TcSimplify.lhs 69.1 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  = do { ev_binds_var <- newTcEvBinds ; zonked_wanteds <- zonkWC wanteds  dimitris committed Jul 19, 2012 68  ; wc_first_go <- solveWantedsWithEvBinds ev_binds_var zonked_wanteds  dimitris committed Jun 08, 2012 69 70 71 72 73 74 75 76 77 78 79 80 81  ; 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 {" $text "zonked_wc =" <+> ppr zonked_wanteds ; simpl_top_loop ev_binds_var wc_for_loop } where simpl_top_loop ev_binds_var wc | isEmptyWC wc = do { traceTc "simpl_top_loop }" empty ; TcRnMonad.getTcEvBinds ev_binds_var } | otherwise  dimitris committed Jul 19, 2012 82  = do { wc_residual <- solveWantedsWithEvBinds ev_binds_var wc  dimitris committed Jun 08, 2012 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145  ; let wc_flat_approximate = approximateWC wc_residual ; (dflt_eqs,_unused_bind) <- runTcS$ 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 Jul 19, 2012 201  <- 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  get_good :: Ct -> Either PredType Ct  dimitris committed Jul 19, 2012 206 207 208 209 210 211  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 212  where p = ctPred ct  simonpj@microsoft.com committed Sep 13, 2010 213   Simon Peyton Jones committed Jan 12, 2012 214 215 216  -- 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 217   simonpj@microsoft.com committed Jan 12, 2011 218 219  ; let min_theta = mkMinimalBySCs (bagToList good) ; return (substTheta subst_skol min_theta) }  simonpj@microsoft.com committed Sep 13, 2010 220 \end{code}  simonpj committed May 03, 2001 221   simonpj@microsoft.com committed Feb 17, 2011 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 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 247 248 249 250 251 252 253 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 254   simonpj@microsoft.com committed Sep 13, 2010 255 256 257 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 258   simonpj@microsoft.com committed Sep 13, 2010 259 260 Notice that this instance (just) satisfies the Paterson termination conditions. Then we *could* derive an instance decl like this:  simonpj committed Feb 10, 2004 261   simonpj@microsoft.com committed Sep 13, 2010 262 263 264 265  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 266   simonpj@microsoft.com committed Sep 13, 2010 267 268 269 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 270   simonpj@microsoft.com committed Sep 13, 2010 271 272 So for now we simply require that the derived instance context should have only type-variable constraints.  simonpj committed Feb 10, 2004 273   simonpj@microsoft.com committed Sep 13, 2010 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 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 303   dreixel committed Nov 11, 2011 304 305 306 307 308 309 310 311 312 313 314 315 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 316 \begin{code}  Simon Peyton Jones committed Aug 16, 2011 317 simplifyInfer :: Bool  simonpj@microsoft.com committed Jan 12, 2011 318 319 320  -> Bool -- Apply monomorphism restriction -> [(Name, TcTauType)] -- Variables to be generalised, -- and their tau-types  dimitris committed Jun 08, 2012 321  -> (Untouchables, WantedConstraints)  simonpj@microsoft.com committed Sep 13, 2010 322 323  -> TcM ([TcTyVar], -- Quantify over these type variables [EvVar], -- ... and these constraints  Simon Peyton Jones committed Aug 16, 2011 324 325 326  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 327  TcEvBinds) -- ... binding these evidence variables  dimitris committed Jun 08, 2012 328 simplifyInfer _top_lvl apply_mr name_taus (untch,wanteds)  simonpj@microsoft.com committed Jan 12, 2011 329 330 331  | isEmptyWC wanteds = do { gbl_tvs <- tcGetGlobalTyVars -- Already zonked ; zonked_taus <- zonkTcTypes (map snd name_taus)  Simon Peyton Jones committed Feb 16, 2012 332  ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus minusVarSet gbl_tvs)  dreixel committed Nov 11, 2011 333 334 335  -- 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 336  ; return (qtvs, [], False, emptyTcEvBinds) }  simonpj committed Feb 10, 2004 337   simonpj@microsoft.com committed Sep 13, 2010 338  | otherwise  Simon Peyton Jones committed Jun 21, 2012 339  = do { runtimeCoercionErrors <- doptM Opt_DeferTypeErrors  simonpj@microsoft.com committed Jan 12, 2011 340  ; gbl_tvs <- tcGetGlobalTyVars  Simon Peyton Jones committed Mar 02, 2012 341  ; zonked_tau_tvs <- zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))  Simon Peyton Jones committed Jun 21, 2012 342  ; zonked_wanteds <- zonkWC wanteds  simonpj@microsoft.com committed Jan 12, 2011 343   simonpj@microsoft.com committed Sep 13, 2010 344  ; traceTc "simplifyInfer {"$ vcat  Simon Peyton Jones committed Aug 16, 2011 345  [ ptext (sLit "names =") <+> ppr (map fst name_taus)  Simon Peyton Jones committed Mar 02, 2012 346 347  , ptext (sLit "taus =") <+> ppr (map snd name_taus) , ptext (sLit "tau_tvs (zonked) =") <+> ppr zonked_tau_tvs  Simon Peyton Jones committed Aug 16, 2011 348 349 350  , 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 351  , ptext (sLit "untch =") <+> ppr untch  simonpj@microsoft.com committed Jan 12, 2011 352  , ptext (sLit "wanted =") <+> ppr zonked_wanteds  simonpj@microsoft.com committed Sep 13, 2010 353 354  ]  Simon Peyton Jones committed Jun 21, 2012 355 356 357 358 359  -- 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 360   Simon Peyton Jones committed Jun 21, 2012 361 362 363 364 365 366 367 368  -- 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.  dimitris committed Jun 08, 2012 369  ; ev_binds_var <- newTcEvBinds  dimitris committed Jul 19, 2012 370  ; wanted_transformed <- solveWantedsWithEvBinds ev_binds_var zonked_wanteds  Simon Peyton Jones committed Jun 21, 2012 371 372  -- Step 3) Fail fast if there is an insoluble constraint,  dimitris committed Jun 08, 2012 373 374 375  -- unless we are deferring errors to runtime ; when (not runtimeCoercionErrors && insolubleWC wanted_transformed) $do { _ev_binds <- reportUnsolved False wanted_transformed; failM }  Simon Peyton Jones committed Jun 21, 2012 376 377  -- Step 4) Candidates for quantification are an approximation of wanted_transformed  dimitris committed Jun 08, 2012 378 379 380 381  ; let quant_candidates = approximateWC wanted_transformed -- NB: Already the fixpoint of any unifications that may have happened -- NB: We do not do any defaulting when inferring a type, this can lead -- to less polymorphic types, see Note [Default while Inferring]  dimitris committed Jul 19, 2012 382 383  -- NB: quant_candidates here are wanted or derived, we filter the wanteds later, anyway  Simon Peyton Jones committed Jun 21, 2012 384  -- Step 5) Minimize the quantification candidates  dimitris committed Jun 08, 2012 385  ; (quant_candidates_transformed, _extra_binds)  dimitris committed Jul 19, 2012 386 387 388  <- solveWanteds$ WC { wc_flat = quant_candidates , wc_impl = emptyBag , wc_insol = emptyBag }  Simon Peyton Jones committed Jun 21, 2012 389 390  -- Step 6) Final candidates for quantification  dimitris committed Jun 08, 2012 391 392 393 394 395 396  ; let final_quant_candidates :: Bag PredType final_quant_candidates = mapBag ctPred $keepWanted (wc_flat quant_candidates_transformed) -- NB: Already the fixpoint of any unifications that may have happened ; gbl_tvs <- tcGetGlobalTyVars -- TODO: can we just use untch instead of gbl_tvs?  Simon Peyton Jones committed Mar 02, 2012 397  ; zonked_tau_tvs <- zonkTyVarsAndFV zonked_tau_tvs  dimitris committed Jun 08, 2012 398 399 400 401 402 403  ; traceTc "simplifyWithApprox"$ 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 404  ; let init_tvs = zonked_tau_tvs minusVarSet gbl_tvs  dimitris committed Jun 08, 2012 405 406 407 408 409 410 411  poly_qtvs = growPreds gbl_tvs id final_quant_candidates init_tvs pbound = filterBag (quantifyMe poly_qtvs id) final_quant_candidates ; traceTc "simplifyWithApprox" $vcat [ ptext (sLit "pbound =") <+> ppr pbound ]  Simon Peyton Jones committed Aug 16, 2011 412  -- Monomorphism restriction  dimitris committed Jun 08, 2012 413 414  ; let mr_qtvs = init_tvs minusVarSet