typecheck/TcSimplify.lhs

\begin{code}

    module TcSimplify(
           simplifyInfer, quantifyPred,
           simplifyAmbiguityCheck,
           simplifyDefault,
           simplifyRule, simplifyTop, simplifyInteractive,
           solveWantedsTcM
      ) where

    #include "HsVersions.h"

    import TcRnTypes
    import TcRnMonad
    import TcErrors
    import TcMType as TcM
    import TcType
    import TcSMonad as TcS
    import TcInteract
    import Kind     ( isKind, defaultKind_maybe )
    import Inst
    import FunDeps  ( growThetaTyVars )
    import Type     ( classifyPredType, PredTree(..), getClassPredTys_maybe )
    import Class    ( Class )
    import Var
    import Unique
    import VarSet
    import VarEnv
    import TcEvidence
    import Name
    import Bag
    import ListSetOps
    import Util
    import PrelInfo
    import PrelNames
    import Control.Monad    ( unless )
    import DynFlags         ( ExtensionFlag( Opt_AllowAmbiguousTypes ) )
    import Class            ( classKey )
    import BasicTypes       ( RuleName )
    import Outputable
    import FastString
    import TrieMap () -- DV: for now

\end{code} ********************************************************************************* * * * External interface * * * ********************************************************************************* \begin{code}

    simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
    -- Simplify top-level constraints
    -- 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
    simplifyTop wanteds
      = do { traceTc "simplifyTop {" $ text "wanted = " <+> ppr wanteds
           ; ev_binds_var <- newTcEvBinds
           ; zonked_final_wc <- solveWantedsTcMWithEvBinds ev_binds_var wanteds simpl_top
           ; binds1 <- TcRnMonad.getTcEvBinds ev_binds_var
           ; traceTc "End simplifyTop }" empty

           ; traceTc "reportUnsolved {" empty
           ; binds2 <- reportUnsolved zonked_final_wc
           ; traceTc "reportUnsolved }" empty

           ; return (binds1 `unionBags` binds2) }

    simpl_top :: WantedConstraints -> TcS WantedConstraints
        -- See Note [Top-level Defaulting Plan]
    simpl_top wanteds
      = do { wc_first_go <- nestTcS (solve_wanteds_and_drop wanteds)
                                -- This is where the main work happens
           ; try_tyvar_defaulting wc_first_go }
      where
        try_tyvar_defaulting :: WantedConstraints -> TcS WantedConstraints
        try_tyvar_defaulting wc
          | isEmptyWC wc
          = return wc
          | otherwise
          = do { free_tvs <- TcS.zonkTyVarsAndFV (tyVarsOfWC wc)
               ; let meta_tvs = varSetElems (filterVarSet isMetaTyVar free_tvs)
                       -- zonkTyVarsAndFV: the wc_first_go is not yet zonked
                       -- filter isMetaTyVar: we might have runtime-skolems in GHCi,
                       -- and we definitely don't want to try to assign to those!

               ; meta_tvs' <- mapM defaultTyVar meta_tvs   -- Has unification side effects
               ; if meta_tvs' == meta_tvs   -- No defaulting took place;
                                            -- (defaulting returns fresh vars)
                 then try_class_defaulting wc
                 else do { wc_residual <- nestTcS (solve_wanteds_and_drop wc)
                                -- See Note [Must simplify after defaulting]
                         ; try_class_defaulting wc_residual } }

        try_class_defaulting :: WantedConstraints -> TcS WantedConstraints
        try_class_defaulting wc
          | isEmptyWC wc || insolubleWC wc
          = return wc  -- Don't do type-class defaulting if there are insolubles
                       -- Doing so is not going to solve the insolubles
          | otherwise
          = do { something_happened <- applyDefaultingRules (approximateWC wc)
                                       -- See Note [Top-level Defaulting Plan]
               ; if something_happened
                 then do { wc_residual <- nestTcS (solve_wanteds_and_drop wc)
                         ; try_class_defaulting wc_residual }
                 else return wc }

\end{code} Note [Must simplify after defaulting] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We may have a deeply buried constraint (t:*) ~ (a:Open) which we couldn't solve because of the kind incompatibility, and 'a' is free. Then when we default 'a' we can solve the constraint. And we want to do that before starting in on type classes. We MUST do it before reporting errors, because it isn't an error! Trac #7967 was due to this. 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 constraints. - Apply defaulting to their kinds More details in Note [DefaultTyVar]. \begin{code}

    ------------------
    simplifyAmbiguityCheck :: Type -> WantedConstraints -> TcM ()
    simplifyAmbiguityCheck ty wanteds
      = do { traceTc "simplifyAmbiguityCheck {" (text "type = " <+> ppr ty $$ text "wanted = " <+> ppr wanteds)
           ; ev_binds_var <- newTcEvBinds
           ; zonked_final_wc <- solveWantedsTcMWithEvBinds ev_binds_var wanteds simpl_top
           ; traceTc "End simplifyAmbiguityCheck }" empty

           -- Normally report all errors; but with -XAllowAmbiguousTypes
           -- report only insoluble ones, since they represent genuinely
           -- inaccessible code
           ; allow_ambiguous <- xoptM Opt_AllowAmbiguousTypes
           ; traceTc "reportUnsolved(ambig) {" empty
           ; unless (allow_ambiguous && not (insolubleWC zonked_final_wc))
                    (discardResult (reportUnsolved zonked_final_wc))
           ; traceTc "reportUnsolved(ambig) }" empty

           ; return () }

    ------------------
    simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
    simplifyInteractive wanteds
      = traceTc "simplifyInteractive" empty >>
        simplifyTop wanteds

    ------------------
    simplifyDefault :: ThetaType    -- Wanted; has no type variables in it
                    -> TcM ()       -- Succeeds iff the constraint is soluble
    simplifyDefault theta
      = do { traceTc "simplifyInteractive" empty
           ; wanted <- newFlatWanteds DefaultOrigin theta
           ; (unsolved, _binds) <- solveWantedsTcM (mkFlatWC wanted)

