TcSimplify.lhs 58 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 committed Sep 13, 2002 17 import TcRnMonad  simonpj@microsoft.com committed Sep 13, 2010 18 import TcErrors  simonpj@microsoft.com committed Jan 02, 2007 19 import TcMType  simonpj@microsoft.com committed Sep 13, 2010 20 21 import TcType import TcSMonad  dimitris committed Nov 16, 2011 22 import TcInteract  simonpj@microsoft.com committed Sep 13, 2010 23 import Inst  24 import Unify ( niFixTvSubst, niSubstTvSet )  Simon Peyton Jones committed Apr 16, 2012 25 import Type ( classifyPredType, PredTree(..) )  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  Simon Peyton Jones committed Dec 05, 2011 29 import TcEvidence  simonpj@microsoft.com committed Nov 12, 2010 30 import TypeRep  simonpj@microsoft.com committed Sep 13, 2010 31 import Name  simonmar committed Dec 10, 2003 32 import Bag  Simon Marlow committed Oct 11, 2006 33 34 import ListSetOps import Util  simonpj@microsoft.com committed Sep 13, 2010 35 36 37 import PrelInfo import PrelNames import Class ( classKey )  Simon Peyton Jones committed Aug 16, 2011 38 import BasicTypes ( RuleName )  simonpj@microsoft.com committed Jan 12, 2011 39 import Control.Monad ( when )  simonpj@microsoft.com committed Sep 13, 2010 40 import Outputable  Ian Lynagh committed Mar 29, 2008 41 import FastString  dimitris committed Mar 28, 2012 42 import TrieMap () -- DV: for now  Simon Peyton Jones committed Jan 12, 2012 43 import DynFlags  dimitris committed Nov 16, 2011 44   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  Simon Peyton Jones committed Mar 02, 2012 100  = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize  simonpj@microsoft.com committed Feb 17, 2011 101 102 103 104  -- 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   Simon Peyton Jones committed Mar 02, 2012 106  ; let 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  ; wanted <- newFlatWanteds orig (substTheta skol_subst theta)  Simon Peyton Jones committed Mar 02, 2012 112  ; traceTc "simplifyDeriv" (pprTvBndrs tvs $$ppr theta$$ ppr wanted)  Simon Peyton Jones committed Jan 12, 2012 113 114 115  ; (residual_wanted, _ev_binds1) <- runTcS (SimplInfer doc) NoUntouchables emptyInert emptyWorkList $solveWanteds$ mkFlatWC wanted  simonpj@microsoft.com committed Sep 13, 2010 116   simonpj@microsoft.com committed Jan 12, 2011 117 118  ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted) -- See Note [Exotic derived instance contexts]  dimitris committed Nov 16, 2011 119 120 121  get_good :: Ct -> Either PredType Ct get_good ct | validDerivPred skol_set p = Left p | otherwise = Right ct  dimitris committed Nov 29, 2011 122  where p = ctPred ct  simonpj@microsoft.com committed Sep 13, 2010 123   Simon Peyton Jones committed Jan 12, 2012 124 125 126  -- We never want to defer these errors because they are errors in the -- compiler! Hence the False below ; _ev_binds2 <- reportUnsolved False (residual_wanted { wc_flat = bad })  simonpj@microsoft.com committed Sep 13, 2010 127   simonpj@microsoft.com committed Jan 12, 2011 128 129  ; let min_theta = mkMinimalBySCs (bagToList good) ; return (substTheta subst_skol min_theta) }  simonpj@microsoft.com committed Sep 13, 2010 130 \end{code}  simonpj committed May 03, 2001 131   simonpj@microsoft.com committed Feb 17, 2011 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 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 157 158 159 160 161 162 163 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 164   simonpj@microsoft.com committed Sep 13, 2010 165 166 167 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 168   simonpj@microsoft.com committed Sep 13, 2010 169 170 Notice that this instance (just) satisfies the Paterson termination conditions. Then we *could* derive an instance decl like this:  simonpj committed Feb 10, 2004 171   simonpj@microsoft.com committed Sep 13, 2010 172 173 174 175  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 176   simonpj@microsoft.com committed Sep 13, 2010 177 178 179 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 180   simonpj@microsoft.com committed Sep 13, 2010 181 182 So for now we simply require that the derived instance context should have only type-variable constraints.  