constrained_tvs constrained_tvs = tyVarsOfBag tyVarsOfType final_quant_candidates  Simon Peyton Jones committed Aug 16, 2011 415 416  mr_bites = apply_mr && not (isEmptyBag pbound)  dimitris committed Jun 08, 2012 417 418 419 420  (qtvs, bound) | mr_bites = (mr_qtvs, emptyBag) | otherwise = (poly_qtvs, pbound)  simonpj@microsoft.com committed Jan 12, 2011 421   Simon Peyton Jones committed Aug 16, 2011 422  ; if isEmptyVarSet qtvs && isEmptyBag bound  Simon Peyton Jones committed Jul 14, 2012 423 424 425 426  then do { traceTc "} simplifyInfer/no quantification" empty ; emitConstraints wanted_transformed -- Includes insolubles (if -fdefer-type-errors) -- as well as flats and implications  dimitris committed Jun 08, 2012 427  ; return ([], [], mr_bites, TcEvBinds ev_binds_var) }  simonpj@microsoft.com committed Jan 12, 2011 428 429  else do  dimitris committed Jun 08, 2012 430 431 432  { traceTc "simplifyApprox"$ ptext (sLit "bound are =") <+> ppr bound  simonpj@microsoft.com committed Jan 12, 2011 433  -- Step 4, zonk quantified variables  dimitris committed Jun 08, 2012 434  ; let minimal_flat_preds = mkMinimalBySCs $bagToList bound  Simon Peyton Jones committed Aug 16, 2011 435 436  skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty) | (name, ty) <- name_taus ]  simonpj@microsoft.com committed Jan 12, 2011 437 438 439 440  -- Don't add the quantified variables here, because -- they are also bound in ic_skols and we want them to be -- tidied uniformly  Simon Peyton Jones committed Feb 16, 2012 441  ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)  simonpj@microsoft.com committed Jan 12, 2011 442   Simon Peyton Jones committed Jun 21, 2012 443  -- Step 7) Emit an implication  simonpj@microsoft.com committed Jan 12, 2011 444 445  ; minimal_bound_ev_vars <- mapM TcMType.newEvVar minimal_flat_preds ; lcl_env <- getLclTypeEnv  dreixel committed Nov 11, 2011 446  ; gloc <- getCtLoc skol_info  dimitris committed Jun 08, 2012 447  ; let implic = Implic { ic_untch = untch  simonpj@microsoft.com committed Jan 12, 2011 448  , ic_env = lcl_env  Simon Peyton Jones committed Jan 12, 2012 449  , ic_skols = qtvs_to_return  simonpj@microsoft.com committed Jan 12, 2011 450  , ic_given = minimal_bound_ev_vars  dimitris committed Jun 08, 2012 451  , ic_wanted = wanted_transformed  simonpj@microsoft.com committed Jan 12, 2011 452 453 454 455  , ic_insol = False , ic_binds = ev_binds_var , ic_loc = gloc } ; emitImplication implic  dimitris committed Jun 08, 2012 456   simonpj@microsoft.com committed Jan 12, 2011 457 458 459  ; traceTc "} simplifyInfer/produced residual implication for quantification"$ vcat [ ptext (sLit "implic =") <+> ppr implic -- ic_skols, ic_given give rest of result  Simon Peyton Jones committed Aug 16, 2011 460  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return  dimitris committed Jun 08, 2012 461  , ptext (sLit "spb =") <+> ppr final_quant_candidates  simonpj@microsoft.com committed Jan 12, 2011 462 463  , ptext (sLit "bound =") <+> ppr bound ]  Simon Peyton Jones committed Aug 16, 2011 464 465  ; return ( qtvs_to_return, minimal_bound_ev_vars , mr_bites, TcEvBinds ev_binds_var) } }  dimitris committed Jun 08, 2012 466  where  simonpj@microsoft.com committed Jan 12, 2011 467 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 468 469   dimitris committed Jun 08, 2012 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 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 506 507 Note [Minimize by Superclasses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Sep 13, 2010 508   simonpj@microsoft.com committed Jan 12, 2011 509 510 511 512 513 514 515 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 516   dimitris committed Nov 16, 2011 517   dimitris committed Jun 08, 2012 518 \begin{code}  simonpj@microsoft.com committed Jan 12, 2011 519   dimitris committed Nov 16, 2011 520   dimitris committed Jun 08, 2012 521 approximateWC :: WantedConstraints -> Cts  dimitris committed Jul 19, 2012 522 -- Postcondition: Wanted or Derived Cts  dimitris committed Jun 08, 2012 523 approximateWC wc = float_wc emptyVarSet