TcInteract.lhs 73.9 KB
 simonpj@microsoft.com committed Sep 13, 2010 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 TcInteract (  dimitris committed Nov 16, 2011 10 11 12  solveInteractWanted, -- Solves [WantedEvVar] solveInteractGiven, -- Solves [EvVar],GivenLoc solveInteractCts, -- Solves [Cts]  simonpj@microsoft.com committed Sep 13, 2010 13 14 15 16  ) where #include "HsVersions.h"  dimitris@microsoft.com committed Oct 04, 2010 17   dimitris committed Nov 16, 2011 18 import BasicTypes ()  simonpj@microsoft.com committed Sep 13, 2010 19 20 21 import TcCanonical import VarSet import Type  dimitris committed May 17, 2011 22 import Unify  simonpj@microsoft.com committed Sep 13, 2010 23 24 25  import Id import Var  dimitris committed Nov 16, 2011 26 import VarEnv ( ) -- unitVarEnv, mkInScopeSet  simonpj@microsoft.com committed Sep 13, 2010 27 28  import TcType  simonpj@microsoft.com committed Nov 12, 2010 29 import HsBinds  simonpj@microsoft.com committed Sep 13, 2010 30   simonpj@microsoft.com committed Nov 12, 2010 31 32 import Class import TyCon  simonpj@microsoft.com committed Sep 13, 2010 33 import Name  dimitris committed Nov 16, 2011 34 import IParam  simonpj@microsoft.com committed Sep 13, 2010 35 36 37 38 39 40  import FunDeps import Coercion import Outputable  simonpj@microsoft.com committed Nov 12, 2010 41 import TcRnTypes  simonpj@microsoft.com committed Sep 13, 2010 42 import TcErrors  simonpj@microsoft.com committed Nov 12, 2010 43 import TcSMonad  Simon Peyton Jones committed Jun 22, 2011 44 import Maybes( orElse )  simonpj@microsoft.com committed Oct 07, 2010 45 import Bag  dimitris@microsoft.com committed Oct 04, 2010 46   dimitris committed Nov 16, 2011 47 48 49 import Control.Monad ( foldM ) import TrieMap  dimitris committed Nov 28, 2011 50 51 52 import VarEnv import qualified Data.Traversable as Traversable  simonpj@microsoft.com committed Jan 12, 2011 53 import Control.Monad( when )  dimitris committed Nov 28, 2011 54 import Pair ( pSnd )  Simon Peyton Jones committed Jul 29, 2011 55 import UniqFM  simonpj@microsoft.com committed Sep 13, 2010 56 57 58 import FastString ( sLit ) import DynFlags \end{code}  dimitris committed Nov 16, 2011 59 60 ********************************************************************** * *  simonpj@microsoft.com committed Sep 13, 2010 61 62 63 64 * Main Interaction Solver * * * **********************************************************************  dimitris committed Nov 16, 2011 65 66 Note [Basic Simplifier Plan] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~  67   dimitris committed Nov 16, 2011 68 69 1. Pick an element from the WorkList if there exists one with depth less thanour context-stack depth.  simonpj@microsoft.com committed Sep 13, 2010 70   dimitris committed Nov 16, 2011 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 2. Run it down the 'stage' pipeline. Stages are: - canonicalization - inert reactions - spontaneous reactions - top-level intreactions Each stage returns a StopOrContinue and may have sideffected the inerts or worklist. The threading of the stages is as follows: - If (Stop) is returned by a stage then we start again from Step 1. - If (ContinueWith ct) is returned by a stage, we feed 'ct' on to the next stage in the pipeline. 4. If the element has survived (i.e. ContinueWith x) the last stage then we add him in the inerts and jump back to Step 1. If in Step 1 no such element exists, we have exceeded our context-stack depth and will simply fail.  simonpj@microsoft.com committed Sep 13, 2010 88 89 \begin{code}  dimitris committed Nov 16, 2011 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 solveInteractCts :: [Ct] -> TcS () solveInteractCts cts = do { evvar_cache <- getTcSEvVarCacheMap ; (cts_thinner, new_evvar_cache) <- add_cts_in_cache evvar_cache cts ; traceTcS "solveInteractCts" (vcat [ text "cts_original =" <+> ppr cts, text "cts_thinner =" <+> ppr cts_thinner ]) ; setTcSEvVarCacheMap new_evvar_cache ; updWorkListTcS (appendWorkListCt cts_thinner) >> solveInteract } where add_cts_in_cache evvar_cache = foldM solve_or_cache ([],evvar_cache) solve_or_cache :: ([Ct],TypeMap (EvVar,CtFlavor)) -> Ct -> TcS ([Ct],TypeMap (EvVar,CtFlavor)) solve_or_cache (acc_cts,acc_cache) ct | isIPPred pty = return (ct:acc_cts,acc_cache) -- Do not use the cache, -- nor update it for IPPreds due to subtle shadowing | Just (ev',fl') <- lookupTM pty acc_cache , fl' canSolve fl , isWanted fl  dimitris committed Nov 28, 2011 111  = do { _ <- setEvBind ev (EvId ev') fl  dimitris committed Nov 16, 2011 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  ; return (acc_cts,acc_cache) } | otherwise -- If it's a given keep it in the work list, even if it exists in the cache! = return (ct:acc_cts, alterTM pty (\_ -> Just (ev,fl)) acc_cache) where fl = cc_flavor ct ev = cc_id ct pty = evVarPred ev solveInteractGiven :: GivenLoc -> [EvVar] -> TcS () solveInteractGiven gloc evs = solveInteractCts (map mk_noncan evs) where mk_noncan ev = CNonCanonical { cc_id = ev , cc_flavor = Given gloc GivenOrig , cc_depth = 0 } solveInteractWanted :: [WantedEvVar] -> TcS () -- Solve these wanteds along with current inerts and wanteds! solveInteractWanted wevs = solveInteractCts (map mk_noncan wevs) where mk_noncan (EvVarX v w) = CNonCanonical { cc_id = v, cc_flavor = Wanted w, cc_depth = 0 } -- The main solver loop implements Note [Basic Simplifier Plan] --------------------------------------------------------------- solveInteract :: TcS () -- Returns the final InertSet in TcS, WorkList will be eventually empty. solveInteract  dimitris committed Nov 28, 2011 140 141  = {-# SCC "solveInteract" #-} do { dyn_flags <- getDynFlags  dimitris committed Nov 16, 2011 142 143  ; let max_depth = ctxtStkDepth dyn_flags solve_loop  dimitris committed Nov 28, 2011 144 145  = {-# SCC "solve_loop" #-} do { sel <- selectNextWorkItem max_depth  dimitris committed Nov 16, 2011 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170  ; case sel of NoWorkRemaining -- Done, successfuly (modulo frozen) -> return () MaxDepthExceeded ct -- Failure, depth exceeded -> solverDepthErrorTcS (cc_depth ct) [ct] NextWorkItem ct -- More work, loop around! -> runSolverPipeline thePipeline ct >> solve_loop } ; solve_loop } type WorkItem = Ct type SimplifierStage = WorkItem -> TcS StopOrContinue continueWith :: WorkItem -> TcS StopOrContinue continueWith work_item = return (ContinueWith work_item) data SelectWorkItem = NoWorkRemaining -- No more work left (effectively we're done!) | MaxDepthExceeded Ct -- More work left to do but this constraint has exceeded -- the max subgoal depth and we must stop | NextWorkItem Ct -- More work left, here's the next item to look at selectNextWorkItem :: SubGoalDepth -- Max depth allowed -> TcS SelectWorkItem selectNextWorkItem max_depth = updWorkListTcS_return pick_next  simonpj@microsoft.com committed Sep 13, 2010 171  where  dimitris committed Nov 16, 2011 172  pick_next :: WorkList -> (SelectWorkItem, WorkList)  dimitris committed Nov 28, 2011 173 174 175 176 177 178 179 180  pick_next wl = case selectWorkItem wl of (Nothing,_) -> (NoWorkRemaining,wl) -- No more work (Just ct, new_wl) | cc_depth ct > max_depth -- Depth exceeded -> (MaxDepthExceeded ct,new_wl) (Just ct, new_wl) -> (NextWorkItem ct, new_wl) -- New workitem and worklist  dimitris committed Nov 16, 2011 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213  runSolverPipeline :: [(String,SimplifierStage)] -- The pipeline -> WorkItem -- The work item -> TcS () -- Run this item down the pipeline, leaving behind new work and inerts runSolverPipeline pipeline workItem = do { initial_is <- getTcSInerts ; traceTcS "Start solver pipeline {" $vcat [ ptext (sLit "work item = ") <+> ppr workItem , ptext (sLit "inerts = ") <+> ppr initial_is] ; final_res <- run_pipeline pipeline (ContinueWith workItem) ; final_is <- getTcSInerts ; case final_res of Stop -> do { traceTcS "End solver pipeline (discharged) }" (ptext (sLit "inerts = ") <+> ppr final_is) ; return () } ContinueWith ct -> do { traceTcS "End solver pipeline (not discharged) }"$ vcat [ ptext (sLit "final_item = ") <+> ppr ct , ptext (sLit "inerts = ") <+> ppr final_is] ; updInertSetTcS ct } } where run_pipeline :: [(String,SimplifierStage)] -> StopOrContinue -> TcS StopOrContinue run_pipeline [] res = return res run_pipeline _ Stop = return Stop run_pipeline ((stg_name,stg):stgs) (ContinueWith ct) = do { traceTcS ("runStage " ++ stg_name ++ " {") (text "workitem = " <+> ppr ct) ; res <- stg ct ; traceTcS ("end stage " ++ stg_name ++ " }") empty ; run_pipeline stgs res }  simonpj@microsoft.com committed Sep 13, 2010 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 \end{code} Example 1: Inert: {c ~ d, F a ~ t, b ~ Int, a ~ ty} (all given) Reagent: a ~ [b] (given) React with (c~d) ==> IR (ContinueWith (a~[b])) True [] React with (F a ~ t) ==> IR (ContinueWith (a~[b])) False [F [b] ~ t] React with (b ~ Int) ==> IR (ContinueWith (a~[Int]) True [] Example 2: Inert: {c ~w d, F a ~g t, b ~w Int, a ~w ty} Reagent: a ~w [b] React with (c ~w d) ==> IR (ContinueWith (a~[b])) True [] React with (F a ~g t) ==> IR (ContinueWith (a~[b])) True [] (can't rewrite given with wanted!) etc. Example 3: Inert: {a ~ Int, F Int ~ b} (given) Reagent: F a ~ b (wanted) React with (a ~ Int) ==> IR (ContinueWith (F Int ~ b)) True [] React with (F Int ~ b) ==> IR Stop True [] -- after substituting we re-canonicalize and get nothing \begin{code}  dimitris committed Nov 16, 2011 240 241 242 243 244 thePipeline :: [(String,SimplifierStage)] thePipeline = [ ("canonicalization", canonicalizationStage) , ("spontaneous solve", spontaneousSolveStage) , ("interact with inerts", interactWithInertsStage) , ("top-level reactions", topReactionsStage) ]  dimitris@microsoft.com committed Dec 09, 2010 245 246 247 248 \end{code} \begin{code}  simonpj@microsoft.com committed Sep 13, 2010 249   dimitris committed Nov 16, 2011 250 251 252 253 -- The canonicalization stage, see TcCanonical for details ---------------------------------------------------------- canonicalizationStage :: SimplifierStage canonicalizationStage = TcCanonical.canonicalize  simonpj@microsoft.com committed Oct 20, 2010 254   simonpj@microsoft.com committed Sep 13, 2010 255 256 257 258 259 260 261 262 \end{code} ********************************************************************************* * * The spontaneous-solve Stage * * *********************************************************************************  263 264 265 266 267 268 Note [Efficient Orientation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There are two cases where we have to be careful about orienting equalities to get better efficiency.  simonpj@microsoft.com committed Nov 12, 2010 269 Case 1: In Rewriting Equalities (function rewriteEqLHS)  270   271 272 273 274 275 276 277 278 279 280  When rewriting two equalities with the same LHS: (a) (tv ~ xi1) (b) (tv ~ xi2) We have a choice of producing work (xi1 ~ xi2) (up-to the canonicalization invariants) However, to prevent the inert items from getting kicked out of the inerts first, we prefer to canonicalize (xi1 ~ xi2) if (b) comes from the inert set, or (xi2 ~ xi1) if (a) comes from the inert set. This choice is implemented using the WhichComesFromInert flag.  281   simonpj@microsoft.com committed Nov 12, 2010 282 283 284 285 286 Case 2: Functional Dependencies Again, we should prefer, if possible, the inert variables on the RHS Case 3: IP improvement work We must always rewrite so that the inert type is on the right.  287   simonpj@microsoft.com committed Sep 13, 2010 288 289 \begin{code} spontaneousSolveStage :: SimplifierStage  dimitris committed Nov 16, 2011 290 spontaneousSolveStage workItem  simonpj@microsoft.com committed Nov 12, 2010 291  = do { mSolve <- trySpontaneousSolve workItem  dimitris committed Nov 16, 2011 292  ; spont_solve mSolve }  dimitris committed Nov 28, 2011 293 294 295 296 297 298 299  where spont_solve SPCantSolve | isCTyEqCan workItem -- Unsolved equality = do { kickOutRewritableInerts workItem -- NB: will add workItem in inerts ; return Stop } | otherwise = continueWith workItem spont_solve (SPSolved workItem') -- Post: workItem' must be equality  dimitris committed Nov 16, 2011 300 301 302 303  = do { bumpStepCountTcS ; traceFireTcS (cc_depth workItem) $ptext (sLit "Spontaneous") <+> parens (ppr (cc_flavor workItem)) <+> ppr workItem  dimitris committed Nov 28, 2011 304 305 306 307  -- NB: will add the item in the inerts ; kickOutRewritableInerts workItem' -- .. and Stop  dimitris committed Nov 16, 2011 308 309 310 311  ; return Stop } kickOutRewritableInerts :: Ct -> TcS () -- Pre: ct is a CTyEqCan  dimitris committed Nov 28, 2011 312 313 314 -- Post: The TcS monad is left with the thinner non-rewritable inerts; but which -- contains the new constraint. -- The rewritable end up in the worklist  dimitris committed Nov 28, 2011 315 kickOutRewritableInerts ct  dimitris committed Nov 28, 2011 316 317 318 319 320 321 322 323 324 325 326 327 328 329  = {-# SCC "kickOutRewritableInerts" #-} do { (wl,ieqs) <- {-# SCC "kick_out_rewritable" #-} modifyInertTcS (kick_out_rewritable ct) -- Step 1: Rewrite as many of the inert_eqs on the spot! -- NB: if it is a solved constraint just use the cached evidence ; let ct_coercion | Just (GivenSolved (Just (EvCoercionBox co))) <- isGiven_maybe (cc_flavor ct) = co | otherwise = mkEqVarLCo (cc_id ct) ; new_ieqs <- {-# SCC "rewriteInertEqsFromInertEq" #-} rewriteInertEqsFromInertEq (cc_tyvar ct,ct_coercion, cc_flavor ct) ieqs  dimitris committed Nov 28, 2011 330 331  ; modifyInertTcS (\is -> ((), is { inert_eqs = new_ieqs }))  dimitris committed Nov 28, 2011 332 333  -- Step 2: Add the new guy in ; updInertSetTcS ct  dimitris committed Nov 16, 2011 334 335 336  ; traceTcS "Kick out" (ppr ct$$ppr wl) ; updWorkListTcS (unionWorkList wl) }  dimitris committed Nov 28, 2011 337   dimitris committed Nov 28, 2011 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 rewriteInertEqsFromInertEq :: (TcTyVar,Coercion, CtFlavor) -- A new substitution -> TyVarEnv (Ct,Coercion) -- All inert equalities -> TcS (TyVarEnv (Ct,Coercion)) -- The new inert equalities rewriteInertEqsFromInertEq (subst_tv,subst_co, subst_fl) ieqs -- The goal: traverse the inert equalities: -- 1) If current inert element cannot itself rewrite subst_fl then: -- a) if it is rewritable by subst fl throw him out -- b) if it is not rewritable by subst keep him in as is -- 2) otherwise -- a) if it is rewritable by subst fl rewrite him on the spot -- b) if it is not rewritable by subst fl then keep him as is = do { mieqs <- Traversable.mapM do_one ieqs -- mieqs :: TyVarEnv (Maybe (Ct,Coercion)) ; case Traversable.sequence mieqs of Nothing -> return emptyVarEnv Just final_ieqs -> return final_ieqs } where do_one (ct,inert_co) | (not (subst_fl canRewrite fl)) || isReflCo co -- If the inert is not rewritable we just keep it = return (Just (ct,inert_co)) -- Inert is definitely rewritable | not (fl canRewrite subst_fl) -- But the inert cannot itself rewrite the subst item -- so there is need for recanonicalization. = do { updWorkListTcS (extendWorkListEq ct) ; return Nothing } | otherwise -- Or the inert can rewrite subst as well, so it's safe to rewrite on-the-spot = do { let rhs' = pSnd (liftedCoercionKind co) ; delCachedEvVar ev fl ; evc <- newEqVar fl (mkTyVarTy tv) rhs' ; let ev' = evc_the_evvar evc ; let evco' = mkEqVarLCo ev' ; fl' <- if isNewEvVar evc then do { case fl of Wanted {} -> setEqBind ev (evco' mkTransCo mkSymCo co) fl Given {} -> setEqBind ev' (mkEqVarLCo ev mkTransCo co) fl Derived {} -> return fl }  dimitris committed Nov 28, 2011 377  else  dimitris committed Nov 28, 2011 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396  if (isWanted fl) then setEqBind ev (evco' mkTransCo mkSymCo