           ; traceTc "reportUnsolved {" empty
           -- See Note [Deferring coercion errors to runtime]
           ; reportAllUnsolved unsolved
             -- Postcondition of solveWantedsTcM is that returned
             -- constraints are zonked. So Precondition of reportUnsolved
             -- is true.
           ; traceTc "reportUnsolved }" empty

           ; return () }

\end{code} ********************************************************************************* * * * Inference * * *********************************************************************************** \begin{code}

    simplifyInfer :: Bool
                  -> Bool                  -- Apply monomorphism restriction
                  -> [(Name, TcTauType)]   -- Variables to be generalised,
                                           -- and their tau-types
                  -> WantedConstraints
                  -> TcM ([TcTyVar],    -- Quantify over these type variables
                          [EvVar],      -- ... and these constraints
                          Bool,         -- The monomorphism restriction did something
                                        --   so the results type is not as general as
                                        --   it could be
                          TcEvBinds)    -- ... binding these evidence variables
    simplifyInfer _top_lvl apply_mr name_taus wanteds
      | isEmptyWC wanteds
      = do { gbl_tvs <- tcGetGlobalTyVars
           ; qtkvs <- quantifyTyVars gbl_tvs (tyVarsOfTypes (map snd name_taus))
           ; traceTc "simplifyInfer: empty WC" (ppr name_taus $$ ppr qtkvs)
           ; return (qtkvs, [], False, emptyTcEvBinds) }

      | otherwise
      = do { traceTc "simplifyInfer {"  $ vcat
                 [ ptext (sLit "binds =") <+> ppr name_taus
                 , ptext (sLit "closed =") <+> ppr _top_lvl
                 , ptext (sLit "apply_mr =") <+> ppr apply_mr
                 , ptext (sLit "(unzonked) wanted =") <+> ppr wanteds
                 ]

                  -- 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!

                  -- 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.

           ; ev_binds_var <- newTcEvBinds
           ; wanted_transformed_incl_derivs
                   <- solveWantedsTcMWithEvBinds ev_binds_var wanteds solve_wanteds
                      -- Post: wanted_transformed_incl_derivs are zonked

                  -- Step 4) Candidates for quantification are an approximation of 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]

           ; tc_lcl_env <- TcRnMonad.getLclEnv
           ; let untch = tcl_untch tc_lcl_env
                 wanted_transformed = dropDerivedWC wanted_transformed_incl_derivs
           ; quant_pred_candidates   -- Fully zonked
               <- if insolubleWC wanted_transformed_incl_derivs
                  then return []   -- See Note [Quantification with errors]
                                   -- NB: must include derived errors in this test,
                                   --     hence "incl_derivs"

                  else do { let quant_cand = approximateWC wanted_transformed
                                meta_tvs   = filter isMetaTyVar (varSetElems (tyVarsOfCts quant_cand))
                          ; gbl_tvs <- tcGetGlobalTyVars
                          ; null_ev_binds_var <- newTcEvBinds
                                -- Miminise quant_cand.  We are not interested in any evidence
                                -- produced, because we are going to simplify wanted_transformed
                                -- again later. All we want here is the predicates over which to
                                -- quantify.
                                --
                                -- If any meta-tyvar unifications take place (unlikely), we'll
                                -- pick that up later.

                          ; (flats, _insols) <- runTcSWithEvBinds null_ev_binds_var $
                            do { mapM_ (promoteAndDefaultTyVar untch gbl_tvs) meta_tvs
                                     -- See Note [Promote _and_ default when inferring]
                               ; _implics <- solveInteract quant_cand
                               ; getInertUnsolved }

                          ; flats' <- zonkFlats null_ev_binds_var untch $
                                      filterBag isWantedCt flats
                               -- The quant_cand were already fully zonked, so this zonkFlats
                               -- really only unflattens the flattening that solveInteract
                               -- may have done (Trac #8889).
                               -- E.g. quant_cand = F a, where F :: * -> Constraint
                               --      We'll flatten to   (alpha, F a ~ alpha)
                               -- fail to make any further progress and must unflatten again

                          ; return (map ctPred $ bagToList flats') }

           -- NB: quant_pred_candidates is already the fixpoint of any
           --     unifications that may have happened
           ; gbl_tvs        <- tcGetGlobalTyVars
           ; zonked_tau_tvs <- TcM.zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))
           ; let poly_qtvs = growThetaTyVars quant_pred_candidates zonked_tau_tvs
                             `minusVarSet` gbl_tvs
                 pbound    = filter (quantifyPred poly_qtvs) quant_pred_candidates

                 -- Monomorphism restriction
                 constrained_tvs = tyVarsOfTypes pbound `unionVarSet` gbl_tvs
                 mr_bites        = apply_mr && not (null pbound)

           ; (qtvs, bound) <- if mr_bites
                              then do { qtvs <- quantifyTyVars constrained_tvs zonked_tau_tvs
                                      ; return (qtvs, []) }
                              else do { qtvs <- quantifyTyVars gbl_tvs poly_qtvs
                                      ; return (qtvs, pbound) }

           ; traceTc "simplifyWithApprox" $
             vcat [ ptext (sLit "quant_pred_candidates =") <+> ppr quant_pred_candidates
                  , ptext (sLit "gbl_tvs=") <+> ppr gbl_tvs
                  , ptext (sLit "zonked_tau_tvs=") <+> ppr zonked_tau_tvs
                  , ptext (sLit "pbound =") <+> ppr pbound
                  , ptext (sLit "bbound =") <+> ppr bound
                  , ptext (sLit "poly_qtvs =") <+> ppr poly_qtvs
                  , ptext (sLit "constrained_tvs =") <+> ppr constrained_tvs
                  , ptext (sLit "mr_bites =") <+> ppr mr_bites
                  , ptext (sLit "qtvs =") <+> ppr qtvs ]

           ; if null qtvs && null bound
             then do { traceTc "} simplifyInfer/no implication needed" empty
                     ; emitConstraints wanted_transformed
                        -- Includes insolubles (if -fdefer-type-errors)
                        -- as well as flats and implications
                     ; return ([], [], mr_bites, TcEvBinds ev_binds_var) }
             else do

