOfType pred intersectsVarSet qtvs  simonpj@microsoft.com committed Sep 13, 2010 468  where  dimitris committed Nov 29, 2011 469  pred = ctPred ct  simonpj@microsoft.com committed Sep 13, 2010 470 \end{code}  simonpj committed Jan 25, 2001 471   simonpj@microsoft.com committed Oct 08, 2010 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 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 490 491 Note [Inheriting implicit parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj committed May 03, 2001 492 493 494 Consider this: f x = (x::Int) + ?y  simonpj committed Jan 25, 2001 495   simonpj committed May 03, 2001 496 497 498 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 499   simonpj committed May 03, 2001 500 501 502  f :: Int -> Int (so we get ?y from the context of f's definition), or  simonpj committed Jan 25, 2001 503 504 505  f :: (?y::Int) => Int -> Int  simonpj committed May 03, 2001 506 507 508 509 510 511 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 512 513 BOTTOM LINE: when *inferring types* you *must* quantify over implicit parameters. See the predicate isFreeWhenInferring.  simonpj committed Jun 25, 2001 514   simonpj committed Oct 25, 2001 515   simonpj@microsoft.com committed Sep 13, 2010 516 517 518 519 520 ********************************************************************************* * * * RULES * * * ***********************************************************************************  simonpj committed May 12, 2004 521   simonpj@microsoft.com committed Sep 13, 2010 522 523 Note [Simplifying RULE lhs constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Dec 13, 2010 524 On the LHS of transformation rules we only simplify only equalities,  simonpj@microsoft.com committed Sep 13, 2010 525 526 527 528 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 529   simonpj@microsoft.com committed Sep 13, 2010 530 531 532 Implementation: the TcSFlags carried by the TcSMonad controls the amount of simplification, so simplifyRuleLhs just sets the flag appropriately.  simonpj committed May 18, 1999 533   simonpj@microsoft.com committed May 19, 2006 534 535 536 537 538 539 540 541 542 543 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 544   simonpj@microsoft.com committed May 19, 2006 545 Here is another example:  simonpj committed Feb 28, 2001 546 547  fromIntegral :: (Integral a, Num b) => a -> b {-# RULES "foo" fromIntegral = id :: Int -> Int #-}  simonpj@microsoft.com committed May 19, 2006 548 549 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 550  forall dIntegralInt.  simonpj@microsoft.com committed May 19, 2006 551  fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int  simonpj committed Mar 09, 2005 552 because the scsel will mess up RULE matching. Instead we want  simonpj committed Feb 28, 2001 553  forall dIntegralInt, dNumInt.  simonpj@microsoft.com committed May 19, 2006 554  fromIntegral Int Int dIntegralInt dNumInt = id Int  simonpj committed Feb 28, 2001 555   simonpj@microsoft.com committed May 19, 2006 556 557 558 559 560 561 562 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 563   simonpj@microsoft.com committed Sep 13, 2010 564 565 In short, simplifyRuleLhs must *only* squash equalities, leaving all dicts unchanged, with absolutely no sharing.  