1. 26 Jun, 2015 1 commit
  2. 21 Jun, 2015 1 commit
  3. 16 Jun, 2015 1 commit
    • eir@cis.upenn.edu's avatar
      Decompose wanted repr. eqs. when no matchable givens. · ff82387d
      eir@cis.upenn.edu authored
      This is pursuant to a conversion with SPJ, where we agreed that
      the logic behind Note [Instance and Given overlap] in TcInteract
      applied to newtype decomposition for representational equality.
      
      There is no bug report or test case, as tickling this kind of thing
      is quite hard to do!
      ff82387d
  4. 15 Jun, 2015 1 commit
  5. 11 Jun, 2015 1 commit
    • Simon Peyton Jones's avatar
      Another major improvement of "improvement" · ddbb97d0
      Simon Peyton Jones authored
      I wasn't very happy with my fix to Trac #10009. This is much better.
      
      The main idea is that the inert set now contains a "model", which
      embodies *all* the (nominal) equalities that we know about, with
      a view to exposing unifications.  This requires a lot fewer iterations
      of the solver than before.
      
      There are extensive comments in
       TcSMonad:  Note [inert_model: the inert model]
                  Note [Adding an inert canonical constraint the InertCans]
      
      The big changes are
      
        * New inert_model field in InertCans
      
        * Functions addInertEq, addInertCan deal with adding a
          constraint, maintaining the model
      
        * A nice improvement is that unification variables can
          unify with fmvs, so that from, say   alpha ~ fmv
          we get              alpha := fmv
          See Note [Orientation of equalities with fmvs] in TcFlatten
          It's still not perfect, as the Note explains
      
      New flag -fconstraint-solver-iterations=n, allows us to control
      the number of constraint solver iterations, and in particular
      will flag up when it's more than a small number.
      
      Performance is generally slightly better:
      T5837 is a lot better for some reason.
      ddbb97d0
  6. 01 Jun, 2015 1 commit
    • Simon Peyton Jones's avatar
      Re-do superclass solving (again); fixes #10423 · 1189196c
      Simon Peyton Jones authored
      TcInstDcls.tcSuperClasses was getting increasingly baroque as a
      succession of tickets (#10423 being the latest) pointed out that
      my cunning plan was not so cunning.
      
      The big issue is how to restrict the evidence that we generate
      for superclass constraints in an instance declaration to avoid
      superclass loops.  See Note [Recursive superclasses] in TcInstDcls
      which explains the plan.
      
      The question is how to implement the plan.  The new implementation is
      much neater, and is described in Note [Solving superclass constraints]
      in TcInstDcls.
      1189196c
  7. 18 May, 2015 2 commits
    • Simon Peyton Jones's avatar
      Make the "matchable-given" check happen first · 228ddb95
      Simon Peyton Jones authored
      This change makes the matchable-given check apply uniformly to
           - constraint tuples
           - natural numbers
           - Typeable
      as well as to vanilla class constraints.
      
      See Note [Instance and Given overlap] in TcInteract
      228ddb95
    • Simon Peyton Jones's avatar
      Refactor tuple constraints · ffc21506
      Simon Peyton Jones authored
      Make tuple constraints be handled by a perfectly ordinary
      type class, with the component constraints being the
      superclasses:
          class (c1, c2) => (c2, c2)
      
      This change was provoked by
      
        #10359  inability to re-use a given tuple
                constraint as a whole
      
        #9858   confusion between term tuples
                and constraint tuples
      
      but it's generally a very nice simplification. We get rid of
       -  In Type, the TuplePred constructor of PredTree,
          and all the code that dealt with TuplePreds
       -  In TcEvidence, the constructors EvTupleMk, EvTupleSel
      
      See Note [How tuples work] in TysWiredIn.
      
      Of course, nothing is ever entirely simple. This one
      proved quite fiddly.
      
      - I did quite a bit of renaming, which makes this patch
        touch a lot of modules. In partiuclar tupleCon -> tupleDataCon.
      