          {     -- Step 7) Emit an implication
             let minimal_flat_preds = mkMinimalBySCs bound
                      -- See Note [Minimize by Superclasses]
                 skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty)
                                       | (name, ty) <- name_taus ]
                            -- Don't add the quantified variables here, because
                            -- they are also bound in ic_skols and we want them to be
                            -- tidied uniformly

           ; minimal_bound_ev_vars <- mapM TcM.newEvVar minimal_flat_preds
           ; let implic = Implic { ic_untch    = pushUntouchables untch
                                 , ic_skols    = qtvs
                                 , ic_no_eqs   = False
                                 , ic_fsks     = []  -- wanted_tansformed arose only from solveWanteds
                                                     -- hence no flatten-skolems (which come from givens)
                                 , ic_given    = minimal_bound_ev_vars
                                 , ic_wanted   = wanted_transformed
                                 , ic_insol    = False
                                 , ic_binds    = ev_binds_var
                                 , ic_info     = skol_info
                                 , ic_env      = tc_lcl_env }
           ; emitImplication implic

           ; traceTc "} simplifyInfer/produced residual implication for quantification" $
                 vcat [ ptext (sLit "implic =") <+> ppr implic
                           -- ic_skols, ic_given give rest of result
                      , ptext (sLit "qtvs =") <+> ppr qtvs
                      , ptext (sLit "spb =") <+> ppr quant_pred_candidates
                      , ptext (sLit "bound =") <+> ppr bound ]

           ; return ( qtvs, minimal_bound_ev_vars
                    , mr_bites,  TcEvBinds ev_binds_var) } }

    quantifyPred :: TyVarSet           -- Quantifying over these
                 -> PredType -> Bool   -- True <=> quantify over this wanted
    quantifyPred qtvs pred
      | isIPPred pred = True  -- Note [Inheriting implicit parameters]
      | otherwise     = tyVarsOfType pred `intersectsVarSet` qtvs

\end{code} Note [Inheriting implicit parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this: f x = (x::Int) + ?y 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: f :: Int -> Int (so we get ?y from the context of f's definition), or f :: (?y::Int) => Int -> Int At first you might think the first was better, because 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. BOTTOM LINE: when *inferring types* you must quantify over implicit parameters, *even if* they don't mention the bound type variables. Reason: because implicit parameters, uniquely, have local instance declarations. See the predicate quantifyPred. Note [Quantification with errors] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we find that the RHS of the definition has some absolutely-insoluble constraints, we abandon all attempts to find a context to quantify over, and instead make the function fully-polymorphic in whatever type we have found. For two reasons a) Minimise downstream errors b) Avoid spurious errors from this function But NB that we must include *derived* errors in the check. Example: (a::*) ~ Int# We get an insoluble derived error *~#, and we don't want to discard it before doing the isInsolubleWC test! (Trac #8262) 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. Note [Minimize by Superclasses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we quantify over a constraint, in simplifyInfer we need to quantify over a constraint that is minimal in some sense: For instance, if the final wanted constraint is (Eq alpha, Ord alpha), we'd like to quantify over Ord alpha, because we can just get Eq alpha from superclass selection from Ord alpha. This minimization is what mkMinimalBySCs does. Then, simplifyInfer uses the minimal constraint to check the original wanted. Note [Avoid unecessary constraint simplification] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -------- 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.) ---------------------------------------- When inferring the type of a let-binding, with simplifyInfer, try to avoid unnecessarily simplifying class constraints. Doing so aids sharing, but it also helps with delicate situations like instance C t => C [t] where .. f :: C [t] => .... f x = let g y = ...(constraint C [t])... in ... When inferring a type for 'g', we don't want to apply the instance decl, because then we can't satisfy (C t). So we just notice that g isn't quantified over 't' and partition the constraints before simplifying. This only half-works, but then let-generalisation only half-works. ********************************************************************************* * * * RULES * * * *********************************************************************************** See note [Simplifying RULE consraints] in TcRule 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. \begin{code}

    simplifyRule :: RuleName
                 -> WantedConstraints       -- Constraints from LHS
                 -> WantedConstraints       -- Constraints from RHS
                 -> TcM ([EvVar], WantedConstraints)   -- LHS evidence varaibles
    -- See Note [Simplifying RULE constraints] in TcRule
    simplifyRule name lhs_wanted rhs_wanted
      = do {         -- We allow ourselves to unify environment
                     -- variables: runTcS runs with NoUntouchables
             (resid_wanted, _) <- solveWantedsTcM (lhs_wanted `andWC` rhs_wanted)
                                  -- Post: these are zonked and unflattened

           ; zonked_lhs_flats <- zonkCts (wc_flat lhs_wanted)
           ; let (q_cts, non_q_cts) = partitionBag quantify_me zonked_lhs_flats
                 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 `tcEqType` t2)
                   | otherwise
                   = True

           ; traceTc "simplifyRule" $
             vcat [ ptext (sLit "LHS of rule") <+> doubleQuotes (ftext name)
                  , text "zonked_lhs_flats" <+> ppr zonked_lhs_flats
                  , text "q_cts"      <+> ppr q_cts
                  , text "non_q_cts"  <+> ppr non_q_cts ]

           ; return ( map (ctEvId . ctEvidence) (bagToList q_cts)
                    , lhs_wanted { wc_flat = non_q_cts }) }

