TcSimplify.lhs 53.5 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,  Simon Peyton Jones committed Sep 03, 2012 12 13  simplifyRule, simplifyTop, simplifyInteractive, solveWantedsTcM  simonpj@microsoft.com committed Sep 13, 2010 14  ) where  partain committed Jan 08, 1996 15   simonm committed Jan 08, 1998 16 #include "HsVersions.h"  partain committed Mar 19, 1996 17   Simon Peyton Jones committed Aug 28, 2012 18 import TcRnTypes  simonpj committed Sep 13, 2002 19 import TcRnMonad  simonpj@microsoft.com committed Sep 13, 2010 20 import TcErrors  Simon Peyton Jones committed Dec 24, 2012 21 import TcMType as TcM  simonpj@microsoft.com committed Sep 13, 2010 22 import TcType  Simon Peyton Jones committed Dec 24, 2012 23 import TcSMonad as TcS  dimitris committed Nov 16, 2011 24 import TcInteract  simonpj@microsoft.com committed Sep 13, 2010 25 import Inst  Simon Peyton Jones committed Aug 28, 2012 26 27 import Type ( classifyPredType, PredTree(..), getClassPredTys_maybe ) import Class ( Class )  Simon Marlow committed Oct 11, 2006 28 import Var  dimitris committed Jun 08, 2012 29 import Unique  simonm committed Dec 02, 1998 30 import VarSet  simonpj@microsoft.com committed Nov 12, 2010 31 import VarEnv  Simon Peyton Jones committed Dec 05, 2011 32 import TcEvidence  simonpj@microsoft.com committed Nov 12, 2010 33 import TypeRep  simonpj@microsoft.com committed Sep 13, 2010 34 import Name  simonmar committed Dec 10, 2003 35 import Bag  Simon Marlow committed Oct 11, 2006 36 37 import ListSetOps import Util  simonpj@microsoft.com committed Sep 13, 2010 38 39 40 import PrelInfo import PrelNames import Class ( classKey )  Simon Peyton Jones committed Aug 16, 2011 41 import BasicTypes ( RuleName )  simonpj@microsoft.com committed Sep 13, 2010 42 import Outputable  Ian Lynagh committed Mar 29, 2008 43 import FastString  dimitris committed Mar 28, 2012 44 import TrieMap () -- DV: for now  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  dimitris committed Sep 06, 2012 60 61 62 63 64 simplifyTop wanteds = do { traceTc "simplifyTop {" $text "wanted = " <+> ppr wanteds ; ev_binds_var <- newTcEvBinds ; zonked_final_wc <- solveWantedsTcMWithEvBinds ev_binds_var wanteds simpl_top ; binds1 <- TcRnMonad.getTcEvBinds ev_binds_var  Simon Peyton Jones committed Sep 01, 2012 65 66 67  ; traceTc "End simplifyTop }" empty ; traceTc "reportUnsolved {" empty  Simon Peyton Jones committed Sep 17, 2012 68  ; binds2 <- reportUnsolved zonked_final_wc  Simon Peyton Jones committed Sep 01, 2012 69  ; traceTc "reportUnsolved }" empty  dimitris committed Sep 06, 2012 70   Simon Peyton Jones committed Sep 01, 2012 71  ; return (binds1 unionBags binds2) }  Simon Peyton Jones committed Aug 28, 2012 72   Simon Peyton Jones committed Sep 01, 2012 73 74  where -- See Note [Top-level Defaulting Plan]  Simon Peyton Jones committed Sep 18, 2012 75  simpl_top :: WantedConstraints -> TcS WantedConstraints  Simon Peyton Jones committed Sep 01, 2012 76  simpl_top wanteds  dimitris committed Sep 06, 2012 77  = do { wc_first_go <- nestTcS (solve_wanteds_and_drop wanteds)  Simon Peyton Jones committed Dec 24, 2012 78 79 80  ; free_tvs <- TcS.zonkTyVarsAndFV (tyVarsOfWC wc_first_go) ; let meta_tvs = filterVarSet isMetaTyVar free_tvs -- zonkTyVarsAndFV: the wc_first_go is not yet zonked  Simon Peyton Jones committed Oct 15, 2012 81 82 83  -- filter isMetaTyVar: we might have runtime-skolems in GHCi, -- and we definitely don't want to try to assign to those!  Simon Peyton Jones committed Dec 24, 2012 84  ; mapM_ defaultTyVar (varSetElems meta_tvs) -- Has unification side effects  Simon Peyton Jones committed Sep 01, 2012 85  ; simpl_top_loop wc_first_go }  dimitris committed Jun 08, 2012 86   Simon Peyton Jones committed Sep 01, 2012 87  simpl_top_loop wc  Simon Peyton Jones committed Oct 04, 2012 88 89 90  | isEmptyWC wc || insolubleWC wc -- Don't do type-class defaulting if there are insolubles -- Doing so is not going to solve the insolubles  Simon Peyton Jones committed Sep 01, 2012 91 92  = return wc | otherwise  dimitris committed Sep 06, 2012 93  = do { wc_residual <- nestTcS (solve_wanteds_and_drop wc)  Simon Peyton Jones committed Sep 01, 2012 94 95 96 97 98 99 100  ; let wc_flat_approximate = approximateWC wc_residual ; something_happened <- applyDefaultingRules wc_flat_approximate -- See Note [Top-level Defaulting Plan] ; if something_happened then simpl_top_loop wc_residual else return wc_residual }  dimitris committed Jun 08, 2012 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 \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 145   Simon Peyton Jones committed Aug 16, 2011 146 147 148 ------------------ simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind) simplifyAmbiguityCheck name wanteds  dimitris committed Jun 08, 2012 149  = traceTc "simplifyAmbiguityCheck" (text "name =" <+> ppr name) >>  Simon Peyton Jones committed Sep 03, 2012 150  simplifyTop wanteds -- NB: must be simplifyTop so that we  151 152  -- do ambiguity resolution. -- See Note [Impedence matching] in TcBinds.  dimitris committed Nov 16, 2011 153   simonpj@microsoft.com committed Sep 13, 2010 154 155 156 ------------------ simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind) simplifyInteractive wanteds  dimitris committed Jun 08, 2012 157 158  = traceTc "simplifyInteractive" empty >> simplifyTop wanteds  simonpj@microsoft.com committed Sep 13, 2010 159 160 161 162 163  ------------------ simplifyDefault :: ThetaType -- Wanted; has no type variables in it -> TcM () -- Succeeds iff the constraint is soluble simplifyDefault theta  dimitris committed Jun 08, 2012 164 165  = do { traceTc "simplifyInteractive" empty ; wanted <- newFlatWanteds DefaultOrigin theta  Simon Peyton Jones committed Sep 03, 2012 166 167 168 169  ; (unsolved, _binds) <- solveWantedsTcM (mkFlatWC wanted) ; traceTc "reportUnsolved {" empty -- See Note [Deferring coercion errors to runtime]  Simon Peyton Jones committed Sep 17, 2012 170  ; reportAllUnsolved unsolved  dimitris committed Sep 06, 2012 171 172 173  -- Postcondition of solveWantedsTcM is that returned -- constraints are zonked. So Precondition of reportUnsolved -- is true.  