/compiler/typecheck/TcInteract.lhs

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\begin{code}
{-# 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

module TcInteract ( 
     solveInteractGiven,  -- Solves [EvVar],GivenLoc
     solveInteract,       -- Solves Cts
  ) where  

#include "HsVersions.h"


import BasicTypes ()
import TcCanonical
import VarSet
import Type
import Unify
import FamInstEnv
import Coercion( mkAxInstRHS )

import Var
import TcType
import PrelNames (singIClassName, ipClassNameKey )

import Class
import TyCon
import Name

import FunDeps

import TcEvidence
import Outputable

import TcMType ( zonkTcPredType )

import TcRnTypes
import TcErrors
import TcSMonad
import Maybes( orElse )
import Bag

import Control.Monad ( foldM )

import VarEnv

import Control.Monad( when, unless )
import Pair ()
import Unique( hasKey )
import UniqFM
import FastString ( sLit ) 
import DynFlags
import Util
\end{code}
**********************************************************************
*                                                                    * 
*                      Main Interaction Solver                       *
*                                                                    *
**********************************************************************

Note [Basic Simplifier Plan] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1. Pick an element from the WorkList if there exists one with depth 
   less thanour context-stack depth. 

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.
\begin{code}
solveInteractGiven :: CtLoc -> [TcTyVar] -> [EvVar] -> TcS ()
-- In principle the givens can kick out some wanteds from the inert
-- resulting in solving some more wanted goals here which could emit
-- implications. That's why I return a bag of implications. Not sure
-- if this can happen in practice though.
solveInteractGiven loc fsks givens
  = do { implics <- solveInteract (fsk_bag `unionBags` given_bag)
       ; ASSERT( isEmptyBag implics )
         return () }  -- We do not decompose *given* polymorphic equalities
                      --    (forall a. t1 ~ forall a. t2)
                      -- What would the evidence look like?!
                      -- See Note [Do not decompose given polytype equalities]
                      -- in TcCanonical
  where 
    given_bag = listToBag [ mkNonCanonical loc $ CtGiven { ctev_evtm = EvId ev_id
                                                         , ctev_pred = evVarPred ev_id }
                          | ev_id <- givens ]

    fsk_bag = listToBag [ mkNonCanonical loc $ CtGiven { ctev_evtm = EvCoercion (mkTcReflCo tv_ty)
                                                       , ctev_pred = pred  }
                        | tv <- fsks
                        , let FlatSkol fam_ty = tcTyVarDetails tv
                              tv_ty = mkTyVarTy tv
                              pred  = mkTcEqPred fam_ty tv_ty
                        ]

-- The main solver loop implements Note [Basic Simplifier Plan]
---------------------------------------------------------------
solveInteract :: Cts -> TcS (Bag Implication)
-- Returns the final InertSet in TcS
-- Has no effect on work-list or residual-iplications
solveInteract cts
  = {-# SCC "solveInteract" #-}
    withWorkList cts $
    do { dyn_flags <- getDynFlags
       ; solve_loop (ctxtStkDepth dyn_flags) }
  where
    solve_loop max_depth
      = {-# SCC "solve_loop" #-}
        do { sel <- selectNextWorkItem max_depth
           ; case sel of 
              NoWorkRemaining     -- Done, successfuly (modulo frozen)
                -> return ()
              MaxDepthExceeded ct -- Failure, depth exceeded
                -> wrapErrTcS $ solverDepthErrorTcS ct
              NextWorkItem ct     -- More work, loop around!
                -> do { runSolverPipeline thePipeline ct; solve_loop max_depth } }

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
  where 
    pick_next :: WorkList -> (SelectWorkItem, WorkList)
    pick_next wl 
      = case selectWorkItem wl of
          (Nothing,_) 
              -> (NoWorkRemaining,wl)           -- No more work
          (Just ct, new_wl) 
              | ctLocDepth (cc_loc ct) > max_depth  -- Depth exceeded
              -> (MaxDepthExceeded ct,new_wl)
          (Just ct, new_wl) 
              -> (NextWorkItem ct, new_wl)      -- New workitem and worklist

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]