simonpj committed Feb 10, 2004 183   simonpj@microsoft.com committed Sep 13, 2010 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 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 213   dreixel committed Nov 11, 2011 214 215 216 217 218 219 220 221 222 223 224 225 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 226 \begin{code}  Simon Peyton Jones committed Aug 16, 2011 227 simplifyInfer :: Bool  simonpj@microsoft.com committed Jan 12, 2011 228 229 230  -> Bool -- Apply monomorphism restriction -> [(Name, TcTauType)] -- Variables to be generalised, -- and their tau-types  simonpj@microsoft.com committed Sep 13, 2010 231 232 233  -> WantedConstraints -> TcM ([TcTyVar], -- Quantify over these type variables [EvVar], -- ... and these constraints  Simon Peyton Jones committed Aug 16, 2011 234 235 236  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 237  TcEvBinds) -- ... binding these evidence variables  Simon Peyton Jones committed Aug 16, 2011 238 simplifyInfer _top_lvl apply_mr name_taus wanteds  simonpj@microsoft.com committed Jan 12, 2011 239 240 241  | isEmptyWC wanteds = do { gbl_tvs <- tcGetGlobalTyVars -- Already zonked ; zonked_taus <- zonkTcTypes (map snd name_taus)  Simon Peyton Jones committed Feb 16, 2012 242  ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus minusVarSet gbl_tvs)  dreixel committed Nov 11, 2011 243 244 245  -- 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 246  ; return (qtvs, [], False, emptyTcEvBinds) }  simonpj committed Feb 10, 2004 247   simonpj@microsoft.com committed Sep 13, 2010 248  | otherwise  simonpj@microsoft.com committed Jan 12, 2011 249 250  = do { zonked_wanteds <- zonkWC wanteds ; gbl_tvs <- tcGetGlobalTyVars  Simon Peyton Jones committed Mar 02, 2012 251  ; zonked_tau_tvs <- zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))  Simon Peyton Jones committed Jan 12, 2012 252  ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors  simonpj@microsoft.com committed Jan 12, 2011 253   simonpj@microsoft.com committed Sep 13, 2010 254  ; traceTc "simplifyInfer {" $vcat  Simon Peyton Jones committed Aug 16, 2011 255  [ ptext (sLit "names =") <+> ppr (map fst name_taus)  Simon Peyton Jones committed Mar 02, 2012 256 257  , ptext (sLit "taus =") <+> ppr (map snd name_taus) , ptext (sLit "tau_tvs (zonked) =") <+> ppr zonked_tau_tvs  Simon Peyton Jones committed Aug 16, 2011 258 259 260  , ptext (sLit "gbl_tvs =") <+> ppr gbl_tvs , ptext (sLit "closed =") <+> ppr _top_lvl , ptext (sLit "apply_mr =") <+> ppr apply_mr  simonpj@microsoft.com committed Jan 12, 2011 261  , ptext (sLit "wanted =") <+> ppr zonked_wanteds  simonpj@microsoft.com committed Sep 13, 2010 262 263  ]  simonpj@microsoft.com committed Jan 12, 2011 264 265  -- Step 1 -- Make a guess at the quantified type variables  simonpj@microsoft.com committed Oct 08, 2010 266 267 268  -- Then split the constraints on the baisis of those tyvars -- to avoid unnecessarily simplifying a class constraint -- See Note [Avoid unecessary constraint simplification]  Simon Peyton Jones committed Mar 02, 2012 269  ; let proto_qtvs = growWanteds gbl_tvs zonked_wanteds$  simonpj@microsoft.com committed Oct 08, 2010 270  zonked_tau_tvs minusVarSet gbl_tvs  simonpj@microsoft.com committed Jan 12, 2011 271 272 273 274 275 276 277 278 279  (perhaps_bound, surely_free) = partitionBag (quantifyMe proto_qtvs) (wc_flat zonked_wanteds) ; traceTc "simplifyInfer proto" $vcat [ ptext (sLit "zonked_tau_tvs =") <+> ppr zonked_tau_tvs , ptext (sLit "proto_qtvs =") <+> ppr proto_qtvs , ptext (sLit "surely_fref =") <+> ppr surely_free ]  Simon Peyton Jones committed Jan 12, 2012 280  ; emitFlats surely_free  281 282 283 284  ; traceTc "sinf"$ vcat [ ptext (sLit "perhaps_bound =") <+> ppr perhaps_bound , ptext (sLit "surely_free =") <+> ppr surely_free ]  simonpj@microsoft.com committed Oct 08, 2010 285   simonpj@microsoft.com committed Jan 12, 2011 286  -- Step 2  Simon Peyton Jones committed Jan 12, 2012 287 288 289 290 291 292 293 294 295 296 297  -- Now simplify the possibly-bound constraints ; let ctxt = SimplInfer (ppr (map fst name_taus)) ; (simpl_results, tc_binds) <- runTcS ctxt NoUntouchables emptyInert emptyWorkList $simplifyWithApprox (zonked_wanteds { wc_flat = perhaps_bound }) -- Fail fast if there is an insoluble constraint, -- unless we are deferring errors to runtime ; when (not runtimeCoercionErrors && insolubleWC simpl_results)$ do { _ev_binds <- reportUnsolved False simpl_results ; failM }  simonpj@microsoft.com committed Jan 12, 2011 298 299 300 301 302  -- Step 3 -- Split again simplified_perhaps_bound, because some unifications -- may have happened, and emit the free constraints. ; gbl_tvs <- tcGetGlobalTyVars  Simon Peyton Jones committed Mar 02, 2012 303  ; zonked_tau_tvs <- zonkTyVarsAndFV zonked_tau_tvs  Simon Peyton Jones committed Jan 12, 2012 304  ; zonked_flats <- zonkCts (wc_flat simpl_results)  simonpj@microsoft.com committed Oct 08, 2010 305  ; let init_tvs = zonked_tau_tvs minusVarSet gbl_tvs  Simon Peyton Jones committed Jan 12, 2012 