\end{code} ********************************************************************************* * * * Main Simplifier * * * *********************************************************************************** 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 a :: Int a = 'a' main = print "b" 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`. 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 -fdefer-type-errors is on, so that we can continue compilation. The errors are turned into warnings in `reportUnsolved`. Note [Zonk after solving] ~~~~~~~~~~~~~~~~~~~~~~~~~ We zonk the result immediately after constraint solving, for two reasons: a) because zonkWC generates evidence, and this is the moment when we have a suitable evidence variable to hand. Note that *after* solving the constraints are typically small, so the overhead is not great. \begin{code}

    solveWantedsTcMWithEvBinds :: EvBindsVar
                               -> WantedConstraints
                               -> (WantedConstraints -> TcS WantedConstraints)
                               -> TcM WantedConstraints
    -- Returns a *zonked* result
    -- We zonk when we finish primarily to un-flatten out any
    -- flatten-skolems etc introduced by canonicalisation of
    -- types involving type funuctions.  Happily the result
    -- is typically much smaller than the input, indeed it is
    -- often empty.
    solveWantedsTcMWithEvBinds ev_binds_var wc tcs_action
      = do { traceTc "solveWantedsTcMWithEvBinds" $ text "wanted=" <+> ppr wc
           ; wc2 <- runTcSWithEvBinds ev_binds_var (tcs_action wc)
           ; zonkWC ev_binds_var wc2 }
             -- See Note [Zonk after solving]

    solveWantedsTcM :: WantedConstraints -> TcM (WantedConstraints, Bag EvBind)
    -- Zonk the input constraints, and simplify them
    -- Return the evidence binds in the BagEvBinds result
    -- Discards all Derived stuff in result
    -- Postcondition: fully zonked and unflattened constraints
    solveWantedsTcM wanted
      = do { ev_binds_var <- newTcEvBinds
           ; wanteds' <- solveWantedsTcMWithEvBinds ev_binds_var wanted solve_wanteds_and_drop
           ; binds <- TcRnMonad.getTcEvBinds ev_binds_var
           ; return (wanteds', binds) }

    solve_wanteds_and_drop :: WantedConstraints -> TcS (WantedConstraints)
    -- Since solve_wanteds returns the residual WantedConstraints,
    -- it should always be called within a runTcS or something similar,
    solve_wanteds_and_drop wanted = do { wc <- solve_wanteds wanted
                                       ; return (dropDerivedWC wc) }

    solve_wanteds :: WantedConstraints -> TcS WantedConstraints
    -- so that the inert set doesn't mindlessly propagate.
    -- NB: wc_flats may be wanted /or/ derived now
    solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols })
      = do { traceTcS "solveWanteds {" (ppr wanted)

             -- 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, and we don't run the danger
             -- of adding Derived insolubles twice; see
             -- TcSMonad Note [Do not add duplicate derived insolubles]
           ; traceTcS "solveFlats {" empty
           ; let all_flats = flats `unionBags` insols
           ; impls_from_flats <- solveInteract all_flats
           ; traceTcS "solveFlats end }" (ppr impls_from_flats)

           -- solve_wanteds iterates when it is able to float equalities
           -- out of one or more of the implications.
           ; unsolved_implics <- simpl_loop 1 (implics `unionBags` impls_from_flats)

           ; (unsolved_flats, insoluble_flats) <- getInertUnsolved

            -- We used to unflatten here but now we only do it once at top-level
            -- during zonking -- see Note [Unflattening while zonking] in TcMType
           ; let wc = WC { wc_flat  = unsolved_flats
                         , wc_impl  = unsolved_implics
                         , wc_insol = insoluble_flats }

           ; bb <- getTcEvBindsMap
           ; tb <- getTcSTyBindsMap
           ; traceTcS "solveWanteds }" $
                     vcat [ text "unsolved_flats   =" <+> ppr unsolved_flats
                          , text "unsolved_implics =" <+> ppr unsolved_implics
                          , text "current evbinds  =" <+> ppr (evBindMapBinds bb)
                          , text "current tybinds  =" <+> vcat (map ppr (varEnvElts tb))
                          , text "final wc =" <+> ppr wc ]

           ; return wc }

    simpl_loop :: Int
               -> Bag Implication
               -> TcS (Bag Implication)
    simpl_loop n implics
      | n > 10
      = traceTcS "solveWanteds: loop!" empty >> return implics
      | otherwise
      = do { traceTcS "simpl_loop, iteration" (int n)
           ; (floated_eqs, unsolved_implics) <- solveNestedImplications implics
           ; if isEmptyBag floated_eqs
             then return unsolved_implics
             else
        do {   -- Put floated_eqs into the current inert set before looping
             (unifs_happened, impls_from_eqs) <- reportUnifications $
                                                 solveInteract floated_eqs
           ; if   -- See Note [Cutting off simpl_loop]
                  isEmptyBag impls_from_eqs &&
                  not unifs_happened &&                 -- (a)
                  not (anyBag isCFunEqCan floated_eqs)  -- (b)
             then return unsolved_implics
             else simpl_loop (n+1) (unsolved_implics `unionBags` impls_from_eqs) } }

    solveNestedImplications :: Bag Implication
                            -> TcS (Cts, Bag Implication)
    -- Precondition: the TcS inerts may contain unsolved flats which have
    -- to be converted to givens before we go inside a nested implication.
    solveNestedImplications implics
      | isEmptyBag implics
      = return (emptyBag, emptyBag)
      | otherwise
      = do { inerts <- getTcSInerts
           ; let thinner_inerts = prepareInertsForImplications inerts
                     -- See Note [Preparing inert set for implications]

           ; traceTcS "solveNestedImplications starting {" $
             vcat [ text "original inerts = " <+> ppr inerts
                  , text "thinner_inerts  = " <+> ppr thinner_inerts ]

           ; (floated_eqs, unsolved_implics)
               <- flatMapBagPairM (solveImplication thinner_inerts) 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.
           ; traceTcS "solveNestedImplications end }" $
                      vcat [ text "all floated_eqs ="  <+> ppr floated_eqs
                           , text "unsolved_implics =" <+> ppr unsolved_implics ]

           ; return (floated_eqs, unsolved_implics) }

    solveImplication :: InertSet
                     -> 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 inerts
         imp@(Implic { ic_untch  = untch
                     , ic_binds  = ev_binds
                     , ic_skols  = skols
                     , ic_fsks   = old_fsks
                     , ic_given  = givens
                     , ic_wanted = wanteds
                     , ic_info   = info
                     , ic_env    = env })
      = do { traceTcS "solveImplication {" (ppr imp)

