TcCanonical.hs 76.1 KB
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{-# LANGUAGE CPP #-}

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module TcCanonical(
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     canonicalize,
     unifyDerived,

     StopOrContinue(..), stopWith, continueWith
  ) where
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#include "HsVersions.h"

import TcRnTypes
import TcType
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import Type
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import TcFlatten
import TcSMonad
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import TcEvidence
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import Class
import TyCon
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import TyCoRep   -- cleverly decomposes types, good for completeness checking
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import Coercion
import FamInstEnv ( FamInstEnvs )
import FamInst ( tcTopNormaliseNewTypeTF_maybe )
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import Var
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import Name( isSystemName )
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import OccName( OccName )
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import Outputable
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import DynFlags( DynFlags )
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import VarSet
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import RdrName
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import Pair
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import Util
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import Bag
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import MonadUtils
import Control.Monad
import Data.List  ( zip4, foldl' )
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import BasicTypes
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import FastString
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#if __GLASGOW_HASKELL__ < 709
bimap :: (a -> b) -> (c -> d) -> Either a c -> Either b d
bimap f _ (Left x)  = Left (f x)
bimap _ f (Right x) = Right (f x)
#else
import Data.Bifunctor ( bimap )
#endif

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{-
************************************************************************
*                                                                      *
*                      The Canonicaliser                               *
*                                                                      *
************************************************************************
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Note [Canonicalization]
~~~~~~~~~~~~~~~~~~~~~~~
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Canonicalization converts a simple constraint to a canonical form. It is
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unary (i.e. treats individual constraints one at a time), does not do
any zonking, but lives in TcS monad because it needs to create fresh
variables (for flattening) and consult the inerts (for efficiency).
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The execution plan for canonicalization is the following:
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  1) Decomposition of equalities happens as necessary until we reach a
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     variable or type family in one side. There is no decomposition step
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     for other forms of constraints.
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  2) If, when we decompose, we discover a variable on the head then we
     look at inert_eqs from the current inert for a substitution for this
     variable and contine decomposing. Hence we lazily apply the inert
     substitution if it is needed.
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  3) If no more decomposition is possible, we deeply apply the substitution
     from the inert_eqs and continue with flattening.
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  4) During flattening, we examine whether we have already flattened some
     function application by looking at all the CTyFunEqs with the same
     function in the inert set. The reason for deeply applying the inert
     substitution at step (3) is to maximise our chances of matching an
     already flattened family application in the inert.
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The net result is that a constraint coming out of the canonicalization
phase cannot be rewritten any further from the inerts (but maybe /it/ can
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rewrite an inert or still interact with an inert in a further phase in the
simplifier.
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Note [Caching for canonicals]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Our plan with pre-canonicalization is to be able to solve a constraint
really fast from existing bindings in TcEvBinds. So one may think that
the condition (isCNonCanonical) is not necessary.  However consider
the following setup:
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InertSet = { [W] d1 : Num t }
WorkList = { [W] d2 : Num t, [W] c : t ~ Int}
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Now, we prioritize equalities, but in our concrete example
(should_run/mc17.hs) the first (d2) constraint is dealt with first,
because (t ~ Int) is an equality that only later appears in the
worklist since it is pulled out from a nested implication
constraint. So, let's examine what happens:
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   - We encounter work item (d2 : Num t)

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   - Nothing is yet in EvBinds, so we reach the interaction with inerts
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     and set:
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              d2 := d1
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    and we discard d2 from the worklist. The inert set remains unaffected.

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   - Now the equation ([W] c : t ~ Int) is encountered and kicks-out
     (d1 : Num t) from the inerts.  Then that equation gets
     spontaneously solved, perhaps. We end up with:
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        InertSet : { [G] c : t ~ Int }
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        WorkList : { [W] d1 : Num t}
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   - Now we examine (d1), we observe that there is a binding for (Num
     t) in the evidence binds and we set:
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             d1 := d2
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     and end up in a loop!

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Now, the constraints that get kicked out from the inert set are always
Canonical, so by restricting the use of the pre-canonicalizer to
NonCanonical constraints we eliminate this danger. Moreover, for
canonical constraints we already have good caching mechanisms
(effectively the interaction solver) and we are interested in reducing
things like superclasses of the same non-canonical constraint being
generated hence I don't expect us to lose a lot by introducing the
(isCNonCanonical) restriction.
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A similar situation can arise in TcSimplify, at the end of the
solve_wanteds function, where constraints from the inert set are
returned as new work -- our substCt ensures however that if they are
not rewritten by subst, they remain canonical and hence we will not
attempt to solve them from the EvBinds. If on the other hand they did
get rewritten and are now non-canonical they will still not match the
EvBinds, so we are again good.
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-}
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-- Top-level canonicalization
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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canonicalize :: Ct -> TcS (StopOrContinue Ct)
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canonicalize ct@(CNonCanonical { cc_ev = ev })
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  = do { traceTcS "canonicalize (non-canonical)" (ppr ct)
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       ; {-# SCC "canEvVar" #-}
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         canEvNC ev }
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canonicalize (CDictCan { cc_ev = ev
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                       , cc_class  = cls
                       , cc_tyargs = xis })
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  = {-# SCC "canClass" #-}
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    canClass ev cls xis -- Do not add any superclasses
canonicalize (CTyEqCan { cc_ev = ev
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                       , cc_tyvar  = tv
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                       , cc_rhs    = xi
                       , cc_eq_rel = eq_rel })
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  = {-# SCC "canEqLeafTyVarEq" #-}
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    canEqNC ev eq_rel (mkTyVarTy tv) xi
      -- NB: Don't use canEqTyVar because that expects flattened types,
      -- and tv and xi may not be flat w.r.t. an updated inert set
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canonicalize (CFunEqCan { cc_ev = ev
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                        , cc_fun    = fn
                        , cc_tyargs = xis1
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                        , cc_fsk    = fsk })
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  = {-# SCC "canEqLeafFunEq" #-}
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    canCFunEqCan ev fn xis1 fsk
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canonicalize (CIrredEvCan { cc_ev = ev })
  = canIrred ev
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canonicalize (CHoleCan { cc_ev = ev, cc_occ = occ, cc_hole = hole })
  = canHole ev occ hole
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canEvNC :: CtEvidence -> TcS (StopOrContinue Ct)
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-- Called only for non-canonical EvVars
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canEvNC ev
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  = case classifyPredType (ctEvPred ev) of
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      ClassPred cls tys     -> do traceTcS "canEvNC:cls" (ppr cls <+> ppr tys)
                                  canClassNC ev cls tys
      EqPred eq_rel ty1 ty2 -> do traceTcS "canEvNC:eq" (ppr ty1 $$ ppr ty2)
                                  canEqNC    ev eq_rel ty1 ty2
      IrredPred {}          -> do traceTcS "canEvNC:irred" (ppr (ctEvPred ev))
                                  canIrred   ev
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{-
************************************************************************
*                                                                      *
*                      Class Canonicalization
*                                                                      *
************************************************************************
-}
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canClass, canClassNC
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   :: CtEvidence
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   -> Class -> [Type] -> TcS (StopOrContinue Ct)
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-- Precondition: EvVar is class evidence
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-- The canClassNC version is used on non-canonical constraints
-- and adds superclasses.  The plain canClass version is used
-- for already-canonical class constraints (but which might have
-- been subsituted or somthing), and hence do not need superclasses

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canClassNC ev cls tys
  = canClass ev cls tys
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    `andWhenContinue` emitSuperclasses

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canClass ev cls tys
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  =   -- all classes do *nominal* matching
    ASSERT2( ctEvRole ev == Nominal, ppr ev $$ ppr cls $$ ppr tys )
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    do { (xis, cos) <- flattenManyNom ev tys
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       ; let co = mkTcTyConAppCo Nominal (classTyCon cls) cos
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             xi = mkClassPred cls xis
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             mk_ct new_ev = CDictCan { cc_ev = new_ev
                                     , cc_tyargs = xis, cc_class = cls }
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       ; mb <- rewriteEvidence ev xi co
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       ; traceTcS "canClass" (vcat [ ppr ev
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                                   , ppr xi, ppr mb ])
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       ; return (fmap mk_ct mb) }
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emitSuperclasses :: Ct -> TcS (StopOrContinue Ct)
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emitSuperclasses ct@(CDictCan { cc_ev = ev , cc_tyargs = xis_new, cc_class = cls })
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            -- Add superclasses of this one here, See Note [Adding superclasses].