       ; bumpStepCountTcS    -- One step for each constraint processed
       ; 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 { traceFireTcS ct (ptext (sLit "Kept as inert:") <+> ppr ct)
                                 ; traceTcS "End solver pipeline (not discharged) }" $
                                       vcat [ ptext (sLit "final_item = ") <+> ppr ct
                                            , pprTvBndrs (varSetElems $ tyVarsOfCt ct)
                                            , ptext (sLit "inerts     = ") <+> ppr final_is]
                                 ; insertInertItemTcS 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 
               }
\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}
thePipeline :: [(String,SimplifierStage)]
thePipeline = [ ("canonicalization",        TcCanonical.canonicalize)
              , ("spontaneous solve",       spontaneousSolveStage)
              , ("interact with inerts",    interactWithInertsStage)
              , ("top-level reactions",     topReactionsStage) ]
\end{code}


*********************************************************************************
*                                                                               * 
                       The spontaneous-solve Stage
*                                                                               *
*********************************************************************************

\begin{code}
spontaneousSolveStage :: SimplifierStage 
-- CTyEqCans are always consumed, returning Stop
spontaneousSolveStage workItem
  = do { mb_solved <- trySpontaneousSolve workItem
       ; case mb_solved of
           SPCantSolve
              | CTyEqCan { cc_tyvar = tv, cc_ev = fl } <- workItem
              -- Unsolved equality
              -> do { n_kicked <- kickOutRewritable (ctEvFlavour fl) tv
                    ; traceFireTcS workItem $
                      ptext (sLit "Kept as inert") <+> ppr_kicked n_kicked <> colon 
                      <+> ppr workItem
                    ; insertInertItemTcS workItem
                    ; return Stop }
              | otherwise
              -> continueWith workItem

           SPSolved new_tv
              -- Post: tv ~ xi is now in TyBinds, no need to put in inerts as well
              -- see Note [Spontaneously solved in TyBinds]
              -> do { n_kicked <- kickOutRewritable Given new_tv
                    ; traceFireTcS workItem $
                      ptext (sLit "Spontaneously solved") <+> ppr_kicked n_kicked <> colon 
                      <+> ppr workItem
                    ; return Stop } }

ppr_kicked :: Int -> SDoc
ppr_kicked 0 = empty
ppr_kicked n = parens (int n <+> ptext (sLit "kicked out")) 
\end{code}
Note [Spontaneously solved in TyBinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we encounter a constraint ([W] alpha ~ tau) which can be spontaneously solved,
we record the equality on the TyBinds of the TcSMonad. In the past, we used to also
add a /given/ version of the constraint ([G] alpha ~ tau) to the inert
canonicals -- and potentially kick out other equalities that mention alpha.

Then, the flattener only had to look in the inert equalities during flattening of a
type (TcCanonical.flattenTyVar).

However it is a bit silly to record these equalities /both/ in the inerts AND the
TyBinds, so we have now eliminated spontaneously solved equalities from the inerts,
and only record them in the TyBinds of the TcS monad. The flattener is now consulting
these binds /and/ the inerts for potentially unsolved or other given equalities.

\begin{code}
kickOutRewritable :: CtFlavour    -- Flavour of the equality that is 
                                  -- being added to the inert set
                  -> TcTyVar      -- The new equality is tv ~ ty
                  -> TcS Int
kickOutRewritable new_flav new_tv
  = do { wl <- modifyInertTcS kick_out
       ; traceTcS "kickOutRewritable" $ 
            vcat [ text "tv = " <+> ppr new_tv  
                 , ptext (sLit "Kicked out =") <+> ppr wl]
       ; updWorkListTcS (appendWorkList wl)
       ; return (workListSize wl)  }
  where
    kick_out :: InertSet -> (WorkList, InertSet)
    kick_out (is@(IS { inert_cans = IC { inert_eqs = tv_eqs
                     , inert_dicts  = dictmap
                     , inert_funeqs = funeqmap
                     , inert_irreds = irreds
                     , inert_insols = insols } }))
       = (kicked_out, is { inert_cans = inert_cans_in })
                -- NB: Notice that don't rewrite 
                -- inert_solved_dicts, and inert_solved_funeqs
                -- optimistically. But when we lookup we have to take the 
                -- subsitution into account
       where
         inert_cans_in = IC { inert_eqs = tv_eqs_in
                            , inert_dicts = dicts_in
                            , inert_funeqs = feqs_in
                            , inert_irreds = irs_in
                            , inert_insols = insols_in }

         kicked_out = WorkList { wl_eqs    = varEnvElts tv_eqs_out
                               , wl_funeqs = foldrBag insertDeque emptyDeque feqs_out
                               , wl_rest   = bagToList (dicts_out `andCts` irs_out 
                                                        `andCts` insols_out) }
  