306 307  poly_qtvs = growWantedEVs gbl_tvs zonked_flats init_tvs (pbound, pfree) = partitionBag (quantifyMe poly_qtvs) zonked_flats  Simon Peyton Jones committed Aug 16, 2011 308 309  -- Monomorphism restriction  simonpj@microsoft.com committed Oct 08, 2010 310  mr_qtvs = init_tvs minusVarSet constrained_tvs  Simon Peyton Jones committed Jan 12, 2012 311  constrained_tvs = tyVarsOfCts zonked_flats  Simon Peyton Jones committed Aug 16, 2011 312 313 314  mr_bites = apply_mr && not (isEmptyBag pbound) (qtvs, (bound, free))  Simon Peyton Jones committed Jan 12, 2012 315  | mr_bites = (mr_qtvs, (emptyBag, zonked_flats))  Simon Peyton Jones committed Aug 16, 2011 316  | otherwise = (poly_qtvs, (pbound, pfree))  Simon Peyton Jones committed Jan 12, 2012 317  ; emitFlats free  simonpj@microsoft.com committed Jan 12, 2011 318   Simon Peyton Jones committed Aug 16, 2011 319  ; if isEmptyVarSet qtvs && isEmptyBag bound  simonpj@microsoft.com committed Jan 12, 2011 320 321 322  then ASSERT( isEmptyBag (wc_insol simpl_results) ) do { traceTc "} simplifyInfer/no quantification" empty ; emitImplications (wc_impl simpl_results)  Simon Peyton Jones committed Jan 12, 2012 323  ; return ([], [], mr_bites, EvBinds tc_binds) }  simonpj@microsoft.com committed Jan 12, 2011 324 325 326  else do -- Step 4, zonk quantified variables  dimitris committed Nov 16, 2011 327  { let minimal_flat_preds = mkMinimalBySCs $ dimitris committed Nov 29, 2011 328  map ctPred$ bagToList bound  Simon Peyton Jones committed Aug 16, 2011 329 330  skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty) | (name, ty) <- name_taus ]  simonpj@microsoft.com committed Jan 12, 2011 331 332 333 334  -- Don't add the quantified variables here, because -- they are also bound in ic_skols and we want them to be -- tidied uniformly  Simon Peyton Jones committed Feb 16, 2012 335  ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)  simonpj@microsoft.com committed Jan 12, 2011 336 337 338 339 340  -- Step 5 -- Minimize bound' and emit an implication ; minimal_bound_ev_vars <- mapM TcMType.newEvVar minimal_flat_preds ; ev_binds_var <- newTcEvBinds  Simon Peyton Jones committed Jan 12, 2012 341 342  ; mapBagM_ (\(EvBind evar etrm) -> addTcEvBind ev_binds_var evar etrm) tc_binds  simonpj@microsoft.com committed Jan 12, 2011 343  ; lcl_env <- getLclTypeEnv  dreixel committed Nov 11, 2011 344  ; gloc <- getCtLoc skol_info  simonpj@microsoft.com committed Jan 12, 2011 345 346  ; let implic = Implic { ic_untch = NoUntouchables , ic_env = lcl_env  Simon Peyton Jones committed Jan 12, 2012 347  , ic_skols = qtvs_to_return  simonpj@microsoft.com committed Jan 12, 2011 348 349 350 351 352 353 354 355 356  , 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 357  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return  Simon Peyton Jones committed Jan 12, 2012 358  , ptext (sLit "spb =") <+> ppr zonked_flats  simonpj@microsoft.com committed Jan 12, 2011 359 360 361 362  , ptext (sLit "bound =") <+> ppr bound ]  Simon Peyton Jones committed Aug 16, 2011 363 364  ; return ( qtvs_to_return, minimal_bound_ev_vars , mr_bites, TcEvBinds ev_binds_var) } }  simonpj@microsoft.com committed Jan 12, 2011 365 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 366 367   simonpj@microsoft.com committed Jan 12, 2011 368 369 Note [Minimize by Superclasses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Sep 13, 2010 370   simonpj@microsoft.com committed Jan 12, 2011 371 372 373 374 375 376 377 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 378   simonpj@microsoft.com committed Jan 12, 2011 379 \begin{code}  dimitris committed Nov 16, 2011 380   simonpj@microsoft.com committed Jan 12, 2011 381 simplifyWithApprox :: WantedConstraints -> TcS WantedConstraints  dimitris committed Nov 17, 2011 382 -- Post: returns only wanteds (no deriveds)  simonpj@microsoft.com committed Jan 12, 2011 383 384 simplifyWithApprox wanted = do { traceTcS "simplifyApproxLoop" (ppr wanted)  simonpj committed Feb 10, 2004 385   dimitris committed Nov 16, 2011 386  ; let all_flats = wc_flat wanted unionBags keepWanted (wc_insol wanted)  dimitris committed Apr 10, 2012 387 388 389  ; implics_from_flats <- solveInteractCts$ bagToList all_flats ; unsolved_implics <- simpl_loop 1 (wc_impl wanted unionBags implics_from_flats)  dimitris committed Nov 16, 2011 390 391 392 393 394 395  ; 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" $text "Calling solve_wanteds!"  