co) fl else return fl ; let ct' = ct { cc_id = ev', cc_flavor = fl', cc_rhs = rhs' } ; return (Just (ct',evco')) } where ev = cc_id ct fl = cc_flavor ct tv = cc_tyvar ct rhs = cc_rhs ct co = liftCoSubstWith [subst_tv] [subst_co] rhs -- (eqs_out, eqs_in) = partitionEqMap -- (\inert_ct -> (not (cc_flavor inert_ct canRewrite fl)) && -- rewritable inert_ct) eqmap -- -- Why not just (rewritable_inert ct)? Check out Note [Delicate equality kick-out]  dimitris committed Nov 28, 2011 397   dimitris committed Nov 28, 2011 398 399 400 401 402 403 404 405 406 407 408 409 410  -- {- -- traceTcS "rewriteInertEqsFromInertEq"$ -- vcat [ text "rewriting equality: " <+> ppr ct -- , text "from: " <+> ppr subst_co <+> text "of flavor: " <+> ppr subst_fl -- , text "can rewrite? " <+> ppr (subst_fl canRewrite fl) ] -- -} kick_out_rewritable :: Ct -> InertSet -> ((WorkList,TyVarEnv (Ct,Coercion)), InertSet) -- Returns ALL equalities, to be dealt with later  dimitris committed Nov 16, 2011 411 412 413 414 415 416 417 418 kick_out_rewritable ct (IS { inert_eqs = eqmap , inert_eq_tvs = inscope , inert_dicts = dictmap , inert_ips = ipmap , inert_funeqs = funeqmap , inert_irreds = irreds , inert_frozen = frozen } )  dimitris committed Nov 28, 2011 419  = ((kicked_out, eqmap), remaining)  dimitris committed Nov 16, 2011 420  where  dimitris committed Nov 28, 2011 421  kicked_out = WorkList { wl_eqs = []  dimitris committed Nov 28, 2011 422 423 424  , wl_funeqs = bagToList feqs_out , wl_rest = bagToList (fro_out andCts dicts_out andCts ips_out andCts irs_out) }  dimitris committed Nov 16, 2011 425   dimitris committed Nov 28, 2011 426  remaining = IS { inert_eqs = emptyVarEnv  dimitris committed Nov 16, 2011 427 428 429 430 431 432 433 434 435 436  , inert_eq_tvs = inscope -- keep the same, safe and cheap , inert_dicts = dicts_in , inert_ips = ips_in , inert_funeqs = feqs_in , inert_irreds = irs_in , inert_frozen = fro_in } fl = cc_flavor ct tv = cc_tyvar ct  dimitris committed Nov 28, 2011 437 438  (ips_out, ips_in) = partitionCCanMap rewritable ipmap  dimitris committed Nov 16, 2011 439   dimitris committed Nov 28, 2011 440 441  (feqs_out, feqs_in) = partitionCtTypeMap rewritable funeqmap (dicts_out, dicts_in) = partitionCCanMap rewritable dictmap  dimitris committed Nov 16, 2011 442 443 444  (irs_out, irs_in) = partitionBag rewritable irreds (fro_out, fro_in) = partitionBag rewritable frozen  dimitris committed Nov 28, 2011 445 446  rewritable ct = (fl canRewrite cc_flavor ct) &&  dimitris committed Nov 16, 2011 447  (tv elemVarSet tyVarsOfCt ct)  dimitris committed Nov 28, 2011 448 \end{code}  dimitris committed Nov 16, 2011 449   dimitris committed Nov 28, 2011 450 451 Note [Delicate equality kick-out] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  dimitris committed Nov 28, 2011 452   dimitris committed Nov 28, 2011 453 454 455 456 457 Delicate: When kicking out rewritable constraints, it would be safe to simply kick out all rewritable equalities, but instead we only kick out those that, when rewritten, may result in occur-check errors. We rewrite the rest on the spot. Example:  dimitris committed Nov 16, 2011 458   dimitris committed Nov 28, 2011 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476  WorkItem = [S] a ~ b Inerts = { [W] b ~ [a] } Now at this point the work item cannot be further rewritten by the inert (due to the weaker inert flavor), so we are examining if we can instead rewrite the inert from the workitem. But if we rewrite it on the spot we have to recanonicalize because of the danger of occurs errors. On the other hand if the inert flavor was just as powerful or more powerful than the workitem flavor, the work-item could not have reached this stage (because it would have already been rewritten by the inert). The coclusion is: we kick out the 'dangerous' equalities that may require recanonicalization (occurs checks) and the rest we rewrite unconditionally without further checks, on-the-spot with function rewriteInertEqsFromInertEq. \begin{code}  dimitris committed Nov 16, 2011 477 478 data SPSolveResult = SPCantSolve | SPSolved WorkItem  dimitris@microsoft.com committed Oct 06, 2010 479   simonpj@microsoft.com committed Nov 12, 2010 480 481 482 -- SPCantSolve means that we can't do the unification because e.g. the variable is untouchable -- SPSolved workItem' gives us a new *given* to go on  simonpj@microsoft.com committed Sep 13, 2010 483 -- @trySpontaneousSolve wi@ solves equalities where one side is a  simonpj@microsoft.com committed Nov 12, 2010 484 -- touchable unification variable.  simonpj@microsoft.com committed Sep 13, 2010 485 -- See Note [Touchables and givens]  simonpj@microsoft.com committed Nov 12, 2010 486 trySpontaneousSolve :: WorkItem -> TcS SPSolveResult  dimitris committed Nov 16, 2011 487 488 trySpontaneousSolve workItem@(CTyEqCan { cc_id = eqv, cc_flavor = gw , cc_tyvar = tv1, cc_rhs = xi, cc_depth = d })  dimitris committed May 17, 2011 489  | isGivenOrSolved gw  simonpj@microsoft.com committed Nov 12, 2010 490  = return SPCantSolve  simonpj@microsoft.com committed Sep 13, 2010 491 492 493 494  | Just tv2 <- tcGetTyVar_maybe xi = do { tch1 <- isTouchableMetaTyVar tv1 ; tch2 <- isTouchableMetaTyVar tv2 ; case (tch1, tch2) of  dimitris committed Nov 16, 2011 495 496 497  (True, True) -> trySpontaneousEqTwoWay d eqv gw tv1 tv2 (True, False) -> trySpontaneousEqOneWay d eqv gw tv1 xi (False, True) -> trySpontaneousEqOneWay d eqv gw tv2 (mkTyVarTy tv1)  simonpj@microsoft.com committed Nov 12, 2010 498  _ -> return SPCantSolve }  simonpj@microsoft.com committed Sep 13, 2010 499 500  | otherwise = do { tch1 <- isTouchableMetaTyVar tv1  dimitris committed Nov 16, 2011 501 502 503  ; if tch1 then trySpontaneousEqOneWay d eqv gw tv1 xi else do { traceTcS "Untouchable LHS, can't spontaneously solve workitem:" $ppr workItem  simonpj@microsoft.com committed Nov 12, 2010 504  ; return SPCantSolve }  505  }  simonpj@microsoft.com committed Sep 13, 2010 506 507 508 509  -- No need for -- trySpontaneousSolve (CFunEqCan ...) = ... -- See Note [No touchables as FunEq RHS] in TcSMonad  simonpj@microsoft.com committed Nov 12, 2010 510 trySpontaneousSolve _ = return SPCantSolve  simonpj@microsoft.com committed Sep 13, 2010 511 512  ----------------  dimitris committed Nov 16, 2011 513 514 trySpontaneousEqOneWay :: SubGoalDepth -> EqVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult  515 -- tv is a MetaTyVar, not untouchable  dimitris committed Nov 16, 2011 516 trySpontaneousEqOneWay d eqv gw tv xi  517  | not (isSigTyVar tv) || isTyVarTy xi  simonpj@microsoft.com committed Nov 12, 2010 518 519  = do { let kxi = typeKind xi -- NB: 'xi' is fully rewritten according to the inerts -- so we have its more specific kind in our hands  dreixel committed Nov 11, 2011 520 521  ; is_sub_kind <- kxi isSubKindTcS tyVarKind tv ; if is_sub_kind then  dimitris committed Nov 16, 2011 522  solveWithIdentity d eqv gw tv xi  simonpj@microsoft.com committed Jan 12, 2011 523  else return SPCantSolve  524  }  525  | otherwise -- Still can't solve, sig tyvar and non-variable rhs  simonpj@microsoft.com committed Nov 12, 2010 526  = return SPCantSolve  simonpj@microsoft.com committed Sep 13, 2010 527 528  ----------------  dimitris committed Nov 16, 2011 529 530 trySpontaneousEqTwoWay :: SubGoalDepth -> EqVar -> CtFlavor -> TcTyVar -> TcTyVar -> TcS SPSolveResult  531 -- Both tyvars are *touchable* MetaTyvars so there is only a chance for kind error here  dimitris committed Nov 16, 2011 532 533  trySpontaneousEqTwoWay d eqv gw tv1 tv2  dreixel committed Nov 11, 2011 534 535  = do { k1_sub_k2 <- k1 isSubKindTcS k2 ; if k1_sub_k2 && nicer_to_update_tv2  dimitris committed Nov 16, 2011 536  then solveWithIdentity d eqv gw tv2 (mkTyVarTy tv1)  dreixel committed Nov 11, 2011 537 538 539  else do { k2_sub_k1 <- k2 isSubKindTcS k1 ; MASSERT( k2_sub_k1 ) -- they were unified in TcCanonical  dimitris committed Nov 16, 2011 540  ; solveWithIdentity d eqv gw tv1 (mkTyVarTy tv2) } }  simonpj@microsoft.com committed Sep 13, 2010 541 542 543 544  where k1 = tyVarKind tv1 k2 = tyVarKind tv2 nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)  dimitris committed Nov 28, 2011 545   simonpj@microsoft.com committed Sep 13, 2010 546 547 \end{code}  548 549 550 551 Note [Kind errors] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider the wanted problem: alpha ~ (# Int, Int #)  dreixel committed Nov 21, 2011 552 where alpha :: ArgKind and (# Int, Int #) :: (#). We can't spontaneously solve this constraint,  553 but we should rather reject the program that give rise to it. If 'trySpontaneousEqTwoWay'  simonpj@microsoft.com committed Nov 12, 2010 554 simply returns @CantSolve@ then that wanted constraint is going to propagate all the way and  555 get quantified over in inference mode. That's bad because we do know at this point that the  simonpj@microsoft.com committed Nov 12, 2010 556 constraint is insoluble. Instead, we call 'recKindErrorTcS' here, which will fail later on.  557 558  The same applies in canonicalization code in case of kind errors in the givens.  