         (tv_eqs_out,  tv_eqs_in) = partitionVarEnv  kick_out_eq tv_eqs
         (feqs_out,   feqs_in)    = partCtFamHeadMap kick_out_ct funeqmap
         (dicts_out,  dicts_in)   = partitionCCanMap kick_out_ct dictmap
         (irs_out,    irs_in)     = partitionBag     kick_out_ct irreds
         (insols_out, insols_in)  = partitionBag     kick_out_ct insols
           -- Kick out even insolubles; see Note [Kick out insolubles]

    kick_out_ct inert_ct = new_flav `canRewrite` (ctFlavour inert_ct) &&
                          (new_tv `elemVarSet` tyVarsOfCt inert_ct) 
                    -- NB: tyVarsOfCt will return the type 
                    --     variables /and the kind variables/ that are 
                    --     directly visible in the type. Hence we will
                    --     have exposed all the rewriting we care about
                    --     to make the most precise kinds visible for 
                    --     matching classes etc. No need to kick out 
                    --     constraints that mention type variables whose
                    --     kinds could contain this variable!

    kick_out_eq (CTyEqCan { cc_tyvar = tv, cc_rhs = rhs, cc_ev = ev })
      =  (new_flav `canRewrite` inert_flav)  -- See Note [Delicate equality kick-out]
      && (new_tv `elemVarSet` kind_vars ||              -- (1)
          (not (inert_flav `canRewrite` new_flav) &&    -- (2)
           new_tv `elemVarSet` (extendVarSet (tyVarsOfType rhs) tv)))
      where
        inert_flav = ctEvFlavour ev
        kind_vars = tyVarsOfType (tyVarKind tv) `unionVarSet`
                    tyVarsOfType (typeKind rhs)

    kick_out_eq other_ct = pprPanic "kick_out_eq" (ppr other_ct)
\end{code}

Note [Kick out insolubles]
~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have an insoluble alpha ~ [alpha], which is insoluble
because an occurs check.  And then we unify alpha := [Int].  
Then we really want to rewrite the insouluble to [Int] ~ [[Int]].
Now it can be decomposed.  Otherwise we end up with a "Can't match
[Int] ~ [[Int]]" which is true, but a bit confusing because the
outer type constructors match. 

Note [Delicate equality kick-out]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
When adding an equality (a ~ xi), we kick out an inert type-variable 
equality (b ~ phi) in two cases

(1) If the new tyvar can rewrite the kind LHS or RHS of the inert
    equality.  Example:
    Work item: [W] k ~ *
    Inert:     [W] (a:k) ~ ty        
               [W] (b:*) ~ c :: k
    We must kick out those blocked inerts so that we rewrite them
    and can subsequently unify.

(2) If the new tyvar can 
          Work item:  [G] a ~ b
          Inert:      [W] b ~ [a]
    Now at this point the work item cannot be further rewritten by the
    inert (due to the weaker inert flavor). But we can't add the work item
    as-is because the inert set would then have a cyclic substitution, 
    when rewriting a wanted type mentioning 'a'. So we must kick the inert out. 

    We have to do this only if the inert *cannot* rewrite the work item;
    it it can, then the work item will have been fully rewritten by the 
    inert during canonicalisation.  So for example:
         Work item: [W] a ~ Int
         Inert:     [W] b ~ [a]
    No need to kick out the inert, beause the inert substitution is not
    necessarily idemopotent.  See Note [Non-idempotent inert substitution].