            -- But only if we are not simplifying the LHS of a rule.
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 = do { newSCWorkFromFlavored ev cls xis_new
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      -- Arguably we should "seq" the coercions if they are derived,
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      -- as we do below for emit_kind_constraint, to allow errors in
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      -- superclasses to be executed if deferred to runtime!
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      ; continueWith ct }
emitSuperclasses _ = panic "emit_superclasses of non-class!"
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{- Note [Adding superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Since dictionaries are canonicalized only once in their lifetime, the
place to add their superclasses is canonicalisation.  See Note [Add
superclasses only during canonicalisation].  Here is what we do:

  Givens:   Add all their superclasses as Givens.
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            They may be needed to prove Wanteds.
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  Wanteds/Derived:
            Add all their superclasses as Derived.
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            The sole reason is to expose functional dependencies
            in superclasses or equality superclasses.
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Examples of how adding superclasses as Derived is useful

    --- Example 1
        class C a b | a -> b
    Suppose we want to solve
         [G] C a b
         [W] C a beta
    Then adding [D] beta~b will let us solve it.

    -- Example 2 (similar but using a type-equality superclass)
        class (F a ~ b) => C a b
    And try to sllve:
         [G] C a b
         [W] C a beta
    Follow the superclass rules to add
         [G] F a ~ b
         [D] F a ~ beta
    Now we we get [D] beta ~ b, and can solve that.

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    -- Example (tcfail138)
      class L a b | a -> b
      class (G a, L a b) => C a b

      instance C a b' => G (Maybe a)
      instance C a b  => C (Maybe a) a
      instance L (Maybe a) a

    When solving the superclasses of the (C (Maybe a) a) instance, we get
      [G] C a b, and hance by superclasses, [G] G a, [G] L a b
      [W] G (Maybe a)
    Use the instance decl to get
      [W] C a beta
    Generate its derived superclass
      [D] L a beta.  Now using fundeps, combine with [G] L a b to get
      [D] beta ~ b
    which is what we want.

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---------- Historical note -----------
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Example of why adding superclass of a Wanted as a Given would
be terrible, see Note [Do not add superclasses of solved dictionaries]
in TcSMonad, which has this example:
        class Ord a => C a where
        instance Ord [a] => C [a] where ...
Suppose we are trying to solve
  [G] d1 : Ord a
  [W] d2 : C [a]
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If we (bogusly) added the superclass of d2 as Given we'd have
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  [G] d1 : Ord a
  [W] d2 : C [a]
  [G] d3 : Ord [a]   -- Superclass of d2, bogus

Then we'll use the instance decl to give
  [G] d1 : Ord a     Solved: d2 : C [a] = $dfCList d4
  [G] d3 : Ord [a]   -- Superclass of d2, bogus
  [W] d4: Ord [a]

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And now we could bogusly solve d4 from d3.
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---------- End of historical note -----------
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Note [Add superclasses only during canonicalisation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We add superclasses only during canonicalisation, on the passage
from CNonCanonical to CDictCan.  A class constraint can be repeatedly
rewritten, and there's no point in repeatedly adding its superclasses.

Here's a serious, but now out-dated example, from Trac #4497:
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   class Num (RealOf t) => Normed t
   type family RealOf x

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Assume the generated wanted constraint is:
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   [W] RealOf e ~ e
   [W] Normed e

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If we were to be adding the superclasses during simplification we'd get:
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   [W] RealOf e ~ e
   [W] Normed e
   [D] RealOf e ~ fuv
   [D] Num fuv
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==>
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   e := fuv, Num fuv, Normed fuv, RealOf fuv ~ fuv
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While looks exactly like our original constraint. If we add the
superclass of (Normed fuv) again we'd loop.  By adding superclasses
definitely only once, during canonicalisation, this situation can't
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happen.
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Mind you, now that Wanteds cannot rewrite Derived, I think this particular
situation can't happen.
  -}
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newSCWorkFromFlavored :: CtEvidence -> Class -> [Xi] -> TcS ()
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-- Returns superclasses, see Note [Adding superclasses]
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newSCWorkFromFlavored flavor cls xis
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  | CtGiven { ctev_evar = evar, ctev_loc = loc } <- flavor
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  = do { given_evs <- newGivenEvVars (mk_given_loc loc)
                                     (mkEvScSelectors (EvId evar) cls xis)
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       ; emitWorkNC given_evs }
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  | isEmptyVarSet (tyCoVarsOfTypes xis)
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  = return () -- Wanteds with no variables yield no deriveds.
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              -- See Note [Improvement from Ground Wanteds]
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  | otherwise -- Wanted/Derived case, just add those SC that can lead to improvement.
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  = do { let sc_rec_theta = transSuperClasses cls xis
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             impr_theta   = filter isImprovementPred sc_rec_theta
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             loc          = ctEvLoc flavor
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       ; traceTcS "newSCWork/Derived" $ text "impr_theta =" <+> ppr impr_theta
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       ; emitNewDeriveds loc impr_theta }
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  where
    size = sizeTypes xis
    mk_given_loc loc
       | isCTupleClass cls
       = loc   -- For tuple predicates, just take them apart, without
               -- adding their (large) size into the chain.  When we
               -- get down to a base predicate, we'll include its size.
