TcSimplify.lhs 69.5 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 Jun 13, 2012 25 import Type ( classifyPredType, PredTree(..), isIPPred_maybe )  Simon Marlow committed Oct 11, 2006 26 import Var  dimitris committed Jun 08, 2012 27 import Unique  simonm committed Dec 02, 1998 28 import VarSet  simonpj@microsoft.com committed Nov 12, 2010 29 import VarEnv  Simon Peyton Jones committed Dec 05, 2011 30 import TcEvidence  simonpj@microsoft.com committed Nov 12, 2010 31 import TypeRep  simonpj@microsoft.com committed Sep 13, 2010 32 import Name  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 Mar 28, 2012 43 import TrieMap () -- DV: for now  Simon Peyton Jones committed Jan 12, 2012 44 import DynFlags  Simon Peyton Jones committed Jun 13, 2012 45 import Data.Maybe ( mapMaybe )  partain committed Jan 08, 1996 46 47 48 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 49 50 51 52 53 ********************************************************************************* * * * External interface * * * *********************************************************************************  simonpj committed Jan 25, 2001 54   dimitris committed Jun 08, 2012 55   simonpj@microsoft.com committed Sep 13, 2010 56 \begin{code}  dimitris committed Jun 08, 2012 57 58   simonpj@microsoft.com committed Sep 13, 2010 59 60 simplifyTop :: WantedConstraints -> TcM (Bag EvBind) -- Simplify top-level constraints  simonpj@microsoft.com committed Dec 13, 2010 61 62 63 -- 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 64 simplifyTop wanteds  dimitris committed Jun 08, 2012 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145  = do { ev_binds_var <- newTcEvBinds ; zonked_wanteds <- zonkWC wanteds ; wc_first_go <- runTcSWithEvBinds ev_binds_var $solveWanteds zonked_wanteds ; cts <- applyTyVarDefaulting wc_first_go -- See Note [Top-level Defaulting Plan] ; let wc_for_loop = wc_first_go { wc_flat = wc_flat wc_first_go unionBags cts } ; traceTc "simpl_top_loop {"$ text "zonked_wc =" <+> ppr zonked_wanteds ; simpl_top_loop ev_binds_var wc_for_loop } where simpl_top_loop ev_binds_var wc | isEmptyWC wc = do { traceTc "simpl_top_loop }" empty ; TcRnMonad.getTcEvBinds ev_binds_var } | otherwise = do { wc_residual <- runTcSWithEvBinds ev_binds_var $solveWanteds wc ; let wc_flat_approximate = approximateWC wc_residual ; (dflt_eqs,_unused_bind) <- runTcS$ applyDefaultingRules wc_flat_approximate -- See Note [Top-level Defaulting Plan] ; if isEmptyBag dflt_eqs then do { traceTc "simpl_top_loop }" empty ; report_and_finish ev_binds_var wc_residual } else simpl_top_loop ev_binds_var $wc_residual { wc_flat = wc_flat wc_residual unionBags dflt_eqs } } report_and_finish ev_binds_var wc_residual = do { eb1 <- TcRnMonad.getTcEvBinds ev_binds_var ; traceTc "reportUnsolved {" empty -- See Note [Deferring coercion errors to runtime] ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors ; eb2 <- reportUnsolved runtimeCoercionErrors wc_residual ; traceTc "reportUnsolved }" empty ; return (eb1 unionBags eb2) } \end{code} Note [Top-level Defaulting Plan] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We have considered two design choices for where/when to apply defaulting. (i) Do it in SimplCheck mode only /whenever/ you try to solve some flat constraints, maybe deep inside the context of implications. This used to be the case in GHC 7.4.1. (ii) Do it in a tight loop at simplifyTop, once all other constraint has finished. This is the current story. Option (i) had many disadvantages: a) First it was deep inside the actual solver, b) Second it was dependent on the context (Infer a type signature, or Check a type signature, or Interactive) since we did not want to always start defaulting when inferring (though there is an exception to this see Note [Default while Inferring]) c) It plainly did not work. Consider typecheck/should_compile/DfltProb2.hs: f :: Int -> Bool f x = const True (\y -> let w :: a -> a w a = const a (y+1) in w y) We will get an implication constraint (for beta the type of y): [untch=beta] forall a. 0 => Num beta which we really cannot default /while solving/ the implication, since beta is untouchable. Instead our new defaulting story is to pull defaulting out of the solver loop and go with option (i), implemented at SimplifyTop. Namely: - First have a go at solving the residual constraint of the whole program - Try to approximate it with a flat constraint - Figure out derived defaulting equations for that flat constraint - Go round the loop again if you did manage to get some equations Now, that has to do with class defaulting. However there exists type variable /kind/ defaulting. Again this is done at the top-level and the plan is: - At the top-level, once you had a go at solving the constraint, do figure out /all/ the touchable unification variables of the wanted contraints. - Apply defaulting to their kinds More details in Note [DefaultTyVar]. \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 146   Simon Peyton Jones committed Aug 16, 2011 147 148 149 ------------------ simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind) simplifyAmbiguityCheck name wanteds  dimitris committed Jun 08, 2012 150 151  = traceTc "simplifyAmbiguityCheck" (text "name =" <+> ppr name) >> simplifyCheck