dimitris committed Nov 17, 2011 396 397 398 399 400  ; wants_or_ders <- solve_wanteds (WC { wc_flat = floats -- They are floated so they are not in the evvar cache , wc_impl = residual_implics , wc_insol = emptyBag }) ; return$ wants_or_ders { wc_flat = keepWanted (wc_flat wants_or_ders) } }  simonpj@microsoft.com committed Jan 12, 2011 401   dimitris committed Nov 16, 2011 402 403  approximateImplications :: Bag Implication -> (Bag Implication, Cts)  simonpj@microsoft.com committed Jan 12, 2011 404 405 406 -- 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 407  where  simonpj@microsoft.com committed Jan 12, 2011 408 409 410 411 412  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 413  float_implic :: TyVarSet -> Implication -> (Bag Implication, Cts)  simonpj@microsoft.com committed Jan 12, 2011 414 415 416  float_implic skols imp = (unitBag (imp { ic_wanted = wanted' }), floats) where  Simon Peyton Jones committed Jan 12, 2012 417  (wanted', floats) = float_wc (skols extendVarSetList ic_skols imp) (ic_wanted imp)  simonpj@microsoft.com committed Jan 12, 2011 418 419 420 421 422 423 424  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 425 426 427 428  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 429 \end{code}  simonpj committed Feb 10, 2004 430   simonpj@microsoft.com committed Sep 13, 2010 431 \begin{code}  simonpj@microsoft.com committed Oct 08, 2010 432 433 -- (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 434 435 -- 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 436   simonpj@microsoft.com committed Jan 12, 2011 437 438 439 growWanteds :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet growWanteds gbl_tvs wc = fixVarSet (growWC gbl_tvs wc)  dimitris committed Nov 16, 2011 440 growWantedEVs :: TyVarSet -> Cts -> TyVarSet -> TyVarSet  simonpj@microsoft.com committed Jan 12, 2011 441 442 growWantedEVs gbl_tvs ws tvs | isEmptyBag ws = tvs  dimitris committed Nov 29, 2011 443  | otherwise = fixVarSet (growPreds gbl_tvs ctPred ws) tvs  simonpj@microsoft.com committed Oct 19, 2010 444   simonpj@microsoft.com committed Jan 12, 2011 445 446 447 -------- 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 448 449  growPreds gbl_tvs ctPred (wc_flat wc) . growPreds gbl_tvs ctPred (wc_insol wc)  simonpj@microsoft.com committed Sep 13, 2010 450   simonpj@microsoft.com committed Jan 12, 2011 451 452 453 454 455 growImplics :: TyVarSet -> Bag Implication -> TyVarSet -> TyVarSet growImplics gbl_tvs implics tvs = foldrBag grow_implic tvs implics where grow_implic implic tvs  Simon Peyton Jones committed Jan 12, 2012 456  = grow tvs delVarSetList ic_skols implic  simonpj@microsoft.com committed Jan 12, 2011 457 458 459 460 461 462 463 464  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 465  where  simonpj@microsoft.com committed Jan 12, 2011 466 467  extend item tvs = tvs unionVarSet (growPredTyVars (get_pred item) tvs minusVarSet gbl_tvs)  simonpj@microsoft.com committed Sep 13, 2010 468 469 470  -------------------- quantifyMe :: TyVarSet -- Quantifying over these  dimitris committed Nov 16, 2011 471  -> Ct  simonpj@microsoft.com committed Sep 13, 2010 472  -> Bool -- True <=> quantify over this wanted  dimitris committed Nov 16, 2011 473 quantifyMe qtvs ct  simonpj@microsoft.com committed Sep 13, 2010 474  | isIPPred pred = True -- Note [Inheriting implicit parameters]  batterseapower committed Sep 06, 2011 475  | otherwise = tyVarsOfType pred intersectsVarSet qtvs  simonpj@microsoft.com committed Sep 13, 2010 476  where  dimitris committed Nov 29, 2011 477  pred = ctPred ct  simonpj@microsoft.com committed Sep 13, 2010 478 \end{code}  simonpj committed Jan 25, 2001 479   simonpj@microsoft.com committed Oct 08, 2010 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 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 498 499 Note [Inheriting implicit parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj committed May 03, 2001 500 501 502 Consider this: f x = (x::Int) + ?y  simonpj committed Jan 25, 2001 503   simonpj committed May 03, 2001 504 505 506 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 507   simonpj committed May 03, 2001 508 509 510  f :: Int -> Int (so we get ?y from the context of f's definition), or  simonpj committed Jan 25, 2001 511 512 513  f :: (?y::Int) => Int -> Int  simonpj committed May 03, 2001 514 515 516 517 518 519 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 520 521 BOTTOM LINE: when *inferring types* you *must* quantify over implicit parameters. See the predicate isFreeWhenInferring.  