lewie committed Mar 02, 2000 566   simonpj@microsoft.com committed Sep 13, 2010 567 568 569 570 571 572 573 574 575 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 576 577  \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 578 579 580 581 582 583 584 585 586 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 587 588 589 590 591 592 593  = 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)  dimitris committed Nov 16, 2011 594  <- solveWanteds (SimplRuleLhs name) untch zonked_lhs  simonpj@microsoft.com committed Jan 12, 2011 595 596 597 598 599 600 601  ; 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 602 603  -- Don't quantify over equalities (judgement call here)  dimitris committed Nov 29, 2011 604  ; let (eqs, dicts) = partitionBag (isEqPred . ctPred)  simonpj@microsoft.com committed Jan 12, 2011 605  (wc_flat lhs_results)  dimitris committed Nov 16, 2011 606  lhs_dicts = map cc_id (bagToList dicts)  simonpj@microsoft.com committed Jan 12, 2011 607 608 609 610 611 612 613 614 615 616 617 618  -- Dicts and implicit parameters -- Fail if we have not got down to unsolved flats ; ev_binds_var <- newTcEvBinds ; emitImplication $Implic { ic_untch = untch , ic_env = emptyNameEnv , ic_skols = mkVarSet tv_bndrs , ic_given = lhs_dicts , ic_wanted = lhs_results { wc_flat = eqs } , ic_insol = insolubleWC lhs_results , ic_binds = ev_binds_var , ic_loc = loc }  simonpj@microsoft.com committed Sep 13, 2010 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634  -- Notice that we simplify the RHS with only the explicitly -- introduced skolems, allowing the RHS to constrain any -- unification variables. -- Then, and only then, we call zonkQuantifiedTypeVariables -- Example foo :: Ord a => a -> a -- foo_spec :: Int -> Int -- {-# RULE "foo" foo = foo_spec #-} -- Here, it's the RHS that fixes the type variable -- So we don't want to make untouchable the type -- variables in the envt of the RHS, because they include -- the template variables of the RULE -- Hence the rather painful ad-hoc treatement here ; rhs_binds_var@(EvBindsVar evb_ref _) <- newTcEvBinds  simonpj committed Apr 19, 2011 635 636  ; let doc = ptext (sLit "rhs of rule") <+> doubleQuotes (ftext name) ; rhs_binds1 <- simplifyCheck (SimplCheck doc)$  simonpj@microsoft.com committed Jan 12, 2011 637 638 639 640 641 642 643 644 645 646 647  WC { wc_flat = emptyBag , wc_insol = emptyBag , wc_impl = unitBag $Implic { ic_untch = NoUntouchables , ic_env = emptyNameEnv , ic_skols = mkVarSet tv_bndrs , ic_given = lhs_dicts , ic_wanted = rhs_wanted , ic_insol = insolubleWC rhs_wanted , ic_binds = rhs_binds_var , ic_loc = loc } }  simonpj@microsoft.com committed Sep 13, 2010 648 649 650 651 652  ; rhs_binds2 <- readTcRef evb_ref ; return ( lhs_dicts , EvBinds lhs_binds , EvBinds (rhs_binds1 unionBags evBindMapBinds rhs_binds2)) }  simonpj committed Sep 13, 2002 653 654 655 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 656 657 658 659 660 ********************************************************************************* * * * Main Simplifier * * * ***********************************************************************************  lewie committed Mar 02, 2000 661 662  \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 simplifyCheck :: SimplContext -> WantedConstraints -- Wanted -> TcM (Bag EvBind) -- Solve a single, top-level implication constraint -- e.g. typically one created from a top-level type signature -- f :: forall a. [a] -> [a] -- f x = rhs -- We do this even if the function has no polymorphism: -- g :: Int -> Int -- g y = rhs -- (whereas for *nested* bindings we would not create -- an implication constraint for g at all.) -- -- Fails if can't solve something in the input wanteds simplifyCheck ctxt wanteds  simonpj@microsoft.com committed Jan 12, 2011 679  = do { wanteds <- zonkWC wanteds  simonpj@microsoft.com committed Sep 13, 2010 680 681 682 683  ; traceTc "simplifyCheck {" (vcat [ ptext (sLit "wanted =") <+> ppr wanteds ])  dimitris committed Nov 16, 2011 684 685  ; (unsolved, ev_binds) <- solveWanteds ctxt NoUntouchables wanteds  simonpj@microsoft.com committed Sep 13, 2010 686 