wanteds  dimitris committed Nov 16, 2011 152   simonpj@microsoft.com committed Sep 13, 2010 153 154 155 ------------------ simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind) simplifyInteractive wanteds  dimitris committed Jun 08, 2012 156 157  = traceTc "simplifyInteractive" empty >> simplifyTop wanteds  simonpj@microsoft.com committed Sep 13, 2010 158 159 160 161 162  ------------------ simplifyDefault :: ThetaType -- Wanted; has no type variables in it -> TcM () -- Succeeds iff the constraint is soluble simplifyDefault theta  dimitris committed Jun 08, 2012 163 164 165  = do { traceTc "simplifyInteractive" empty ; wanted <- newFlatWanteds DefaultOrigin theta ; _ignored_ev_binds <- simplifyCheck (mkFlatWC wanted)  simonpj@microsoft.com committed Sep 13, 2010 166 167  ; return () } \end{code}  simonpj committed Jan 25, 2001 168   simonpj committed May 03, 2001 169   dimitris committed Nov 16, 2011 170 ***********************************************************************************  simonpj@microsoft.com committed Sep 13, 2010 171 * *  dimitris committed Nov 16, 2011 172 * Deriving *  simonpj@microsoft.com committed Sep 13, 2010 173 174 * * ***********************************************************************************  simonpj committed May 03, 2001 175   simonpj@microsoft.com committed Sep 13, 2010 176 177 \begin{code} simplifyDeriv :: CtOrigin  simonpj committed Apr 19, 2011 178 179 180 181  -> PredType -> [TyVar] -> ThetaType -- Wanted -> TcM ThetaType -- Needed  simonpj@microsoft.com committed Sep 13, 2010 182 183 -- Given instance (wanted) => C inst_ty -- Simplify 'wanted' as much as possibles  simonpj@microsoft.com committed Jan 12, 2011 184 -- Fail if not possible  simonpj committed Apr 19, 2011 185 simplifyDeriv orig pred tvs theta  Simon Peyton Jones committed Mar 02, 2012 186  = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize  simonpj@microsoft.com committed Feb 17, 2011 187 188 189 190  -- 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 191   Simon Peyton Jones committed Mar 02, 2012 192  ; let subst_skol = zipTopTvSubst tvs_skols$ map mkTyVarTy tvs  Simon Peyton Jones committed Jul 23, 2011 193  skol_set = mkVarSet tvs_skols  dimitris committed Jun 08, 2012 194  doc = ptext (sLit "deriving") <+> parens (ppr pred)  simonpj@microsoft.com committed Jan 12, 2011 195 196 197  ; wanted <- newFlatWanteds orig (substTheta skol_subst theta)  dimitris committed Jun 08, 2012 198 199  ; traceTc "simplifyDeriv" $vcat [ pprTvBndrs tvs $$ppr theta$$ ppr wanted, doc ]  Simon Peyton Jones committed Jan 12, 2012 200  ; (residual_wanted, _ev_binds1)  dimitris committed Jun 08, 2012 201  <- runTcS$ solveWanteds (mkFlatWC wanted)  simonpj@microsoft.com committed Sep 13, 2010 202   simonpj@microsoft.com committed Jan 12, 2011 203 204  ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted) -- See Note [Exotic derived instance contexts]  dimitris committed Nov 16, 2011 205 206 207  get_good :: Ct -> Either PredType Ct get_good ct | validDerivPred skol_set p = Left p | otherwise = Right ct  dimitris committed Nov 29, 2011 208  where p = ctPred ct  simonpj@microsoft.com committed Sep 13, 2010 209   Simon Peyton Jones committed Jan 12, 2012 210 211 212  -- 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 213   simonpj@microsoft.com committed Jan 12, 2011 214 215  ; let min_theta = mkMinimalBySCs (bagToList good) ; return (substTheta subst_skol min_theta) }  simonpj@microsoft.com committed Sep 13, 2010 216 \end{code}  simonpj committed May 03, 2001 217   simonpj@microsoft.com committed Feb 17, 2011 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 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 243 244 245 246 247 248 249 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 250   simonpj@microsoft.com committed Sep 13, 2010 251 252 253 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 254   simonpj@microsoft.com committed Sep 13, 2010 255 256 Notice that this instance (just) satisfies the Paterson termination conditions. Then we *could* derive an instance decl like this:  simonpj committed Feb 10, 2004 257   simonpj@microsoft.com committed Sep 13, 2010 258 259 260 261  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 262   simonpj@microsoft.com committed Sep 13, 2010 263 264 265 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 266   simonpj@microsoft.com committed Sep 13, 2010 267 268 So for now we simply require that the derived instance context should have only type-variable constraints.  