               -- Trac #10335

       | GivenOrigin skol_info <- ctLocOrigin loc
         -- See Note [Solving superclass constraints] in TcInstDcls
         -- for explantation of this transformation for givens
       = case skol_info of
            InstSkol -> loc { ctl_origin = GivenOrigin (InstSC size) }
            InstSC n -> loc { ctl_origin = GivenOrigin (InstSC (n `max` size)) }
            _        -> loc

       | otherwise  -- Probably doesn't happen, since this function
       = loc        -- is only used for Givens, but does no harm
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{-
************************************************************************
*                                                                      *
*                      Irreducibles canonicalization
*                                                                      *
************************************************************************
-}
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canIrred :: CtEvidence -> TcS (StopOrContinue Ct)
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-- Precondition: ty not a tuple and no other evidence form
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canIrred old_ev
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  = do { let old_ty = ctEvPred old_ev
       ; traceTcS "can_pred" (text "IrredPred = " <+> ppr old_ty)
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       ; (xi,co) <- flatten FM_FlattenAll old_ev old_ty -- co :: xi ~ old_ty
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       ; rewriteEvidence old_ev xi co `andWhenContinue` \ new_ev ->
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    do { -- Re-classify, in case flattening has improved its shape
       ; case classifyPredType (ctEvPred new_ev) of
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           ClassPred cls tys     -> canClassNC new_ev cls tys
           EqPred eq_rel ty1 ty2 -> canEqNC new_ev eq_rel ty1 ty2
           _                     -> continueWith $
                                    CIrredEvCan { cc_ev = new_ev } } }
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canHole :: CtEvidence -> OccName -> HoleSort -> TcS (StopOrContinue Ct)
canHole ev occ hole_sort
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  = do { let ty = ctEvPred ev
       ; (xi,co) <- flatten FM_SubstOnly ev ty -- co :: xi ~ ty
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       ; rewriteEvidence ev xi co `andWhenContinue` \ new_ev ->
    do { emitInsoluble (CHoleCan { cc_ev = new_ev
                                 , cc_occ = occ
                                 , cc_hole = hole_sort })
       ; stopWith new_ev "Emit insoluble hole" } }
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{-
************************************************************************
*                                                                      *
*        Equalities
*                                                                      *
************************************************************************
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Note [Canonicalising equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to canonicalise an equality, we look at the structure of the
two types at hand, looking for similarities. A difficulty is that the
types may look dissimilar before flattening but similar after flattening.
However, we don't just want to jump in and flatten right away, because
this might be wasted effort. So, after looking for similarities and failing,
we flatten and then try again. Of course, we don't want to loop, so we
track whether or not we've already flattened.

It is conceivable to do a better job at tracking whether or not a type
is flattened, but this is left as future work. (Mar '15)
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-}
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canEqNC :: CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqNC ev eq_rel ty1 ty2
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  = do { result <- zonk_eq_types ty1 ty2
       ; case result of
           Left (Pair ty1' ty2') -> can_eq_nc False ev eq_rel ty1' ty1 ty2' ty2
           Right ty              -> canEqReflexive ev eq_rel ty }
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can_eq_nc
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   :: Bool            -- True => both types are flat
   -> CtEvidence
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   -> EqRel
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   -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
   -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
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   -> TcS (StopOrContinue Ct)
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can_eq_nc flat ev eq_rel ty1 ps_ty1 ty2 ps_ty2
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  = do { traceTcS "can_eq_nc" $
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         vcat [ ppr ev, ppr eq_rel, ppr ty1, ppr ps_ty1, ppr ty2, ppr ps_ty2 ]
       ; rdr_env <- getGlobalRdrEnvTcS
       ; fam_insts <- getFamInstEnvs
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       ; can_eq_nc' flat rdr_env fam_insts ev eq_rel ty1 ps_ty1 ty2 ps_ty2 }
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can_eq_nc'
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   :: Bool           -- True => both input types are flattened
   -> GlobalRdrEnv   -- needed to see which newtypes are in scope
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   -> FamInstEnvs    -- needed to unwrap data instances
   -> CtEvidence
   -> EqRel
   -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
   -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
   -> TcS (StopOrContinue Ct)
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-- Expand synonyms first; see Note [Type synonyms and canonicalization]
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can_eq_nc' flat _rdr_env _envs ev eq_rel ty1 ps_ty1 ty2 ps_ty2
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  | Just ty1' <- coreView ty1 = can_eq_nc flat ev eq_rel ty1' ps_ty1 ty2  ps_ty2
  | Just ty2' <- coreView ty2 = can_eq_nc flat ev eq_rel ty1  ps_ty1 ty2' ps_ty2
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-- need to check for reflexivity in the ReprEq case.
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-- See Note [Eager reflexivity check]
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-- Check only when flat because the zonk_eq_types check in canEqNC takes
-- care of the non-flat case.
can_eq_nc' True _rdr_env _envs ev ReprEq ty1 _ ty2 _
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  | ty1 `eqType` ty2
  = canEqReflexive ev ReprEq ty1

-- When working with ReprEq, unwrap newtypes.
