[commit: ghc] wip/rae-new-coercible: Rewrite `Coercible` solver (1d29739)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : wip/rae-new-coercible
Link       : http://ghc.haskell.org/trac/ghc/changeset/1d29739aa690a43befd98b30223dabd93168ba5c/ghc

>---------------------------------------------------------------

commit 1d29739aa690a43befd98b30223dabd93168ba5c
Author: Richard Eisenberg <eir at cis.upenn.edu>
Date:   Mon Dec 1 10:28:34 2014 -0500

    Rewrite `Coercible` solver
    
    Summary:
    This is a rewrite of the algorithm to solve for Coercible "instances".
    
    A preliminary form of these ideas is at
    https://ghc.haskell.org/trac/ghc/wiki/Design/NewCoercibleSolver
    
    The basic idea here is that the `EqPred` constructor of `PredTree`
    now is parameterised by a new type `EqRel` (where
    `data EqRel = NomEq | ReprEq`). Thus, every equality constraint can
    now talk about nominal equality (the usual case) or representational
    equality (the `Coercible` case).
    
    This is a change from the previous
    behavior where `Coercible` was just considered a regular class with
    a special case in `matchClassInst`.
    
    Because of this change, representational equalities are now
    canonicalized just like nominal ones, allowing more equalities
    to be solved -- in particular, the case at the top of #9117.
    
    A knock-on effect is that the flattener must be aware of the
    choice of equality relation, because the inert set now stores
    both representational inert equalities alongside the nominal
    inert equalities. Of course, we can use representational equalities
    to rewrite only within another representational equality --
    thus the parameterization of the flattener.
    
    A nice side effect of this change is that I've introduced a new
    type `CtFlavour`, which tracks G vs. W vs. D, removing some ugliness
    in the flattener.
    
    Test Plan: testsuite, a new T9117, and others TBD
    
    Reviewers: simonpj, austin, nomeata
    
    Subscribers: carter, thomie
    
    Differential Revision: https://phabricator.haskell.org/D546
    
    GHC Trac Issues: #9117


>---------------------------------------------------------------

1d29739aa690a43befd98b30223dabd93168ba5c
 compiler/typecheck/TcCanonical.lhs |  0
 compiler/typecheck/TcErrors.lhs    |  0
 compiler/typecheck/TcFlatten.lhs   |  0
 compiler/typecheck/TcSMonad.lhs    | 11 ++++++++---
 compiler/typecheck/TcValidity.lhs  |  0
 compiler/types/Coercion.hs         |  2 --
 6 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/compiler/typecheck/TcSMonad.lhs b/compiler/typecheck/TcSMonad.lhs
index 80ff73b..c9b979d 100644
--- a/compiler/typecheck/TcSMonad.lhs
+++ b/compiler/typecheck/TcSMonad.lhs
@@ -818,7 +818,8 @@ splitInertCans iCans = (given,derived,wanted)
   where
     allCts   = foldDicts  (:) (inert_dicts iCans)
              $ foldFunEqs (:) (inert_funeqs iCans)
-             $ concat (varEnvElts (inert_eqs iCans))
+             $ concat (varEnvElts (inert_eqs iCans) ++
+                       varEnvElts (inert_repr_eqs iCans))
 
     (derived,other) = partition isDerivedCt allCts
     (wanted,given)  = partition isWantedCt  other
@@ -838,8 +839,12 @@ removeInertCt is ct =
     CFunEqCan { cc_fun  = tf,  cc_tyargs = tys } ->
       is { inert_funeqs = delFunEq (inert_funeqs is) tf tys }
 
-    CTyEqCan  { cc_tyvar = x,  cc_rhs    = ty  } ->
-      is { inert_eqs = delTyEq (inert_eqs is) x ty }
+    CTyEqCan  { cc_tyvar = x,  cc_rhs    = ty, cc_eq_rel = NomEq } ->
+      is { inert_eqs      = delTyEq (inert_eqs is) x ty
+         , inert_repr_eqs = delTyEq (inert_repr_eqs is) x ty }
+
+    CTyEqCan  { cc_tyvar = x,  cc_rhs    = ty, cc_eq_rel = ReprEq } ->
+      is { inert_repr_eqs = delTyEq (inert_repr_eqs is) x ty }
 
     CIrredEvCan {}   -> panic "removeInertCt: CIrredEvCan"
     CNonCanonical {} -> panic "removeInertCt: CNonCanonical"
diff --git a/compiler/types/Coercion.hs b/compiler/types/Coercion.hs
index bb39207..2b5081a 100644
--- a/compiler/types/Coercion.hs
+++ b/compiler/types/Coercion.hs
@@ -1994,5 +1994,3 @@ instance IsCoercion Coercion where
 gMkUnbranchedAxInstCo :: IsCoercion co
                       => Role -> CoAxiom Unbranched -> [Type] -> co
 gMkUnbranchedAxInstCo r ax = gMkAxInstCo r ax 0
-
-\end{code}