             -- Solve the nested constraints
           ; (no_given_eqs, new_fsks, residual_wanted)
                <- nestImplicTcS ev_binds untch inerts $
                   do { (no_eqs, new_fsks) <- solveInteractGiven (mkGivenLoc info env)
                                                                 old_fsks givens

                      ; residual_wanted <- solve_wanteds wanteds
                            -- solve_wanteds, *not* solve_wanteds_and_drop, because
                            -- we want to retain derived equalities so we can float
                            -- them out in floatEqualities

                      ; return (no_eqs, new_fsks, residual_wanted) }

           ; (floated_eqs, final_wanted)
                 <- floatEqualities (skols ++ new_fsks) no_given_eqs residual_wanted

           ; let res_implic | isEmptyWC final_wanted && no_given_eqs
                            = emptyBag  -- Reason for the no_given_eqs: we don't want to
                                        -- lose the "inaccessible code" error message
                            | otherwise
                            = unitBag (imp { ic_fsks   = new_fsks
                                           , ic_no_eqs = no_given_eqs
                                           , ic_wanted = dropDerivedWC final_wanted
                                           , ic_insol  = insolubleWC final_wanted })

           ; evbinds <- getTcEvBindsMap
           ; traceTcS "solveImplication end }" $ vcat
                 [ text "no_given_eqs =" <+> ppr no_given_eqs
                 , text "floated_eqs =" <+> ppr floated_eqs
                 , text "new_fsks =" <+> ppr new_fsks
                 , text "res_implic =" <+> ppr res_implic
                 , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds) ]

           ; return (floated_eqs, res_implic) }

\end{code} Note [Cutting off simpl_loop] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It is very important not to iterate in simpl_loop unless there is a chance of progress. Trac #8474 is a classic example: * There's a deeply-nested chain of implication constraints. ?x:alpha => ?y1:beta1 => ... ?yn:betan => [W] ?x:Int * From the innermost one we get a [D] alpha ~ Int, but alpha is untouchable until we get out to the outermost one * We float [D] alpha~Int out (it is in floated_eqs), but since alpha is untouchable, the solveInteract in simpl_loop makes no progress * So there is no point in attempting to re-solve ?yn:betan => [W] ?x:Int because we'll just get the same [D] again * If we *do* re-solve, we'll get an ininite loop. It is cut off by the fixed bound of 10, but solving the next takes 10*10*...*10 (ie exponentially many) iterations! Conclusion: we should iterate simpl_loop iff we will get more 'givens' in the inert set when solving the nested implications. That is the result of prepareInertsForImplications is larger. How can we tell this? Consider floated_eqs (all wanted or derived): (a) [W/D] CTyEqCan (a ~ ty). This can give rise to a new given only by causing a unification. So we count those unifications. (b) [W] CFunEqCan (F tys ~ xi). Even though these are wanted, they are pushed in as givens by prepareInertsForImplications. See Note [Preparing inert set for implications] in TcSMonad. But because of that very fact, we won't generate another copy if we iterate simpl_loop. So we iterate if there any of these \begin{code}

    floatEqualities :: [TcTyVar] -> Bool -> WantedConstraints
                    -> TcS (Cts, WantedConstraints)
    -- Post: The returned floated constraints (Cts) are only Wanted or Derived
    -- and come from the input wanted ev vars or deriveds
    -- Also performs some unifications, adding to monadically-carried ty_binds
    -- These will be used when processing floated_eqs later
    floatEqualities skols no_given_eqs wanteds@(WC { wc_flat = flats })
      | not no_given_eqs  -- There are some given equalities, so don't float
      = return (emptyBag, wanteds)   -- Note [Float Equalities out of Implications]
      | otherwise
      = do { let (float_eqs, remaining_flats) = partitionBag is_floatable flats
           ; untch <- TcS.getUntouchables
           ; mapM_ (promoteTyVar untch) (varSetElems (tyVarsOfCts float_eqs))
                 -- See Note [Promoting unification variables]
           ; ty_binds <- getTcSTyBindsMap
           ; traceTcS "floatEqualities" (vcat [ text "Flats =" <+> ppr flats
                                              , text "Floated eqs =" <+> ppr float_eqs
                                              , text "Ty binds =" <+> ppr ty_binds])
           ; return (float_eqs, wanteds { wc_flat = remaining_flats }) }
      where
          -- See Note [Float equalities from under a skolem binding]
        skol_set = fixVarSet mk_next (mkVarSet skols)
        mk_next tvs = foldrBag grow_one tvs flats
        grow_one (CFunEqCan { cc_tyargs = xis, cc_rhs = rhs }) tvs
           | intersectsVarSet tvs (tyVarsOfTypes xis)
           = tvs `unionVarSet` tyVarsOfType rhs
        grow_one _ tvs = tvs

        is_floatable :: Ct -> Bool
        is_floatable ct = isEqPred pred && skol_set `disjointVarSet` tyVarsOfType pred
           where
             pred = ctPred ct

    promoteTyVar :: Untouchables -> TcTyVar  -> TcS ()
    -- When we float a constraint out of an implication we must restore
    -- invariant (MetaTvInv) in Note [Untouchable type variables] in TcType
    -- See Note [Promoting unification variables]
    promoteTyVar untch tv
      | isFloatedTouchableMetaTyVar untch tv
      = do { cloned_tv <- TcS.cloneMetaTyVar tv
           ; let rhs_tv = setMetaTyVarUntouchables cloned_tv untch
           ; setWantedTyBind tv (mkTyVarTy rhs_tv) }
      | otherwise
      = return ()

    promoteAndDefaultTyVar :: Untouchables -> TcTyVarSet -> TyVar -> TcS ()
    -- See Note [Promote _and_ default when inferring]
    promoteAndDefaultTyVar untch gbl_tvs tv
      = do { tv1 <- if tv `elemVarSet` gbl_tvs
                    then return tv
                    else defaultTyVar tv
           ; promoteTyVar untch tv1 }