wc  simonpj@microsoft.com committed Sep 13, 2010 524  where  dimitris committed Jun 08, 2012 525 526 527 528 529 530  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 531  float_implic skols imp  dimitris committed Jun 08, 2012 532 533 534 535 536  = float_wc (skols extendVarSetList ic_skols imp) (ic_wanted imp) float_flat :: TcTyVarSet -> Ct -> Cts float_flat skols ct | tyVarsOfCt ct disjointVarSet skols  dimitris committed Jul 19, 2012 537  = singleCt ct  dimitris committed Jun 08, 2012 538 539 540 541  | 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 542 543   simonpj@microsoft.com committed Sep 13, 2010 544 \end{code}  simonpj committed Feb 10, 2004 545   simonpj@microsoft.com committed Sep 13, 2010 546 \begin{code}  simonpj@microsoft.com committed Jan 12, 2011 547 548 549 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 550  where  simonpj@microsoft.com committed Jan 12, 2011 551 552  extend item tvs = tvs unionVarSet (growPredTyVars (get_pred item) tvs minusVarSet gbl_tvs)  simonpj@microsoft.com committed Sep 13, 2010 553 554 555  -------------------- quantifyMe :: TyVarSet -- Quantifying over these  dimitris committed Jun 08, 2012 556 557 558  -> (a -> PredType) -> a -> Bool -- True <=> quantify over this wanted quantifyMe qtvs toPred ct  simonpj@microsoft.com committed Sep 13, 2010 559  | isIPPred pred = True -- Note [Inheriting implicit parameters]  batterseapower committed Sep 06, 2011 560  | otherwise = tyVarsOfType pred intersectsVarSet qtvs  simonpj@microsoft.com committed Sep 13, 2010 561  where  dimitris committed Jun 08, 2012 562  pred = toPred ct  simonpj@microsoft.com committed Sep 13, 2010 563 \end{code}  simonpj committed Jan 25, 2001 564   simonpj@microsoft.com committed Oct 08, 2010 565 566 Note [Avoid unecessary constraint simplification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  Simon Peyton Jones committed Jun 21, 2012 567 568 569 570  -------- 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 571 When inferring the type of a let-binding, with simplifyInfer,  Simon Peyton Jones committed Jun 21, 2012 572 try to avoid unnecessarily simplifying class constraints.  simonpj@microsoft.com committed Oct 08, 2010 573 574 Doing so aids sharing, but it also helps with delicate situations like  dimitris committed Jun 08, 2012 575   simonpj@microsoft.com committed Oct 08, 2010 576  instance C t => C [t] where ..  dimitris committed Jun 08, 2012 577   simonpj@microsoft.com committed Oct 08, 2010 578 579 580 581 582 583 584 585 586 587 588  f :: C [t] => .... f x = let g y = ...(constraint C [t])... in ... When inferring a type for 'g', we don't want to apply the instance decl, because then we can't satisfy (C t). So we just notice that g isn't quantified over 't' and partition the contraints before simplifying. This only half-works, but then let-generalisation only half-works.  simonpj@microsoft.com committed Nov 24, 2006 589 590 Note [Inheriting implicit parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj committed May 03, 2001 591 592 593 Consider this: f x = (x::Int) + ?y  simonpj committed Jan 25, 2001 594   simonpj committed May 03, 2001 595 596 597 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 598   simonpj committed May 03, 2001 599 600 601  f :: Int -> Int (so we get ?y from the context of f's definition), or  simonpj committed Jan 25, 2001 602 603 604  f :: (?y::Int) => Int -> Int  simonpj committed May 03, 2001 605 606 607 608 609 610 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 611 612 BOTTOM LINE: when *inferring types* you *must* quantify over implicit parameters. See the predicate isFreeWhenInferring.  