559   560 However, when we canonicalize givens we only check for compatibility (@compatKind@).  simonpj@microsoft.com committed Nov 12, 2010 561 If there were a kind error in the givens, this means some form of inconsistency or dead code.  562   simonpj@microsoft.com committed Nov 12, 2010 563 564 565 566 567 You may think that when we spontaneously solve wanteds we may have to look through the bindings to determine the right kind of the RHS type. E.g one may be worried that xi is @alpha@ where alpha :: ? and a previous spontaneous solving has set (alpha := f) with (f :: *). But we orient our constraints so that spontaneously solved ones can rewrite all other constraint so this situation can't happen.  568   569 570 Note [Spontaneous solving and kind compatibility] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  simonpj@microsoft.com committed Jan 12, 2011 571 572 573 Note that our canonical constraints insist that *all* equalities (tv ~ xi) or (F xis ~ rhs) require the LHS and the RHS to have *compatible* the same kinds. ("compatible" means one is a subKind of the other.)  574   simonpj@microsoft.com committed Jan 12, 2011 575 576 577 578 579 580 581 582 583 584 585 586 587 588  - It can't be *equal* kinds, because b) wanted constraints don't necessarily have identical kinds eg alpha::? ~ Int b) a solved wanted constraint becomes a given - SPJ thinks that *given* constraints (tv ~ tau) always have that tau has a sub-kind of tv; and when solving wanted constraints in trySpontaneousEqTwoWay we re-orient to achieve this. - Note that the kind invariant is maintained by rewriting. Eg wanted1 rewrites wanted2; if both were compatible kinds before, wanted2 will be afterwards. Similarly givens. Caveat:  589 590 591 592 593 594 595 596 597  - Givens from higher-rank, such as: type family T b :: * -> * -> * type instance T Bool = (->) f :: forall a. ((T a ~ (->)) => ...) -> a -> ... flop = f (...) True Whereas we would be able to apply the type instance, we would not be able to use the given (T Bool ~ (->)) in the body of 'flop'  simonpj@microsoft.com committed Sep 13, 2010 598 599 600 601 602 603 604  Note [Avoid double unifications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The spontaneous solver has to return a given which mentions the unified unification variable *on the left* of the equality. Here is what happens if not: Original wanted: (a ~ alpha), (alpha ~ Int) We spontaneously solve the first wanted, without changing the order!  simonpj@microsoft.com committed Oct 07, 2010 605  given : a ~ alpha [having unified alpha := a]  simonpj@microsoft.com committed Sep 13, 2010 606 607 608 Now the second wanted comes along, but he cannot rewrite the given, so we simply continue. At the end we spontaneously solve that guy, *reunifying* [alpha := Int]  simonpj@microsoft.com committed Nov 12, 2010 609 We avoid this problem by orienting the resulting given so that the unification  simonpj@microsoft.com committed Oct 07, 2010 610 611 variable is on the left. [Note that alternatively we could attempt to enforce this at canonicalization]  simonpj@microsoft.com committed Sep 13, 2010 612   simonpj@microsoft.com committed Oct 07, 2010 613 614 615 See also Note [No touchables as FunEq RHS] in TcSMonad; avoiding double unifications is the main reason we disallow touchable unification variables as RHS of type family equations: F xis ~ alpha.  simonpj@microsoft.com committed Sep 13, 2010 616 617 618  \begin{code} ----------------  simonpj@microsoft.com committed Nov 12, 2010 619   dimitris committed Nov 16, 2011 620 621 solveWithIdentity :: SubGoalDepth -> EqVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult  simonpj@microsoft.com committed Sep 13, 2010 622 623 -- Solve with the identity coercion -- Precondition: kind(xi) is a sub-kind of kind(tv)  simonpj@microsoft.com committed Oct 07, 2010 624 625 626 -- Precondition: CtFlavor is Wanted or Derived -- See [New Wanted Superclass Work] to see why solveWithIdentity -- must work for Derived as well as Wanted  simonpj@microsoft.com committed Nov 12, 2010 627 -- Returns: workItem where  628 -- workItem = the new Given constraint  dimitris committed Nov 16, 2011 629 solveWithIdentity d eqv wd tv xi  simonpj@microsoft.com committed Nov 12, 2010 630  = do { traceTcS "Sneaky unification:"$  dimitris committed Nov 28, 2011 631  vcat [text "Coercion variable: " <+> ppr eqv <+> ppr wd,  dimitris@microsoft.com committed Oct 04, 2010 632 633 634  text "Coercion: " <+> pprEq (mkTyVarTy tv) xi, text "Left Kind is : " <+> ppr (typeKind (mkTyVarTy tv)), text "Right Kind is : " <+> ppr (typeKind xi)  dimitris committed Nov 16, 2011 635  ]  636   simonpj@microsoft.com committed Jan 12, 2011 637  ; setWantedTyBind tv xi  638  ; let refl_xi = mkReflCo xi  dimitris committed Nov 16, 2011 639   dimitris committed Nov 28, 2011 640 641  ; let solved_fl = mkSolvedFlavor wd UnkSkol (EvCoercionBox refl_xi) ; (_,eqv_given) <- newGivenEqVar solved_fl (mkTyVarTy tv) xi refl_xi  simonpj@microsoft.com committed Nov 12, 2010 642   dimitris committed Nov 28, 2011 643  ; when (isWanted wd) $do { _ <- setEqBind eqv refl_xi wd; return () }  simonpj@microsoft.com committed Jan 12, 2011 644  -- We don't want to do this for Derived, that's why we use 'when (isWanted wd)'  dimitris committed Nov 16, 2011 645 646 647 648 649  ; return$ SPSolved (CTyEqCan { cc_id = eqv_given , cc_flavor = solved_fl , cc_tyvar = tv, cc_rhs = xi, cc_depth = d }) } \end{code}  simonpj@microsoft.com committed Sep 13, 2010 650 651 652 653 654 655 656  ********************************************************************************* * * The interact-with-inert Stage * * *********************************************************************************  simonpj@microsoft.com committed Feb 21, 2011 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 Note [The Solver Invariant] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ We always add Givens first. So you might think that the solver has the invariant If the work-item is Given, then the inert item must Given But this isn't quite true. Suppose we have, c1: [W] beta ~ [alpha], c2 : [W] blah, c3 :[W] alpha ~ Int After processing the first two, we get c1: [G] beta ~ [alpha], c2 : [W] blah Now, c3 does not interact with the the given c1, so when we spontaneously solve c3, we must re-react it with the inert set. So we can attempt a reaction between inert c2 [W] and work-item c3 [G]. It *is* true that [Solver Invariant] If the work-item is Given, AND there is a reaction then the inert item must Given or, equivalently, If the work-item is Given, and the inert item is Wanted/Derived then there is no reaction  simonpj@microsoft.com committed Sep 13, 2010 682 683 684 \begin{code} -- Interaction result of WorkItem <~> AtomicInert  dimitris committed Nov 16, 2011 685 686 687 688 data InteractResult = IRWorkItemConsumed { ir_fire :: String } | IRInertConsumed { ir_fire :: String } | IRKeepGoing { ir_fire :: String }  simonpj@microsoft.com committed Feb 21, 2011 689   dimitris committed Nov 16, 2011 690 691 irWorkItemConsumed :: String -> TcS InteractResult irWorkItemConsumed str = return (IRWorkItemConsumed str)  simonpj@microsoft.com committed Sep 13, 2010 692   dimitris committed Nov 16, 2011 693 694 irInertConsumed :: String -> TcS InteractResult irInertConsumed str = return (IRInertConsumed str)  simonpj@microsoft.com committed Sep 13, 2010 695   dimitris committed Nov 16, 2011 696 697 698 699 irKeepGoing :: String -> TcS InteractResult irKeepGoing str = return (IRKeepGoing str) -- You can't discard neither workitem or inert, but you must keep -- going. It's possible that new work is waiting in the TcS worklist.  