simonpj committed Feb 10, 2004 269   simonpj@microsoft.com committed Sep 13, 2010 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 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 299   dreixel committed Nov 11, 2011 300 301 302 303 304 305 306 307 308 309 310 311 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 312 \begin{code}  Simon Peyton Jones committed Aug 16, 2011 313 simplifyInfer :: Bool  simonpj@microsoft.com committed Jan 12, 2011 314 315 316  -> Bool -- Apply monomorphism restriction -> [(Name, TcTauType)] -- Variables to be generalised, -- and their tau-types  dimitris committed Jun 08, 2012 317  -> (Untouchables, WantedConstraints)  simonpj@microsoft.com committed Sep 13, 2010 318 319  -> TcM ([TcTyVar], -- Quantify over these type variables [EvVar], -- ... and these constraints  Simon Peyton Jones committed Aug 16, 2011 320 321 322  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 323  TcEvBinds) -- ... binding these evidence variables  dimitris committed Jun 08, 2012 324 simplifyInfer _top_lvl apply_mr name_taus (untch,wanteds)  simonpj@microsoft.com committed Jan 12, 2011 325 326 327  | isEmptyWC wanteds = do { gbl_tvs <- tcGetGlobalTyVars -- Already zonked ; zonked_taus <- zonkTcTypes (map snd name_taus)  Simon Peyton Jones committed Feb 16, 2012 328  ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus minusVarSet gbl_tvs)  dreixel committed Nov 11, 2011 329 330 331  -- 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 332  ; return (qtvs, [], False, emptyTcEvBinds) }  simonpj committed Feb 10, 2004 333   simonpj@microsoft.com committed Sep 13, 2010 334  | otherwise  simonpj@microsoft.com committed Jan 12, 2011 335 336  = do { zonked_wanteds <- zonkWC wanteds ; gbl_tvs <- tcGetGlobalTyVars  Simon Peyton Jones committed Mar 02, 2012 337  ; zonked_tau_tvs <- zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))  Simon Peyton Jones committed Jan 12, 2012 338  ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors  simonpj@microsoft.com committed Jan 12, 2011 339   simonpj@microsoft.com committed Sep 13, 2010 340  ; traceTc "simplifyInfer {" $vcat  Simon Peyton Jones committed Aug 16, 2011 341  [ ptext (sLit "names =") <+> ppr (map fst name_taus)  Simon Peyton Jones committed Mar 02, 2012 342 343  , ptext (sLit "taus =") <+> ppr (map snd name_taus) , ptext (sLit "tau_tvs (zonked) =") <+> ppr zonked_tau_tvs  Simon Peyton Jones committed Aug 16, 2011 344 345 346  , ptext (sLit "gbl_tvs =") <+> ppr gbl_tvs , ptext (sLit "closed =") <+> ppr _top_lvl , ptext (sLit "apply_mr =") <+> ppr apply_mr  dimitris committed Jun 08, 2012 347  , ptext (sLit "untch =") <+> ppr untch  simonpj@microsoft.com committed Jan 12, 2011 348  , ptext (sLit "wanted =") <+> ppr zonked_wanteds  simonpj@microsoft.com committed Sep 13, 2010 349 350  ]  simonpj@microsoft.com committed Jan 12, 2011 351 352  -- Step 1 -- Make a guess at the quantified type variables  simonpj@microsoft.com committed Oct 08, 2010 353 354 355  -- Then split the constraints on the baisis of those tyvars -- to avoid unnecessarily simplifying a class constraint -- See Note [Avoid unecessary constraint simplification]  Simon Peyton Jones committed Mar 02, 2012 356  ; let proto_qtvs = growWanteds gbl_tvs zonked_wanteds$  simonpj@microsoft.com committed Oct 08, 2010 357  zonked_tau_tvs minusVarSet gbl_tvs  simonpj@microsoft.com committed Jan 12, 2011 358  (perhaps_bound, surely_free)  dimitris committed Jun 08, 2012 359  = partitionBag (quantifyMe proto_qtvs ctPred) (wc_flat zonked_wanteds)  simonpj@microsoft.com committed Jan 12, 2011 360 361 362 363 364 365 366  ; traceTc "simplifyInfer proto" $vcat [ ptext (sLit "zonked_tau_tvs =") <+> ppr zonked_tau_tvs , ptext (sLit "proto_qtvs =") <+> ppr proto_qtvs , ptext (sLit "surely_fref =") <+> ppr surely_free ]  367 368 369 370  ; traceTc "sinf"$ vcat [ ptext (sLit "perhaps_bound =") <+> ppr perhaps_bound , ptext (sLit "surely_free =") <+> ppr surely_free ]  dimitris committed Jun 08, 2012 371 372 373  ; emitFlats surely_free  simonpj@microsoft.com committed Jan 12, 2011 374  -- Step 2  Simon Peyton Jones committed Jan 12, 2012 375  -- Now simplify the possibly-bound constraints  dimitris committed Jun 08, 2012 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405  ; let wanteds_to_approximate = zonked_wanteds { wc_flat = perhaps_bound } ; traceTc "simplifyWithApprox" $ptext (sLit "wanteds_to_approximate = ") <+> ppr wanteds_to_approximate -- 2.1) First try full-blown solving ; ev_binds_var <- newTcEvBinds ; wanted_transformed <- runTcSWithEvBinds ev_binds_var$ solveWanteds wanteds_to_approximate -- 2.2) Fail fast if there is an insoluble constraint, -- unless we are deferring errors to runtime ; when (not runtimeCoercionErrors && insolubleWC wanted_transformed) $do { _ev_binds <- reportUnsolved False wanted_transformed; failM } -- 2.3) Candidates for quantification are an approximation of wanted_transformed ; let quant_candidates = approximateWC wanted_transformed -- NB: Already the fixpoint of any unifications that may have happened -- NB: We do not do any defaulting when inferring a type, this can lead -- to less polymorphic types, see Note [Default while Inferring] -- 2.4) Minimize the quantification candidates ; (quant_candidates_transformed, _extra_binds) <- runTcS$ solveWanteds $WC { wc_flat = quant_candidates , wc_impl = emptyBag , wc_insol = emptyBag } -- 2.5) Final candidates for quantification ; let final_quant_candidates :: Bag PredType final_quant_candidates = mapBag ctPred$ keepWanted (wc_flat quant_candidates_transformed) -- NB: Already the fixpoint of any unifications that may have happened ; gbl_tvs <- tcGetGlobalTyVars -- TODO: can we just use untch instead of gbl_tvs?  