See also point (8) of Note [Detailed InertCans Invariants] 

\begin{code}
data SPSolveResult = SPCantSolve
                   | SPSolved TcTyVar
                     -- We solved this /unification/ variable to some type using reflexivity

-- 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 

-- @trySpontaneousSolve wi@ solves equalities where one side is a
-- touchable unification variable.
--     	    See Note [Touchables and givens] 
trySpontaneousSolve :: WorkItem -> TcS SPSolveResult
trySpontaneousSolve workItem@(CTyEqCan { cc_ev = gw
                                       , cc_tyvar = tv1, cc_rhs = xi, cc_loc = d })
  | isGiven gw
  = return SPCantSolve
  | Just tv2 <- tcGetTyVar_maybe xi
  = do { tch1 <- isTouchableMetaTyVarTcS tv1
       ; tch2 <- isTouchableMetaTyVarTcS tv2
       ; case (tch1, tch2) of
           (True,  True)  -> trySpontaneousEqTwoWay d gw tv1 tv2
           (True,  False) -> trySpontaneousEqOneWay d gw tv1 xi
           (False, True)  -> trySpontaneousEqOneWay d gw tv2 (mkTyVarTy tv1)
	   _ -> return SPCantSolve }
  | otherwise
  = do { tch1 <- isTouchableMetaTyVarTcS tv1
       ; if tch1 then trySpontaneousEqOneWay d gw tv1 xi
                 else do { untch <- getUntouchables
                         ; traceTcS "Untouchable LHS, can't spontaneously solve workitem" $
                           vcat [text "Untouchables =" <+> ppr untch
                                , text "Workitem =" <+> ppr workItem ]
                         ; return SPCantSolve }
       }

  -- No need for 
  --      trySpontaneousSolve (CFunEqCan ...) = ...
  -- See Note [No touchables as FunEq RHS] in TcSMonad
trySpontaneousSolve _ = return SPCantSolve

----------------
trySpontaneousEqOneWay :: CtLoc -> CtEvidence 
                       -> TcTyVar -> Xi -> TcS SPSolveResult
-- tv is a MetaTyVar, not untouchable
trySpontaneousEqOneWay d gw tv xi
  | not (isSigTyVar tv) || isTyVarTy xi
  , typeKind xi `tcIsSubKind` tyVarKind tv
  = solveWithIdentity d gw tv xi
  | otherwise -- Still can't solve, sig tyvar and non-variable rhs
  = return SPCantSolve

----------------
trySpontaneousEqTwoWay :: CtLoc -> CtEvidence 
                       -> TcTyVar -> TcTyVar -> TcS SPSolveResult
-- Both tyvars are *touchable* MetaTyvars so there is only a chance for kind error here

trySpontaneousEqTwoWay d gw tv1 tv2
  | k1 `tcIsSubKind` k2 && nicer_to_update_tv2
  = solveWithIdentity d gw tv2 (mkTyVarTy tv1)
  | k2 `tcIsSubKind` k1
  = solveWithIdentity d gw tv1 (mkTyVarTy tv2)
  | otherwise
  = return SPCantSolve
  where
    k1 = tyVarKind tv1
    k2 = tyVarKind tv2
    nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)
\end{code}