700 701   dimitris committed Nov 16, 2011 702 703 704 705 interactWithInertsStage :: WorkItem -> TcS StopOrContinue -- Precondition: if the workitem is a CTyEqCan then it will not be able to -- react with anything at this stage. interactWithInertsStage wi  simonpj@microsoft.com committed Feb 21, 2011 706  = do { ctxt <- getTcSContext  dimitris committed Nov 16, 2011 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737  ; if simplEqsOnly ctxt then return (ContinueWith wi) else extractRelevantInerts wi >>= foldlBagM interact_next (ContinueWith wi) } where interact_next Stop atomic_inert = updInertSetTcS atomic_inert >> return Stop interact_next (ContinueWith wi) atomic_inert = do { ir <- doInteractWithInert atomic_inert wi ; let mk_msg rule keep_doc = text rule <+> keep_doc <+> vcat [ ptext (sLit "Inert =") <+> ppr atomic_inert , ptext (sLit "Work =") <+> ppr wi ] ; case ir of IRWorkItemConsumed { ir_fire = rule } -> do { bumpStepCountTcS ; traceFireTcS (cc_depth wi) (mk_msg rule (text "WorkItemConsumed")) ; updInertSetTcS atomic_inert ; return Stop } IRInertConsumed { ir_fire = rule } -> do { bumpStepCountTcS ; traceFireTcS (cc_depth atomic_inert) (mk_msg rule (text "InertItemConsumed")) ; return (ContinueWith wi) } IRKeepGoing {} -- Should we do a bumpStepCountTcS? No for now. -> do { updInertSetTcS atomic_inert ; return (ContinueWith wi) } }  simonpj@microsoft.com committed Sep 13, 2010 738 --------------------------------------------  dimitris committed Nov 16, 2011 739 data WhichComesFromInert = LeftComesFromInert | RightComesFromInert  simonpj@microsoft.com committed Sep 13, 2010 740   dimitris committed Nov 16, 2011 741 742 doInteractWithInert :: Ct -> Ct -> TcS InteractResult -- Identical class constraints.  simonpj@microsoft.com committed Jan 12, 2011 743 doInteractWithInert  simonpj@microsoft.com committed Feb 21, 2011 744  inertItem@(CDictCan { cc_id = d1, cc_flavor = fl1, cc_class = cls1, cc_tyargs = tys1 })  dimitris committed Nov 16, 2011 745  workItem@(CDictCan { cc_id = _d2, cc_flavor = fl2, cc_class = cls2, cc_tyargs = tys2 })  simonpj@microsoft.com committed Sep 13, 2010 746   dimitris committed Jun 08, 2011 747  | cls1 == cls2  batterseapower committed Sep 06, 2011 748 749  = do { let pty1 = mkClassPred cls1 tys1 pty2 = mkClassPred cls2 tys2  simonpj@microsoft.com committed Nov 12, 2010 750  inert_pred_loc = (pty1, pprFlavorArising fl1)  simonpj@microsoft.com committed Feb 17, 2011 751  work_item_pred_loc = (pty2, pprFlavorArising fl2)  dimitris committed Jun 08, 2011 752   dimitris committed Nov 16, 2011 753 754 755  ; traceTcS "doInteractWithInert" (vcat [ text "inertItem = " <+> ppr inertItem , text "workItem = " <+> ppr workItem ])  dimitris committed Jun 08, 2011 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770  ; any_fundeps <- if isGivenOrSolved fl1 && isGivenOrSolved fl2 then return Nothing -- NB: We don't create fds for given (and even solved), have not seen a useful -- situation for these and even if we did we'd have to be very careful to only -- create Derived's and not Wanteds. else let fd_eqns = improveFromAnother inert_pred_loc work_item_pred_loc wloc = get_workitem_wloc fl2 in rewriteWithFunDeps fd_eqns tys2 wloc -- See Note [Efficient Orientation], [When improvement happens] ; case any_fundeps of -- No Functional Dependencies Nothing | eqTypes tys1 tys2 -> solveOneFromTheOther "Cls/Cls" (EvId d1,fl1) workItem  dimitris committed Nov 16, 2011 771  | otherwise -> irKeepGoing "NOP"  dimitris committed Jun 08, 2011 772 773  -- Actual Functional Dependencies  dimitris committed Nov 16, 2011 774 775  Just (_rewritten_tys2,_cos2,fd_work) -- Standard thing: create derived fds and keep on going. Importantly we don't  dimitris committed Jun 08, 2011 776  -- throw workitem back in the worklist because this can cause loops. See #5236.  dimitris committed Nov 16, 2011 777 778  -> do { emitFDWorkAsDerived fd_work (cc_depth workItem) ; irKeepGoing "Cls/Cls (new fundeps)" } -- Just keep going without droping the inert  dimitris committed Jun 08, 2011 779 780 781 782 783  } where get_workitem_wloc (Wanted wl) = wl get_workitem_wloc (Derived wl) = wl get_workitem_wloc (Given {}) = panic "Unexpected given!"  simonpj@microsoft.com committed Sep 13, 2010 784   batterseapower committed Sep 10, 2011 785 786 787 788 789 790 791 792 -- Two pieces of irreducible evidence: if their types are *exactly identical* we can -- rewrite them. We can never improve using this: if we want ty1 :: Constraint and have -- ty2 :: Constraint it clearly does not mean that (ty1 ~ ty2) doInteractWithInert (CIrredEvCan { cc_id = id1, cc_flavor = ifl, cc_ty = ty1 }) workItem@(CIrredEvCan { cc_ty = ty2 }) | ty1 eqType ty2 = solveOneFromTheOther "Irred/Irred" (EvId id1,ifl) workItem  simonpj@microsoft.com committed Sep 13, 2010 793 794 795 796 797 -- Two implicit parameter constraints. If the names are the same, -- but their types are not, we generate a wanted type equality -- that equates the type (this is "improvement"). -- However, we don't actually need the coercion evidence, -- so we just generate a fresh coercion variable that isn't used anywhere.  simonpj@microsoft.com committed Jan 12, 2011 798 doInteractWithInert (CIPCan { cc_id = id1, cc_flavor = ifl, cc_ip_nm = nm1, cc_ip_ty = ty1 })  simonpj@microsoft.com committed Sep 13, 2010 799  workItem@(CIPCan { cc_flavor = wfl, cc_ip_nm = nm2, cc_ip_ty = ty2 })  dimitris committed May 17, 2011 800  | nm1 == nm2 && isGivenOrSolved wfl && isGivenOrSolved ifl  simonpj@microsoft.com committed Sep 17, 2010 801 802 803  = -- See Note [Overriding implicit parameters] -- Dump the inert item, override totally with the new one -- Do not require type equality  simonpj@microsoft.com committed Feb 17, 2011 804 805  -- For example, given let ?x::Int = 3 in let ?x::Bool = True in ... -- we must *override* the outer one with the inner one  dimitris committed Nov 16, 2011 806  irInertConsumed "IP/IP (override inert)"  simonpj@microsoft.com committed Sep 17, 2010 807   808  | nm1 == nm2 && ty1 eqType ty2  simonpj@microsoft.com committed Feb 21, 2011 809  = solveOneFromTheOther "IP/IP" (EvId id1,ifl) workItem  simonpj@microsoft.com committed Sep 13, 2010 810   simonpj@microsoft.com committed Sep 17, 2010 811  | nm1 == nm2  simonpj@microsoft.com committed Sep 13, 2010 812  = -- See Note [When improvement happens]  dimitris committed Nov 16, 2011 813 814 815 816 817 818 819 820  do { let flav = Wanted (combineCtLoc ifl wfl) ; eqv <- newEqVar flav ty2 ty1 -- See Note [Efficient Orientation] ; when (isNewEvVar eqv) $(let ct = CNonCanonical { cc_id = evc_the_evvar eqv , cc_flavor = flav , cc_depth = cc_depth workItem } in updWorkListTcS (extendWorkListEq ct))  U-EUROPE\dimitris committed Apr 27, 2011 821 822 823 824  ; case wfl of Given {} -> pprPanic "Unexpected given IP" (ppr workItem) Derived {} -> pprPanic "Unexpected derived IP" (ppr workItem) Wanted {} ->  dimitris committed Nov 28, 2011 825 826  do { _ <- setEvBind (cc_id workItem) (mkEvCast id1 (mkSymCo (mkTyConAppCo (ipTyCon nm1) [mkEqVarLCo (evc_the_evvar eqv)]))) wfl  dimitris committed Nov 16, 2011 827  ; irWorkItemConsumed "IP/IP (solved by rewriting)" } }  simonpj@microsoft.com committed Sep 13, 2010 828   batterseapower committed Sep 06, 2011 829 doInteractWithInert (CFunEqCan { cc_id = eqv1, cc_flavor = fl1, cc_fun = tc1  dimitris committed Nov 16, 2011 830 831 832 833  , cc_tyargs = args1, cc_rhs = xi1, cc_depth = d1 }) (CFunEqCan { cc_id = eqv2, cc_flavor = fl2, cc_fun = tc2 , cc_tyargs = args2, cc_rhs = xi2, cc_depth = d2 }) | lhss_match  dimitris committed Nov 28, 2011 834  , Just (GivenSolved {}) <- isGiven_maybe fl1 -- Inert is solved and we can simply ignore it  dimitris committed Nov 16, 2011 835 836 837 838  -- when workitem is given/solved , isGivenOrSolved fl2 = irInertConsumed "FunEq/FunEq" | lhss_match  dimitris committed Nov 28, 2011 839 840  , Just (GivenSolved {}) <- isGiven_maybe fl2 -- Workitem is solved and we can ignore it when -- the inert is given/solved  dimitris committed Nov 16, 2011 841 842  , isGivenOrSolved fl1 = irWorkItemConsumed "FunEq/FunEq"  843  | fl1 canSolve fl2 && lhss_match  dimitris committed Nov 16, 2011 844 845 846  = do { rewriteEqLHS LeftComesFromInert (eqv1,xi1) (eqv2,d2,fl2,xi2) ; irWorkItemConsumed "FunEq/FunEq" }  847  | fl2 canSolve fl1 && lhss_match  dimitris committed Nov 16, 2011 848 849  = do { rewriteEqLHS RightComesFromInert (eqv2,xi2) (eqv1,d1,fl1,xi1) ; irInertConsumed "FunEq/FunEq"}  simonpj@microsoft.com committed Sep 13, 2010 850  where  851  lhss_match = tc1 == tc2 && eqTypes args1 args2  simonpj@microsoft.com committed Sep 13, 2010 852 853   dimitris committed Nov 16, 2011 854 855 856 857 doInteractWithInert _ _ = irKeepGoing "NOP" rewriteEqLHS :: WhichComesFromInert -> (EqVar,Xi) -> (EqVar,SubGoalDepth,CtFlavor,Xi) -> TcS ()  858 -- Used to ineract two equalities of the following form:  simonpj@microsoft.com committed Sep 13, 2010 859 860 -- First Equality: co1: (XXX ~ xi1) -- Second Equality: cv2: (XXX ~ xi2)  simonpj@microsoft.com committed Feb 21, 2011 861 -- Where the cv1 canRewrite cv2 equality  862 863 -- We have an option of creating new work (xi1 ~ xi2) OR (xi2 ~ xi1), -- See Note [Efficient Orientation] for that  dimitris committed Nov 16, 2011 864 rewriteEqLHS LeftComesFromInert (eqv1,xi1) (eqv2,d,gw,xi2)  dimitris committed Nov 28, 2011 865  = do { delCachedEvVar eqv2 gw -- Similarly to canonicalization!  dimitris committed Nov 16, 2011 866 867  ; evc <- newEqVar gw xi2 xi1 ; let eqv2' = evc_the_evvar evc  dimitris committed Nov 28, 2011 868  ; gw' <- case gw of  dimitris committed Nov 16, 2011 869  Wanted {}  dimitris committed Nov 28, 2011 870 871 872 873 874  -> setEqBind eqv2 (mkEqVarLCo eqv1 mkTransCo mkSymCo (mkEqVarLCo eqv2')) gw Given {} -> setEqBind eqv2' (mkSymCo (mkEqVarLCo eqv2) mkTransCo mkEqVarLCo eqv1) gw  dimitris committed Nov 16, 2011 875  Derived {}  dimitris committed Nov 28, 2011 876  -> return gw  dimitris committed Nov 16, 2011 877 878  ; when (isNewEvVar evc)$ updWorkListTcS (extendWorkListEq (CNonCanonical { cc_id = eqv2'  dimitris committed Nov 28, 2011 879  , cc_flavor = gw'  dimitris committed Nov 16, 2011 880 881 882  , cc_depth = d } ) ) } rewriteEqLHS RightComesFromInert (eqv1,xi1) (eqv2,d,gw,xi2)  dimitris committed Nov 28, 2011 883  = do { delCachedEvVar eqv2 gw -- Similarly to canonicalization!  dimitris committed Nov 16, 2011 884 885  ; evc <- newEqVar gw xi1 xi2 ; let eqv2' = evc_the_evvar evc  dimitris committed Nov 28, 2011 886  ; gw' <- case gw of  dimitris committed Nov 16, 2011 887  Wanted {}  dimitris committed Nov 28, 2011 888 889  -> setEqBind eqv2 (mkEqVarLCo eqv1 mkTransCo mkEqVarLCo eqv2') gw  dimitris committed Nov 16, 2011 890  Given {}  dimitris committed Nov 28, 2011 891 892  -> setEqBind eqv2' (mkSymCo (mkEqVarLCo eqv1) mkTransCo mkEqVarLCo eqv2) gw  dimitris committed Nov 16, 2011 893  Derived {}  dimitris committed Nov 28, 2011 894  -> return gw  dimitris committed Nov 16, 2011 895 896 897  ; when (isNewEvVar evc) $updWorkListTcS (extendWorkListEq (CNonCanonical { cc_id = eqv2'  dimitris committed Nov 28, 2011 898  , cc_flavor = gw'  dimitris committed Nov 16, 2011 899 900 901 902 903 904 905 906 907 908  , cc_depth = d } ) ) } solveOneFromTheOther :: String -- Info -> (EvTerm, CtFlavor) -- Inert -> Ct -- WorkItem -> TcS InteractResult -- Preconditions: -- 1) inert and work item represent evidence for the /same/ predicate -- 2) ip/class/irred evidence (no coercions) only solveOneFromTheOther info (ev_term,ifl) workItem  simonpj@microsoft.com committed Feb 17, 2011 909  | isDerived wfl  dimitris committed Nov 16, 2011 910  = irWorkItemConsumed ("Solved[DW] " ++ info)  simonpj@microsoft.com committed Feb 17, 2011 911   simonpj@microsoft.com committed Feb 21, 2011 912 913 914  | isDerived ifl -- The inert item is Derived, we can just throw it away, -- The workItem is inert wrt earlier inert-set items, -- so it's safe to continue on from this point  dimitris committed Nov 16, 2011 915  = irInertConsumed ("Solved[DI] " ++ info)  simonpj@microsoft.com committed Jan 12, 2011 916   dimitris committed Nov 28, 2011 917  | Just (GivenSolved {}) <- isGiven_maybe ifl, isGivenOrSolved wfl  dimitris committed May 17, 2011 918  -- Same if the inert is a GivenSolved -- just get rid of it  dimitris committed Nov 16, 2011 919  = irInertConsumed ("Solved[SI] " ++ info)  dimitris committed May 17, 2011 920   simonpj@microsoft.com committed Feb 21, 2011 921 922 923  | otherwise = ASSERT( ifl canSolve wfl ) -- Because of Note [The Solver Invariant], plus Derived dealt with  dimitris committed Nov 28, 2011 924  do { when (isWanted wfl)$ do { _ <- setEvBind wid ev_term wfl; return () }  simonpj@microsoft.com committed Feb 21, 2011 925 926  -- Overwrite the binding, if one exists -- If both are Given, we already have evidence; no need to duplicate  dimitris committed Nov 16, 2011 927  ; irWorkItemConsumed ("Solved " ++ info) }  simonpj@microsoft.com committed Sep 13, 2010 928 929 930  where wfl = cc_flavor workItem wid = cc_id workItem  dimitris committed Jun 08, 2011 931   simonpj@microsoft.com committed Sep 13, 2010 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 \end{code} Note [Superclasses and recursive dictionaries] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Overlaps with Note [SUPERCLASS-LOOP 1] Note [SUPERCLASS-LOOP 2] Note [Recursive instances and superclases] ToDo: check overlap and delete redundant stuff Right before adding a given into the inert set, we must produce some more work, that will bring the superclasses of the given into scope. The superclass constraints go into our worklist. When we simplify a wanted constraint, if we first see a matching instance, we may produce new wanted work. To (1) avoid doing this work twice in the future and (2) to handle recursive dictionaries we may cache''  simonpj@microsoft.com committed Jan 12, 2011 949 950 951 this item as given into our inert set WITHOUT adding its superclass constraints, otherwise we'd be in danger of creating a loop [In fact this was the exact reason for doing the isGoodRecEv check in an older version of the type checker].  