687  ; traceTc "simplifyCheck }"$  simonpj@microsoft.com committed Jan 12, 2011 688  ptext (sLit "unsolved =") <+> ppr unsolved  simonpj@microsoft.com committed Sep 13, 2010 689   simonpj@microsoft.com committed Jan 12, 2011 690  ; reportUnsolved unsolved  simonpj@microsoft.com committed Sep 13, 2010 691 692 693 694  ; return ev_binds } ----------------  dimitris committed Nov 16, 2011 695 696 solveWanteds :: SimplContext -> Untouchables  simonpj@microsoft.com committed Jan 12, 2011 697  -> WantedConstraints  dimitris committed Nov 16, 2011 698 699 700 701 702 703 704 705 706 707 708 709 710  -> TcM (WantedConstraints, Bag EvBind) -- Returns: residual constraints, plus evidence bindings -- NB: When we are called from TcM there are no inerts to pass down to TcS solveWanteds ctxt untch wanted = do { (wc_out, ev_binds) <- runTcS ctxt untch emptyInert emptyWorkList $solve_wanteds wanted ; let wc_ret = wc_out { wc_flat = keepWanted (wc_flat wc_out) } -- Discard Derived ; return (wc_ret, ev_binds) } solve_wanteds :: WantedConstraints -> TcS WantedConstraints -- NB: wc_flats may be wanted *or* derived now solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols })  simonpj@microsoft.com committed Jan 12, 2011 711 712 713  = do { traceTcS "solveWanteds {" (ppr wanted) -- Try the flat bit  simonpj@microsoft.com committed Jan 13, 2011 714 715 716 717 718  -- Discard from insols all the derived/given constraints -- because they will show up again when we try to solve -- everything else. Solving them a second time is a bit -- of a waste, but the code is simple, and the program is -- wrong anyway!  dimitris committed Nov 16, 2011 719   simonpj@microsoft.com committed Jan 12, 2011 720  ; let all_flats = flats unionBags keepWanted insols  dimitris committed Nov 16, 2011 721  ; solveInteractCts$ bagToList all_flats  simonpj@microsoft.com committed Jan 12, 2011 722   dimitris committed Nov 16, 2011 723 724 725  -- solve_wanteds iterates when it is able to float equalities -- out of one or more of the implications. ; unsolved_implics <- simpl_loop 1 implics  simonpj@microsoft.com committed Jan 12, 2011 726   dimitris committed Nov 16, 2011 727 728 729  ; (insoluble_flats,unsolved_flats) <- extractUnsolvedTcS ; bb <- getTcEvBindsMap  simonpj@microsoft.com committed Nov 12, 2010 730  ; tb <- getTcSTyBindsMap  dimitris committed Nov 16, 2011 731   simonpj@microsoft.com committed Sep 13, 2010 732  ; traceTcS "solveWanteds }" $ simonpj@microsoft.com committed Nov 12, 2010 733  vcat [ text "unsolved_flats =" <+> ppr unsolved_flats  simonpj@microsoft.com committed Jan 12, 2011 734  , text "unsolved_implics =" <+> ppr unsolved_implics  dimitris committed Nov 16, 2011 735  , text "current evbinds =" <+> ppr (evBindMapBinds bb)  simonpj@microsoft.com committed Nov 12, 2010 736 737 738  , text "current tybinds =" <+> vcat (map ppr (varEnvElts tb)) ]  dimitris committed Nov 16, 2011 739  ; (subst, remaining_unsolved_flats) <- solveCTyFunEqs unsolved_flats  simonpj@microsoft.com committed Nov 12, 2010 740  -- See Note [Solving Family Equations]  simonpj@microsoft.com committed Jan 12, 2011 741 742  -- NB: remaining_flats has already had subst applied  dimitris committed Nov 16, 2011 743 744 745 746 747 748 749 750 751 752 753 754 755 756  ; return$ WC { wc_flat = mapBag (substCt subst) remaining_unsolved_flats , wc_impl = mapBag (substImplication subst) unsolved_implics , wc_insol = mapBag (substCt subst) insoluble_flats } } simpl_loop :: Int -> Bag Implication -> TcS (Bag Implication) simpl_loop n implics | n > 10 = traceTcS "solveWanteds: loop!" empty >> return implics | otherwise = do { (implic_eqs, unsolved_implics) <- solveNestedImplications implics  simonpj@microsoft.com