simonpj committed Jun 25, 2001 522   simonpj committed Oct 25, 2001 523   simonpj@microsoft.com committed Sep 13, 2010 524 525 526 527 528 ********************************************************************************* * * * RULES * * * ***********************************************************************************  simonpj committed May 12, 2004 529   Simon Peyton Jones committed Apr 16, 2012 530 See note [Simplifying RULE consraints] in TcRule  simonpj committed Jun 28, 1999 531   Simon Peyton Jones committed Apr 16, 2012 532 533 534 535 536 537 538 539 540 541 542 543 544 545 Note [RULE quanfification over equalities] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Decideing which equalities to quantify over is tricky: * We do not want to quantify over insoluble equalities (Int ~ Bool) (a) because we prefer to report a LHS type error (b) because if such things end up in 'givens' we get a bogus "inaccessible code" error * But we do want to quantify over things like (a ~ F b), where F is a type function. The difficulty is that it's hard to tell what is insoluble! So we see whether the simplificaiotn step yielded any type errors, and if so refrain from quantifying over *any* equalites.  simonpj committed Sep 13, 2002 546 547  \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 548 549 550 simplifyRule :: RuleName -> WantedConstraints -- Constraints from LHS -> WantedConstraints -- Constraints from RHS  Simon Peyton Jones committed Apr 16, 2012 551 552 553 554 555 556 557 558 559 560 561 562  -> TcM ([EvVar], WantedConstraints) -- LHS evidence varaibles -- See Note [Simplifying RULE constraints] in TcRule simplifyRule name lhs_wanted rhs_wanted = do { zonked_all <- zonkWC (lhs_wanted andWC rhs_wanted) ; let doc = ptext (sLit "LHS of rule") <+> doubleQuotes (ftext name) untch = NoUntouchables -- We allow ourselves to unify environment -- variables; hence NoUntouchables ; (resid_wanted, _) <- runTcS (SimplInfer doc) untch emptyInert emptyWorkList $solveWanteds zonked_all  simonpj@microsoft.com committed Jan 12, 2011 563 564  ; zonked_lhs <- zonkWC lhs_wanted  Simon Peyton Jones committed Apr 16, 2012 565 566 567 568 569 570 571 572 573 574 575 576 577  ; let (q_cts, non_q_cts) = partitionBag quantify_me (wc_flat zonked_lhs) quantify_me -- Note [RULE quantification over equalities] | insolubleWC resid_wanted = quantify_insol | otherwise = quantify_normal quantify_insol ct = not (isEqPred (ctPred ct)) quantify_normal ct | EqPred t1 t2 <- classifyPredType (ctPred ct) = not (t1 eqType t2) | otherwise = True  simonpj@microsoft.com committed Jan 12, 2011 578  ; traceTc "simplifyRule"$  Simon Peyton Jones committed Apr 16, 2012 579 580 581 582  vcat [ text "zonked_lhs" <+> ppr zonked_lhs , text "q_cts" <+> ppr q_cts ] ; return (map ctId (bagToList q_cts), zonked_lhs { wc_flat = non_q_cts }) }  simonpj committed Sep 13, 2002 583 584 585 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 586 587 588 589 590 ********************************************************************************* * * * Main Simplifier * * * ***********************************************************************************  lewie committed Mar 02, 2000 591 592  \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 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 609  = do { wanteds <- zonkWC wanteds  simonpj@microsoft.com committed Sep 13, 2010 610 611 612 613  ; traceTc "simplifyCheck {" (vcat [ ptext (sLit "wanted =") <+> ppr wanteds ])  Simon Peyton Jones committed Jan 12, 2012 614 615 616 617 618 619  ; (unsolved, eb1) <- runTcS ctxt NoUntouchables emptyInert emptyWorkList $solveWanteds wanteds ; traceTc "simplifyCheck }"$ ptext (sLit "unsolved =") <+> ppr unsolved  dimitris committed Mar 30, 2012 620  ; traceTc "reportUnsolved {" empty  Simon Peyton Jones committed Jan 12, 2012 621 622 623  -- See Note [Deferring coercion errors to runtime] ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors ; eb2 <- reportUnsolved runtimeCoercionErrors unsolved  dimitris committed Mar 30, 2012 624 625  ; traceTc "reportUnsolved }" empty  Simon Peyton Jones committed Jan 12, 2012 626 627 628 629 630 631 632 633 634 635  ; return (eb1 unionBags eb2) } \end{code} Note [Deferring coercion errors to runtime] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ While developing, sometimes it is desirable to allow compilation to succeed even if there are type errors in the code. Consider the following case: module Main where  simonpj@microsoft.com committed Sep 13, 2010 636   Simon Peyton Jones committed Jan 12, 2012 637 638  a :: Int a = 'a'  simonpj@microsoft.com committed Sep 13, 2010 639   Simon Peyton Jones committed Jan 12, 2012 640  main = print "b"  simonpj@microsoft.com committed Sep 13, 2010 641   Simon Peyton Jones committed Jan 12, 2012 642 643 Even though a is ill-typed, it is not used in the end, so if all that we're interested in is main it is handy to be able