Simon Peyton Jones committed Sep 03, 2012 174 175  ; traceTc "reportUnsolved }" empty  simonpj@microsoft.com committed Sep 13, 2010 176 177  ; return () } \end{code}  simonpj committed Jan 25, 2001 178   simonpj committed May 03, 2001 179   dimitris committed Nov 16, 2011 180 ***********************************************************************************  simonpj@microsoft.com committed Sep 13, 2010 181 * *  dimitris committed Nov 16, 2011 182 * Deriving *  simonpj@microsoft.com committed Sep 13, 2010 183 184 * * ***********************************************************************************  simonpj committed May 03, 2001 185   simonpj@microsoft.com committed Sep 13, 2010 186 187 \begin{code} simplifyDeriv :: CtOrigin  simonpj committed Apr 19, 2011 188 189 190 191  -> PredType -> [TyVar] -> ThetaType -- Wanted -> TcM ThetaType -- Needed  simonpj@microsoft.com committed Sep 13, 2010 192 193 -- Given instance (wanted) => C inst_ty -- Simplify 'wanted' as much as possibles  simonpj@microsoft.com committed Jan 12, 2011 194 -- Fail if not possible  simonpj committed Apr 19, 2011 195 simplifyDeriv orig pred tvs theta  Simon Peyton Jones committed Mar 02, 2012 196  = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize  simonpj@microsoft.com committed Feb 17, 2011 197 198 199 200  -- 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 201   Simon Peyton Jones committed Mar 02, 2012 202  ; let subst_skol = zipTopTvSubst tvs_skols$ map mkTyVarTy tvs  Simon Peyton Jones committed Jul 23, 2011 203  skol_set = mkVarSet tvs_skols  dimitris committed Jun 08, 2012 204  doc = ptext (sLit "deriving") <+> parens (ppr pred)  simonpj@microsoft.com committed Jan 12, 2011 205 206 207  ; wanted <- newFlatWanteds orig (substTheta skol_subst theta)  dimitris committed Jun 08, 2012 208 209  ; traceTc "simplifyDeriv" $vcat [ pprTvBndrs tvs $$ppr theta$$ ppr wanted, doc ]  Simon Peyton Jones committed Jan 12, 2012 210  ; (residual_wanted, _ev_binds1)  Simon Peyton Jones committed Sep 01, 2012 211  <- solveWantedsTcM (mkFlatWC wanted)  dimitris committed Sep 06, 2012 212  -- Post: residual_wanted are already zonked  simonpj@microsoft.com committed Sep 13, 2010 213   simonpj@microsoft.com committed Jan 12, 2011 214 215  ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted) -- See Note [Exotic derived instance contexts]  dimitris committed Nov 16, 2011 216  get_good :: Ct -> Either PredType Ct  dimitris committed Jul 19, 2012 217 218 219 220 221 222  get_good ct | validDerivPred skol_set p , isWantedCt ct = Left p -- NB: residual_wanted may contain unsolved -- Derived and we stick them into the bad set -- so that reportUnsolved may decide what to do with them | otherwise = Right ct  dimitris committed Nov 29, 2011 223  where p = ctPred ct  simonpj@microsoft.com committed Sep 13, 2010 224   Simon Peyton Jones committed Jan 12, 2012 225 226  -- We never want to defer these errors because they are errors in the -- compiler! Hence the False below  Simon Peyton Jones committed Sep 17, 2012 227  ; reportAllUnsolved (residual_wanted { wc_flat = bad })  simonpj@microsoft.com committed Sep 13, 2010 228   simonpj@microsoft.com committed Jan 12, 2011 229 230  ; let min_theta = mkMinimalBySCs (bagToList good) ; return (substTheta subst_skol min_theta) }  simonpj@microsoft.com committed Sep 13, 2010 231 \end{code}  simonpj committed May 03, 2001 232   simonpj@microsoft.com committed Feb 17, 2011 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 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 258 259 260 261 262 263 264 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 265   simonpj@microsoft.com committed Sep 13, 2010 266 267 268 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 269   simonpj@microsoft.com committed Sep 13, 2010 270 271 Notice that this instance (just) satisfies the Paterson termination conditions. Then we *could* derive an instance decl like this:  simonpj committed Feb 10, 2004 272   simonpj@microsoft.com committed Sep 13, 2010 273 274 275 276  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 277   simonpj@microsoft.com committed Sep 13, 2010 278 279 280 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 281   simonpj@microsoft.com committed Sep 13, 2010 282 283 So for now we simply require that the derived instance context should have only type-variable constraints.  