Simon Peyton Jones committed Mar 02, 2012 406  ; zonked_tau_tvs <- zonkTyVarsAndFV zonked_tau_tvs  dimitris committed Jun 08, 2012 407 408 409 410 411 412  ; traceTc "simplifyWithApprox" $vcat [ ptext (sLit "final_quant_candidates =") <+> ppr final_quant_candidates , ptext (sLit "gbl_tvs=") <+> ppr gbl_tvs , ptext (sLit "zonked_tau_tvs=") <+> ppr zonked_tau_tvs ]  simonpj@microsoft.com committed Oct 08, 2010 413  ; let init_tvs = zonked_tau_tvs minusVarSet gbl_tvs  dimitris committed Jun 08, 2012 414 415 416 417 418 419 420  poly_qtvs = growPreds gbl_tvs id final_quant_candidates init_tvs pbound = filterBag (quantifyMe poly_qtvs id) final_quant_candidates ; traceTc "simplifyWithApprox"$ vcat [ ptext (sLit "pbound =") <+> ppr pbound ]  Simon Peyton Jones committed Aug 16, 2011 421  -- Monomorphism restriction  dimitris committed Jun 08, 2012 422 423  ; let mr_qtvs = init_tvs minusVarSet constrained_tvs constrained_tvs = tyVarsOfBag tyVarsOfType final_quant_candidates  Simon Peyton Jones committed Aug 16, 2011 424 425  mr_bites = apply_mr && not (isEmptyBag pbound)  dimitris committed Jun 08, 2012 426 427 428 429  (qtvs, bound) | mr_bites = (mr_qtvs, emptyBag) | otherwise = (poly_qtvs, pbound)  simonpj@microsoft.com committed Jan 12, 2011 430   Simon Peyton Jones committed Aug 16, 2011 431  ; if isEmptyVarSet qtvs && isEmptyBag bound  dimitris committed Jun 08, 2012 432 433 434 435  then ASSERT( isEmptyBag (wc_insol wanted_transformed) ) do { traceTc "} simplifyInfer/no quantification" empty ; emitWC wanted_transformed ; return ([], [], mr_bites, TcEvBinds ev_binds_var) }  simonpj@microsoft.com committed Jan 12, 2011 436 437  else do  dimitris committed Jun 08, 2012 438 439 440  { traceTc "simplifyApprox" $ptext (sLit "bound are =") <+> ppr bound  simonpj@microsoft.com committed Jan 12, 2011 441  -- Step 4, zonk quantified variables  dimitris committed Jun 08, 2012 442  ; let minimal_flat_preds = mkMinimalBySCs$ bagToList bound  Simon Peyton Jones committed Aug 16, 2011 443 444  skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty) | (name, ty) <- name_taus ]  simonpj@microsoft.com committed Jan 12, 2011 445 446 447 448  -- 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 449  ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)  simonpj@microsoft.com committed Jan 12, 2011 450 451 452 453  -- Step 5 -- Minimize bound' and emit an implication ; minimal_bound_ev_vars <- mapM TcMType.newEvVar minimal_flat_preds  dimitris committed Jun 08, 2012 454   simonpj@microsoft.com committed Jan 12, 2011 455  ; lcl_env <- getLclTypeEnv  dreixel committed Nov 11, 2011 456  ; gloc <- getCtLoc skol_info  dimitris committed Jun 08, 2012 457 458  ; let implic = Implic { ic_untch = untch  simonpj@microsoft.com committed Jan 12, 2011 459  , ic_env = lcl_env  Simon Peyton Jones committed Jan 12, 2012 460  , ic_skols = qtvs_to_return  simonpj@microsoft.com committed Jan 12, 2011 461  , ic_given = minimal_bound_ev_vars  dimitris committed Jun 08, 2012 462  , ic_wanted = wanted_transformed  simonpj@microsoft.com committed Jan 12, 2011 463 464 465  , ic_insol = False , ic_binds = ev_binds_var , ic_loc = gloc }  dimitris committed Jun 08, 2012 466   simonpj@microsoft.com committed Jan 12, 2011 467  ; emitImplication implic  dimitris committed Jun 08, 2012 468   simonpj@microsoft.com committed Jan 12, 2011 469 470 471  ; 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 472  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return  dimitris committed Jun 08, 2012 473  , ptext (sLit "spb =") <+> ppr final_quant_candidates  simonpj@microsoft.com committed Jan 12, 2011 474 475  , ptext (sLit "bound =") <+> ppr bound ]  Simon Peyton Jones committed Aug 16, 2011 476 477  ; return ( qtvs_to_return, minimal_bound_ev_vars , mr_bites, TcEvBinds ev_binds_var) } }  dimitris committed Jun 08, 2012 478  where  simonpj@microsoft.com committed Jan 12, 2011 479 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 480 481   dimitris committed Jun 08, 2012 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 Note [Note [Default while Inferring] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Our current plan is that defaulting only happens at simplifyTop and not simplifyInfer. This may lead to some insoluble deferred constraints Example: instance D g => C g Int b constraint inferred = (forall b. 0 => C gamma alpha b) /\ Num alpha type inferred = gamma -> gamma Now, if we try to default (alpha := Int) we will be able to refine the implication to (forall b. 0 => C gamma Int b) which can then be simplified further to (forall b. 0 => D gamma) Finally we /can/ approximate this implication with (D gamma) and infer the quantified type: forall g. D g => g -> g Instead what will currently happen is that we will get a quantified type (forall g. g -> g) and an implication: forall g. 0 => (forall b. 0 => C g alpha b) /\ Num alpha which, even if the simplifyTop defaults (alpha := Int) we will still be left with an unsolvable implication: forall g. 0 => (forall b. 0 => D g) The concrete example would be: h :: C g a s => g -> a -> ST s a f (x::gamma) = (\_ -> x) (runST (h x (undefined::alpha)) + 1) But it is quite tedious to do defaulting and resolve the implication constraints and we have not observed code breaking because of the lack of defaulting in inference so we don't do it for now.  simonpj@microsoft.com committed Jan 12, 2011 518 519 Note [Minimize by Superclasses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Sep 13, 2010 520   simonpj@microsoft.com committed Jan 12, 2011 521 522 523 