    defaultTyVar :: TcTyVar -> TcS TcTyVar
    -- Precondition: MetaTyVars only
    -- See Note [DefaultTyVar]
    defaultTyVar the_tv
      | Just default_k <- defaultKind_maybe (tyVarKind the_tv)
      = do { tv' <- TcS.cloneMetaTyVar the_tv
           ; let new_tv = setTyVarKind tv' default_k
           ; traceTcS "defaultTyVar" (ppr the_tv <+> ppr new_tv)
           ; setWantedTyBind the_tv (mkTyVarTy new_tv)
           ; return new_tv }
                 -- Why not directly derived_pred = mkTcEqPred k default_k?
                 -- See Note [DefaultTyVar]
                 -- We keep the same Untouchables on tv'

      | otherwise = return the_tv    -- The common case

    approximateWC :: WantedConstraints -> Cts
    -- Postcondition: Wanted or Derived Cts
    -- See Note [ApproximateWC]
    approximateWC wc
      = float_wc emptyVarSet wc
      where
        float_wc :: TcTyVarSet -> WantedConstraints -> Cts
        float_wc trapping_tvs (WC { wc_flat = flats, wc_impl = implics })
          = filterBag is_floatable flats `unionBags`
            do_bag (float_implic new_trapping_tvs) implics
          where
            new_trapping_tvs = fixVarSet grow trapping_tvs
            is_floatable ct = tyVarsOfCt ct `disjointVarSet` new_trapping_tvs

            grow tvs = foldrBag grow_one tvs flats
            grow_one ct tvs | ct_tvs `intersectsVarSet` tvs = tvs `unionVarSet` ct_tvs
                            | otherwise                     = tvs
                            where
                              ct_tvs = tyVarsOfCt ct

        float_implic :: TcTyVarSet -> Implication -> Cts
        float_implic trapping_tvs imp
          | ic_no_eqs imp                 -- No equalities, so float
          = float_wc new_trapping_tvs (ic_wanted imp)
          | otherwise                     -- Don't float out of equalities
          = emptyCts                      -- See Note [ApproximateWC]
          where
            new_trapping_tvs = trapping_tvs `extendVarSetList` ic_skols imp
                                            `extendVarSetList` ic_fsks imp
        do_bag :: (a -> Bag c) -> Bag a -> Bag c
        do_bag f = foldrBag (unionBags.f) emptyBag