Note [Kind errors] 
~~~~~~~~~~~~~~~~~~
Consider the wanted problem: 
      alpha ~ (# Int, Int #) 
where alpha :: ArgKind and (# Int, Int #) :: (#). We can't spontaneously solve this constraint, 
but we should rather reject the program that give rise to it. If 'trySpontaneousEqTwoWay' 
simply returns @CantSolve@ then that wanted constraint is going to propagate all the way and 
get quantified over in inference mode. That's bad because we do know at this point that the 
constraint is insoluble. Instead, we call 'recKindErrorTcS' here, which will fail later on.

The same applies in canonicalization code in case of kind errors in the givens. 

However, when we canonicalize givens we only check for compatibility (@compatKind@). 
If there were a kind error in the givens, this means some form of inconsistency or dead code.

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. 

Note [Spontaneous solving and kind compatibility] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.)

  - 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:
  - 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' 


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! 
      given : a ~ alpha      [having unified alpha := a] 
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] 

We avoid this problem by orienting the resulting given so that the unification
variable is on the left.  [Note that alternatively we could attempt to
enforce this at canonicalization]

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.

\begin{code}
----------------

solveWithIdentity :: CtLoc -> CtEvidence -> TcTyVar -> Xi -> TcS SPSolveResult
-- Solve with the identity coercion 
-- Precondition: kind(xi) is a sub-kind of kind(tv)
-- Precondition: CtEvidence is Wanted or Derived
-- See [New Wanted Superclass Work] to see why solveWithIdentity 
--     must work for Derived as well as Wanted
-- Returns: workItem where 
--        workItem = the new Given constraint
--
-- NB: No need for an occurs check here, because solveWithIdentity always 
--     arises from a CTyEqCan, a *canonical* constraint.  Its invariants
--     say that in (a ~ xi), the type variable a does not appear in xi.
--     See TcRnTypes.Ct invariants.
solveWithIdentity _d wd tv xi 
  = do { let tv_ty = mkTyVarTy tv
       ; traceTcS "Sneaky unification:" $ 
                       vcat [text "Unifies:" <+> ppr tv <+> ptext (sLit ":=") <+> ppr xi,
                             text "Coercion:" <+> pprEq tv_ty xi,
                             text "Left Kind is:" <+> ppr (typeKind tv_ty),
                             text "Right Kind is:" <+> ppr (typeKind xi) ]

       ; let xi' = defaultKind xi      
               -- We only instantiate kind unification variables
               -- with simple kinds like *, not OpenKind or ArgKind
               -- cf TcUnify.uUnboundKVar

       ; setWantedTyBind tv xi'
       ; let refl_evtm = EvCoercion (mkTcReflCo xi')

       ; when (isWanted wd) $ 
              setEvBind (ctev_evar wd) refl_evtm

       ; return (SPSolved tv) }
\end{code}


*********************************************************************************
*                                                                               * 
                       The interact-with-inert Stage
*                                                                               *
*********************************************************************************

Note [

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

\begin{code}
-- Interaction result of  WorkItem <~> Ct

data InteractResult 
    = IRWorkItemConsumed { ir_fire :: String }    -- Work item discharged by interaction; stop
    | IRReplace          { ir_fire :: String }    -- Inert item replaced by work item; stop
    | IRInertConsumed    { ir_fire :: String }    -- Inert item consumed, keep going with work item 
    | IRKeepGoing        { ir_fire :: String }    -- Inert item remains, keep going with work item

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 
  = do { traceTcS "interactWithInerts" $ text "workitem = " <+> ppr wi
       ; rels <- extractRelevantInerts wi 
       ; traceTcS "relevant inerts are:" $ ppr rels
       ; foldlBagM interact_next (ContinueWith wi) rels }

  where interact_next Stop atomic_inert 
          = do { insertInertItemTcS atomic_inert; return Stop }
        interact_next (ContinueWith wi) atomic_inert 
          = do { ir <- doInteractWithInert atomic_inert wi
               ; let mk_msg rule keep_doc 
                       = vcat [ text rule <+> keep_doc
                              , ptext (sLit "InertItem =") <+> ppr atomic_inert
                              , ptext (sLit "WorkItem  =") <+> ppr wi ]
               ; case ir of 
                   IRWorkItemConsumed { ir_fire = rule } 
                       -> do { traceFireTcS wi (mk_msg rule (text "WorkItemConsumed"))