simonpj committed Jun 25, 2001 613   simonpj committed Oct 25, 2001 614   simonpj@microsoft.com committed Sep 13, 2010 615 616 617 618 619 ********************************************************************************* * * * RULES * * * ***********************************************************************************  simonpj committed May 12, 2004 620   Simon Peyton Jones committed Apr 16, 2012 621 See note [Simplifying RULE consraints] in TcRule  simonpj committed Jun 28, 1999 622   Simon Peyton Jones committed Apr 16, 2012 623 624 625 626 627 628 629 630 631 632 633 634 635 636 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 637 638  \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 639 640 641 simplifyRule :: RuleName -> WantedConstraints -- Constraints from LHS -> WantedConstraints -- Constraints from RHS  Simon Peyton Jones committed Apr 16, 2012 642 643 644 645 646  -> 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 647   Simon Peyton Jones committed Apr 16, 2012 648  -- We allow ourselves to unify environment  dimitris committed Jun 08, 2012 649  -- variables: runTcS runs with NoUntouchables  dimitris committed Jul 19, 2012 650  ; (resid_wanted, _) <- solveWanteds zonked_all  Simon Peyton Jones committed Apr 16, 2012 651   simonpj@microsoft.com committed Jan 12, 2011 652 653  ; zonked_lhs <- zonkWC lhs_wanted  Simon Peyton Jones committed Apr 16, 2012 654 655 656 657 658 659 660 661 662 663 664 665 666  ; 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 667  ; traceTc "simplifyRule" $ dimitris committed Jun 08, 2012 668 669  vcat [ doc , text "zonked_lhs" <+> ppr zonked_lhs  Simon Peyton Jones committed Apr 16, 2012 670 671  , text "q_cts" <+> ppr q_cts ]  Simon Peyton Jones committed May 07, 2012 672 673  ; return ( map (ctEvId . ctEvidence) (bagToList q_cts) , zonked_lhs { wc_flat = non_q_cts }) }  simonpj committed Sep 13, 2002 674 675 676 \end{code}  simonpj@microsoft.com committed Sep 13, 2010 677 678 679 680 681 ********************************************************************************* * * * Main Simplifier * * * ***********************************************************************************  lewie committed Mar 02, 2000 682 683  \begin{code}  dimitris committed Jun 08, 2012 684 simplifyCheck :: WantedConstraints -- Wanted  simonpj@microsoft.com committed Sep 13, 2010 685 686 687 688 689 690 691 692 693 694 695 696 697  -> 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 698 simplifyCheck wanteds  simonpj@microsoft.com committed Jan 12, 2011 699  = do { wanteds <- zonkWC wanteds  simonpj@microsoft.com committed Sep 13, 2010 700 701 702 703  ; traceTc "simplifyCheck {" (vcat [ ptext (sLit "wanted =") <+> ppr wanteds ])  dimitris committed Jul 19, 2012 704  ; (unsolved, eb1) <- solveWanteds wanteds  Simon Peyton Jones committed Jan 12, 2012 705 706 707  ; traceTc "simplifyCheck }"$ ptext (sLit "unsolved =") <+> ppr unsolved  dimitris committed Mar 30, 2012 708  ; traceTc "reportUnsolved {" empty  Simon Peyton Jones committed Jan 12, 2012 709 710 711  -- See Note [Deferring coercion errors to runtime] ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors ; eb2 <- reportUnsolved runtimeCoercionErrors unsolved  dimitris committed Mar 30, 2012 712 713  ; traceTc "reportUnsolved }" empty  Simon Peyton Jones committed Jan 12, 2012 714 715 716 717 718 719 720 721 722 723  ; 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 724   Simon Peyton Jones committed Jan 12, 2012 725 726  a :: Int a = 'a'  simonpj@microsoft.com committed Sep 13, 2010 727   Simon Peyton Jones committed Jan 12, 2012 728  main = print "b"  simonpj@microsoft.com committed Sep 13, 2010 729   Simon Peyton Jones committed Jan 12, 2012 730 731 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 732   Simon Peyton Jones committed Jan 12, 2012 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 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}  dimitris committed Jul 19, 2012 756 757 758 759 760 761 762 763 764 765 766 767 768  solveWanteds :: WantedConstraints -> TcM (WantedConstraints, Bag EvBind) -- Return the evidence binds in the BagEvBinds result solveWanteds wanted = runTcS $solve_wanteds wanted solveWantedsWithEvBinds :: EvBindsVar -> WantedConstraints -> TcM WantedConstraints -- Side-effect the EvBindsVar argument to add new bindings from solving solveWantedsWithEvBinds ev_binds_var wanted = runTcSWithEvBinds ev_binds_var$ solve_wanteds wanted solve_wanteds :: WantedConstraints -> TcS WantedConstraints -- NB: wc_flats may be wanted /or/ derived now  dimitris committed Nov 16, 2011 769 solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols })  simonpj@microsoft.com committed Jan 12, 2011 770 771  = do { traceTcS "solveWanteds {" (ppr wanted)  dimitris committed Jul 19, 2012 772 773 774 775  -- Try the flat bit, including insolubles. Solving insolubles a -- second time round is a bit of a waste but the code is simple -- and the program is wrong anyway. -- Why keepWanted insols? See Note [KeepWanted in SolveWanteds]  simonpj@microsoft.com committed Jan 12, 2011 776  ; let all_flats = flats unionBags keepWanted insols  dimitris committed Jul 19, 2012 777  -- DV: Used to be 'keepWanted insols' but just insols is  dimitris committed Jun 08, 2012 778   dimitris committed Apr 10, 2012 779  ; impls_from_flats <- solveInteractCts $bagToList all_flats  simonpj@microsoft.com committed Jan 12, 2011 780   dimitris committed Nov 16, 2011 781 782  -- solve_wanteds iterates when it is able to float equalities -- out of one or more of the implications.  dimitris committed Apr 10, 2012 783  ; unsolved_implics <- simpl_loop 1 (implics unionBags impls_from_flats)  simonpj@microsoft.com committed Jan 12, 2011 784   dimitris committed Jul 19, 2012 785 786 787  ; is <- getTcSInerts ; let insoluble_flats = getInertInsols is unsolved_flats = getInertUnsolved is  dimitris committed Nov 16, 2011 788 789  ; bb <- getTcEvBindsMap  simonpj@microsoft.com committed Nov 12, 2010 790  ; tb <- getTcSTyBindsMap  dimitris committed Nov 16, 2011 791   simonpj@microsoft.com committed Sep 13, 2010 792  ; traceTcS "solveWanteds }"$  simonpj@microsoft.com committed Nov 12, 2010 793  vcat [ text "unsolved_flats =" <+> ppr unsolved_flats  simonpj@microsoft.com committed Jan 12, 2011 794  , text "unsolved_implics =" <+> ppr unsolved_implics  dimitris committed Nov 16, 2011 795  , text "current evbinds =" <+> ppr (evBindMapBinds bb)  simonpj@microsoft.com committed Nov 12, 2010 796 797 798  , text "current tybinds =" <+> vcat (map ppr (varEnvElts tb)) ]  dimitris committed Jul 19, 2012 799 800 801 802  ; let wc = WC { wc_flat = unsolved_flats , wc_impl = unsolved_implics , wc_insol = insoluble_flats }  simonpj@microsoft.com committed Jan 12, 2011 803   Simon Peyton Jones committed Mar 02, 2012 804  ; traceTcS "solveWanteds finished with" $ dimitris committed Jul 19, 2012 805 806 807 808  vcat [ text "wc (unflattened) =" <+> ppr wc ] ; unFlattenWC wc }  Simon Peyton Jones committed Mar 02, 2012 809   dimitris committed Nov 16, 2011 810 811 812 813 814 815 816 817 818  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 819   dimitris committed Jun 08, 2012 820 821 822 823  ; 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 Jul 19, 2012 824 825  ; unsolved_flats <- getTcSInerts >>= (return . getInertUnsolved)  dimitris committed Nov 16, 2011 826 827 828 829  ; traceTcS "solveWanteds: simpl_loop end"$ vcat [ text "improve_eqs =" <+> ppr improve_eqs , text "unsolved_flats =" <+> ppr unsolved_flats , text "unsolved_implics =" <+> ppr unsolved_implics ]  830   dimitris committed Jul 19, 2012 831   dimitris committed Nov 16, 2011 832  ; if isEmptyBag improve_eqs then return unsolved_implics  dimitris committed Apr 10, 2012 833 834 835  else do { impls_from_eqs <- solveInteractCts $bagToList improve_eqs ; simpl_loop (n+1) (unsolved_implics unionBags impls_from_eqs)} }  836   dimitris committed Jul 19, 2012 837   dimitris committed Nov 16, 2011 838 839 840 841 842 843 844 845 846 solveNestedImplications :: Bag Implication -> TcS (Cts, Bag Implication) -- Precondition: the TcS inerts may contain unsolved flats which have -- to be converted to givens before we go inside a nested implication. solveNestedImplications implics | isEmptyBag implics = return (emptyBag, emptyBag) | otherwise = do { inerts <- getTcSInerts  dimitris committed Jul 19, 2012 847 848 849  ; traceTcS "solveNestedImplications starting, inerts are:"$ ppr inerts ; let (pushed_givens, thinner_inerts) = splitInertsForImplications inerts  dimitris committed Jun 08, 2012 850  ; traceTcS "solveNestedImplications starting, more info:" $ dimitris