committed Jan 12, 2011 757   dimitris committed Nov 16, 2011 758 759  ; inerts <- getTcSInerts ; let ((_,unsolved_flats),_) = extractUnsolved inerts  simonpj@microsoft.com committed Jan 12, 2011 760   dimitris committed Nov 16, 2011 761 762 763  ; ecache_pre <- getTcSEvVarCacheMap ; let pr = ppr ((\k z m -> foldTM k m z) (:) [] ecache_pre) ; traceTcS "ecache_pre" $pr  dimitris committed May 17, 2011 764   dimitris committed Nov 16, 2011 765 766 767  ; improve_eqs <- if not (isEmptyBag implic_eqs) then return implic_eqs else applyDefaultingRules unsolved_flats  simonpj@microsoft.com committed Jan 12, 2011 768   dimitris committed Nov 16, 2011 769 770 771  ; ecache_post <- getTcSEvVarCacheMap ; let po = ppr ((\k z m -> foldTM k m z) (:) [] ecache_post) ; traceTcS "ecache_po"$ po  dimitris committed May 17, 2011 772   dimitris committed Nov 16, 2011 773 774 775 776  ; traceTcS "solveWanteds: simpl_loop end" $vcat [ text "improve_eqs =" <+> ppr improve_eqs , text "unsolved_flats =" <+> ppr unsolved_flats , text "unsolved_implics =" <+> ppr unsolved_implics ]  777   dimitris committed Nov 16, 2011 778 779 780  ; if isEmptyBag improve_eqs then return unsolved_implics else do { solveInteractCts$ bagToList improve_eqs ; simpl_loop (n+1) unsolved_implics } }  781   dimitris committed Nov 16, 2011 782 783 784 785 786 787 788 789 790 791 solveNestedImplications :: Bag Implication -> TcS (Cts, Bag Implication) -- Precondition: the TcS inerts may contain unsolved flats which have -- to be converted to givens before we go inside a nested implication. solveNestedImplications implics | isEmptyBag implics = return (emptyBag, emptyBag) | otherwise = do { inerts <- getTcSInerts ; let ((_insoluble_flats, unsolved_flats),thinner_inerts) = extractUnsolved inerts  792   dimitris committed Nov 16, 2011 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808  ; (implic_eqs, unsolved_implics) <- doWithInert thinner_inerts $do { let pushed_givens = givens_from_wanteds unsolved_flats tcs_untouchables = filterVarSet isFlexiTcsTv$ tyVarsOfCts unsolved_flats -- See Note [Preparing inert set for implications] -- Push the unsolved wanteds inwards, but as givens ; traceTcS "solveWanteds: preparing inerts for implications {" $vcat [ppr tcs_untouchables, ppr pushed_givens] ; solveInteractCts pushed_givens ; traceTcS "solveWanteds: } now doing nested implications {" empty ; flatMapBagPairM (solveImplication tcs_untouchables) implics } -- ... and we are back in the original TcS inerts -- Notice that the original includes the _insoluble_flats so it was safe to ignore -- them in the beginning of this function.  809 810 811 812 813 814  ; traceTcS "solveWanteds: done nested implications }"$ vcat [ text "implic_eqs =" <+> ppr implic_eqs , text "unsolved_implics =" <+> ppr unsolved_implics ] ; return (implic_eqs, unsolved_implics) }  dimitris committed Nov 16, 2011 815 816 817 818 819 820 821 822 823 824  where givens_from_wanteds = foldrBag get_wanted [] get_wanted cc rest_givens | pushable_wanted cc = let this_given = cc { cc_flavor = mkGivenFlavor (cc_flavor cc) UnkSkol } in this_given : rest_givens | otherwise = rest_givens pushable_wanted :: Ct -> Bool pushable_wanted cc | isWantedCt cc  dimitris committed Nov 29, 2011 825  = isEqPred (ctPred cc) -- see Note [Preparing inert set for implications]  dimitris committed Nov 16, 2011 826 827 828 829 830 831 832 833 834 835  | otherwise = False solveImplication :: TcTyVarSet -- Untouchable TcS unification variables -> Implication -- Wanted -> TcS (Cts, -- All wanted or derived floated equalities: var = type Bag Implication) -- Unsolved rest (always empty or singleton) -- Precondition: The TcS monad contains an empty worklist and given-only