can_eq_nc' _flat rdr_env envs ev ReprEq ty1 _ ty2 ps_ty2
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  | Just (co, ty1') <- tcTopNormaliseNewTypeTF_maybe envs rdr_env ty1
  = can_eq_newtype_nc rdr_env ev NotSwapped co ty1 ty1' ty2 ps_ty2
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can_eq_nc' _flat rdr_env envs ev ReprEq ty1 ps_ty1 ty2 _
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  | Just (co, ty2') <- tcTopNormaliseNewTypeTF_maybe envs rdr_env ty2
  = can_eq_newtype_nc rdr_env ev IsSwapped  co ty2 ty2' ty1 ps_ty1
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-- Then, get rid of casts
can_eq_nc' flat _rdr_env _envs ev eq_rel (CastTy ty1 co1) _ ty2 ps_ty2
  = canEqCast flat ev eq_rel NotSwapped ty1 co1 ty2 ps_ty2
can_eq_nc' flat _rdr_env _envs ev eq_rel ty1 ps_ty1 (CastTy ty2 co2) _
  = canEqCast flat ev eq_rel IsSwapped ty2 co2 ty1 ps_ty1

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----------------------
-- Otherwise try to decompose
----------------------

-- Literals
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can_eq_nc' _flat _rdr_env _envs ev eq_rel ty1@(LitTy l1) _ (LitTy l2) _
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 | l1 == l2
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  = do { setEqIfWanted ev (mkReflCo (eqRelRole eq_rel) ty1)
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       ; stopWith ev "Equal LitTy" }
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-- Try to decompose type constructor applications
-- Including FunTy (s -> t)
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can_eq_nc' _flat _rdr_env _envs ev eq_rel ty1 _ ty2 _
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  | Just (tc1, tys1) <- tcRepSplitTyConApp_maybe ty1
  , Just (tc2, tys2) <- tcRepSplitTyConApp_maybe ty2
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  , not (isTypeFamilyTyCon tc1)
  , not (isTypeFamilyTyCon tc2)
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  = canTyConApp ev eq_rel tc1 tys1 tc2 tys2
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can_eq_nc' _flat _rdr_env _envs ev eq_rel
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           s1@(ForAllTy (Named {}) _) _ s2@(ForAllTy (Named {}) _) _
 | CtWanted { ctev_loc = loc, ctev_dest = orig_dest } <- ev
 = do { let (bndrs1,body1) = tcSplitNamedPiTys s1
            (bndrs2,body2) = tcSplitNamedPiTys s2
      ; if not (equalLength bndrs1 bndrs2)
           || not (map binderVisibility bndrs1 == map binderVisibility bndrs2)
        then canEqHardFailure ev s1 s2
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        else
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          do { traceTcS "Creating implication for polytype equality" $ ppr ev
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             ; kind_cos <- zipWithM (unifyWanted loc Nominal)
                             (map binderType bndrs1) (map binderType bndrs2)
             ; all_co <- deferTcSForAllEq (eqRelRole eq_rel) loc
                                           kind_cos (bndrs1,body1) (bndrs2,body2)
             ; setWantedEq orig_dest all_co
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             ; stopWith ev "Deferred polytype equality" } }
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 | otherwise
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 = do { traceTcS "Omitting decomposition of given polytype equality" $
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        pprEq s1 s2    -- See Note [Do not decompose given polytype equalities]
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      ; stopWith ev "Discard given polytype equality" }
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-- See Note [Canonicalising type applications] about why we require flat types
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can_eq_nc' True _rdr_env _envs ev eq_rel (AppTy t1 s1) _ ty2 _
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  | Just (t2, s2) <- tcSplitAppTy_maybe ty2
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  = can_eq_app ev eq_rel t1 s1 t2 s2
can_eq_nc' True _rdr_env _envs ev eq_rel ty1 _ (AppTy t2 s2) _
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  | Just (t1, s1) <- tcSplitAppTy_maybe ty1
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  = can_eq_app ev eq_rel t1 s1 t2 s2
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-- No similarity in type structure detected. Flatten and try again.
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can_eq_nc' False rdr_env envs ev eq_rel _ ps_ty1 _ ps_ty2
  = do { (xi1, co1) <- flatten FM_FlattenAll ev ps_ty1
       ; (xi2, co2) <- flatten FM_FlattenAll ev ps_ty2
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       ; rewriteEqEvidence ev NotSwapped xi1 xi2 co1 co2
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         `andWhenContinue` \ new_ev ->
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         can_eq_nc' True rdr_env envs new_ev eq_rel xi1 xi1 xi2 xi2 }

-- Type variable on LHS or RHS are last. We want only flat types sent
-- to canEqTyVar.
-- See also Note [No top-level newtypes on RHS of representational equalities]
can_eq_nc' True _rdr_env _envs ev eq_rel (TyVarTy tv1) _ _ ps_ty2
  = canEqTyVar ev eq_rel NotSwapped tv1 ps_ty2
can_eq_nc' True _rdr_env _envs ev eq_rel _ ps_ty1 (TyVarTy tv2) _
  = canEqTyVar ev eq_rel IsSwapped  tv2 ps_ty1

-- We've flattened and the types don't match. Give up.
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can_eq_nc' True _rdr_env _envs ev _eq_rel _ ps_ty1 _ ps_ty2
  = do { traceTcS "can_eq_nc' catch-all case" (ppr ps_ty1 $$ ppr ps_ty2)
       ; canEqHardFailure ev ps_ty1 ps_ty2 }

---------------------------------
-- | Compare types for equality, while zonking as necessary. Gives up
-- as soon as it finds that two types are not equal.
-- This is quite handy when some unification has made two
-- types in an inert wanted to be equal. We can discover the equality without
-- flattening, which is sometimes very expensive (in the case of type functions).
-- In particular, this function makes a ~20% improvement in test case
-- perf/compiler/T5030.
--
-- Returns either the (partially zonked) types in the case of
-- inequality, or the one type in the case of equality. canEqReflexive is
-- a good next step in the 'Right' case. Returning 'Left' is always safe.
--
-- NB: This does *not* look through type synonyms. In fact, it treats type
-- synonyms as rigid constructors. In the future, it might be convenient
-- to look at only those arguments of type synonyms that actually appear
-- in the synonym RHS. But we're not there yet.
zonk_eq_types :: TcType -> TcType -> TcS (Either (Pair TcType) TcType)
zonk_eq_types = go
  where
    go (TyVarTy tv1) (TyVarTy tv2) = tyvar_tyvar tv1 tv2
    go (TyVarTy tv1) ty2           = tyvar NotSwapped tv1 ty2
    go ty1 (TyVarTy tv2)           = tyvar IsSwapped  tv2 ty1

    go ty1 ty2
      | Just (tc1, tys1) <- tcRepSplitTyConApp_maybe ty1
      , Just (tc2, tys2) <- tcRepSplitTyConApp_maybe ty2
      , tc1 == tc2
      = tycon tc1 tys1 tys2

    go ty1 ty2
      | Just (ty1a, ty1b) <- tcRepSplitAppTy_maybe ty1
      , Just (ty2a, ty2b) <- tcRepSplitAppTy_maybe ty2
      = do { res_a <- go ty1a ty2a
           ; res_b <- go ty1b ty2b
           ; return $ combine_rev mkAppTy res_b res_a }

    go ty1@(LitTy lit1) (LitTy lit2)
      | lit1 == lit2
      = return (Right ty1)

    go ty1 ty2 = return $ Left (Pair ty1 ty2)
      -- we don't handle more complex forms here

    tyvar :: SwapFlag -> TcTyVar -> TcType
          -> TcS (Either (Pair TcType) TcType)
      -- try to do as little as possible, as anything we do here is redundant
      -- with flattening. In particular, no need to zonk kinds. That's why
      -- we don't use the already-defined zonking functions
    tyvar swapped tv ty
      = case tcTyVarDetails tv of
          MetaTv { mtv_ref = ref }
            -> do { cts <- readTcRef ref
                  ; case cts of
                      Flexi        -> give_up
                      Indirect ty' -> unSwap swapped go ty' ty }
          _ -> give_up
      where
        give_up = return $ Left $ unSwap swapped Pair (mkTyVarTy tv) ty

    tyvar_tyvar tv1 tv2
      | tv1 == tv2 = return (Right (mkTyVarTy tv1))
      | otherwise  = do { (ty1', progress1) <- quick_zonk tv1
                        ; (ty2', progress2) <- quick_zonk tv2
                        ; if progress1 || progress2
                          then go ty1' ty2'
                          else return $ Left (Pair (TyVarTy tv1) (TyVarTy tv2)) }

    quick_zonk tv = case tcTyVarDetails tv of
      MetaTv { mtv_ref = ref }
        -> do { cts <- readTcRef ref
              ; case cts of
                  Flexi        -> return (TyVarTy tv, False)
                  Indirect ty' -> return (ty', True) }
      _ -> return (TyVarTy tv, False)

      -- This happens for type families, too. But recall that failure
      -- here just means to try harder, so it's OK if the type function
      -- isn't injective.