committed Jul 19, 2012 851 852  vcat [ text "original inerts = " <+> ppr inerts , text "pushed_givens = " <+> ppr pushed_givens  dimitris committed Jun 08, 2012 853 854  , text "thinner_inerts = " <+> ppr thinner_inerts ]  dimitris committed Nov 16, 2011 855 856  ; (implic_eqs, unsolved_implics) <- doWithInert thinner_inerts$  dimitris committed Jul 19, 2012 857  do { let tcs_untouchables  dimitris committed Jun 08, 2012 858 859 860 861 862 863  = 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 864 865 866 867  -- See Note [Preparing inert set for implications] -- Push the unsolved wanteds inwards, but as givens ; traceTcS "solveWanteds: preparing inerts for implications {" $vcat [ppr tcs_untouchables, ppr pushed_givens]  dimitris committed Jun 08, 2012 868 869  ; impls_from_givens <- solveInteractCts pushed_givens  dimitris committed Apr 10, 2012 870  ; MASSERT (isEmptyBag impls_from_givens)  dimitris committed Jun 08, 2012 871 872 873 874  -- impls_from_givens must be empty, since we are reacting givens -- with givens, and they can never generate extra implications -- from decomposition of ForAll types. (Whereas wanteds can, see -- TcCanonical, canEq ForAll-ForAll case)  dimitris committed Apr 10, 2012 875   dimitris committed Nov 16, 2011 876 877 878 879 880 881  ; traceTcS "solveWanteds: } now doing nested implications {" empty ; flatMapBagPairM (solveImplication tcs_untouchables) implics } -- ... and we are back in the original TcS inerts -- Notice that the original includes the _insoluble_flats so it was safe to ignore -- them in the beginning of this function.  882 883 884 885 886 887  ; traceTcS "solveWanteds: done nested implications }"$ vcat [ text "implic_eqs =" <+> ppr implic_eqs , text "unsolved_implics =" <+> ppr unsolved_implics ] ; return (implic_eqs, unsolved_implics) }  dimitris committed Nov 16, 2011 888 889 890 891 892 893 894 895 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 896 897 898  , ic_binds = ev_binds , ic_skols = skols , ic_given = givens  simonpj@microsoft.com committed Sep 13, 2010 899  , ic_wanted = wanteds  simonpj@microsoft.com committed Oct 06, 2010 900  , ic_loc = loc })  Simon Peyton Jones committed Jun 13, 2012 901 902  = shadowIPs givens $-- See Note [Shadowing of Implicit Parameters] nestImplicTcS ev_binds (untch, tcs_untouchables)$  simonpj@microsoft.com committed Oct 19, 2010 903 904  recoverTcS (return (emptyBag, emptyBag)) $-- Recover from nested failures. Even the top level is  dimitris committed Nov 16, 2011 905  -- just a bunch of implications, so failing at the first one is bad  simonpj@microsoft.com committed Sep 13, 2010 906 907 908  do { traceTcS "solveImplication {" (ppr imp) -- Solve flat givens  dimitris committed Apr 10, 2012 909 910 911  ; impls_from_givens <- solveInteractGiven loc givens ; MASSERT (isEmptyBag impls_from_givens)  simonpj@microsoft.com committed Sep 13, 2010 912  -- Simplify the wanteds  dimitris committed Jul 19, 2012 913 914 915  ; WC { wc_flat = unsolved_flats , wc_impl = unsolved_implics , wc_insol = insols } <- solve_wanteds wanteds  simonpj@microsoft.com committed Jan 12, 2011 916 917 918  ; let (res_flat_free, res_flat_bound) = floatEqualities skols givens unsolved_flats  simonpj@microsoft.com committed Sep 13, 2010 919   dimitris committed Jul 19, 2012 920 921 922 923  ; let res_wanted = WC { wc_flat = keepWanted$ res_flat_bound -- I think this keepWanted must eventually go away, but it is -- a real code-breaking change. -- See Note [KeepWanted in SolveImplication]  dimitris committed Nov 16, 2011 924  , wc_impl = unsolved_implics  simonpj@microsoft.com committed Jan 12, 2011 925  , wc_insol = insols }  dimitris committed Nov 16, 2011 926   simonpj@microsoft.com committed Jan 12, 2011 927 928 929  res_implic = unitImplication $imp { ic_wanted = res_wanted , ic_insol = insolubleWC res_wanted }  simonpj@microsoft.com committed Sep 13, 2010 930   dimitris committed Nov 16, 2011 931 932  ; evbinds <- getTcEvBindsMap  simonpj@microsoft.com committed Sep 13, 2010 933 934  ; traceTcS "solveImplication end }"$ vcat [ text "res_flat_free =" <+> ppr