\end{code} Note [ApproximateWC] ~~~~~~~~~~~~~~~~~~~~ approximateWC takes a constraint, typically arising from the RHS of a let-binding whose type we are *inferring*, and extracts from it some *flat* constraints that we might plausibly abstract over. Of course the top-level flat constraints are plausible, but we also float constraints out from inside, if they are not captured by skolems. The same function is used when doing type-class defaulting (see the call to applyDefaultingRules) to extract constraints that that might be defaulted. There are two caveats: 1. We do *not* float anything out if the implication binds equality constraints, because that defeats the OutsideIn story. Consider data T a where TInt :: T Int MkT :: T a f TInt = 3::Int We get the implication (a ~ Int => res ~ Int), where so far we've decided f :: T a -> res We don't want to float (res~Int) out because then we'll infer f :: T a -> Int which is only on of the possible types. (GHC 7.6 accidentally *did* float out of such implications, which meant it would happily infer non-principal types.) 2. We do not float out an inner constraint that shares a type variable (transitively) with one that is trapped by a skolem. Eg forall a. F a ~ beta, Integral beta We don't want to float out (Integral beta). Doing so would be bad when defaulting, because then we'll default beta:=Integer, and that makes the error message much worse; we'd get Can't solve F a ~ Integer rather than Can't solve Integral (F a) Moreover, floating out these "contaminated" constraints doesn't help when generalising either. If we generalise over (Integral b), we still can't solve the retained implication (forall a. F a ~ b). Indeed, arguably that too would be a harder error to understand. Note [DefaultTyVar] ~~~~~~~~~~~~~~~~~~~ defaultTyVar is used on any un-instantiated meta type variables to default the kind of OpenKind and ArgKind etc to *. This is important to ensure that instance declarations match. For example consider instance Show (a->b) foo x = show (\_ -> True) Then we'll get a constraint (Show (p ->q)) where p has kind ArgKind, and that won't match the typeKind (*) in the instance decl. See tests tc217 and tc175. We look only at touchable type variables. No further constraints are going to affect these type variables, so it's time to do it by hand. However we aren't ready to default them fully to () or whatever, because the type-class defaulting rules have yet to run. An important point is that if the type variable tv has kind k and the default is default_k we do not simply generate [D] (k ~ default_k) because: (1) k may be ArgKind and default_k may be * so we will fail (2) We need to rewrite all occurrences of the tv to be a type variable with the right kind and we choose to do this by rewriting the type variable /itself/ by a new variable which does have the right kind. Note [Promote _and_ default when inferring] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we are inferring a type, we simplify the constraint, and then use approximateWC to produce a list of candidate constraints. Then we MUST a) Promote any meta-tyvars that have been floated out by approximateWC, to restore invariant (MetaTvInv) described in Note [Untouchable type variables] in TcType. b) Default the kind of any meta-tyyvars that are not mentioned in in the environment. To see (b), suppose the constraint is (C ((a :: OpenKind) -> Int)), and we have an instance (C ((x:*) -> Int)). The instance doesn't match -- but it should! If we don't solve the constraint, we'll stupidly quantify over (C (a->Int)) and, worse, in doing so zonkQuantifiedTyVar will quantify over (b:*) instead of (a:OpenKind), which can lead to disaster; see Trac #7332. Trac #7641 is a simpler example. Note [Float Equalities out of Implications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For ordinary pattern matches (including existentials) we float equalities out of implications, for instance: data T where MkT :: Eq a => a -> T f x y = case x of MkT _ -> (y::Int) We get the implication constraint (x::T) (y::alpha): forall a. [untouchable=alpha] Eq a => alpha ~ Int We want to float out the equality into a scope where alpha is no longer untouchable, to solve the implication! But we cannot float equalities out of implications whose givens may yield or contain equalities: data T a where T1 :: T Int T2 :: T Bool T3 :: T a h :: T a -> a -> Int f x y = case x of T1 -> y::Int T2 -> y::Bool T3 -> h x y We generate constraint, for (x::T alpha) and (y :: beta): [untouchables = beta] (alpha ~ Int => beta ~ Int) -- From 1st branch [untouchables = beta] (alpha ~ Bool => beta ~ Bool) -- From 2nd branch (alpha ~ beta) -- From 3rd branch If we float the equality (beta ~ Int) outside of the first implication and the equality (beta ~ Bool) out of the second we get an insoluble constraint. But if we just leave them inside the implications we unify alpha := beta and solve everything. Principle: We do not want to float equalities out which may need the given *evidence* to become soluble. Consequence: classes with functional dependencies don't matter (since there is no evidence for a fundep equality), but equality superclasses do matter (since they carry evidence). Note [When does an implication have given equalities?] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider an implication beta => alpha ~ Int where beta is a unification variable that has already been unified to () in an outer scope. Then we can float the (alpha ~ Int) out just fine. So when deciding whether the givens contain an equality, we should canonicalise first, rather than just looking at the original givens (Trac #8644). This is the entire reason for the inert_no_eqs field in InertCans. We initialise it to False before processing the Givens of an implication; and set it to True when adding an inert equality in addInertCan. However, when flattening givens, we generate given equalities like : F [a] ~ f, with Refl evidence, and we *don't* want those to count as an equality in the givens! After all, the entire flattening business is just an internal matter, and the evidence does not mention any of the 'givens' of this implication. So we set the flag to False when adding an equality (TcSMonad.addInertCan) whose evidence whose CtOrigin is FlatSkolOrigin; see TcSMonad.isFlatSkolEv. Note that we may transform the original flat-skol equality before adding it to the inerts, so it's important that the transformation preserves origin (which xCtEvidence and rewriteEvidence both do). Example instance F [a] = Maybe a implication: C (F [a]) => blah We flatten (C (F [a])) to C fsk, with : F [a] ~ fsk Then we reduce the F [a] LHS, giving g22 = ax7 ; g22 : Maybe a ~ fsk And before adding g22 we'll re-orient it to an ordinary tyvar equality. None of this should count as "adding a given equality". This really happens (Trac #8651). An alternative we considered was to * Accumulate the new inert equalities (in TcSMonad.addInertCan) * In solveInteractGiven, check whether the evidence for the new equalities mentions any of the ic_givens of this implication. This seems like the Right Thing, but it's more code, and more work at runtime, so we are using the FlatSkolOrigin idea intead. It's less obvious that it works, but I htink it does, and it's simple and efficient. Note [Float equalities from under a skolem binding] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You might worry about skolem escape with all this floating. For example, consider [2] forall a. (a ~ F beta[2] delta, Maybe beta[2] ~ gamma[1]) The (Maybe beta ~ gamma) doesn't mention 'a', so we float it, and solve with gamma := beta. But what if later delta:=Int, and F b Int = b. Then we'd get a ~ beta[2], and solve to get beta:=a, and now the skolem has escaped! But it's ok: when we float (Maybe beta[2] ~ gamma[1]), we promote beta[2] to beta[1], and that means the (a ~ beta[1]) will be stuck, as it should be. Previously we tried to "grow" the skol_set with the constraints, to get all the tyvars that could *conceivably* unify with the skolems, but that was far too conservative (Trac #7804). Example: this should be fine: f :: (forall a. a -> Proxy x -> Proxy (F x)) -> Int f = error "Urk" :: (forall a. a -> Proxy x -> Proxy (F x)) -> Int BUT (sigh) we have to be careful. Here are some edge cases: a) [2]forall a. (F a delta[1] ~ beta[2], delta[1] ~ Maybe beta[2]) b) [2]forall a. (F b ty ~ beta[2], G beta[2] ~ gamma[2]) c) [2]forall a. (F a ty ~ beta[2], delta[1] ~ Maybe beta[2]) In (a) we *must* float out the second equality, else we can't solve at all (Trac #7804). In (b) we *must not* float out the second equality. It will ultimately be solved (by flattening) in situ, but if we float it we'll promote beta,gamma, and render the first equality insoluble. In (c) it would be OK to float the second equality but better not to. If we flatten we see (delta[1] ~ Maybe (F a ty)), which is a skolem-escape problem. If we float the secodn equality we'll end up with (F a ty ~ beta'[1]), which is a less explicable error. Hence we start with the skolems, grow them by the CFunEqCans, and float ones that don't mention the grown variables. Seems very ad hoc. Note [Promoting unification variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we float an equality out of an implication we must "promote" free unification variables of the equality, in order to maintain Invariant (MetaTvInv) from Note [Untouchable type variables] in TcType. for the leftover implication. This is absolutely necessary. Consider the following example. We start with two implications and a class with a functional dependency. class C x y | x -> y instance C [a] [a] (I1) [untch=beta]forall b. 0 => F Int ~ [beta] (I2) [untch=beta]forall c. 0 => F Int ~ [[alpha]] /\ C beta [c] We float (F Int ~ [beta]) out of I1, and we float (F Int ~ [[alpha]]) out of I2. They may react to yield that (beta := [alpha]) which can then be pushed inwards the leftover of I2 to get (C [alpha] [a]) which, using the FunDep, will mean that (alpha := a). In the end we will have the skolem 'b' escaping in the untouchable beta! Concrete example is in indexed_types/should_fail/ExtraTcsUntch.hs: class C x y | x -> y where op :: x -> y -> () instance C [a] [a] type family F a :: * h :: F Int -> () h = undefined data TEx where TEx :: a -> TEx f (x::beta) = let g1 :: forall b. b -> () g1 _ = h [x] g2 z = case z of TEx y -> (h [[undefined]], op x [y]) in (g1 '3', g2 undefined) Note [Solving Family Equations] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ After we are done with simplification we may be left with constraints of the form: [Wanted] F xis ~ beta If 'beta' is a touchable unification variable not already bound in the TyBinds then we'd like to create a binding for it, effectively "defaulting" it to be 'F xis'. When is it ok to do so? 1) 'beta' must not already be defaulted to something. Example: [Wanted] F Int ~ beta <~ Will default [beta := F Int] [Wanted] F Char ~ beta <~ Already defaulted, can't default again. We have to report this as unsolved. 2) However, we must still do an occurs check when defaulting (F xis ~ beta), to set [beta := F xis] only if beta is not among the free variables of xis. 3) Notice that 'beta' can't be bound in ty binds already because we rewrite RHS of type family equations. See Inert Set invariants in TcInteract. This solving is now happening during zonking, see Note [Unflattening while zonking] in TcMType. ********************************************************************************* * * * Defaulting and disamgiguation * * * ********************************************************************************* \begin{code}

    applyDefaultingRules :: Cts -> TcS Bool
      -- True <=> I did some defaulting, reflected in ty_binds