to ignore the problems in a.  simonpj@microsoft.com committed Sep 13, 2010 644   Simon Peyton Jones committed Jan 12, 2012 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 Since we treat type equalities as evidence, this is relatively simple. Whenever we run into a type mismatch in TcUnify, we normally just emit an error. But it is always safe to defer the mismatch to the main constraint solver. If we do that, a will get transformed into co :: Int ~ Char co = ... a :: Int a = 'a' cast co The constraint solver would realize that co is an insoluble constraint, and emit an error with reportUnsolved. But we can also replace the right-hand side of co with error "Deferred type error: Int ~ Char". This allows the program to compile, and it will run fine unless we evaluate a. This is what deferErrorsToRuntime does. It does this by keeping track of which errors correspond to which coercion in TcErrors (with ErrEnv). TcErrors.reportTidyWanteds does not print the errors and does not fail if -fwarn-type-errors is on, so that we can continue compilation. The errors are turned into warnings in reportUnsolved. \begin{code} solveWanteds :: WantedConstraints -> TcS WantedConstraints  dimitris committed Nov 16, 2011 669 670 -- Returns: residual constraints, plus evidence bindings -- NB: When we are called from TcM there are no inerts to pass down to TcS  Simon Peyton Jones committed Jan 12, 2012 671 672 solveWanteds wanted = do { wc_out <- solve_wanteds wanted  dimitris committed Nov 16, 2011 673 674  ; let wc_ret = wc_out { wc_flat = keepWanted (wc_flat wc_out) } -- Discard Derived  Simon Peyton Jones committed Jan 12, 2012 675  ; return wc_ret }  dimitris committed Nov 16, 2011 676 677 678 679  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 680 681 682  = do { traceTcS "solveWanteds {" (ppr wanted) -- Try the flat bit  simonpj@microsoft.com committed Jan 13, 2011 683 684 685 686 687  -- 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 688   simonpj@microsoft.com committed Jan 12, 2011 689  ; let all_flats = flats unionBags keepWanted insols  dimitris committed Apr 10, 2012 690  ; impls_from_flats <- solveInteractCts $bagToList all_flats  simonpj@microsoft.com committed Jan 12, 2011 691   dimitris committed Nov 16, 2011 692 693  -- solve_wanteds iterates when it is able to float equalities -- out of one or more of the implications.  dimitris committed Apr 10, 2012 694  ; unsolved_implics <- simpl_loop 1 (implics unionBags impls_from_flats)  simonpj@microsoft.com committed Jan 12, 2011 695   dimitris committed Nov 16, 2011 696 697 698  ; (insoluble_flats,unsolved_flats) <- extractUnsolvedTcS ; bb <- getTcEvBindsMap  simonpj@microsoft.com committed Nov 12, 2010 699  ; tb <- getTcSTyBindsMap  dimitris committed Nov 16, 2011 700   simonpj@microsoft.com committed Sep 13, 2010 701  ; traceTcS "solveWanteds }"$  simonpj@microsoft.com committed Nov 12, 2010 702  vcat [ text "unsolved_flats =" <+> ppr unsolved_flats  simonpj@microsoft.com committed Jan 12, 2011 703  , text "unsolved_implics =" <+> ppr unsolved_implics  dimitris committed Nov 16, 2011 704  , text "current evbinds =" <+> ppr (evBindMapBinds bb)  simonpj@microsoft.com committed Nov 12, 2010 705 706 707  , text "current tybinds =" <+> vcat (map ppr (varEnvElts tb)) ]  dimitris committed Nov 16, 2011 708  ; (subst, remaining_unsolved_flats) <- solveCTyFunEqs unsolved_flats  simonpj@microsoft.com committed Nov 12, 2010 709  -- See Note [Solving Family Equations]  simonpj@microsoft.com committed Jan 12, 2011 710 711  -- NB: remaining_flats has already had subst applied  Simon Peyton Jones committed Mar 02, 2012 712 713 714 715 716  ; traceTcS "solveWanteds finished with" $vcat [ text "remaining_unsolved_flats =" <+> ppr remaining_unsolved_flats , text "subst =" <+> ppr subst ]  dimitris committed Nov 16, 2011 717 718 719 720 721 722 723 724 725 726 727 728 729 730  ; return$ 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 731   dimitris committed Nov 16, 2011 732 733  ; inerts <- getTcSInerts ; let ((_,unsolved_flats),_) = extractUnsolved inerts  dimitris committed Mar 28, 2012 734   dimitris committed Nov 16, 2011 735 736 737  ; improve_eqs <- if not (isEmptyBag implic_eqs) then return implic_eqs else applyDefaultingRules unsolved_flats  simonpj@microsoft.com committed Jan 12, 2011 738   dimitris committed Nov 16, 2011 739 740 741 742  ; traceTcS "solveWanteds: simpl_loop end" $vcat [ text "improve_eqs =" <+> ppr improve_eqs , text "unsolved_flats =" <+> ppr unsolved_flats , text "unsolved_implics =" <+> ppr unsolved_implics ]  743   dimitris committed Nov 16, 2011 744  ; if isEmptyBag improve_eqs then return