inerts -- which after trying to solve this implication we must restore to their original value solveImplication tcs_untouchables imp@(Implic { ic_untch = untch  simonpj@microsoft.com committed Oct 06, 2010 836 837 838  , ic_binds = ev_binds , ic_skols = skols , ic_given = givens  simonpj@microsoft.com committed Sep 13, 2010 839  , ic_wanted = wanteds  simonpj@microsoft.com committed Oct 06, 2010 840  , ic_loc = loc })  841  = nestImplicTcS ev_binds (untch, tcs_untouchables) $ simonpj@microsoft.com committed Oct 19, 2010 842 843  recoverTcS (return (emptyBag, emptyBag))$ -- Recover from nested failures. Even the top level is  dimitris committed Nov 16, 2011 844  -- just a bunch of implications, so failing at the first one is bad  simonpj@microsoft.com committed Sep 13, 2010 845 846 847  do { traceTcS "solveImplication {" (ppr imp) -- Solve flat givens  dimitris committed Nov 16, 2011 848  ; solveInteractGiven loc givens  simonpj@microsoft.com committed Sep 13, 2010 849 850  -- Simplify the wanteds  dimitris committed Nov 16, 2011 851 852 853  ; WC { wc_flat = unsolved_flats , wc_impl = unsolved_implics , wc_insol = insols } <- solve_wanteds wanteds  simonpj@microsoft.com committed Jan 12, 2011 854 855 856 857  ; 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 858   dimitris committed Nov 16, 2011 859 860  ; let res_wanted = WC { wc_flat = final_flat , wc_impl = unsolved_implics  simonpj@microsoft.com committed Jan 12, 2011 861  , wc_insol = insols }  dimitris committed Nov 16, 2011 862   simonpj@microsoft.com committed Jan 12, 2011 863 864 865  res_implic = unitImplication $imp { ic_wanted = res_wanted , ic_insol = insolubleWC res_wanted }  simonpj@microsoft.com committed Sep 13, 2010 866   dimitris committed Nov 16, 2011 867 868  ; evbinds <- getTcEvBindsMap  simonpj@microsoft.com committed Sep 13, 2010 869 870  ; traceTcS "solveImplication end }"$ vcat [ text "res_flat_free =" <+> ppr res_flat_free  dimitris committed Nov 16, 2011 871  , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds)  simonpj@microsoft.com committed Jan 12, 2011 872  , text "res_implic =" <+> ppr res_implic ]  simonpj@microsoft.com committed Sep 13, 2010 873   simonpj@microsoft.com committed Jan 12, 2011 874  ; return (res_flat_free, res_implic) }  dimitris committed Nov 16, 2011 875  -- and we are back to the original inerts  simonpj@microsoft.com committed Sep 13, 2010 876 877   dimitris committed Nov 16, 2011 878 floatEqualities :: TcTyVarSet -> [EvVar] -> Cts -> (Cts, Cts)  simonpj@microsoft.com committed Jan 12, 2011 879 880 881 882 -- 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 883  -- Note [Float Equalities out of Implications]  simonpj@microsoft.com committed Jan 12, 2011 884 885  | otherwise = partitionBag is_floatable wantders  dimitris committed Nov 16, 2011 886 887  where is_floatable :: Ct -> Bool is_floatable ct  dimitris committed Nov 29, 2011 888  | ct_predty <- ctPred ct  dimitris committed Nov 16, 2011 889 890 891  , isEqPred ct_predty = skols disjointVarSet tvs_under_fsks ct_predty is_floatable _ct = False  simonpj@microsoft.com committed Nov 12, 2010 892 893 894 895 896 897 898 899 900 901 902  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  dimitris committed Nov 16, 2011 903  -- can mention type variables!  