    tycon :: TyCon -> [TcType] -> [TcType]
          -> TcS (Either (Pair TcType) TcType)
    tycon tc tys1 tys2
      = do { results <- zipWithM go tys1 tys2
           ; return $ case combine_results results of
               Left tys  -> Left (mkTyConApp tc <$> tys)
               Right tys -> Right (mkTyConApp tc tys) }

    combine_results :: [Either (Pair TcType) TcType]
                    -> Either (Pair [TcType]) [TcType]
    combine_results = bimap (fmap reverse) reverse .
                      foldl' (combine_rev (:)) (Right [])

      -- combine (in reverse) a new result onto an already-combined result
    combine_rev :: (a -> b -> c)
                -> Either (Pair b) b
                -> Either (Pair a) a
                -> Either (Pair c) c
    combine_rev f (Left list) (Left elt) = Left (f <$> elt     <*> list)
    combine_rev f (Left list) (Right ty) = Left (f <$> pure ty <*> list)
    combine_rev f (Right tys) (Left elt) = Left (f <$> elt     <*> pure tys)
    combine_rev f (Right tys) (Right ty) = Right (f ty tys)
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{-
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Note [Newtypes can blow the stack]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)
  newtype Y = MkY (Int -> Y)

and now wish to prove

  [W] X ~R Y

This Wanted will loop, expanding out the newtypes ever deeper looking
for a solid match or a solid discrepancy. Indeed, there is something
appropriate to this looping, because X and Y *do* have the same representation,
in the limit -- they're both (Fix ((->) Int)). However, no finitely-sized
coercion will ever witness it. This loop won't actually cause GHC to hang,
though, because we check our depth when unwrapping newtypes.

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Note [Eager reflexivity check]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)

and

  [W] X ~R X
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Naively, we would start unwrapping X and end up in a loop. Instead,
we do this eager reflexivity check. This is necessary only for representational
equality because the flattener technology deals with the similar case
(recursive type families) for nominal equality.

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Note that this check does not catch all cases, but it will catch the cases
we're most worried about, types like X above that are actually inhabited.
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Here's another place where this reflexivity check is key:
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Consider trying to prove (f a) ~R (f a). The AppTys in there can't
be decomposed, because representational equality isn't congruent with respect
to AppTy. So, when canonicalising the equality above, we get stuck and
would normally produce a CIrredEvCan. However, we really do want to
be able to solve (f a) ~R (f a). So, in the representational case only,
we do a reflexivity check.

(This would be sound in the nominal case, but unnecessary, and I [Richard
E.] am worried that it would slow down the common case.)
-}

------------------------
-- | We're able to unwrap a newtype. Update the bits accordingly.
can_eq_newtype_nc :: GlobalRdrEnv
                  -> CtEvidence           -- ^ :: ty1 ~ ty2
                  -> SwapFlag
                  -> TcCoercion           -- ^ :: ty1 ~ ty1'
                  -> TcType               -- ^ ty1
                  -> TcType               -- ^ ty1'
                  -> TcType               -- ^ ty2
                  -> TcType               -- ^ ty2, with type synonyms
                  -> TcS (StopOrContinue Ct)
can_eq_newtype_nc rdr_env ev swapped co ty1 ty1' ty2 ps_ty2
  = do { traceTcS "can_eq_newtype_nc" $
         vcat [ ppr ev, ppr swapped, ppr co, ppr ty1', ppr ty2 ]

         -- check for blowing our stack:
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         -- See Note [Newtypes can blow the stack]
       ; checkReductionDepth (ctEvLoc ev) ty1
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       ; addUsedDataCons rdr_env (tyConAppTyCon ty1)
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           -- we have actually used the newtype constructor here, so
           -- make sure we don't warn about importing it!

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       ; rewriteEqEvidence ev swapped ty1' ps_ty2
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                           (mkTcSymCo co) (mkTcReflCo Representational ps_ty2)
         `andWhenContinue` \ new_ev ->
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         can_eq_nc False new_ev ReprEq ty1' ty1' ty2 ps_ty2 }
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---------
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-- ^ Decompose a type application.
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-- All input types must be flat. See Note [Canonicalising type applications]
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can_eq_app :: CtEvidence       -- :: s1 t1 ~r s2 t2
           -> EqRel            -- r
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           -> Xi -> Xi         -- s1 t1
           -> Xi -> Xi         -- s2 t2
           -> TcS (StopOrContinue Ct)
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-- AppTys only decompose for nominal equality, so this case just leads
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-- to an irreducible constraint; see typecheck/should_compile/T10494
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-- See Note [Decomposing equality], note {4}
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can_eq_app ev ReprEq _ _ _ _
  = do { traceTcS "failing to decompose representational AppTy equality" (ppr ev)
       ; continueWith (CIrredEvCan { cc_ev = ev }) }
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          -- no need to call canEqFailure, because that flattens, and the
          -- types involved here are already flat
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can_eq_app ev NomEq s1 t1 s2 t2
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  | CtDerived { ctev_loc = loc } <- ev
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  = do { unifyDeriveds loc [Nominal, Nominal] [s1, t1] [s2, t2]
       ; stopWith ev "Decomposed [D] AppTy" }
  | CtWanted { ctev_dest = dest, ctev_loc = loc } <- ev
  = do { co_s <- unifyWanted loc Nominal s1 s2
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       ; co_t <- unifyWanted loc Nominal t1 t2
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       ; let co = mkAppCo co_s co_t
       ; setWantedEq dest co
       ; stopWith ev "Decomposed [W] AppTy" }
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  | CtGiven { ctev_evar = evar, ctev_loc = loc } <- ev
  = do { let co   = mkTcCoVarCo evar
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             co_s = mkTcLRCo CLeft  co
             co_t = mkTcLRCo CRight co
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       ; evar_s <- newGivenEvVar loc ( mkTcEqPredLikeEv ev s1 s2
                                     , EvCoercion co_s )
       ; evar_t <- newGivenEvVar loc ( mkTcEqPredLikeEv ev t1 t2
                                     , EvCoercion co_t )
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       ; emitWorkNC [evar_t]
       ; canEqNC evar_s NomEq s1 s2 }
  | otherwise  -- Can't happen
  = error "can_eq_app"
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-----------------------
-- | Break apart an equality over a casted type
canEqCast :: Bool         -- are both types flat?