simonpj committed Feb 10, 2004 284   simonpj@microsoft.com committed Sep 13, 2010 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 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 314   dreixel committed Nov 11, 2011 315 316 317 318 319 320 321 322 323 324 325 326 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 327 \begin{code}  Simon Peyton Jones committed Aug 16, 2011 328 simplifyInfer :: Bool  simonpj@microsoft.com committed Jan 12, 2011 329 330 331  -> Bool -- Apply monomorphism restriction -> [(Name, TcTauType)] -- Variables to be generalised, -- and their tau-types  Simon Peyton Jones committed Aug 29, 2012 332  -> WantedConstraints  simonpj@microsoft.com committed Sep 13, 2010 333 334  -> TcM ([TcTyVar], -- Quantify over these type variables [EvVar], -- ... and these constraints  Simon Peyton Jones committed Aug 16, 2011 335 336 337  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 338  TcEvBinds) -- ... binding these evidence variables  Simon Peyton Jones committed Aug 29, 2012 339 simplifyInfer _top_lvl apply_mr name_taus wanteds  simonpj@microsoft.com committed Jan 12, 2011 340 341 342  | isEmptyWC wanteds = do { gbl_tvs <- tcGetGlobalTyVars -- Already zonked ; zonked_taus <- zonkTcTypes (map snd name_taus)  Simon Peyton Jones committed Feb 16, 2012 343  ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus minusVarSet gbl_tvs)  dreixel committed Nov 11, 2011 344 345 346  -- 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 347  ; return (qtvs, [], False, emptyTcEvBinds) }  simonpj committed Feb 10, 2004 348   simonpj@microsoft.com committed Sep 13, 2010 349  | otherwise  Simon Peyton Jones committed Oct 15, 2012 350 351  = do { traceTc "simplifyInfer {"$ vcat [ ptext (sLit "binds =") <+> ppr name_taus  Simon Peyton Jones committed Aug 16, 2011 352 353  , ptext (sLit "closed =") <+> ppr _top_lvl , ptext (sLit "apply_mr =") <+> ppr apply_mr  dimitris committed Sep 06, 2012 354  , ptext (sLit "(unzonked) wanted =") <+> ppr wanteds  simonpj@microsoft.com committed Sep 13, 2010 355 356  ]  Simon Peyton Jones committed Jun 21, 2012 357 358 359 360 361  -- Historical note: Before step 2 we used to have a -- HORRIBLE HACK described in Note [Avoid unecessary -- constraint simplification] but, as described in Trac -- #4361, we have taken in out now. That's why we start -- with step 2!  simonpj@microsoft.com committed Jan 12, 2011 362   Simon Peyton Jones committed Jun 21, 2012 363 364 365 366 367 368 369 370  -- Step 2) First try full-blown solving -- NB: we must gather up all the bindings from doing -- this solving; hence (runTcSWithEvBinds ev_binds_var). -- And note that since there are nested implications, -- calling solveWanteds will side-effect their evidence -- bindings, so we can't just revert to the input -- constraint.  Simon Peyton Jones committed Oct 15, 2012 371 372  ; ev_binds_var <- newTcEvBinds  dimitris committed Sep 06, 2012 373 374 375  ; wanted_transformed <- solveWantedsTcMWithEvBinds ev_binds_var wanteds $solve_wanteds_and_drop -- Post: wanted_transformed are zonked  Simon Peyton Jones committed Jun 21, 2012 376 377  -- Step 4) Candidates for quantification are an approximation of wanted_transformed  dimitris committed Jun 08, 2012 378 379 380  -- NB: Already the fixpoint of any unifications that may have happened -- NB: We do not do any defaulting when inferring a type, this can lead -- to less polymorphic types, see Note [Default while Inferring]  dimitris committed Jul 19, 2012 381   Simon Peyton Jones committed Jun 21, 2012 382 383  -- Step 5) Minimize the quantification candidates -- Step 6) Final candidates for quantification  Simon Peyton Jones committed Sep 01, 2012 384  -- We discard bindings, insolubles etc, because all we are  dimitris committed Sep 06, 2012 385 386  -- care aout it  Simon Peyton Jones committed Oct 15, 2012 387 388 389 390 391 392 393 394 395 396 397 398 399  ; tc_lcl_env <- TcRnMonad.getLclEnv ; let untch = tcl_untch tc_lcl_env ; quant_pred_candidates <- if insolubleWC wanted_transformed then return [] -- See Note [Quantification with errors] else do { gbl_tvs <- tcGetGlobalTyVars ; let quant_cand = approximateWC wanted_transformed meta_tvs = filter isMetaTyVar (varSetElems (tyVarsOfCts quant_cand)) ; ((flats, _insols), _extra_binds) <- runTcS$ do { mapM_ (promoteAndDefaultTyVar untch gbl_tvs) meta_tvs ; _implics <- solveInteract quant_cand ; getInertUnsolved } ; return (map ctPred $filter isWantedCt (bagToList flats)) }  Simon Peyton Jones committed Sep 17, 2012 400 401 402 403 404  -- NB: Dimitrios is slightly worried that we will get -- family equalities (F Int ~ alpha) in the quantification -- candidates, as we have performed no further unflattening -- at this point. Nothing bad, but inferred contexts might -- look complicated.  Simon Peyton Jones committed Sep 01, 2012 405   Simon Peyton Jones committed Oct 15, 2012 406 407 408  -- NB: quant_pred_candidates is already the fixpoint of any -- unifications that may have happened ; gbl_tvs <- tcGetGlobalTyVars  Simon Peyton Jones committed Dec 24, 2012 409  ; zonked_tau_tvs <- TcM.zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))  410  ; let init_tvs = zonked_tau_tvs minusVarSet gbl_tvs  Simon Peyton Jones committed Sep 01, 2012 411  poly_qtvs = growThetaTyVars quant_pred_candidates init_tvs  412  minusVarSet gbl_tvs  Simon Peyton Jones committed Sep 01, 2012 413  pbound = filter (quantifyPred poly_qtvs) quant_pred_candidates  dimitris committed Jun 08, 2012 414   Simon Peyton Jones committed Aug 16, 2011 415  -- Monomorphism restriction  Simon Peyton Jones committed Sep 01, 2012 416 417  mr_qtvs = init_tvs minusVarSet constrained_tvs constrained_tvs = tyVarsOfTypes quant_pred_candidates  418  mr_bites = apply_mr && not (null pbound)  Simon Peyton Jones committed Aug 16, 2011 419   Simon Peyton Jones committed Sep 01, 2012 420 421  (qtvs, bound) | mr_bites = (mr_qtvs, []) | otherwise = (poly_qtvs, pbound)  dimitris committed Jun 08, 2012 422   Simon Peyton Jones committed Sep 01, 2012 423 424 425 426 427 428 429  ; traceTc "simplifyWithApprox"$ vcat [ ptext (sLit "quant_pred_candidates =") <+> ppr quant_pred_candidates , ptext (sLit "gbl_tvs=") <+> ppr gbl_tvs , ptext (sLit "zonked_tau_tvs=") <+> ppr zonked_tau_tvs , ptext (sLit "pbound =") <+> ppr pbound , ptext (sLit "init_qtvs =") <+> ppr init_tvs , ptext (sLit "poly_qtvs =") <+> ppr poly_qtvs ]  simonpj@microsoft.com committed