      - I made constraint tuples known-key rather than wired-in.
        This is different to boxed/unboxed tuples, but it proved
        awkward to have all the superclass selectors wired-in.
        Easier just to use the standard mechanims.
      
      - While I was fiddling with known-key names, I split the TH Name
        definitions out of DsMeta into a new module THNames.  That meant
        that the known-key names can all be gathered in PrelInfo, without
        causing module loops.
      
      - I found that the parser was parsing an import item like
            T( .. )
        as a *data constructor* T, and then using setRdrNameSpace to
        fix it.  Stupid!  So I changed the parser to parse a *type
        constructor* T, which means less use of setRdrNameSpace.
      
        I also improved setRdrNameSpace to behave better on Exact Names.
        Largely on priciple; I don't think it matters a lot.
      
      - When compiling a data type declaration for a wired-in thing like
        tuples (,), or lists, we don't really need to look at the
        declaration.  We have the wired-in thing!  And not doing so avoids
        having to line up the uniques for data constructor workers etc.
        See Note [Declarations for wired-in things]
      
      - I found that FunDeps.oclose wasn't taking superclasses into
        account; easily fixed.
      
      - Some error message refactoring for invalid constraints in TcValidity
      
      - Haddock needs to absorb the change too; so there is a submodule update
      ffc21506
  8. 14 May, 2015 1 commit
    • Austin Seipp's avatar
      Revert multiple commits · 3cf8ecdc
      Austin Seipp authored
      This reverts multiple commits from Simon:
      
        - 04a484ea Test Trac #10359
        - a9ccd37a Test Trac #10403
        - c0aae6f6 Test Trac #10248
        - eb6ca851 Make the "matchable-given" check happen first
        - ca173aa3 Add a case to checkValidTyCon
        - 51cbad15 Update haddock submodule
        - 6e1174da Separate transCloVarSet from fixVarSet
        - a8493e03 Fix imports in HscMain (stage2)
        - a154944b Two wibbles to fix the build
        - 5910a1bc Change in capitalisation of error msg
        - 130e93aa Refactor tuple constraints
        - 8da785d5 Delete commented-out line
      
      These break the build by causing Haddock to fail mysteriously when
      trying to examine GHC.Prim it seems.
      3cf8ecdc
  9. 13 May, 2015 3 commits
    • Simon Peyton Jones's avatar
      Make the "matchable-given" check happen first · eb6ca851
      Simon Peyton Jones authored
      This change makes the matchable-given check apply uniformly to
           - constraint tuples
           - natural numbers
           - Typeable
      as well as to vanilla class constraints.
      
      See Note [Instance and Given overlap] in TcInteract
      eb6ca851
    • Simon Peyton Jones's avatar
      Two wibbles to fix the build · a154944b
      Simon Peyton Jones authored
      ...following the constraint-tuple patch.
      
      * There was interaction with the recent Safe Haskell change
      * Haddock comoplained about constraint tuples defined but not used
      a154944b
    • Simon Peyton Jones's avatar
      Refactor tuple constraints · 130e93aa
      Simon Peyton Jones authored
      Make tuple constraints be handled by a perfectly ordinary
      type class, with the component constraints being the
      superclasses:
          class (c1, c2) => (c2, c2)
      
      This change was provoked by
      
        #10359  inability to re-use a given tuple
                constraint as a whole
      
        #9858   confusion between term tuples
                and constraint tuples
      
      but it's generally a very nice simplification. We get rid of
       -  In Type, the TuplePred constructor of PredTree,
          and all the code that dealt with TuplePreds
       -  In TcEvidence, the constructors EvTupleMk, EvTupleSel
      
      See Note [How tuples work] in TysWiredIn.
      
      Of course, nothing is ever entirely simple. This one
      proved quite fiddly.
      
      - I did quite a bit of renaming, which makes this patch
        touch a lot of modules. In partiuclar tupleCon -> tupleDataCon.
      
      - I made constraint tuples known-key rather than wired-in.
        This is different to boxed/unboxed tuples, but it proved
        awkward to have all the superclass selectors wired-in.
        Easier just to use the standard mechanims.
      