simonpj@microsoft.com committed Sep 13, 2010 952 953 954 955 956 957 958 959 960 961  But now we have added partially solved constraints to the worklist which may interact with other wanteds. Consider the example: Example 1: class Eq b => Foo a b --- 0-th selector instance Eq a => Foo [a] a --- fooDFun and wanted (Foo [t] t). We are first going to see that the instance matches  simonpj@microsoft.com committed Jan 12, 2011 962 and create an inert set that includes the solved (Foo [t] t) but not its superclasses:  simonpj@microsoft.com committed Sep 13, 2010 963 964 965 966  d1 :_g Foo [t] t d1 := EvDFunApp fooDFun d3 Our work list is going to contain a new *wanted* goal d3 :_w Eq t  simonpj@microsoft.com committed Jan 12, 2011 967 Ok, so how do we get recursive dictionaries, at all:  simonpj@microsoft.com committed Sep 13, 2010 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 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 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281  Example 2: data D r = ZeroD | SuccD (r (D r)); instance (Eq (r (D r))) => Eq (D r) where ZeroD == ZeroD = True (SuccD a) == (SuccD b) = a == b _ == _ = False; equalDC :: D [] -> D [] -> Bool; equalDC = (==); We need to prove (Eq (D [])). Here's how we go: d1 :_w Eq (D []) by instance decl, holds if d2 :_w Eq [D []] where d1 = dfEqD d2 *BUT* we have an inert set which gives us (no superclasses): d1 :_g Eq (D []) By the instance declaration of Eq we can show the 'd2' goal if d3 :_w Eq (D []) where d2 = dfEqList d3 d1 = dfEqD d2 Now, however this wanted can interact with our inert d1 to set: d3 := d1 and solve the goal. Why was this interaction OK? Because, if we chase the evidence of d1 ~~> dfEqD d2 ~~-> dfEqList d3, so by setting d3 := d1 we are really setting d3 := dfEqD2 (dfEqList d3) which is FINE because the use of d3 is protected by the instance function applications. So, our strategy is to try to put solved wanted dictionaries into the inert set along with their superclasses (when this is meaningful, i.e. when new wanted goals are generated) but solve a wanted dictionary from a given only in the case where the evidence variable of the wanted is mentioned in the evidence of the given (recursively through the evidence binds) in a protected way: more instance function applications than superclass selectors. Here are some more examples from GHC's previous type checker Example 3: This code arises in the context of "Scrap Your Boilerplate with Class" class Sat a class Data ctx a instance Sat (ctx Char) => Data ctx Char -- dfunData1 instance (Sat (ctx [a]), Data ctx a) => Data ctx [a] -- dfunData2 class Data Maybe a => Foo a instance Foo t => Sat (Maybe t) -- dfunSat instance Data Maybe a => Foo a -- dfunFoo1 instance Foo a => Foo [a] -- dfunFoo2 instance Foo [Char] -- dfunFoo3 Consider generating the superclasses of the instance declaration instance Foo a => Foo [a] So our problem is this d0 :_g Foo t d1 :_w Data Maybe [t] We may add the given in the inert set, along with its superclasses [assuming we don't fail because there is a matching instance, see tryTopReact, given case ] Inert: d0 :_g Foo t WorkList d01 :_g Data Maybe t -- d2 := EvDictSuperClass d0 0 d1 :_w Data Maybe [t] Then d2 can readily enter the inert, and we also do solving of the wanted Inert: d0 :_g Foo t d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3 WorkList d2 :_w Sat (Maybe [t]) d3 :_w Data Maybe t d01 :_g Data Maybe t Now, we may simplify d2 more: Inert: d0 :_g Foo t d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3 d1 :_g Data Maybe [t] d2 :_g Sat (Maybe [t]) d2 := dfunSat d4 WorkList: d3 :_w Data Maybe t d4 :_w Foo [t] d01 :_g Data Maybe t Now, we can just solve d3. Inert d0 :_g Foo t d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3 d2 :_g Sat (Maybe [t]) d2 := dfunSat d4 WorkList d4 :_w Foo [t] d01 :_g Data Maybe t And now we can simplify d4 again, but since it has superclasses we *add* them to the worklist: Inert d0 :_g Foo t d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3 d2 :_g Sat (Maybe [t]) d2 := dfunSat d4 d4 :_g Foo [t] d4 := dfunFoo2 d5 WorkList: d5 :_w Foo t d6 :_g Data Maybe [t] d6 := EvDictSuperClass d4 0 d01 :_g Data Maybe t Now, d5 can be solved! (and its superclass enter scope) Inert d0 :_g Foo t d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3 d2 :_g Sat (Maybe [t]) d2 := dfunSat d4 d4 :_g Foo [t] d4 := dfunFoo2 d5 d5 :_g Foo t d5 := dfunFoo1 d7 WorkList: d7 :_w Data Maybe t d6 :_g Data Maybe [t] d8 :_g Data Maybe t d8 := EvDictSuperClass d5 0 d01 :_g Data Maybe t Now, two problems: [1] Suppose we pick d8 and we react him with d01. Which of the two givens should we keep? Well, we *MUST NOT* drop d01 because d8 contains recursive evidence that must not be used (look at case interactInert where both inert and workitem are givens). So we have several options: - Drop the workitem always (this will drop d8) This feels very unsafe -- what if the work item was the "good" one that should be used later to solve another wanted? - Don't drop anyone: the inert set may contain multiple givens! [This is currently implemented] The "don't drop anyone" seems the most safe thing to do, so now we come to problem 2: [2] We have added both d6 and d01 in the inert set, and we are interacting our wanted d7. Now the [isRecDictEv] function in the ineration solver [case inert-given workitem-wanted] will prevent us from interacting d7 := d8 precisely because chasing the evidence of d8 leads us to an unguarded use of d7. So, no interaction happens there. Then we meet d01 and there is no recursion problem there [isRectDictEv] gives us the OK to interact and we do solve d7 := d01! Note [SUPERCLASS-LOOP 1] ~~~~~~~~~~~~~~~~~~~~~~~~ We have to be very, very careful when generating superclasses, lest we accidentally build a loop. Here's an example: class S a class S a => C a where { opc :: a -> a } class S b => D b where { opd :: b -> b } instance C Int where opc = opd instance D Int where opd = opc From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int} Simplifying, we may well get: $dfCInt = :C ds1 (opd dd) dd =$dfDInt ds1 = $p1 dd Notice that we spot that we can extract ds1 from dd. Alas! Alack! We can do the same for (instance D Int):$dfDInt = :D ds2 (opc dc) dc = $dfCInt ds2 =$p1 dc And now we've defined the superclass in terms of itself. Two more nasty cases are in tcrun021 tcrun033 Solution: - Satisfy the superclass context *all by itself* (tcSimplifySuperClasses) - And do so completely; i.e. no left-over constraints to mix with the constraints arising from method declarations Note [SUPERCLASS-LOOP 2] ~~~~~~~~~~~~~~~~~~~~~~~~ We need to be careful when adding "the constaint we are trying to prove". Suppose we are *given* d1:Ord a, and want to deduce (d2:C [a]) where class Ord a => C a where instance Ord [a] => C [a] where ... Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the superclasses of C [a] to avails. But we must not overwrite the binding for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just build a loop! Here's another variant, immortalised in tcrun020 class Monad m => C1 m class C1 m => C2 m x instance C2 Maybe Bool For the instance decl we need to build (C1 Maybe), and it's no good if we run around and add (C2 Maybe Bool) and its superclasses to the avails before we search for C1 Maybe. Here's another example class Eq b => Foo a b instance Eq a => Foo [a] a If we are reducing (Foo [t] t) we'll first deduce that it holds (via the instance decl). We must not then overwrite the Eq t constraint with a superclass selection! At first I had a gross hack, whereby I simply did not add superclass constraints in addWanted, though I did for addGiven and addIrred. This was sub-optimal, becuase it lost legitimate superclass sharing, and it still didn't do the job: I found a very obscure program (now tcrun021) in which improvement meant the simplifier got two bites a the cherry... so something seemed to be an Stop first time, but reducible next time. Now we implement the Right Solution, which is to check for loops directly when adding superclasses. It's a bit like the occurs check in unification. Note [Recursive instances and superclases] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this code, which arises in the context of "Scrap Your Boilerplate with Class". class Sat a class Data ctx a instance Sat (ctx Char) => Data ctx Char instance (Sat (ctx [a]), Data ctx a) => Data ctx [a] class Data Maybe a => Foo a instance Foo t => Sat (Maybe t) instance Data Maybe a => Foo a instance Foo a => Foo [a] instance Foo [Char] In the instance for Foo [a], when generating evidence