                             ; insertInertItemTcS atomic_inert
                             ; return Stop } 
                   IRReplace { ir_fire = rule }
                       -> do { traceFireTcS atomic_inert 
                                            (mk_msg rule (text "InertReplace"))
                             ; insertInertItemTcS wi
                             ; return Stop } 
                   IRInertConsumed { ir_fire = rule }
                       -> do { traceFireTcS atomic_inert 
                                            (mk_msg rule (text "InertItemConsumed"))
                             ; return (ContinueWith wi) }
                   IRKeepGoing {}
                       -> do { insertInertItemTcS atomic_inert
                             ; return (ContinueWith wi) }
               }
\end{code}

\begin{code}
--------------------------------------------

doInteractWithInert :: Ct -> Ct -> TcS InteractResult
-- Identical class constraints.
doInteractWithInert inertItem@(CDictCan { cc_ev = fl1, cc_class = cls1, cc_tyargs = tys1, cc_loc = loc1 })
                     workItem@(CDictCan { cc_ev = fl2, cc_class = cls2, cc_tyargs = tys2, cc_loc = loc2 })
  | cls1 == cls2  
  = do { let pty1 = mkClassPred cls1 tys1
             pty2 = mkClassPred cls2 tys2
             inert_pred_loc     = (pty1, pprArisingAt loc1)
             work_item_pred_loc = (pty2, pprArisingAt loc2)

       ; let fd_eqns = improveFromAnother inert_pred_loc work_item_pred_loc
       ; fd_work <- rewriteWithFunDeps fd_eqns loc2
                -- We don't really rewrite tys2, see below _rewritten_tys2, so that's ok
                -- NB: We do create FDs for given to report insoluble equations that arise
                -- from pairs of Givens, and also because of floating when we approximate
                -- implications. The relevant test is: typecheck/should_fail/FDsFromGivens.hs
                -- Also see Note [When improvement happens]
       
       ; traceTcS "doInteractWithInert:dict" 
                  (vcat [ text "inertItem =" <+> ppr inertItem
                        , text "workItem  =" <+> ppr workItem
                        , text "fundeps =" <+> ppr fd_work ])
 
       ; case fd_work of
           -- No Functional Dependencies 
           []  | eqTypes tys1 tys2 -> solveOneFromTheOther "Cls/Cls" fl1 workItem
               | otherwise         -> return (IRKeepGoing "NOP")

           -- Actual Functional Dependencies
           _   | cls1 `hasKey` ipClassNameKey
               , isGiven fl1, isGiven fl2  -- See Note [Shadowing of Implicit Parameters]
               -> return (IRReplace ("Replace IP"))

               -- Standard thing: create derived fds and keep on going. Importantly we don't
               -- throw workitem back in the worklist because this can cause loops. See #5236.
               | otherwise 
               -> do { updWorkListTcS (extendWorkListEqs fd_work)
                     ; return (IRKeepGoing "Cls/Cls (new fundeps)") } -- Just keep going without droping the inert 
       }
 
-- 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_ev = ifl })
           workItem@(CIrredEvCan { cc_ev = wfl })
  | ctEvPred ifl `eqType` ctEvPred wfl
  = solveOneFromTheOther "Irred/Irred" ifl workItem

doInteractWithInert ii@(CFunEqCan { cc_ev = ev1, cc_fun = tc1
                                  , cc_tyargs = args1, cc_rhs = xi1, cc_loc = d1 }) 
                    wi@(CFunEqCan { cc_ev = ev2, cc_fun = tc2
                                  , cc_tyargs = args2, cc_rhs = xi2, cc_loc = d2 })
  | i_solves_w && (not (w_solves_i && isMetaTyVarTy xi1))
               -- See Note [Carefully solve the right CFunEqCan]
  = ASSERT( lhss_match )   -- extractRelevantInerts ensures this
    do { traceTcS "interact with inerts: FunEq/FunEq" $ 
         vcat [ text "workItem =" <+> ppr wi
              , text "inertItem=" <+> ppr ii ]

       ; let xev = XEvTerm xcomp xdecomp
             -- xcomp : [(xi2 ~ xi1)] -> (F args ~ xi2) 
             xcomp [x] = EvCoercion (co1 `mkTcTransCo` mk_sym_co x)
             xcomp _   = panic "No more goals!"
             -- xdecomp : (F args ~ xi2) -> [(xi2 ~ xi1)]                 
             xdecomp x = [EvCoercion (mk_sym_co x `mkTcTransCo` co1)]

       ; ctevs <- xCtFlavor ev2 [mkTcEqPred xi2 xi1] xev
             -- No caching!  See Note [Cache-caused loops]
             -- Why not (mkTcEqPred xi1 xi2)? See Note [Efficient orientation]
       ; emitWorkNC d2 ctevs 
       ; return (IRWorkItemConsumed "FunEq/FunEq") }

  | fl2 `canSolve` fl1
  = ASSERT( lhss_match )   -- extractRelevantInerts ensures this
    do { traceTcS "interact with inerts: FunEq/FunEq" $ 
         vcat [ text "workItem =" <+> ppr wi
              , text "inertItem=" <+> ppr ii ]

       ; let xev = XEvTerm xcomp xdecomp
              -- xcomp : [(xi2 ~ xi1)] -> [(F args ~ xi1)]