Jan 12, 2011 430   431  ; if isEmptyVarSet qtvs && null bound  Simon Peyton Jones committed Jul 14, 2012 432 433 434 435  then do { traceTc "} simplifyInfer/no quantification" empty ; emitConstraints wanted_transformed -- Includes insolubles (if -fdefer-type-errors) -- as well as flats and implications  dimitris committed Jun 08, 2012 436  ; return ([], [], mr_bites, TcEvBinds ev_binds_var) }  simonpj@microsoft.com committed Jan 12, 2011 437 438  else do  dimitris committed Jun 08, 2012 439 440 441  { traceTc "simplifyApprox" $ptext (sLit "bound are =") <+> ppr bound  simonpj@microsoft.com committed Jan 12, 2011 442  -- Step 4, zonk quantified variables  443  ; let minimal_flat_preds = mkMinimalBySCs bound  Simon Peyton Jones committed Aug 16, 2011 444 445  skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty) | (name, ty) <- name_taus ]  simonpj@microsoft.com committed Jan 12, 2011 446 447 448 449  -- Don't add the quantified variables here, because -- they are also bound in ic_skols and we want them to be -- tidied uniformly  Simon Peyton Jones committed Feb 16, 2012 450  ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)  simonpj@microsoft.com committed Jan 12, 2011 451   Simon Peyton Jones committed Jun 21, 2012 452  -- Step 7) Emit an implication  Simon Peyton Jones committed Dec 24, 2012 453  ; minimal_bound_ev_vars <- mapM TcM.newEvVar minimal_flat_preds  Simon Peyton Jones committed Oct 15, 2012 454  ; let implic = Implic { ic_untch = pushUntouchables untch  Simon Peyton Jones committed Jan 12, 2012 455  , ic_skols = qtvs_to_return  Simon Peyton Jones committed Aug 28, 2012 456 457  , ic_fsks = [] -- wanted_tansformed arose only from solveWanteds -- hence no flatten-skolems (which come from givens)  simonpj@microsoft.com committed Jan 12, 2011 458  , ic_given = minimal_bound_ev_vars  dimitris committed Jun 08, 2012 459  , ic_wanted = wanted_transformed  simonpj@microsoft.com committed Jan 12, 2011 460 461  , ic_insol = False , ic_binds = ev_binds_var  Simon Peyton Jones committed Sep 17, 2012 462  , ic_info = skol_info  Simon Peyton Jones committed Oct 15, 2012 463  , ic_env = tc_lcl_env }  simonpj@microsoft.com committed Jan 12, 2011 464  ; emitImplication implic  dimitris committed Jun 08, 2012 465   simonpj@microsoft.com committed Jan 12, 2011 466 467 468  ; traceTc "} simplifyInfer/produced residual implication for quantification"$ vcat [ ptext (sLit "implic =") <+> ppr implic -- ic_skols, ic_given give rest of result  Simon Peyton Jones committed Aug 16, 2011 469  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return  Simon Peyton Jones committed Sep 01, 2012 470  , ptext (sLit "spb =") <+> ppr quant_pred_candidates  simonpj@microsoft.com committed Jan 12, 2011 471 472  , ptext (sLit "bound =") <+> ppr bound ]  Simon Peyton Jones committed Aug 16, 2011 473 474  ; return ( qtvs_to_return, minimal_bound_ev_vars , mr_bites, TcEvBinds ev_binds_var) } }  simonpj@microsoft.com committed Jan 12, 2011 475 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 476   Simon Peyton Jones committed Sep 17, 2012 477 478 479 480 481 482 483 484 485 Note [Quantification with errors] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we find that the RHS of the definition has some absolutely-insoluble constraints, we abandon all attempts to find a context to quantify over, and instead make the function fully-polymorphic in whatever type we have found. For two reasons a) Minimise downstream errors b) Avoid spurious errors from this function  simonpj@microsoft.com committed Sep 13, 2010 486   487 488 Note [Default while Inferring] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  dimitris committed Jun 08, 2012 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 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 523 524 525 526 527 528 529 530 531 Note [Minimize by Superclasses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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 532   simonpj committed Jan 25, 2001 533   simonpj@microsoft.com committed Oct 08, 2010 534 535 Note [Avoid unecessary constraint simplification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  Simon Peyton Jones committed Jun 21, 2012 536 537 538 539  -------- NB NB NB (Jun 12) ------------- This note not longer applies; see the notes with Trac #4361. But I'm leaving it in here so we remember the issue.) ----------------------------------------  simonpj@microsoft.com committed Oct 08, 2010 540 When inferring the type of a let-binding, with simplifyInfer,  Simon Peyton Jones committed Jun 21, 2012 541 try to avoid unnecessarily simplifying class constraints.  simonpj@microsoft.com committed Oct 08, 2010 542 543 Doing so aids sharing, but it also helps with delicate situations like  dimitris committed Jun 08, 2012 544   simonpj@microsoft.com committed Oct 08, 2010 545  instance C t => C [t] where ..  