res_flat_free  dimitris committed Nov 16, 2011 935  , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds)  simonpj@microsoft.com committed Jan 12, 2011 936  , text "res_implic =" <+> ppr res_implic ]  simonpj@microsoft.com committed Sep 13, 2010 937   simonpj@microsoft.com committed Jan 12, 2011 938  ; return (res_flat_free, res_implic) }  dimitris committed Nov 16, 2011 939  -- and we are back to the original inerts  simonpj@microsoft.com committed Sep 13, 2010 940   dimitris committed Jul 19, 2012 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 \end{code} Note [KeepWanted in SolveWanteds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Why do we have: let all_flats = flats unionBags keepWanted insols instead of the simpler: let all_flats = flats unionBags insols in solve_wanteds? Assume a top-level class and instance declaration: class D a b | a -> b instance D [a] [a] Assume we have started with an implication: forall c. Eq c => { wc_flat = D [c] c [W] } which we have simplified to: forall c. Eq c => { wc_flat = D [c] c [W] , wc_insols = (c ~ [c]) [D] } For some reason, e.g. because we floated an equality somewhere else, we might try to re-solve this implication. If we do not do a keepWanted, then we will end up trying to solve the following constraints the second time: (D [c] c) [W] (c ~ [c]) [D] which will result in two Deriveds to end up in the insoluble set: wc_flat = D [c] c [W] wc_insols = (c ~ [c]) [D], (c ~ [c]) [D] which can result in reporting the same error twice. So, do we /lose/ some potentially useful information by doing this? No, because the insoluble Derived/Given are going to be equalities, which are going to be derivable anyway from the rest of the flat constraints. Note [KeepWanted in SolveImplication] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is a real example, stripped off from libraries/utf8-string/Codec/Binary/UTF8/Generic.hs class C a b | a -> b g :: C a b => a -> b -> () f :: C a b => a -> b -> () f xa xb = let loop = g xa in loop xb We will first try to infer a type for loop, and we will succeed: C a b' => b' -> () Subsequently, we will type check (loop xb) and all is good. But, recall that we have to solve a final implication constraint: C a b => (C a b' => .... cts from body of loop .... )) And now we have a problem as we will generate an equality b ~ b' and fail to solve it. I actually think this is a legitimate behaviour (to fail). After all, if we had given the inferred signature to foo we would have failed as well, but we have to find a workaround because library code breaks. For now I keep the 'keepWanted' though it seems problematic e.g. we might discard a useful Derived! \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 1017   Simon Peyton Jones committed Jan 12, 2012 1018 floatEqualities :: [TcTyVar] -> [EvVar] -> Cts -> (Cts, Cts)  simonpj@microsoft.com committed Jan 12, 2011 1019 1020 1021 1022 -- 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 1023  -- Note [Float Equalities out of Implications]  simonpj@microsoft.com committed Jan 12, 2011 1024  | otherwise = partitionBag is_floatable wantders  Simon Peyton Jones committed Jan 12, 2012 1025 1026  where skol_set = mkVarSet skols is_floatable :: Ct -> Bool  dimitris committed Nov 16, 2011 1027  is_floatable ct  dimitris committed Nov 29, 2011 1028  | ct_predty <- ctPred ct  dimitris committed Nov 16, 2011 1029  , isEqPred ct_predty  Simon Peyton Jones committed Jan 12, 2012 1030  = skol_set disjointVarSet tvs_under_fsks ct_predty  dimitris committed Nov 16, 2011 1031  is_floatable _ct = False  simonpj@microsoft.com committed Nov 12, 2010 1032 1033 1034 1035 1036 1037 1038 1039  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 1040  tvs_under_fsks (LitTy {}) = emptyVarSet  simonpj@microsoft.com committed Nov 12, 2010 1041 1042 1043  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 1044  -- can mention type variables!  simonpj@microsoft.com committed Nov 12, 2010 1045 1046 1047 1048 1049  | 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 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068