             xcomp [x] = EvCoercion (co2 `mkTcTransCo` evTermCoercion x)
             xcomp _ = panic "No more goals!"
             -- xdecomp : (F args ~ xi1) -> [(xi2 ~ xi1)]
             xdecomp x = [EvCoercion (mkTcSymCo co2 `mkTcTransCo` evTermCoercion x)]

       ; ctevs <- xCtFlavor ev1 [mkTcEqPred xi2 xi1] xev 
             -- Why not (mkTcEqPred xi1 xi2)? See Note [Efficient orientation]

       ; emitWorkNC d1 ctevs 
       ; return (IRInertConsumed "FunEq/FunEq") }
  where
    lhss_match = tc1 == tc2 && eqTypes args1 args2 
    co1 = evTermCoercion $ ctEvTerm ev1
    co2 = evTermCoercion $ ctEvTerm ev2
    mk_sym_co x = mkTcSymCo (evTermCoercion x)
    fl1 = ctEvFlavour ev1
    fl2 = ctEvFlavour ev2

    i_solves_w = fl1 `canSolve` fl2 
    w_solves_i = fl2 `canSolve` fl1 
    

doInteractWithInert _ _ = return (IRKeepGoing "NOP")
\end{code}

Note [Efficient Orientation] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we are interacting two FunEqCans with the same LHS:
          (inert)  ci :: (F ty ~ xi_i) 
          (work)   cw :: (F ty ~ xi_w) 
We prefer to keep the inert (else we pass the work item on down
the pipeline, which is a bit silly).  If we keep the inert, we
will (a) discharge 'cw' 
     (b) produce a new equality work-item (xi_w ~ xi_i)
Notice the orientation (xi_w ~ xi_i) NOT (xi_i ~ xi_w):
    new_work :: xi_w ~ xi_i
    cw := ci ; sym new_work
Why?  Consider the simplest case when xi1 is a type variable.  If
we generate xi1~xi2, porcessing that constraint will kick out 'ci'.
If we generate xi2~xi1, there is less chance of that happening.
Of course it can and should still happen if xi1=a, xi1=Int, say.
But we want to avoid it happening needlessly.

Similarly, if we *can't* keep the inert item (because inert is Wanted,
and work is Given, say), we prefer to orient the new equality (xi_i ~
xi_w).

Note [Carefully solve the right CFunEqCan]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the constraints
  c1 :: F Int ~ a      -- Arising from an application line 5
  c2 :: F Int ~ Bool   -- Arising from an application line 10
Suppose that 'a' is a unification variable, arising only from 
flattening.  So there is no error on line 5; it's just a flattening
variable.  But there is (or might be) an error on line 10.

Two ways to combine them, leaving either (Plan A)
  c1 :: F Int ~ a      -- Arising from an application line 5
  c3 :: a ~ Bool       -- Arising from an application line 10
or (Plan B)
  c2 :: F Int ~ Bool   -- Arising from an application line 10
  c4 :: a ~ Bool       -- Arising from an application line 5

Plan A will unify c3, leaving c1 :: F Int ~ Bool as an error
on the *totally innocent* line 5.  An example is test SimpleFail16
where the expected/actual message comes out backwards if we use
the wrong plan.

The second is the right thing to do.  Hence the isMetaTyVarTy
test when solving pairwise CFunEqCan.

Note [Shadowing of Implicit Parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Consider the following example:

f :: (?x :: Char) => Char
f = let ?x = 'a' in ?x

The "let ?x = ..." generates an implication constraint of the form:

?x :: Char => ?x :: Char

Furthermore, the signature for `f` also generates an implication
constraint, so we end up with the following nested implication:

?x :: Char => (?x :: Char => ?x :: Char)

Note that the wanted (?x :: Char) constraint may be solved in
two incompatible ways:  either by using the parameter from the
signature, or by using the local definition.  Our intention is
that the local definition should "shadow" the parameter of the
signature, and we implement this as follows: when we add a new
given implicit parameter to the inert set, it replaces any existing
givens for the same implicit parameter.

This works for the normal cases but it has an odd side effect
in some pathological programs like this:

-- This is accepted, the second parameter shadows
f1 :: (?x :: Int, ?x :: Char) => Char
f1 = ?x

-- This is rejected, the second parameter shadows
f2 :: (?x :: Int, ?x :: Char) => Int
f2 = ?x

Both of these are actually wrong:  when we try to use either one,
we'll get two incompatible wnated constraints (?x :: Int, ?x :: Char),
which would lead to an error.

I can think of two ways to fix this:

  1. Simply disallow multiple constratits for the same implicit
    parameter---this is never useful, and it can be detected completely
    syntactically.

  2. Move the shadowing machinery to the location where we nest
     implications, and add some code here that will produce an
     error if we get multiple givens for the same implicit parameter.


Note [Cache-caused loops]
~~~~~~~~~~~~~~~~~~~~~~~~~
It is very dangerous to cache a rewritten wanted family equation as 'solved' in our 
solved cache (which is the default behaviour or xCtFlavor), because the interaction 
may not be contributing towards a solution. Here is an example:

Initial inert set:
  [W] g1 : F a ~ beta1
Work item:
  [W] g2 : F a ~ beta2
The work item will react with the inert yielding the _same_ inert set plus:
    i)   Will set g2 := g1 `cast` g3   
    ii)  Will add to our solved cache that [S] g2 : F a ~ beta2
    iii) Will emit [W] g3 : beta1 ~ beta2 
Now, the g3 work item will be spontaneously solved to [G] g3 : beta1 ~ beta2
and then it will react the item in the inert ([W] g1 : F a ~ beta1). So it 
will set 
      g1 := g ; sym g3 
and what is g? Well it would ideally be a new goal of type (F a ~ beta2) but
remember that we have this in our solved cache, and it is ... g2! In short we 
created the evidence loop:

        g2 := g1 ; g3 
        g3 := refl
        g1 := g2 ; sym g3 

To avoid this situation we do not cache as solved any workitems (or inert) 
which did not really made a 'step' towards proving some goal. Solved's are 
just an optimization so we don't lose anything in terms of completeness of 
solving.

\begin{code}
solveOneFromTheOther :: String    -- Info 
                     -> CtEvidence  -- 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 ifl workItem
  | isDerived wfl
  = return (IRWorkItemConsumed ("Solved[DW] " ++ info))

  | 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
  = return (IRInertConsumed ("Solved[DI] " ++ info))
  
  | CtWanted { ctev_evar = ev_id } <- wfl
  = do { setEvBind ev_id (ctEvTerm ifl); return (IRWorkItemConsumed ("Solved(w) " ++ info)) }

  | CtWanted { ctev_evar = ev_id } <- ifl
  = do { setEvBind ev_id (ctEvTerm wfl); return (IRInertConsumed ("Solved(g) " ++ info)) }

  | otherwise	   -- If both are Given, we already have evidence; no need to duplicate
                   -- But the work item *overrides* the inert item (hence IRReplace)
                   -- See Note [Shadowing of Implicit Parameters]
  = return (IRReplace ("Replace(gg) " ++ info))
  where 
     wfl = cc_ev workItem
\end{code}

Note [Shadowing of Implicit Parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the following example:

f :: (?x :: Char) => Char
f = let ?x = 'a' in ?x

The "let ?x = ..." generates an implication constraint of the form:

?x :: Char => ?x :: Char


Furthermore, the signature for `f` also generates an implication
constraint, so we end up with the following nested implication:

?x :: Char => (?x :: Char => ?x :: Char)

Note that the wanted (?x :: Char) constraint may be solved in
two incompatible ways:  either by using the parameter from the
signature, or by using the local definition.  Our intention is
that the local definition should "shadow" the parameter of the
signature, and we implement this as follows: when we nest implications,
we remove any implicit parameters in the outer implication, that
have the same name as givens of the inner implication.

Here is another variation of the example:

f :: (?x :: Int) => Char
f = let ?x = 'x' in ?x

This program should also be accepted: the two constraints `?x :: Int`
and `?x :: Char` never exist in the same context, so they don't get to
interact to cause failure.

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'' 
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]. 

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 
and create an inert set that includes the solved (Foo [t] t) but not its superclasses:
       d1 :_g Foo [t] t                 d1 := EvDFunApp fooDFun d3 
Our work list is going to contain a new *wanted* goal
       d3 :_w Eq t 

Ok, so how do we get recursive dictionaries, at all: 

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 
 topReactionsStage, 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,
because 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…