for the superclasses (ie in tcSimplifySuperClasses) we need a superclass (Data Maybe [a]). Using the instance for Data, we therefore need (Sat (Maybe [a], Data Maybe a) But we are given (Foo a), and hence its superclass (Data Maybe a). So that leaves (Sat (Maybe [a])). Using the instance for Sat means we need (Foo [a]). And that is the very dictionary we are bulding an instance for! So we must put that in the "givens". So in this case we have Given: Foo a, Foo [a] Wanted: Data Maybe [a] BUT we must *not not not* put the *superclasses* of (Foo [a]) in the givens, which is what 'addGiven' would normally do. Why? Because (Data Maybe [a]) is the superclass, so we'd "satisfy" the wanted by selecting a superclass from Foo [a], which simply makes a loop. On the other hand we *must* put the superclasses of (Foo a) in the givens, as you can see from the derivation described above. Conclusion: in the very special case of tcSimplifySuperClasses we have one 'given' (namely the "this" dictionary) whose superclasses must not be added to 'givens' by addGiven. There is a complication though. Suppose there are equalities instance (Eq a, a~b) => Num (a,b) Then we normalise the 'givens' wrt the equalities, so the original given "this" dictionary is cast to one of a different type. So it's a bit trickier than before to identify the "special" dictionary whose superclasses must not be added. See test indexed-types/should_run/EqInInstance We need a persistent property of the dictionary to record this special-ness. Current I'm using the InstLocOrigin (a bit of a hack, but cool), which is maintained by dictionary normalisation. Specifically, the InstLocOrigin is NoScOrigin then the no-superclass thing kicks in. WATCH OUT if you fiddle with InstLocOrigin! Note [MATCHING-SYNONYMS] ~~~~~~~~~~~~~~~~~~~~~~~~ When trying to match a dictionary (D tau) to a top-level instance, or a type family equation (F taus_1 ~ tau_2) to a top-level family instance, we do *not* need to expand type synonyms because the matcher will do that for us. Note [RHS-FAMILY-SYNONYMS] ~~~~~~~~~~~~~~~~~~~~~~~~~~ The RHS of a family instance is represented as yet another constructor which is like a type synonym for the real RHS the programmer declared. Eg: type instance F (a,a) = [a] Becomes: :R32 a = [a] -- internal type synonym introduced F (a,a) ~ :R32 a -- instance When we react a family instance with a type family equation in the work list we keep the synonym-using RHS without expansion. ********************************************************************************* * * The top-reaction Stage * * ********************************************************************************* \begin{code}  dimitris committed Nov 16, 2011 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304  topReactionsStage :: SimplifierStage topReactionsStage workItem = tryTopReact workItem tryTopReact :: WorkItem -> TcS StopOrContinue tryTopReact wi = do { inerts <- getTcSInerts ; ctxt <- getTcSContext ; if simplEqsOnly ctxt then return (ContinueWith wi) -- or Stop? else do { tir <- doTopReact inerts wi ; case tir of NoTopInt -> return (ContinueWith wi) SomeTopInt rule what_next -> do { bumpStepCountTcS ; traceFireTcS (cc_depth wi) $ptext (sLit "Top react:") <+> text rule ; return what_next } } }  simonpj@microsoft.com committed Sep 13, 2010 1305 data TopInteractResult  dimitris committed Nov 16, 2011 1306 1307  = NoTopInt | SomeTopInt { tir_rule :: String, tir_new_item :: StopOrContinue }  simonpj@microsoft.com committed Sep 13, 2010 1308 1309   dimitris committed May 17, 2011 1310 doTopReact :: InertSet -> WorkItem -> TcS TopInteractResult  dimitris committed Nov 16, 2011 1311 1312 1313 1314 1315 1316 1317  -- The work item does not react with the inert set, so try interaction -- with top-level instances -- NB: The place to add superclasses in *not* -- in doTopReact stage. Instead superclasses are added in the worklist -- as part of the canonicalisation process. See Note [Adding superclasses].  simonpj@microsoft.com committed Oct 20, 2010 1318   simonpj@microsoft.com committed Nov 18, 2010 1319 -- Given dictionary  simonpj@microsoft.com committed Oct 20, 2010 1320 -- See Note [Given constraint that matches an instance declaration]  dimitris committed May 17, 2011 1321 doTopReact _inerts (CDictCan { cc_flavor = Given {} })  simonpj@microsoft.com committed Nov 18, 2010 1322  = return NoTopInt -- NB: Superclasses already added since it's canonical  simonpj@microsoft.com committed Oct 20, 2010 1323   simonpj@microsoft.com committed Nov 18, 2010 1324 -- Derived dictionary: just look for functional dependencies  dimitris committed Jun 08, 2011 1325 doTopReact _inerts workItem@(CDictCan { cc_flavor = Derived loc  dimitris committed May 17, 2011 1326  , cc_class = cls, cc_tyargs = xis })  simonpj@microsoft.com committed Feb 17, 2011 1327 1328  = do { instEnvs <- getInstEnvs ; let fd_eqns = improveFromInstEnv instEnvs  dimitris committed Nov 16, 2011 1329  (mkClassPred cls xis, pprArisingAt loc)  dimitris committed Jun 08, 2011 1330  ; m <- rewriteWithFunDeps fd_eqns xis loc  simonpj@microsoft.com committed Feb 17, 2011 1331 1332 1333 1334 1335  ; case m of Nothing -> return NoTopInt Just (xis',_,fd_work) -> let workItem' = workItem { cc_tyargs = xis' } -- Deriveds are not supposed to have identity (cc_id is unused!)  dimitris committed Nov 16, 2011 1336 1337 1338 1339  in do { emitFDWorkAsDerived fd_work (cc_depth workItem) ; return$ SomeTopInt { tir_rule = "Derived Cls fundeps" , tir_new_item = ContinueWith workItem' } }  dimitris committed Jun 08, 2011 1340 1341  }  simonpj@microsoft.com committed Nov 18, 2010 1342 -- Wanted dictionary  dimitris committed Jun 08, 2011 1343 doTopReact inerts workItem@(CDictCan { cc_flavor = fl@(Wanted loc)  dimitris committed May 17, 2011 1344  , cc_class = cls, cc_tyargs = xis })  dimitris committed Jun 08, 2011 1345 1346 1347  -- See Note [MATCHING-SYNONYMS] = do { traceTcS "doTopReact" (ppr workItem) ; instEnvs <- getInstEnvs  dimitris committed Nov 16, 2011 1348 1349  ; let fd_eqns = improveFromInstEnv instEnvs (mkClassPred cls xis, pprArisingAt loc)  dimitris committed Jun 08, 2011 1350 1351 1352 1353 1354 1355 1356 1357  ; any_fundeps <- rewriteWithFunDeps fd_eqns xis loc ; case any_fundeps of -- No Functional Dependencies Nothing -> do { lkup_inst_res <- matchClassInst inerts cls xis loc ; case lkup_inst_res of GenInst wtvs ev_term  dimitris committed Nov 16, 2011 1358  -> doSolveFromInstance wtvs ev_term workItem  dimitris committed Jun 08, 2011 1359 1360 1361 1362  NoInstance -> return NoTopInt } -- Actual Functional Dependencies  dimitris committed Nov 16, 2011 1363 1364 1365 1366  Just (_xis',_cos,fd_work) -> do { emitFDWorkAsDerived fd_work (cc_depth workItem) ; return SomeTopInt { tir_rule = "Dict/Top (fundeps)" , tir_new_item = ContinueWith workItem } } }  dimitris committed Jun 08, 2011 1367 1368 1369  where doSolveFromInstance :: [WantedEvVar] -> EvTerm  dimitris committed Nov 16, 2011 1370 1371  -> Ct -> TcS TopInteractResult  dimitris committed Jun 08, 2011 1372  -- Precondition: evidence term matches the predicate of cc_id of workItem  dimitris committed Nov 16, 2011 1373  doSolveFromInstance wtvs ev_term workItem  dimitris committed Jun 08, 2011 1374 1375  | null wtvs = do { traceTcS "doTopReact/found nullary instance for" (ppr (cc_id workItem))  dimitris committed Nov 28, 2011 1376  ; _ <- setEvBind (cc_id workItem) ev_term fl  dimitris committed Nov 16, 2011 1377 1378 1379  ; return $SomeTopInt { tir_rule = "Dict/Top (solved, no new work)" , tir_new_item = Stop } } -- Don't put him in the inerts  dimitris committed Jun 08, 2011 1380  | otherwise  dimitris committed Nov 16, 2011 1381 1382  = do { traceTcS "doTopReact/found non-nullary instance for"$ ppr (cc_id workItem)  dimitris committed Nov 28, 2011 1383  ; _ <- setEvBind (cc_id workItem) ev_term fl  simonpj@microsoft.com committed Feb 17, 2011 1384