dimitris committed Jun 08, 2012 546   simonpj@microsoft.com committed Oct 08, 2010 547 548 549 550 551 552 553 554 555 556 557  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 Sep 13, 2010 558 559 560 561 562 ********************************************************************************* * * * RULES * * * ***********************************************************************************  simonpj committed May 12, 2004 563   Simon Peyton Jones committed Apr 16, 2012 564 See note [Simplifying RULE consraints] in TcRule  simonpj committed Jun 28, 1999 565   Simon Peyton Jones committed Apr 16, 2012 566 567 568 569 570 571 572 573 574 575 576 577 578 579 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 580 581  \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 582 583 584 simplifyRule :: RuleName -> WantedConstraints -- Constraints from LHS -> WantedConstraints -- Constraints from RHS  Simon Peyton Jones committed Apr 16, 2012 585 586 587  -> TcM ([EvVar], WantedConstraints) -- LHS evidence varaibles -- See Note [Simplifying RULE constraints] in TcRule simplifyRule name lhs_wanted rhs_wanted  Simon Peyton Jones committed Sep 03, 2012 588  = do { -- We allow ourselves to unify environment  dimitris committed Jun 08, 2012 589  -- variables: runTcS runs with NoUntouchables  Simon Peyton Jones committed Sep 03, 2012 590  (resid_wanted, _) <- solveWantedsTcM (lhs_wanted andWC rhs_wanted)  dimitris committed Sep 06, 2012 591  -- Post: these are zonked and unflattened  Simon Peyton Jones committed Apr 16, 2012 592   Simon Peyton Jones committed Sep 17, 2012 593 594  ; zonked_lhs_flats <- zonkCts (wc_flat lhs_wanted) ; let (q_cts, non_q_cts) = partitionBag quantify_me zonked_lhs_flats  Simon Peyton Jones committed Apr 16, 2012 595 596 597 598 599 600 601 602 603 604 605 606  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 607  ; traceTc "simplifyRule" $ Simon Peyton Jones committed Sep 03, 2012 608  vcat [ ptext (sLit "LHS of rule") <+> doubleQuotes (ftext name)  Simon Peyton Jones committed Sep 17, 2012 609  , text "zonked_lhs_flats" <+> ppr zonked_lhs_flats  Simon Peyton Jones committed Apr 16, 2012 610 611  , text "q_cts" <+> ppr q_cts ]  Simon Peyton Jones committed May 07, 2012 612  ; return ( map (ctEvId . ctEvidence) (bagToList q_cts)  Simon Peyton Jones committed Sep 17, 2012 613  , lhs_wanted { wc_flat = non_q_cts }) }  simonpj committed Sep 13, 2002 614 615 616 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 617 618 619 620 621 ********************************************************************************* * * * Main Simplifier * * * ***********************************************************************************  lewie committed Mar 02, 2000 622   Simon Peyton Jones committed Jan 12, 2012 623 624 625 626 627 628 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 629   Simon Peyton Jones committed Jan 12, 2012 630 631  a :: Int a = 'a'  simonpj@microsoft.com committed Sep 13, 2010 632   Simon Peyton Jones committed Jan 12, 2012 633  main = print "b"  simonpj@microsoft.com committed Sep 13, 2010 634   Simon Peyton Jones committed Jan 12, 2012 635 636 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 637   Simon Peyton Jones committed Jan 12, 2012 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 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.  Simon Peyton Jones committed Sep 17, 2012 660 661 662 663 664 665 666 667 668 669 Note [Zonk after solving] ~~~~~~~~~~~~~~~~~~~~~~~~~ We zonk the result immediately after constraint solving, for two reasons: a) because zonkWC generates evidence, and this is the moment when we have a suitable evidence variable to hand. Note that *after* solving the constraints are typically small, so the overhead is not great.  Simon Peyton Jones committed Jan 12, 2012 670 \begin{code}  dimitris committed Sep 06, 2012 671 672 673 674 solveWantedsTcMWithEvBinds :: EvBindsVar -> WantedConstraints -> (WantedConstraints -> TcS WantedConstraints) -> TcM WantedConstraints  Simon Peyton Jones committed Sep 17, 2012 675 676 677 678 679 680 -- Returns a *zonked* result -- We zonk when we finish primarily to un-flatten out any -- flatten-skolems etc introduced by canonicalisation of -- types involving type funuctions. Happily the result -- is typically much smaller than the input, indeed it is -- often empty.  dimitris committed Sep 06, 2012 681 solveWantedsTcMWithEvBinds ev_binds_var wc tcs_action  Simon Peyton Jones committed Sep 17, 2012 682 683  = do { traceTc "solveWantedsTcMWithEvBinds"$ text "wanted=" <+> ppr wc ; wc2 <- runTcSWithEvBinds ev_binds_var (tcs_action wc)  dimitris committed Sep 06, 2012 684  ; zonkWC ev_binds_var wc2 }  Simon Peyton Jones committed Sep 17, 2012 685  -- See Note [Zonk after solving]  dimitris committed Sep 06, 2012 686   Simon Peyton Jones committed Sep 01, 2012 687 solveWantedsTcM :: WantedConstraints -> TcM (WantedConstraints, Bag EvBind)  Simon Peyton Jones committed Sep 03, 2012 688 -- Zonk the input constraints, and simplify them  dimitris committed Jul 19, 2012 689 -- Return the evidence binds in the BagEvBinds result  Simon Peyton Jones committed Aug 28, 2012 690 -- Discards all Derived stuff in result  dimitris committed Sep 06, 2012 691 -- Postcondition: fully zonked and unflattened constraints  Simon Peyton Jones committed Sep 03, 2012 692 solveWantedsTcM wanted  dimitris committed Sep 06, 2012 693 694 695  = do { ev_binds_var <- newTcEvBinds ; wanteds' <- solveWantedsTcMWithEvBinds ev_binds_var wanted solve_wanteds_and_drop ; binds <- TcRnMonad.getTcEvBinds ev_binds_var  Simon Peyton Jones committed Sep 03, 2012 696  ; return (wanteds', binds) }  dimitris committed Jul 19, 2012 697   Simon Peyton Jones committed Sep 01, 2012 698 699 700 701 702 solve_wanteds_and_drop :: WantedConstraints -> TcS (WantedConstraints) -- Since solve_wanteds returns the residual WantedConstraints, -- it should alway be called within a runTcS or something similar, solve_wanteds_and_drop wanted = do { wc <- solve_wanteds wanted ; return (dropDerivedWC wc) }  dimitris committed Jul 19, 2012 703 704  solve_wanteds :: WantedConstraints -> TcS WantedConstraints  Simon Peyton Jones committed Sep 01, 2012 705 -- so that the inert set doesn't mindlessly propagate.  