simonpj@microsoft.com committed Nov 12, 2010 904 905 906 907 908  | 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 909 \end{code}  partain committed Jan 08, 1996 910   simonpj@microsoft.com committed Sep 13, 2010 911 912 913 914 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:  915 916  a) In checking mode, suppresses unnecessary errors. We already have  simonpj@microsoft.com committed Sep 13, 2010 917  on unsolved-wanted error; adding it to the givens prevents any  simonpj@microsoft.com committed Feb 11, 2011 918  consequential errors from showing up  919   simonpj@microsoft.com committed Sep 13, 2010 920 921 922 923  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 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946  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 947 948 949 950 951 952 953 954 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  955 956 'solveInteract' step first.  dimitris committed May 17, 2011 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 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.  993 994 Note [Extra TcsTv untouchables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Feb 11, 2011 995 996 997 998 999 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):  1000 1001 1002  (F Int ~ uf) /\ (forall a. C a => F Int ~ beta)  simonpj@microsoft.com committed Feb 11, 2011 1003 1004 1005 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  1006  uf ~ beta  simonpj@microsoft.com committed Feb 11, 2011 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 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.  1023   simonpj@microsoft.com committed Feb 14, 2011 1024 1025 1026 1027 Note [Float Equalities out of Implications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We want to float equalities out of vanilla existentials, but *not* out of GADT pattern matches.  1028   simonpj@microsoft.com committed Sep 13, 2010 1029   simonpj@microsoft.com committed Nov 12, 2010 1030 1031 \begin{code}  dimitris committed Nov 16, 2011 1032 solveCTyFunEqs :: Cts -> TcS (TvSubst, Cts)  simonpj@microsoft.com committed Nov 12, 2010 1033 1034 -- Default equalities (F xi ~ alpha) by setting (alpha := F xi), whenever possible -- See Note [Solving Family Equations]  dimitris committed Nov 16, 2011 1035 -- Returns: a bunch of unsolved constraints from the original Cts and implications  simonpj@microsoft.com committed Nov 12, 2010 1036 -- where the newly generated equalities (alpha := F xi) have been substituted through.  simonpj@microsoft.com committed Jan 12, 2011 1037 solveCTyFunEqs cts  simonpj@microsoft.com committed Nov 12, 2010 1038  = do { untch <- getUntouchables  simonpj@microsoft.com committed Jan 12, 2011 1039 1040  ; let (unsolved_can_cts, (ni_subst, cv_binds)) = getSolvableCTyFunEqs untch cts  simonpj@microsoft.com committed Nov 12, 2010 1041  ; traceTcS "defaultCTyFunEqs" (vcat [text "Trying to default family equations:"  simonpj@microsoft.com committed Jan 12, 2011 1042  , ppr ni_subst, ppr cv_binds  simonpj@microsoft.com committed Nov 12, 2010 1043  ])  simonpj@microsoft.com committed Jan 12, 2011 1044 1045 1046 1047  ; mapM_ solve_one cv_binds ; return (niFixTvSubst ni_subst, unsolved_can_cts) } where  1048  solve_one (cv,tv,ty) = do { setWantedTyBind tv ty  dimitris committed Nov 28, 2011 1049 1050 1051 1052 1053  ; _ <- setEqBind cv (mkReflCo ty) $(Wanted$ panic "Met an already solved function equality!") ; return () -- Don't care about flavors etc this is -- the last thing happening }  simonpj@microsoft.com committed Nov 12, 2010 1054   simonpj@microsoft.com committed Jan 12, 2011 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 ------------ 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) ------------  1067 getSolvableCTyFunEqs :: TcsUntouchables  dimitris committed Nov 16, 2011 1068 1069  -> Cts -- Precondition: all Wanteds or Derived! -> (Cts, FunEqBinds) -- Postcondition: returns the unsolvables  simonpj@microsoft.com committed Nov 12, 2010 1070 getSolvableCTyFunEqs untch cts  dimitris committed Nov 16, 2011 1071  = Bag.foldlBag dflt_funeq (emptyCts, emptyFunEqBinds) cts `