524 525 526 527 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 528   dimitris committed Nov 16, 2011 529   dimitris committed Jun 08, 2012 530 \begin{code}  simonpj@microsoft.com committed Jan 12, 2011 531   dimitris committed Nov 16, 2011 532   dimitris committed Jun 08, 2012 533 534 approximateWC :: WantedConstraints -> Cts approximateWC wc = float_wc emptyVarSet wc  simonpj@microsoft.com committed Sep 13, 2010 535  where  dimitris committed Jun 08, 2012 536 537 538 539 540 541  float_wc :: TcTyVarSet -> WantedConstraints -> Cts float_wc skols (WC { wc_flat = flat, wc_impl = implic }) = floats1 unionBags floats2 where floats1 = do_bag (float_flat skols) flat floats2 = do_bag (float_implic skols) implic float_implic :: TcTyVarSet -> Implication -> Cts  simonpj@microsoft.com committed Jan 12, 2011 542  float_implic skols imp  dimitris committed Jun 08, 2012 543 544 545 546 547 548 549 550 551 552  = float_wc (skols extendVarSetList ic_skols imp) (ic_wanted imp) float_flat :: TcTyVarSet -> Ct -> Cts float_flat skols ct | tyVarsOfCt ct disjointVarSet skols , isWantedCt ct = singleCt ct | otherwise = emptyCts do_bag :: (a -> Bag c) -> Bag a -> Bag c do_bag f = foldrBag (unionBags.f) emptyBag  simonpj@microsoft.com committed Jan 12, 2011 553 554   simonpj@microsoft.com committed Sep 13, 2010 555 \end{code}  simonpj committed Feb 10, 2004 556   simonpj@microsoft.com committed Sep 13, 2010 557 \begin{code}  simonpj@microsoft.com committed Oct 08, 2010 558 559 -- (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 560 561 -- 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 562   dimitris committed Jun 08, 2012 563   simonpj@microsoft.com committed Jan 12, 2011 564 565 566 growWanteds :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet growWanteds gbl_tvs wc = fixVarSet (growWC gbl_tvs wc)  simonpj@microsoft.com committed Oct 19, 2010 567   simonpj@microsoft.com committed Jan 12, 2011 568 569 570 -------- 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 571 572  growPreds gbl_tvs ctPred (wc_flat wc) . growPreds gbl_tvs ctPred (wc_insol wc)  simonpj@microsoft.com committed Sep 13, 2010 573   simonpj@microsoft.com committed Jan 12, 2011 574 575 576 577 578 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 579  = grow tvs delVarSetList ic_skols implic  simonpj@microsoft.com committed Jan 12, 2011 580 581 582 583 584  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  dimitris committed Jun 08, 2012 585   simonpj@microsoft.com committed Jan 12, 2011 586 587 588 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 589  where  simonpj@microsoft.com committed Jan 12, 2011 590 591  extend item tvs = tvs unionVarSet (growPredTyVars (get_pred item) tvs minusVarSet gbl_tvs)  simonpj@microsoft.com committed Sep 13, 2010 592 593 594  -------------------- quantifyMe :: TyVarSet -- Quantifying over these  dimitris committed Jun 08, 2012 595 596 597  -> (a -> PredType) -> a -> Bool -- True <=> quantify over this wanted quantifyMe qtvs toPred ct  simonpj@microsoft.com committed Sep 13, 2010 598  | isIPPred pred = True -- Note [Inheriting implicit parameters]  batterseapower committed Sep 06, 2011 599  | otherwise = tyVarsOfType pred intersectsVarSet qtvs  simonpj@microsoft.com committed Sep 13, 2010 600  where  dimitris committed Jun 08, 2012 601  pred = toPred ct  simonpj@microsoft.com committed Sep 13, 2010 602 \end{code}  simonpj committed Jan 25, 2001 603   simonpj@microsoft.com committed Oct 08, 2010 604 605 606 607 608 609 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  dimitris committed Jun 08, 2012 610   simonpj@microsoft.com committed Oct 08, 2010 611  instance C t => C [t] where ..  dimitris committed Jun 08, 2012 612   simonpj@microsoft.com committed Oct 08, 2010 613 614 615 616 617 618 619 620 621  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.  dimitris committed Jun 08, 2012 622 623 624 The example that needs this is: typecheck/should_compile/T4361.hs  simonpj@microsoft.com committed Oct 08, 2010 625 626   simonpj@microsoft.com committed Nov 24, 2006 627 628 Note [Inheriting implicit parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj committed May 03, 2001 629 630 631 Consider this: f x = (x::Int) + ?y  simonpj committed Jan 25, 2001 632   simonpj committed May 03, 2001 633 634 635 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 636   simonpj committed May 03, 2001 637 638 639  f :: Int -> Int (so we get ?y from the context of f's definition), or  simonpj committed Jan 25, 2001 640 641 642  f :: (?y::Int) => Int -> Int  simonpj committed May 03, 2001 643 644 645 646 647 648 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 649 650 BOTTOM LINE: when *inferring types* you *must* quantify over implicit parameters. See the predicate isFreeWhenInferring.  