dimitris committed Jul 19, 2012 706 -- NB: wc_flats may be wanted /or/ derived now  dimitris committed Nov 16, 2011 707 solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols })  simonpj@microsoft.com committed Jan 12, 2011 708 709  = do { traceTcS "solveWanteds {" (ppr wanted)  dimitris committed Jul 19, 2012 710  -- Try the flat bit, including insolubles. Solving insolubles a  Simon Peyton Jones committed Sep 28, 2012 711  -- second time round is a bit of a waste; but the code is simple  Simon Peyton Jones committed Jul 23, 2012 712 713 714  -- and the program is wrong anyway, and we don't run the danger -- of adding Derived insolubles twice; see -- TcSMonad Note [Do not add duplicate derived insolubles]  Simon Peyton Jones committed Aug 28, 2012 715  ; traceTcS "solveFlats {" empty  Simon Peyton Jones committed Jul 23, 2012 716  ; let all_flats = flats unionBags insols  Simon Peyton Jones committed Aug 28, 2012 717 718  ; impls_from_flats <- solveInteract all_flats ; traceTcS "solveFlats end }" (ppr impls_from_flats)  simonpj@microsoft.com committed Jan 12, 2011 719   dimitris committed Nov 16, 2011 720 721  -- solve_wanteds iterates when it is able to float equalities -- out of one or more of the implications.  dimitris committed Apr 10, 2012 722  ; unsolved_implics <- simpl_loop 1 (implics unionBags impls_from_flats)  simonpj@microsoft.com committed Jan 12, 2011 723   Simon Peyton Jones committed Aug 28, 2012 724 725  ; (unsolved_flats, insoluble_flats) <- getInertUnsolved  dimitris committed Sep 06, 2012 726 727 728 729 730 731  -- We used to unflatten here but now we only do it once at top-level -- during zonking -- see Note [Unflattening while zonking] in TcMType ; let wc = WC { wc_flat = unsolved_flats , wc_impl = unsolved_implics , wc_insol = insoluble_flats }  dimitris committed Nov 16, 2011 732  ; bb <- getTcEvBindsMap  simonpj@microsoft.com committed Nov 12, 2010 733  ; tb <- getTcSTyBindsMap  simonpj@microsoft.com committed Sep 13, 2010 734  ; traceTcS "solveWanteds }" $ simonpj@microsoft.com committed Nov 12, 2010 735  vcat [ text "unsolved_flats =" <+> ppr unsolved_flats  simonpj@microsoft.com committed Jan 12, 2011 736  , text "unsolved_implics =" <+> ppr unsolved_implics  dimitris committed Nov 16, 2011 737  , text "current evbinds =" <+> ppr (evBindMapBinds bb)  simonpj@microsoft.com committed Nov 12, 2010 738  , text "current tybinds =" <+> vcat (map ppr (varEnvElts tb))  Simon Peyton Jones committed Aug 28, 2012 739  , text "final wc =" <+> ppr wc ]  Simon Peyton Jones committed Mar 02, 2012 740   Simon Peyton Jones committed Aug 28, 2012 741  ; return wc }  dimitris committed Nov 16, 2011 742 743 744 745 746 747 748 749  simpl_loop :: Int -> Bag Implication -> TcS (Bag Implication) simpl_loop n implics | n > 10 = traceTcS "solveWanteds: loop!" empty >> return implics | otherwise  Simon Peyton Jones committed Aug 28, 2012 750 751 752 753 754 755 756  = do { (floated_eqs, unsolved_implics) <- solveNestedImplications implics ; if isEmptyBag floated_eqs then return unsolved_implics else do { -- Put floated_eqs into the current inert set before looping impls_from_eqs <- solveInteract floated_eqs ; simpl_loop (n+1) (unsolved_implics unionBags impls_from_eqs)} }  757   dimitris committed Jul 19, 2012 758   dimitris committed Nov 16, 2011 759 760 761 762 763 764 765 766 767 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  Simon Peyton Jones committed Aug 28, 2012 768 769  ; let thinner_inerts = prepareInertsForImplications inerts -- See Note [Preparing inert set for implications]  dimitris committed Jul 19, 2012 770   Simon Peyton Jones committed Aug 28, 2012 771  ; traceTcS "solveNestedImplications starting {"$  dimitris committed Jul 19, 2012 772  vcat [ text "original inerts = " <+> ppr inerts  dimitris committed Jun 08, 2012 773 774  , text "thinner_inerts = " <+> ppr thinner_inerts ]  Simon Peyton Jones committed Aug 28, 2012 775  ; (floated_eqs, unsolved_implics)  Simon Peyton Jones committed Aug 28, 2012 776  <- flatMapBagPairM (solveImplication thinner_inerts) implics  dimitris committed Nov 16, 2011 777 778 779 780  -- ... 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.  Simon Peyton Jones committed Aug 28, 2012 781  ; traceTcS "solveNestedImplications end }" $ Simon Peyton Jones committed Aug 28, 2012 782  vcat [ text "all floated_eqs =" <+> ppr floated_eqs  783 784  , text "unsolved_implics =" <+> ppr unsolved_implics ]  Simon Peyton Jones committed Aug 28, 2012 785  ; return (floated_eqs, unsolved_implics) }  786   Simon Peyton Jones committed Aug 28, 2012 787 solveImplication :: InertSet  dimitris committed Nov 16, 2011 788 789 790 791 792  -> 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  Simon Peyton Jones committed Aug 28, 2012 793 solveImplication inerts  dimitris committed Nov 16, 2011 794  imp@(Implic { ic_untch = untch  simonpj@microsoft.com committed Oct 06, 2010 795 796  , ic_binds = ev_binds , ic_skols = skols  Simon Peyton Jones committed Aug 28, 2012 797  , ic_fsks = old_fsks  simonpj@microsoft.com committed Oct 06, 2010 798  , ic_given = givens  simonpj@microsoft.com committed Sep 13, 2010 799  , ic_wanted = wanteds  Simon Peyton Jones committed Sep 17, 2012 800 801  , ic_info = info , ic_env = env })  Simon Peyton Jones committed Jan 01, 2013 802  = do { traceTcS "solveImplication {" (ppr imp)  simonpj@microsoft.com committed Sep 13, 2010 803   Simon Peyton Jones committed Aug 28, 2012 804  -- Solve the nested constraints  Simon Peyton Jones committed Sep 01, 2012 805 806 807  -- NB: 'inerts' has empty inert_fsks ; (new_fsks, residual_wanted) <- nestImplicTcS ev_binds untch inerts$  Simon Peyton Jones committed Sep 17, 2012 808  do { solveInteractGiven (mkGivenLoc info env) old_fsks givens  Simon Peyton Jones committed Sep 01, 2012 809  ; residual_wanted <- solve_wanteds