          -> CtEvidence
          -> EqRel
          -> SwapFlag
          -> TcType -> Coercion   -- LHS (res. RHS), the casted type
          -> TcType -> TcType     -- RHS (res. LHS), both normal and pretty
          -> TcS (StopOrContinue Ct)
canEqCast flat ev eq_rel swapped ty1 co1 ty2 ps_ty2
  = do { traceTcS "Decomposing cast" (vcat [ ppr ev
                                           , ppr ty1 <+> text "|>" <+> ppr co1
                                           , ppr ps_ty2 ])
       ; rewriteEqEvidence ev swapped ty1 ps_ty2
                           (mkTcReflCo role ty1
                              `mkTcCoherenceRightCo` co1)
                           (mkTcReflCo role ps_ty2)
         `andWhenContinue` \ new_ev ->
         can_eq_nc flat new_ev eq_rel ty1 ty1 ty2 ps_ty2 }
  where
    role = eqRelRole eq_rel

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------------------------
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canTyConApp :: CtEvidence -> EqRel
            -> TyCon -> [TcType]
            -> TyCon -> [TcType]
            -> TcS (StopOrContinue Ct)
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-- See Note [Decomposing TyConApps]
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canTyConApp ev eq_rel tc1 tys1 tc2 tys2
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  | tc1 == tc2
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  , length tys1 == length tys2
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  = do { inerts <- getTcSInerts
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       ; if can_decompose inerts
         then do { traceTcS "canTyConApp"
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                       (ppr ev $$ ppr eq_rel $$ ppr tc1 $$ ppr tys1 $$ ppr tys2)
                 ; canDecomposableTyConAppOK ev eq_rel tc1 tys1 tys2
                 ; stopWith ev "Decomposed TyConApp" }
         else canEqFailure ev eq_rel ty1 ty2 }
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  -- Fail straight away for better error messages
  -- See Note [Use canEqFailure in canDecomposableTyConApp]
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  | eq_rel == ReprEq && not (isGenerativeTyCon tc1 Representational &&
                             isGenerativeTyCon tc2 Representational)
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  = canEqFailure ev eq_rel ty1 ty2
  | otherwise
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  = canEqHardFailure ev ty1 ty2
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  where
    ty1 = mkTyConApp tc1 tys1
    ty2 = mkTyConApp tc2 tys2

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    loc  = ctEvLoc ev
    pred = ctEvPred ev

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     -- See Note [Decomposing equality]
    can_decompose inerts
      =  isInjectiveTyCon tc1 (eqRelRole eq_rel)
      || (ctEvFlavour ev /= Given && isEmptyBag (matchableGivens loc pred inerts))

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{-
Note [Use canEqFailure in canDecomposableTyConApp]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We must use canEqFailure, not canEqHardFailure here, because there is
the possibility of success if working with a representational equality.
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Here is one case:
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  type family TF a where TF Char = Bool
  data family DF a
  newtype instance DF Bool = MkDF Int

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Suppose we are canonicalising (Int ~R DF (TF a)), where we don't yet
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know `a`. This is *not* a hard failure, because we might soon learn
that `a` is, in fact, Char, and then the equality succeeds.
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Here is another case:

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  [G] Age ~R Int
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where Age's constructor is not in scope. We don't want to report
an "inaccessible code" error in the context of this Given!

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For example, see typecheck/should_compile/T10493, repeated here:

  import Data.Ord (Down)  -- no constructor

  foo :: Coercible (Down Int) Int => Down Int -> Int
  foo = coerce

That should compile, but only because we use canEqFailure and not
canEqHardFailure.

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Note [Decomposing equality]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have a constraint (of any flavour and role) that looks like
T tys1 ~ T tys2, what can we conclude about tys1 and tys2? The answer,
of course, is "it depends". This Note spells it all out.

In this Note, "decomposition" refers to taking the constraint
  [fl] (T tys1 ~X T tys2)
(for some flavour fl and some role X) and replacing it with
  [fls'] (tys1 ~Xs' tys2)
where that notation indicates a list of new constraints, where the
new constraints may have different flavours and different roles.

The key property to consider is injectivity. When decomposing a Given the
decomposition is sound if and only if T is injective in all of its type
arguments. When decomposing a Wanted, the decomposition is sound (assuming the
correct roles in the produced equality constraints), but it may be a guess --
that is, an unforced decision by the constraint solver. Decomposing Wanteds
over injective TyCons does not entail guessing. But sometimes we want to
decompose a Wanted even when the TyCon involved is not injective! (See below.)

So, in broad strokes, we want this rule:

(*) Decompose a constraint (T tys1 ~X T tys2) if and only if T is injective
at role X.

Pursuing the details requires exploring three axes:
* Flavour: Given vs. Derived vs. Wanted
* Role: Nominal vs. Representational
* TyCon species: datatype vs. newtype vs. data family vs. type family vs. type variable

(So a type variable isn't a TyCon, but it's convenient to put the AppTy case
in the same table.)

Right away, we can say that Derived behaves just as Wanted for the purposes
of decomposition. The difference between Derived and Wanted is the handling of
evidence. Since decomposition in these cases isn't a matter of soundness but of
guessing, we want the same behavior regardless of evidence.

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Here is a table (discussion following) detailing where decomposition of
   (T s1 ... sn) ~r (T t1 .. tn)
is allowed.  The first four lines (Data types ... type family) refer
to TyConApps with various TyCons T; the last line is for AppTy, where
there is presumably a type variable at the head, so it's actually
   (s s1 ... sn) ~r (t t1 .. tn)
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NOMINAL               GIVEN                       WANTED

Datatype               YES                         YES
Newtype                YES                         YES
Data family            YES                         YES
Type family            YES, in injective args{1}   YES, in injective args{1}
Type variable          YES                         YES

REPRESENTATIONAL      GIVEN                       WANTED

Datatype               YES                         YES
Newtype                NO{2}                      MAYBE{2}
Data family            NO{3}                      MAYBE{3}
Type family             NO                          NO
Type variable          NO{4}                       NO{4}

{1}: Type families can be injective in some, but not all, of their arguments,
so we want to do partial decomposition. This is quite different than the way
other decomposition is done, where the decomposed equalities replace the original
one. We thus proceed much like we do with superclasses: emitting new Givens
when "decomposing" a partially-injective type family Given and new Deriveds
when "decomposing" a partially-injective type family Wanted. (As of the time of
writing, 13 June 2015, the implementation of injective type families has not
been merged, but it should be soon. Please delete this parenthetical if the
implementation is indeed merged.)