simonpj committed Jun 25, 2001 651   simonpj committed Oct 25, 2001 652   simonpj@microsoft.com committed Sep 13, 2010 653 654 655 656 657 ********************************************************************************* * * * RULES * * * ***********************************************************************************  simonpj committed May 12, 2004 658   Simon Peyton Jones committed Apr 16, 2012 659 See note [Simplifying RULE consraints] in TcRule  simonpj committed Jun 28, 1999 660   Simon Peyton Jones committed Apr 16, 2012 661 662 663 664 665 666 667 668 669 670 671 672 673 674 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 675 676  \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 677 678 679 simplifyRule :: RuleName -> WantedConstraints -- Constraints from LHS -> WantedConstraints -- Constraints from RHS  Simon Peyton Jones committed Apr 16, 2012 680 681 682 683 684  -> 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)  dimitris committed Jun 08, 2012 685   Simon Peyton Jones committed Apr 16, 2012 686  -- We allow ourselves to unify environment  dimitris committed Jun 08, 2012 687 688  -- variables: runTcS runs with NoUntouchables ; (resid_wanted, _) <- runTcS (solveWanteds zonked_all)  Simon Peyton Jones committed Apr 16, 2012 689   simonpj@microsoft.com committed Jan 12, 2011 690 691  ; zonked_lhs <- zonkWC lhs_wanted  Simon Peyton Jones committed Apr 16, 2012 692 693 694 695 696 697 698 699 700 701 702 703 704  ; 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 705  ; traceTc "simplifyRule"$  dimitris committed Jun 08, 2012 706 707  vcat [ doc , text "zonked_lhs" <+> ppr zonked_lhs  Simon Peyton Jones committed Apr 16, 2012 708 709  , text "q_cts" <+> ppr q_cts ]  Simon Peyton Jones committed May 07, 2012 710 711  ; return ( map (ctEvId . ctEvidence) (bagToList q_cts) , zonked_lhs { wc_flat = non_q_cts }) }  simonpj committed Sep 13, 2002 712 713 714 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 715 716 717 718 719 ********************************************************************************* * * * Main Simplifier * * * ***********************************************************************************  lewie committed Mar 02, 2000 720 721  \begin{code}  dimitris committed Jun 08, 2012 722 simplifyCheck :: WantedConstraints -- Wanted  simonpj@microsoft.com committed Sep 13, 2010 723 724 725 726 727 728 729 730 731 732 733 734 735  -> 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  dimitris committed Jun 08, 2012 736 simplifyCheck wanteds  simonpj@microsoft.com committed Jan 12, 2011 737  = do { wanteds <- zonkWC wanteds  simonpj@microsoft.com committed Sep 13, 2010 738 739 740 741  ; traceTc "simplifyCheck {" (vcat [ ptext (sLit "wanted =") <+> ppr wanteds ])  dimitris committed Jun 08, 2012 742  ; (unsolved, eb1) <- runTcS (solveWanteds wanteds)  Simon Peyton Jones committed Jan 12, 2012 743 744 745  ; traceTc "simplifyCheck }" $ptext (sLit "unsolved =") <+> ppr unsolved  dimitris committed Mar 30, 2012 746  ; traceTc "reportUnsolved {" empty  Simon Peyton Jones committed Jan 12, 2012 747 748 749  -- See Note [Deferring coercion errors to runtime] ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors ; eb2 <- reportUnsolved runtimeCoercionErrors unsolved  dimitris committed Mar 30, 2012 750 751  ; traceTc "reportUnsolved }" empty  Simon Peyton Jones committed Jan 12, 2012 752 753 754 755 756 757 758 759 760 761  ; 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 762   Simon Peyton Jones committed Jan 12, 2012 763 764  a :: Int a = 'a'  simonpj@microsoft.com committed Sep 13, 2010 765   Simon Peyton Jones committed Jan 12, 2012 766  main = print "b"  simonpj@microsoft.com committed Sep 13, 2010 767   Simon Peyton Jones committed Jan 12, 2012 768 769 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 770   Simon Peyton Jones committed Jan 12, 2012 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 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 795 796 -- 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 797 solveWanteds wanted  dimitris committed Jun 08, 2012 798  = do { (_,wc_out) <- solve_wanteds wanted  dimitris committed Nov 16, 2011 799 800  ; let wc_ret = wc_out { wc_flat = keepWanted (wc_flat wc_out) } -- Discard Derived  Simon Peyton Jones committed Jan 12, 2012 801  ; return wc_ret }  dimitris committed Nov 16, 2011 802 803  solve_wanteds :: WantedConstraints  dimitris committed Jun 08, 2012 804 805 806  -> TcS (TvSubst, WantedConstraints) -- NB: wc_flats may be wanted *or* derived now -- Returns the flattening substitution as well in case we need to apply it  dimitris committed Nov 16, 2011 807 solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols })  simonpj@microsoft.com committed Jan 12, 2011 808 809 810  = do { traceTcS "solveWanteds {" (ppr wanted) -- Try the flat bit  simonpj@microsoft.com committed Jan 13, 2011 811 812 813 814 815  -- 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 816   dimitris committed Jun 08, 2012 817 818 819 820 821 822 823  -- DV: why only keepWanted? We make sure that we never float out -- whatever constraints can yield equalities, including class -- constraints with functional dependencies and hence all the derived -- that were potentially insoluble will be re-generated. -- (It would not hurt though to just keep the wanted and the derived) -- See Note [The HasEqualities Predicate] in Inst.lhs  simonpj@microsoft.com committed Jan 12, 2011 824  ; let all_flats = flats unionBags keepWanted insols  dimitris committed Jun 08, 2012 825   dimitris committed Apr 10, 2012 826  ; impls_from_flats <- solveInteractCts$ bagToList all_flats  simonpj@microsoft.com committed Jan 12, 2011 827   dimitris committed Nov 16, 2011 828 829  -- solve_wanteds iterates when it is able to float equalities -- out of one or more of the implications.  