wanteds  Simon Peyton Jones committed Sep 17, 2012 810 811 812  -- solve_wanteds, *not* solve_wanteds_and_drop, because -- we want to retain derived equalities so we can float -- them out in floatEqualities  Simon Peyton Jones committed Sep 01, 2012 813 814 815 816 817 818 819  ; more_fsks <- getFlattenSkols ; return (more_fsks ++ old_fsks, residual_wanted) } ; (floated_eqs, final_wanted) <- floatEqualities (skols ++ new_fsks) givens residual_wanted ; let res_implic | isEmptyWC final_wanted  Simon Peyton Jones committed Aug 28, 2012 820 821  = emptyBag | otherwise  Simon Peyton Jones committed Sep 01, 2012 822 823 824  = unitBag (imp { ic_fsks = new_fsks , ic_wanted = dropDerivedWC final_wanted , ic_insol = insolubleWC final_wanted })  simonpj@microsoft.com committed Sep 13, 2010 825   dimitris committed Nov 16, 2011 826  ; evbinds <- getTcEvBindsMap  simonpj@microsoft.com committed Sep 13, 2010 827  ; traceTcS "solveImplication end }" \$ vcat  Simon Peyton Jones committed Aug 28, 2012 828  [ text "floated_eqs =" <+> ppr floated_eqs  Simon Peyton Jones committed Sep 01, 2012 829  , text "new_fsks =" <+> ppr new_fsks  Simon Peyton Jones committed Aug 28, 2012 830 831  , text "res_implic =" <+> ppr res_implic , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds) ]  simonpj@microsoft.com committed Sep 13, 2010 832   Simon Peyton Jones committed Aug 28, 2012 833  ; return (floated_eqs, res_implic) }  dimitris committed Jul 19, 2012 834 835 836 837 \end{code} \begin{code}  Simon Peyton Jones committed Sep 01, 2012 838 839 floatEqualities :: [TcTyVar] -> [EvVar] -> WantedConstraints -> TcS (Cts, WantedConstraints)  simonpj@microsoft.com committed Jan 12, 2011 840 841 -- Post: The returned FlavoredEvVar's are only Wanted or Derived -- and come from the input wanted ev vars or deriveds  Simon Peyton Jones committed Sep 01, 2012 842 843 -- Also performs some unifications, adding to monadically-carried ty_binds -- These will be used when processing floated_eqs later  Simon Peyton Jones committed Aug 28, 2012 844 845 floatEqualities skols can_given wanteds@(WC { wc_flat = flats }) | hasEqualities can_given  Simon Peyton Jones committed Sep 01, 2012 846  = return (emptyBag, wanteds) -- Note [Float Equalities out of Implications]  Simon Peyton Jones committed Aug 28, 2012 847  | otherwise  Simon Peyton Jones committed Sep 01, 2012 848  = do { let (float_eqs, remaining_flats) = partitionBag is_floatable flats  Simon Peyton Jones committed Dec 24, 2012 849  ; untch <- TcS.getUntouchables  Simon Peyton Jones committed Oct 15, 2012 850  ; mapM_ (promoteTyVar untch) (varSetElems (tyVarsOfCts float_eqs))  Simon Peyton Jones committed Sep 01, 2012 851  ; ty_binds <- getTcSTyBindsMap  Simon Peyton Jones committed Sep 01, 2012 852  ; traceTcS "floatEqualities" (vcat [ text "Floated eqs =" <+> ppr float_eqs  Simon Peyton Jones committed Sep 01, 2012 853 854  , text "Ty binds =" <+> ppr ty_binds]) ; return (float_eqs, wanteds { wc_flat = remaining_flats }) }  Simon Peyton Jones committed Aug 28, 2012 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872  where skol_set = growSkols wanteds (mkVarSet skols) is_floatable :: Ct -> Bool is_floatable ct = isEqPred pred && skol_set disjointVarSet tyVarsOfType pred where pred = ctPred ct growSkols :: WantedConstraints -> VarSet -> VarSet -- Find all the type variables that might possibly be unified -- with a type that mentions a skolem. This test is very conservative. -- I don't *think* we need look inside the implications, because any -- relevant unification variables in there are untouchable. growSkols (WC { wc_flat = flats }) skols = growThetaTyVars theta skols where theta = foldrBag ((:) . ctPred) [] flats  Simon Peyton Jones committed Sep 01, 2012 873   Simon Peyton Jones committed Oct 15, 2012 874 875 876 877 878 promoteTyVar :: Untouchables -> TcTyVar -> TcS () -- When we float a constraint out of an implication we must restore -- invariant (MetaTvInv) in Note [Untouchable type variables] in TcType promoteTyVar untch tv | isFloatedTouchableMetaTyVar untch tv  Simon Peyton Jones committed Dec 24, 2012 879  = do { cloned_tv <- TcS.cloneMetaTyVar tv  Simon Peyton Jones committed Oct 15, 2012 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897  ; let rhs_tv = setMetaTyVarUntouchables cloned_tv untch ; setWantedTyBind tv (mkTyVarTy rhs_tv) } | otherwise = return () promoteAndDefaultTyVar :: Untouchables -> TcTyVarSet -> TyVar -> TcS () -- See Note [Promote _and_ default when inferring] promoteAndDefaultTyVar untch gbl_tvs tv = do { tv1 <- if tv elemVarSet gbl_tvs then return tv else defaultTyVar tv ; promoteTyVar untch tv1 } defaultTyVar :: TcTyVar -> TcS TcTyVar -- Precondition: MetaTyVars only -- See Note [DefaultTyVar] defaultTyVar the_tv | not (k eqKind default_k)  Simon Peyton Jones committed Dec 24, 2012 898  = do { tv' <- TcS.cloneMetaTyVar the_tv  Simon Peyton Jones committed Oct 15, 2012 899 900 901 902 903 904 905 906 907 908 909 910 911  ; let new_tv = setTyVarKind tv' default_k ; traceTcS "defaultTyVar" (ppr the_tv <+> ppr new_tv) ; setWantedTyBind the_tv (mkTyVarTy new_tv) ; return new_tv } -- Why not directly derived_pred = mkTcEqPred k default_k? -- See Note [DefaultTyVar] -- We keep the same Untouchables on tv' | otherwise = return the_tv -- The common case where k = tyVarKind the_tv default_k = defaultKind k  Simon Peyton Jones committed Sep 01, 2012 912 913 approximateWC :: WantedConstraints -> Cts -- Postcondition: Wanted or Derived Cts  Simon Peyton Jones committed Oct 15, 2012 914 915 approximateWC wc = float_wc emptyVarSet wc  Simon Peyton Jones committed Sep 01, 2012 916 917  where float_wc :: TcTyVarSet -> WantedConstraints -> Cts  Simon Peyton Jones committed Oct 15, 2012 918 919 920  float_wc skols (WC { wc_flat = flats, wc_impl = implics }) = do_bag (float_flat skols) flats unionBags do_bag (float_implic skols) implics  Simon Peyton Jones committed Sep 01, 2012 921 922 923  float_implic :: TcTyVarSet -> Implication -> Cts float_implic skols imp  924 925 926  | hasEqualities (ic_given imp) -- Don't float out of equalities = emptyCts -- cf floatEqualities | otherwise -- See Note [approximateWC]  Simon Peyton Jones committed Sep 01, 2012 927 928 929 930 931 932 933 934 935 936 937 938 939  = float_wc skols' (ic_wanted imp) where skols' = skols extendVarSetList ic_skols imp extendVarSetList ic_fsks imp float_flat :: TcTyVarSet -> Ct -> Cts float_flat skols ct | tyVarsOfCt ct disjointVarSet skols = singleCt ct | otherwise = emptyCts do_bag :: (a -> Bag c) -> Bag a -> Bag c do_bag f = foldrBag (unionBags.f) emptyBag \end{code}  partain committed Jan 08, 1996 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 Note [ApproximateWC] ~~~~~~~~~~~~~~~~~~~~ approximateWC takes a constraint, typically arising from the RHS of a let-binding whose type we are *inferring*, and extracts from it some *flat* constraints that we might plausibly abstract over. Of course the top-level flat constraints are plausible, but we also float constraints out from inside, if the are not captured by skolems. However we do *not* float anything out if the implication binds equality constriants, because that defeats the OutsideIn story. Consider data T a where TInt :: T Int MkT :: T a f TInt = 3::Int We get the implication (a ~ Int => res ~ Int), where so far we've decided f :: T a -> res We don't want to float (res~Int) out because then we'll infer f :: T a -> Int which is only on of the possible types. (GHC 7.6 accidentally *did* float out of such implications, which meant it would happily infer non-principal types.)  Simon Peyton Jones committed Oct 15, 2012 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 Note [DefaultTyVar] ~~~~~~~~~~~~~~~~~~~ defaultTyVar is used on any un-instantiated meta type variables to default the kind of OpenKind and ArgKind etc to *. This is important to ensure that instance declarations match. For example consider instance Show (a->b) foo x = show (\_ -> True) Then we'll get a constraint (Show (p ->q)) where p has kind ArgKind, and that won't match the typeKind (*) in the instance decl. See tests tc217 and tc175. We look only at touchable type variables. No further constraints are going to affect these type variables, so it's time to do it by hand. However we aren't ready to default them fully to () or whatever, because the type-class defaulting rules have yet to run. An important point is that if the type variable tv has kind k and the default is default_k we do not simply generate [D] (k ~ default_k) because: (1) k may be ArgKind and default_k may be * so we will fail (2) We need to rewrite all occurrences of the tv to be a type variable with the right kind and we choose to do this by rewriting the type variable /itself/ by a new variable which does have the right kind. Note [Promote _and_ default when inferring] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we are inferring a type, we simplify the constraint, and then use approximateWC to produce a list of candidate constraints. Then we MUST a) Promote any meta-tyvars that have been floated out by approximateWC, to restore invariant (MetaTvInv) described in Note [Untouchable type variables] in TcType. b) Default the kind of any meta-tyyvars that are not mentioned in in the environment. To see (b), suppose the constraint is (C ((a :: OpenKind) -> Int)), and we have an instance (C ((x:*) -> Int)). The instance doesn't match -- but it should! If we don't solve the constraint, we'll stupidly quantify over (C (a->Int)) and, worse, in doing so zonkQuantifiedTyVar will quantify over (b:*) instead of (a:OpenKind), which can lead to disaster; see Trac #7332.  simonpj@microsoft.com committed Feb 14, 2011 1011 1012 Note [Float Equalities out of Implications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  dimitris committed Jun 08, 2012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 For ordinary pattern matches (including existentials) we float equalities out of implications, for instance: data T where MkT :: Eq a => a -> T f x y = case x of MkT _ -> (y::Int) We get the implication constraint (x::T) (y::alpha): forall a. [untouchable=alpha] Eq a => alpha ~ Int We want to float out the equality into a scope where alpha is no longer untouchable, to solve the implication! But we cannot float equalities out of implications whose givens may yield or contain equalities: data T a where T1 :: T Int T2 :: T Bool T3 :: T a h :: T a -> a -> Int f x y = case x of T1 -> y::Int T2 -> y::Bool T3 -> h x y We generate constraint, for (x::T alpha) and (y :: beta): [untouchables = beta] (alpha ~ Int => beta ~ Int) -- From 1st branch [untouchables = beta] (alpha ~ Bool => beta ~ Bool) -- From 2nd branch (alpha ~ beta) -- From 3rd branch If we float the equality (beta ~ Int) outside of the first implication and the equality (beta ~ Bool) out of the second we get an insoluble constraint. But if we just leave them inside the implications we unify alpha := beta and solve everything. Principle: We do not want to float equalities out which may need the given *evidence* to become soluble. Consequence: classes with functional dependencies don't matter (since there is no evidence for a fundep equality), but equality superclasses do matter (since they carry evidence).  Simon Peyton Jones committed Sep 17, 2012 1056 1057 1058