    -- Return some extra derived equalities, which express the
    -- type-class default choice.
    applyDefaultingRules wanteds
      | isEmptyBag wanteds
      = return False
      | otherwise
      = do { traceTcS "applyDefaultingRules { " $
                      text "wanteds =" <+> ppr wanteds

           ; info@(default_tys, _) <- getDefaultInfo
           ; let groups = findDefaultableGroups info wanteds
           ; traceTcS "findDefaultableGroups" $ vcat [ text "groups=" <+> ppr groups
                                                     , text "info=" <+> ppr info ]
           ; something_happeneds <- mapM (disambigGroup default_tys) groups

           ; traceTcS "applyDefaultingRules }" (ppr something_happeneds)

           ; return (or something_happeneds) }

\end{code} \begin{code}

    findDefaultableGroups
        :: ( [Type]
           , (Bool,Bool) )  -- (Overloaded strings, extended default rules)
        -> Cts              -- Unsolved (wanted or derived)
        -> [[(Ct,Class,TcTyVar)]]
    findDefaultableGroups (default_tys, (ovl_strings, extended_defaults)) wanteds
      | null default_tys = []
      | otherwise        = defaultable_groups
      where
        defaultable_groups = filter is_defaultable_group groups
        groups             = equivClasses cmp_tv unaries
        unaries     :: [(Ct, Class, TcTyVar)]  -- (C tv) constraints
        non_unaries :: [Ct]             -- and *other* constraints

        (unaries, non_unaries) = partitionWith find_unary (bagToList wanteds)
            -- Finds unary type-class constraints
            -- But take account of polykinded classes like Typeable,
            -- which may look like (Typeable * (a:*))   (Trac #8931)
        find_unary cc
            | Just (cls,tys)   <- getClassPredTys_maybe (ctPred cc)
            , Just (kinds, ty) <- snocView tys
            , all isKind kinds
            , Just tv <- tcGetTyVar_maybe ty
            , isMetaTyVar tv  -- We might have runtime-skolems in GHCi, and
                              -- we definitely don't want to try to assign to those!
            = Left (cc, cls, tv)
        find_unary cc = Right cc  -- Non unary or non dictionary

        bad_tvs :: TcTyVarSet  -- TyVars mentioned by non-unaries
        bad_tvs = foldr (unionVarSet . tyVarsOfCt) emptyVarSet non_unaries

        cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2

        is_defaultable_group ds@((_,_,tv):_)
            = let b1 = isTyConableTyVar tv  -- Note [Avoiding spurious errors]
                  b2 = not (tv `elemVarSet` bad_tvs)
                  b4 = defaultable_classes [cls | (_,cls,_) <- ds]
              in (b1 && b2 && b4)
        is_defaultable_group [] = panic "defaultable_group"

        defaultable_classes clss
            | extended_defaults = any isInteractiveClass clss
            | otherwise         = all is_std_class clss && (any is_num_class clss)

        -- In interactive mode, or with -XExtendedDefaultRules,
        -- we default Show a to Show () to avoid graututious errors on "show []"
        isInteractiveClass cls
            = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])

        is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
        -- is_num_class adds IsString to the standard numeric classes,
        -- when -foverloaded-strings is enabled

        is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
        -- Similarly is_std_class

    ------------------------------
    disambigGroup :: [Type]                  -- The default types
                  -> [(Ct, Class, TcTyVar)]  -- All classes of the form (C a)
                                             --  sharing same type variable
                  -> TcS Bool   -- True <=> something happened, reflected in ty_binds

    disambigGroup []  _grp
      = return False
    disambigGroup (default_ty:default_tys) group
      = do { traceTcS "disambigGroup {" (ppr group $$ ppr default_ty)
           ; success <- tryTcS $ -- Why tryTcS? If this attempt fails, we want to
                                 -- discard all side effects from the attempt
                        do { setWantedTyBind the_tv default_ty
                           ; implics_from_defaulting <- solveInteract wanteds
                           ; MASSERT(isEmptyBag implics_from_defaulting)
                               -- I am not certain if any implications can be generated
                               -- but I am letting this fail aggressively if this ever happens.

                           ; checkAllSolved }

           ; if success then
                 -- Success: record the type variable binding, and return
                 do { setWantedTyBind the_tv default_ty
                    ; wrapWarnTcS $ warnDefaulting wanteds default_ty
                    ; traceTcS "disambigGroup succeeded }" (ppr default_ty)
                    ; return True }
             else
                 -- Failure: try with the next type
                 do { traceTcS "disambigGroup failed, will try other default types }"
                               (ppr default_ty)
                    ; disambigGroup default_tys group } }
      where
        ((_,_,the_tv):_) = group
        wanteds          = listToBag (map fstOf3 group)

\end{code} Note [Avoiding spurious errors] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When doing the unification for defaulting, we check for skolem type variables, and simply don't default them. For example: f = (*) -- Monomorphic g :: Num a => a -> a g x = f x x Here, we get a complaint when checking the type signature for g, that g isn't polymorphic enough; but then we get another one when dealing with the (Num a) context arising from f's definition; we try to unify a with Int (to default it), but find that it's already been unified with the rigid variable from g's type sig