unsolved_implics  dimitris committed Apr 10, 2012 745 746 747  else do { impls_from_eqs <- solveInteractCts$ bagToList improve_eqs ; simpl_loop (n+1) (unsolved_implics unionBags impls_from_eqs)} }  748   dimitris committed Nov 16, 2011 749 750 751 752 753 754 755 756 757 758 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  759   dimitris committed Nov 16, 2011 760 761 762 763 764 765 766 767 768  ; (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 {" $vcat [ppr tcs_untouchables, ppr pushed_givens]  dimitris committed Apr 10, 2012 769 770 771  ; impls_from_givens <- solveInteractCts pushed_givens ; MASSERT (isEmptyBag impls_from_givens)  dimitris committed Nov 16, 2011 772 773 774 775 776 777  ; traceTcS "solveWanteds: } now doing nested implications {" empty ; flatMapBagPairM (solveImplication tcs_untouchables) implics } -- ... and we are back in the original TcS inerts -- Notice that the original includes the _insoluble_flats so it was safe to ignore -- them in the beginning of this function.  778 779 780 781 782 783  ; traceTcS "solveWanteds: done nested implications }"$ vcat [ text "implic_eqs =" <+> ppr implic_eqs , text "unsolved_implics =" <+> ppr unsolved_implics ] ; return (implic_eqs, unsolved_implics) }  dimitris committed Nov 16, 2011 784 785 786  where givens_from_wanteds = foldrBag get_wanted [] get_wanted cc rest_givens | pushable_wanted cc  dimitris committed Mar 28, 2012 787 788 789 790  = let fl = cc_flavor cc wloc = flav_wloc fl gfl = Given (mkGivenLoc wloc UnkSkol) (flav_evar fl) this_given = cc { cc_flavor = gfl }  dimitris committed Nov 16, 2011 791 792 793 794 795 796  in this_given : rest_givens | otherwise = rest_givens pushable_wanted :: Ct -> Bool pushable_wanted cc | isWantedCt cc  dimitris committed Nov 29, 2011 797  = isEqPred (ctPred cc) -- see Note [Preparing inert set for implications]  dimitris committed Nov 16, 2011 798 799 800 801 802 803 804 805 806 807  | 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 808 809 810  , ic_binds = ev_binds , ic_skols = skols , ic_given = givens  simonpj@microsoft.com committed Sep 13, 2010 811  , ic_wanted = wanteds  simonpj@microsoft.com committed Oct 06, 2010 812  , ic_loc = loc })  813  = nestImplicTcS ev_binds (untch, tcs_untouchables) $ simonpj@microsoft.com committed Oct 19, 2010 814 815  recoverTcS (return (emptyBag, emptyBag))$ -- Recover from nested failures. Even the top level is  dimitris committed Nov 16, 2011 816  -- just a bunch of implications, so failing at the first one is bad  simonpj@microsoft.com committed Sep 13, 2010 817 818 819  do { traceTcS "solveImplication {" (ppr imp) -- Solve flat givens  dimitris committed Apr 10, 2012 820 821 822  ; impls_from_givens <- solveInteractGiven loc givens ; MASSERT (isEmptyBag impls_from_givens)  simonpj@microsoft.com committed Sep 13, 2010 823  -- Simplify the wanteds  dimitris committed Nov 16, 2011 824 825 826  ; WC { wc_flat = unsolved_flats , wc_impl = unsolved_implics , wc_insol = insols } <- solve_wanteds wanteds  simonpj@microsoft.com committed Jan 12, 2011 827 828 829 830  ; 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 831   dimitris committed Nov 16, 2011 832 833  ; let res_wanted = WC { wc_flat = final_flat , wc_impl = unsolved_implics  simonpj@microsoft.com committed Jan 12, 2011 834  , wc_insol = insols }  dimitris committed Nov 16, 2011 835   simonpj@microsoft.com committed Jan 12, 2011 836 837 838  res_implic = unitImplication $imp { ic_wanted = res_wanted , ic_insol = insolubleWC res_wanted }  simonpj@microsoft.com committed Sep 13, 2010 839   dimitris committed Nov 16, 2011 840 841  ; evbinds <- getTcEvBindsMap  simonpj@microsoft.com committed Sep 13, 2010 842 843  ; traceTcS "solveImplication end }"$ vcat [ text "res_flat_free =" <+> ppr res_flat_free  dimitris committed Nov 16, 2011 844  , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds)  simonpj@microsoft.com committed Jan 12, 2011 845  , text "res_implic =" <+> ppr res_implic ]  simonpj@microsoft.com committed Sep 13, 2010 846   simonpj@microsoft.com committed Jan 12, 2011 847  ; return (res_flat_free, res_implic) }  dimitris committed Nov 16, 2011 848  -- and we are back to the original inerts  simonpj@microsoft.com committed Sep 13, 2010 849 850   Simon Peyton Jones committed Jan 12, 2012 851 floatEqualities :: [TcTyVar] -> [EvVar] -> Cts -> (Cts, Cts)  simonpj@microsoft.com committed Jan 12, 2011 852 853 854 855 -- 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 856  -- Note [Float Equalities out of Implications]  simonpj@microsoft.com committed Jan 12, 2011 857  | otherwise = partitionBag is_floatable wantders  dimitris committed Mar 30, 2012 858 859 860 861  -- TODO: Maybe we should try out /not/ floating constraints that contain touchables only, -- since they are inert and not going to interact with anything more in a more global scope.  