dimitris committed Apr 10, 2012 830  ; unsolved_implics <- simpl_loop 1 (implics unionBags impls_from_flats)  simonpj@microsoft.com committed Jan 12, 2011 831   dimitris committed Nov 16, 2011 832 833 834  ; (insoluble_flats,unsolved_flats) <- extractUnsolvedTcS ; bb <- getTcEvBindsMap  simonpj@microsoft.com committed Nov 12, 2010 835  ; tb <- getTcSTyBindsMap  dimitris committed Nov 16, 2011 836   simonpj@microsoft.com committed Sep 13, 2010 837  ; traceTcS "solveWanteds }" $ simonpj@microsoft.com committed Nov 12, 2010 838  vcat [ text "unsolved_flats =" <+> ppr unsolved_flats  simonpj@microsoft.com committed Jan 12, 2011 839  , text "unsolved_implics =" <+> ppr unsolved_implics  dimitris committed Nov 16, 2011 840  , text "current evbinds =" <+> ppr (evBindMapBinds bb)  simonpj@microsoft.com committed Nov 12, 2010 841 842 843  , text "current tybinds =" <+> vcat (map ppr (varEnvElts tb)) ]  dimitris committed Nov 16, 2011 844  ; (subst, remaining_unsolved_flats) <- solveCTyFunEqs unsolved_flats  simonpj@microsoft.com committed Nov 12, 2010 845  -- See Note [Solving Family Equations]  simonpj@microsoft.com committed Jan 12, 2011 846 847  -- NB: remaining_flats has already had subst applied  Simon Peyton Jones committed Mar 02, 2012 848 849 850 851 852  ; traceTcS "solveWanteds finished with"$ vcat [ text "remaining_unsolved_flats =" <+> ppr remaining_unsolved_flats , text "subst =" <+> ppr subst ]  dimitris committed Nov 16, 2011 853  ; return $ dimitris committed Jun 08, 2012 854 855 856  (subst, WC { wc_flat = mapBag (substCt subst) remaining_unsolved_flats , wc_impl = mapBag (substImplication subst) unsolved_implics , wc_insol = mapBag (substCt subst) insoluble_flats })  dimitris committed Nov 16, 2011 857 858 859 860 861 862 863 864 865 866  } 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 867   dimitris committed Nov 16, 2011 868 869  ; inerts <- getTcSInerts ; let ((_,unsolved_flats),_) = extractUnsolved inerts  dimitris committed Mar 28, 2012 870   dimitris committed Jun 08, 2012 871 872 873 874 875  ; let improve_eqs = implic_eqs -- NB: improve_eqs used to contain defaulting equations HERE but -- defaulting now happens only at simplifyTop and not deep inside -- simpl_loop! See Note [Top-level Defaulting Plan]  dimitris committed Nov 16, 2011 876 877 878 879  ; traceTcS "solveWanteds: simpl_loop end"$ vcat [ text "improve_eqs =" <+> ppr improve_eqs , text "unsolved_flats =" <+> ppr unsolved_flats , text "unsolved_implics =" <+> ppr unsolved_implics ]  880   dimitris committed Nov 16, 2011 881  ; if isEmptyBag improve_eqs then return unsolved_implics  dimitris committed Apr 10, 2012 882 883 884  else do { impls_from_eqs <- solveInteractCts $bagToList improve_eqs ; simpl_loop (n+1) (unsolved_implics unionBags impls_from_eqs)} }  885   dimitris committed Nov 16, 2011 886 887 888 889 890 891 892 893 894 solveNestedImplications :: Bag Implication -> TcS (Cts, Bag Implication) -- Precondition: the TcS inerts may contain unsolved flats which have -- to be converted to givens before we go inside a nested implication. solveNestedImplications implics | isEmptyBag implics = return (emptyBag, emptyBag) | otherwise = do { inerts <- getTcSInerts  dimitris committed Jun 08, 2012 895 896  ; traceTcS "solveNestedImplications starting, inerts are:"$ ppr inerts  dimitris committed Nov 16, 2011 897  ; let ((_insoluble_flats, unsolved_flats),thinner_inerts) = extractUnsolved inerts  dimitris committed Jun 08, 2012 898 899 900 901 902 903  ; traceTcS "solveNestedImplications starting, more info:" $vcat [ text "inerts = " <+> ppr inerts , text "insoluble_flats = " <+> ppr _insoluble_flats , text "unsolved_flats = " <+> ppr unsolved_flats , text "thinner_inerts = " <+> ppr thinner_inerts ]  dimitris committed Nov 16, 2011 904 905 906  ; (implic_eqs, unsolved_implics) <- doWithInert thinner_inerts$ do { let pushed_givens = givens_from_wanteds unsolved_flats  dimitris committed Jun 08, 2012 907 908 909 910 911 912 913  tcs_untouchables = foldr (unionVarSet . tyVarsOfCt) emptyVarSet pushed_givens -- Typically pushed_givens is very small, consists -- only of unsolved equalities, so no inefficiency -- danger.  dimitris committed Nov 16, 2011 914 915 916 917  -- See Note [Preparing inert set for implications] -- Push the unsolved wanteds inwards, but as givens ; traceTcS "solveWanteds: preparing inerts for implications {" $vcat [ppr tcs_untouchables, ppr pushed_givens]  dimitris committed Jun 08, 2012 918 919  ; impls_from_givens <- solveInteractCts pushed_givens  dimitris committed Apr 10, 2012 920  ; MASSERT (isEmptyBag impls_from_givens)  dimitris committed Jun 08, 2012 921 922 923 924  -- impls_from_givens must be empty, since we are reacting givens -- with givens, and they can never generate extra implications -- from decomposition of ForAll types. (Whereas wanteds can, see -- TcCanonical, canEq ForAll-ForAll case)  dimitris committed Apr 10, 2012 925   dimitris committed Nov 16, 2011 926 927 928 929 930 931  ; traceTcS "solveWanteds: } now doing nested implications {" empty ; flatMapBagPairM (solveImplication tcs_untouchables) implics } -- ... and we are back in the original TcS inerts -- Notice that the original includes the _insoluble_flats so it was safe to ignore -- them in the beginning of this function.  