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{2}: See Note [Decomposing newtypes at representational role]
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{3}: Because of the possibility of newtype instances, we must treat
data families like newtypes. See also Note [Decomposing newtypes at
representational role]. See #10534 and test case
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typecheck/should_fail/T10534.
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{4}: Because type variables can stand in for newtypes, we conservatively do not
decompose AppTys over representational equality.

In the implementation of can_eq_nc and friends, we don't directly pattern
match using lines like in the tables above, as those tables don't cover
all cases (what about PrimTyCon? tuples?). Instead we just ask about injectivity,
boiling the tables above down to rule (*). The exceptions to rule (*) are for
injective type families, which are handled separately from other decompositions,
and the MAYBE entries above.

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Note [Decomposing newtypes at representational role]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This note discusses the 'newtype' line in the REPRESENTATIONAL table
in Note [Decomposing equality]. (At nominal role, newtypes are fully
decomposable.)

Here is a representative example of why representational equality over
newtypes is tricky:

  newtype Nt a = Mk Bool         -- NB: a is not used in the RHS,
  type role Nt representational  -- but the user gives it an R role anyway

If we have [W] Nt alpha ~R Nt beta, we *don't* want to decompose to
[W] alpha ~R beta, because it's possible that alpha and beta aren't
representationally equal. Here's another example.

  newtype Nt a = MkNt (Id a)
  type family Id a where Id a = a

  [W] Nt Int ~R Nt Age

Because of its use of a type family, Nt's parameter will get inferred to have
a nominal role. Thus, decomposing the wanted will yield [W] Int ~N Age, which
is unsatisfiable. Unwrapping, though, leads to a solution.

Conclusion:
 * Unwrap newtypes before attempting to decompose them.
   This is done in can_eq_nc'.

It all comes from the fact that newtypes aren't necessarily injective
w.r.t. representational equality.

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Furthermore, as explained in Note [NthCo and newtypes] in TyCoRep, we can't use
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NthCo on representational coercions over newtypes. NthCo comes into play
only when decomposing givens.

Conclusion:
 * Do not decompose [G] N s ~R N t

Is it sensible to decompose *Wanted* constraints over newtypes?  Yes!
It's the only way we could ever prove (IO Int ~R IO Age), recalling
that IO is a newtype.

However we must be careful.  Consider

  type role Nt representational

  [G] Nt a ~R Nt b       (1)
  [W] NT alpha ~R Nt b   (2)
  [W] alpha ~ a          (3)

If we focus on (3) first, we'll substitute in (2), and now it's
identical to the given (1), so we succeed.  But if we focus on (2)
first, and decompose it, we'll get (alpha ~R b), which is not soluble.
This is exactly like the question of overlapping Givens for class
constraints: see Note [Instance and Given overlap] in TcInteract.

Conclusion:
  * Decompose [W] N s ~R N t  iff there no given constraint that could
    later solve it.
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-}

canDecomposableTyConAppOK :: CtEvidence -> EqRel
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                          -> TyCon -> [TcType] -> [TcType]
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                          -> TcS ()
-- Precondition: tys1 and tys2 are the same length, hence "OK"
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canDecomposableTyConAppOK ev eq_rel tc tys1 tys2
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  = case ev of
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     CtDerived {}
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        -> unifyDeriveds loc tc_roles tys1 tys2
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     CtWanted { ctev_dest = dest }
        -> do { cos <- zipWith4M unifyWanted new_locs tc_roles tys1 tys2
              ; setWantedEq dest (mkTyConAppCo role tc cos) }
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     CtGiven { ctev_evar = evar }
        -> do { let ev_co = mkCoVarCo evar
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              ; given_evs <- newGivenEvVars loc $
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                             [ ( mkPrimEqPredRole r ty1 ty2
                               , EvCoercion (mkNthCo i ev_co) )
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                             | (r, ty1, ty2, i) <- zip4 tc_roles tys1 tys2 [0..]
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                             , r /= Phantom
                             , not (isCoercionTy ty1) && not (isCoercionTy ty2) ]
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              ; emitWorkNC given_evs }
  where
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    loc        = ctEvLoc ev
    role       = eqRelRole eq_rel
    tc_roles   = tyConRolesX role tc

      -- the following makes a better distinction between "kind" and "type"
      -- in error messages
    (bndrs, _) = splitPiTys (tyConKind tc)
    kind_loc   = toKindLoc loc
    is_kinds   = map isNamedBinder bndrs
    new_locs | Just KindLevel <- ctLocTypeOrKind_maybe loc
             = repeat loc
             | otherwise
             = map (\is_kind -> if is_kind then kind_loc else loc) is_kinds

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-- | Call when canonicalizing an equality fails, but if the equality is
-- representational, there is some hope for the future.
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-- Examples in Note [Use canEqFailure in canDecomposableTyConApp]
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canEqFailure :: CtEvidence -> EqRel
             -> TcType -> TcType -> TcS (StopOrContinue Ct)
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canEqFailure ev NomEq ty1 ty2
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  = canEqHardFailure ev ty1 ty2
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canEqFailure ev ReprEq ty1 ty2
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  = do { (xi1, co1) <- flatten FM_FlattenAll ev ty1
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       ; (xi2, co2) <- flatten FM_FlattenAll ev ty2
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            -- We must flatten the types before putting them in the
            -- inert set, so that we are sure to kick them out when
            -- new equalities become available
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       ; traceTcS "canEqFailure with ReprEq" $
         vcat [ ppr ev, ppr ty1, ppr ty2, ppr xi1, ppr xi2 ]
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       ; rewriteEqEvidence ev NotSwapped xi1 xi2 co1 co2
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         `andWhenContinue` \ new_ev ->
         continueWith (CIrredEvCan { cc_ev = new_ev }) }
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-- | Call when canonicalizing an equality fails with utterly no hope.