Simon Peyton Jones committed Jan 12, 2012 862 863  where skol_set = mkVarSet skols is_floatable :: Ct -> Bool  dimitris committed Nov 16, 2011 864  is_floatable ct  dimitris committed Nov 29, 2011 865  | ct_predty <- ctPred ct  dimitris committed Nov 16, 2011 866  , isEqPred ct_predty  Simon Peyton Jones committed Jan 12, 2012 867  = skol_set disjointVarSet tvs_under_fsks ct_predty  dimitris committed Nov 16, 2011 868  is_floatable _ct = False  simonpj@microsoft.com committed Nov 12, 2010 869 870 871 872 873 874 875 876  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)  Simon Peyton Jones committed Jan 13, 2012 877  tvs_under_fsks (LitTy {}) = emptyVarSet  simonpj@microsoft.com committed Nov 12, 2010 878 879 880  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 881  -- can mention type variables!  simonpj@microsoft.com committed Nov 12, 2010 882 883 884 885 886  | 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 887 \end{code}  partain committed Jan 08, 1996 888   simonpj@microsoft.com committed Sep 13, 2010 889 890 891 892 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:  893 894  a) In checking mode, suppresses unnecessary errors. We already have  simonpj@microsoft.com committed Sep 13, 2010 895  on unsolved-wanted error; adding it to the givens prevents any  simonpj@microsoft.com committed Feb 11, 2011 896  consequential errors from showing up  897   simonpj@microsoft.com committed Sep 13, 2010 898 899 900 901  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 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924  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 925 926 927 928 929 930 931 932 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  933 934 'solveInteract' step first.  dimitris committed May 17, 2011 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 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.  971 972 Note [Extra TcsTv untouchables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Feb 11, 2011 973 974 975 976 977 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):  978 979 980  (F Int ~ uf) /\ (forall a. C a => F Int ~ beta)  simonpj@microsoft.com committed Feb 11, 2011 981 982 983 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  984  uf ~ beta  simonpj@microsoft.com committed Feb 11, 2011 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 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.  1001   simonpj@microsoft.com committed Feb 14, 2011 1002 1003 1004 1005 Note [Float Equalities out of Implications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We want to float equalities out of vanilla existentials, but *not* out of GADT pattern matches.  1006   dimitris committed Mar 30, 2012 1007 1008 ---> TODO Expand in accordance to our discussion  simonpj@microsoft.com committed Sep 13, 2010 1009   simonpj@microsoft.com committed Nov 12, 2010 1010 1011 \begin{code}  dimitris committed Nov 16, 2011 1012 solveCTyFunEqs :: Cts -> TcS (TvSubst, Cts)  simonpj@microsoft.com committed Nov 12, 2010 1013 1014 -- Default equalities (F xi ~ alpha) by setting (alpha := F xi), whenever possible -- See Note [Solving Family Equations]  dimitris committed Nov 16, 2011 1015 -- Returns: a bunch of unsolved constraints from the original Cts and implications  simonpj@microsoft.com committed Nov 12, 2010 1016 -- where the newly generated equalities (alpha := F xi) have been substituted through.  simonpj@microsoft.com committed Jan 12, 2011 1017 solveCTyFunEqs cts  simonpj@microsoft.com committed Nov 12, 2010 1018  = do { untch <- getUntouchables  simonpj@microsoft.com committed Jan 12, 2011 1019 1020  ; let (unsolved_can_cts, (ni_subst, cv_binds)) = getSolvableCTyFunEqs untch cts  simonpj@microsoft.com committed Nov 12, 2010 1021  ; traceTcS "defaultCTyFunEqs" (vcat [text "Trying to default family equations:"  simonpj@microsoft.com committed Jan 12, 2011 1022  , ppr ni_subst, ppr cv_binds  simonpj@microsoft.com committed Nov 12, 2010 1023  ])  simonpj@microsoft.com committed Jan 12, 2011 1024 1025 1026 1027  ; mapM_ solve_one cv_binds ; return (niFixTvSubst ni_subst, unsolved_can_cts) } where  dimitris committed Mar 28, 2012 1028 1029 1030 1031 1032 1033  solve_one (Wanted _ cv,tv,ty) = setWantedTyBind tv ty >> setEvBind cv (EvCoercion (mkTcReflCo ty)) solve_one (Derived {}, tv, ty) = setWantedTyBind tv ty solve_one arg = pprPanic "solveCTyFunEqs: can't solve a /given/ family equation!" \$ ppr arg `