932 933 934 935 936 937  ; traceTcS "solveWanteds: done nested implications }"$ vcat [ text "implic_eqs =" <+> ppr implic_eqs , text "unsolved_implics =" <+> ppr unsolved_implics ] ; return (implic_eqs, unsolved_implics) }  dimitris committed Nov 16, 2011 938 939 940  where givens_from_wanteds = foldrBag get_wanted [] get_wanted cc rest_givens | pushable_wanted cc  Simon Peyton Jones committed May 07, 2012 941 942 943 944 945  = let fl = ctEvidence cc gfl = Given { ctev_gloc = setCtLocOrigin (ctev_wloc fl) UnkSkol , ctev_evtm = EvId (ctev_evar fl) , ctev_pred = ctev_pred fl } this_given = cc { cc_ev = gfl }  dimitris committed Nov 16, 2011 946 947 948 949 950 951  in this_given : rest_givens | otherwise = rest_givens pushable_wanted :: Ct -> Bool pushable_wanted cc | isWantedCt cc  dimitris committed Nov 29, 2011 952  = isEqPred (ctPred cc) -- see Note [Preparing inert set for implications]  dimitris committed Nov 16, 2011 953 954 955 956 957 958 959 960 961 962  | 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 963 964 965  , ic_binds = ev_binds , ic_skols = skols , ic_given = givens  simonpj@microsoft.com committed Sep 13, 2010 966  , ic_wanted = wanteds  simonpj@microsoft.com committed Oct 06, 2010 967  , ic_loc = loc })  Simon Peyton Jones committed Jun 13, 2012 968 969  = shadowIPs givens $-- See Note [Shadowing of Implicit Parameters] nestImplicTcS ev_binds (untch, tcs_untouchables)$  simonpj@microsoft.com committed Oct 19, 2010 970 971  recoverTcS (return (emptyBag, emptyBag)) $-- Recover from nested failures. Even the top level is  dimitris committed Nov 16, 2011 972  -- just a bunch of implications, so failing at the first one is bad  simonpj@microsoft.com committed Sep 13, 2010 973 974 975  do { traceTcS "solveImplication {" (ppr imp) -- Solve flat givens  dimitris committed Apr 10, 2012 976 977 978  ; impls_from_givens <- solveInteractGiven loc givens ; MASSERT (isEmptyBag impls_from_givens)  simonpj@microsoft.com committed Sep 13, 2010 979  -- Simplify the wanteds  dimitris committed Jun 08, 2012 980 981 982 983 984 985 986  ; (_flat_subst, WC { wc_flat = unsolved_flats , wc_impl = unsolved_implics , wc_insol = insols }) <- solve_wanteds wanteds -- NB: Not solveWanteds because we need the derived equalities, -- which may not be solvable (due to touchability) in this implication -- but may become solvable by spontantenous unification outside.  simonpj@microsoft.com committed Jan 12, 2011 987 988 989 990  ; let (res_flat_free, res_flat_bound) = floatEqualities skols givens unsolved_flats final_flat = keepWanted res_flat_bound  simonpj@microsoft.com committed Sep 13, 2010 991   dimitris committed Nov 16, 2011 992 993  ; let res_wanted = WC { wc_flat = final_flat , wc_impl = unsolved_implics  simonpj@microsoft.com committed Jan 12, 2011 994  , wc_insol = insols }  dimitris committed Nov 16, 2011 995   simonpj@microsoft.com committed Jan 12, 2011 996 997 998  res_implic = unitImplication$ imp { ic_wanted = res_wanted , ic_insol = insolubleWC res_wanted }  simonpj@microsoft.com committed Sep 13, 2010 999   dimitris committed Nov 16, 2011 1000 1001  ; evbinds <- getTcEvBindsMap  simonpj@microsoft.com committed Sep 13, 2010 1002 1003  ; traceTcS "solveImplication end }" \$ vcat [ text "res_flat_free =" <+> ppr res_flat_free  dimitris committed Nov 16, 2011 1004  , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds)  simonpj@microsoft.com committed Jan 12, 2011 1005  , text "res_implic =" <+> ppr res_implic ]  simonpj@microsoft.com committed Sep 13, 2010 1006   simonpj@microsoft.com committed Jan 12, 2011 1007  ; return (res_flat_free, res_implic) }  dimitris committed Nov 16, 2011 1008  -- and we are back to the original inerts  simonpj@microsoft.com committed Sep 13, 2010 1009 1010   Simon Peyton Jones committed Jan 12, 2012 1011 floatEqualities :: [TcTyVar] -> [EvVar] -> Cts -> (Cts, Cts)  simonpj@microsoft.com committed Jan 12, 2011 1012 1013 1014 1015 -- 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 1016  -- Note [Float Equalities out of Implications]  simonpj@microsoft.com committed Jan 12, 2011 1017  | otherwise = partitionBag is_floatable wantders  Simon Peyton Jones committed Jan 12, 2012 1018 1019  where skol_set = mkVarSet skols is_floatable :: Ct -> Bool  dimitris committed Nov 16, 2011 1020  is_floatable ct  dimitris committed Nov 29, 2011 1021  | ct_predty <- ctPred ct  dimitris committed Nov 16, 2011 1022  , isEqPred ct_predty  Simon Peyton Jones committed Jan 12, 2012 1023  = skol_set disjointVarSet tvs_under_fsks ct_predty  dimitris committed Nov 16, 2011 1024  is_floatable _ct = False  simonpj@microsoft.com committed Nov 12, 2010 1025 1026 1027 1028 1029 1030 1031 1032  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 1033  tvs_under_fsks (LitTy {}) = emptyVarSet  simonpj@microsoft.com committed Nov 12, 2010 1034 1035 1036  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 1037  -- can mention type variables!  simonpj@microsoft.com committed Nov 12, 2010 1038 1039 1040 1041 1042  | 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 ` Simon Peyton Jones committed Jun 13, 2012 1043 1044 1045 1046