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canEqHardFailure :: CtEvidence
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                 -> TcType -> TcType -> TcS (StopOrContinue Ct)
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-- See Note [Make sure that insolubles are fully rewritten]
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canEqHardFailure ev ty1 ty2
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  = do { (s1, co1) <- flatten FM_SubstOnly ev ty1
       ; (s2, co2) <- flatten FM_SubstOnly ev ty2
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       ; rewriteEqEvidence ev NotSwapped s1 s2 co1 co2
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         `andWhenContinue` \ new_ev ->
    do { emitInsoluble (mkNonCanonical new_ev)
       ; stopWith new_ev "Definitely not equal" }}
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{-
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Note [Decomposing TyConApps]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we see (T s1 t1 ~ T s2 t2), then we can just decompose to
  (s1 ~ s2, t1 ~ t2)
and push those back into the work list.  But if
  s1 = K k1    s2 = K k2
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then we will just decomopose s1~s2, and it might be better to
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do so on the spot.  An important special case is where s1=s2,
and we get just Refl.

So canDecomposableTyCon is a fast-path decomposition that uses
unifyWanted etc to short-cut that work.

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Note [Canonicalising type applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Given (s1 t1) ~ ty2, how should we proceed?
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The simple things is to see if ty2 is of form (s2 t2), and
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decompose.  By this time s1 and s2 can't be saturated type
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function applications, because those have been dealt with
by an earlier equation in can_eq_nc, so it is always sound to
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decompose.

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However, over-eager decomposition gives bad error messages
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for things like
   a b ~ Maybe c
   e f ~ p -> q
Suppose (in the first example) we already know a~Array.  Then if we
decompose the application eagerly, yielding
   a ~ Maybe
   b ~ c
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we get an error        "Can't match Array ~ Maybe",
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but we'd prefer to get "Can't match Array b ~ Maybe c".

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So instead can_eq_wanted_app flattens the LHS and RHS, in the hope of
replacing (a b) by (Array b), before using try_decompose_app to
decompose it.
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Note [Make sure that insolubles are fully rewritten]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When an equality fails, we still want to rewrite the equality
all the way down, so that it accurately reflects
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 (a) the mutable reference substitution in force at start of solving
 (b) any ty-binds in force at this point in solving
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See Note [Kick out insolubles] in TcSMonad.
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And if we don't do this there is a bad danger that
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TcSimplify.applyTyVarDefaulting will find a variable
that has in fact been substituted.

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Note [Do not decompose Given polytype equalities]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider [G] (forall a. t1 ~ forall a. t2).  Can we decompose this?
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No -- what would the evidence look like?  So instead we simply discard
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this given evidence.
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Note [Combining insoluble constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As this point we have an insoluble constraint, like Int~Bool.

 * If it is Wanted, delete it from the cache, so that subsequent
   Int~Bool constraints give rise to separate error messages

 * But if it is Derived, DO NOT delete from cache.  A class constraint
   may get kicked out of the inert set, and then have its functional
   dependency Derived constraints generated a second time. In that
   case we don't want to get two (or more) error messages by
   generating two (or more) insoluble fundep constraints from the same
   class constraint.
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Note [No top-level newtypes on RHS of representational equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we're in this situation:

 work item:  [W] c1 : a ~R b
     inert:  [G] c2 : b ~R Id a

where
  newtype Id a = Id a

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We want to make sure canEqTyVar sees [W] a ~R a, after b is flattened
and the Id newtype is unwrapped. This is assured by requiring only flat
types in canEqTyVar *and* having the newtype-unwrapping check above
the tyvar check in can_eq_nc.
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Note [Occurs check error]
~~~~~~~~~~~~~~~~~~~~~~~~~
If we have an occurs check error, are we necessarily hosed? Say our
tyvar is tv1 and the type it appears in is xi2. Because xi2 is function
free, then if we're computing w.r.t. nominal equality, then, yes, we're
hosed. Nothing good can come from (a ~ [a]). If we're computing w.r.t.
representational equality, this is a little subtler. Once again, (a ~R [a])
is a bad thing, but (a ~R N a) for a newtype N might be just fine. This
means also that (a ~ b a) might be fine, because `b` might become a newtype.

So, we must check: does tv1 appear in xi2 under any type constructor that
is generative w.r.t. representational equality? That's what isTyVarUnderDatatype
does. (The other name I considered, isTyVarUnderTyConGenerativeWrtReprEq was
a bit verbose. And the shorter name gets the point across.)

See also #10715, which induced this addition.

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Note [No derived kind equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we're working with a heterogeneous derived equality

  [D] (t1 :: k1) ~ (t2 :: k2)

we want to homogenise to establish the kind invariant on CTyEqCans.
But we can't emit [D] k1 ~ k2 because we wouldn't then be able to
use the evidence in the homogenised types. So we emit a wanted
constraint, because we do really need the evidence here.

Thus: no derived kind equalities.

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-}
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canCFunEqCan :: CtEvidence
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             -> TyCon -> [TcType]   -- LHS
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             -> TcTyVar             -- RHS
             -> TcS (StopOrContinue Ct)
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-- ^ Canonicalise a CFunEqCan.  We know that
--     the arg types are already flat,
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-- and the RHS is a fsk, which we must *not* substitute.
-- So just substitute in the LHS
canCFunEqCan ev fn tys fsk
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  = do { (tys', cos) <- flattenManyNom ev tys
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                        -- cos :: tys' ~ tys
       ; let lhs_co  = mkTcTyConAppCo Nominal fn cos
                        -- :: F tys' ~ F tys
             new_lhs = mkTyConApp fn tys'
             fsk_ty  = mkTyVarTy fsk
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       ; rewriteEqEvidence ev NotSwapped new_lhs fsk_ty
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                           lhs_co (mkTcNomReflCo fsk_ty)
         `andWhenContinue` \ ev' ->
    do { extendFlatCache fn tys' (ctEvCoercion ev', fsk_ty, ctEvFlavour ev')
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       ; continueWith (CFunEqCan { cc_ev = ev', cc_fun = fn
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                                 , cc_tyargs = tys', cc_fsk = fsk }) } }
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---------------------
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canEqTyVar :: CtEvidence -> EqRel -> SwapFlag
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           -> TcTyVar             -- already flat
           -> TcType              -- already flat
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           -> TcS (StopOrContinue Ct)
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-- A TyVar on LHS, but so far un-zonked
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canEqTyVar ev eq_rel swapped tv1 ps_ty2              -- ev :: tv ~ s2
  = do { traceTcS "canEqTyVar" (ppr tv1 $$ ppr ps_ty2 $$ ppr swapped)
         -- FM_Avoid commented out: see Note [Lazy flattening] in TcFlatten
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         -- let fmode = FE { fe_ev = ev, fe_mode = FM_Avoid tv1' True }
         -- Flatten the RHS less vigorously, to avoid gratuitous flattening
         -- True <=> xi2 should not itself be a type-function application
       ; dflags <- getDynFlags
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       ; canEqTyVar2 dflags ev eq_rel swapped tv1 ps_ty2 }
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canEqTyVar2 :: DynFlags
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