      - While I was fiddling with known-key names, I split the TH Name
        definitions out of DsMeta into a new module THNames.  That meant
        that the known-key names can all be gathered in PrelInfo, without
        causing module loops.
      
      - I found that the parser was parsing an import item like
            T( .. )
        as a *data constructor* T, and then using setRdrNameSpace to
        fix it.  Stupid!  So I changed the parser to parse a *type
        constructor* T, which means less use of setRdrNameSpace.
      
        I also improved setRdrNameSpace to behave better on Exact Names.
        Largely on priciple; I don't think it matters a lot.
      
      - When compiling a data type declaration for a wired-in thing like
        tuples (,), or lists, we don't really need to look at the
        declaration.  We have the wired-in thing!  And not doing so avoids
        having to line up the uniques for data constructor workers etc.
        See Note [Declarations for wired-in things]
      
      - I found that FunDeps.oclose wasn't taking superclasses into
        account; easily fixed.
      
      - Some error message refactoring for invalid constraints in TcValidity
      130e93aa
  10. 12 May, 2015 2 commits
    • Erik de Castro Lopo's avatar
      Use fmap instead of <$> (Fixes #10407) · c119a802
      Erik de Castro Lopo authored
      
      
      The <$> operator is only available in the standard Prelude in
      ghc 7.10 and later.
      Signed-off-by: Erik de Castro Lopo's avatarErik de Castro Lopo <erikd@mega-nerd.com>
      
      Test Plan: build with ghc-7.6
      
      Reviewers: dterei, ezyang, austin
      
      Subscribers: bgamari, thomie
      
      Differential Revision: https://phabricator.haskell.org/D886
      
      GHC Trac Issues: #10407
      c119a802
    • David Terei's avatar
      New handling of overlapping inst in Safe Haskell · 4fffbc34
      David Terei authored
      We do much better now due to the newish per-instance flags. Rather than
      mark any module that uses `-XOverlappingInstances`,
      `-XIncoherentInstances` or the new `OVERLAP*` pragmas as unsafe, we
      regard them all as safe and defer the check until an overlap occurs.
      
      An type-class method call that involves overlapping instances is
      considered _unsafe_ when:
      
      1) The most specific instance, Ix, is from a module marked `-XSafe`
      2) Ix is an orphan instance or a MPTC
      3) At least one instance that Ix overlaps, Iy, is:
         a) from a different module than Ix
         AND
         b) Iy is not marked `OVERLAPPABLE`
      
      This check is only enforced in modules compiled with `-XSafe` or
      `-XTrustworthy`.
      
      This fixes Safe Haskell to work with the latest overlapping instance
      pragmas, and also brings consistent behavior. Previously, Safe Inferred
      modules behaved differently than `-XSafe` modules.
      4fffbc34
  11. 30 Apr, 2015 1 commit
  12. 29 Apr, 2015 1 commit
    • Simon Peyton Jones's avatar
      Improve improvement in the constraint solver · a1275a76
      Simon Peyton Jones authored
      This regrettably-big patch substantially improves the way in which
      "improvement" happens in the constraint solver.  It was triggered by
      trying to crack Trac #10009, but it turned out to solve #10340 as
      well.
      
      The big picture, with several of the trickiest examples, is described
      in Note [The improvement story] in TcInteract.
      
      The major change is this:
      
      * After solving we explicitly try "improvement", by
           - making the unsolved Wanteds into Deriveds
           - allowing Deriveds to rewrite Deriveds
        This more aggressive rewriting "unlocks" some extra
        guess-free unifications.
      
      * The main loop is in TcInteract.solveSimpleWanteds, but I also ended
        up refactoring TcSimplify.simpl_loop, and its surrounding code.
      
        Notably, any insolubles from the Givens are pulled out
        and treated separately, rather than staying in the inert set
        during the solveSimpleWanteds loop.
      
      There are a lot of follow-on changes
      
      * Do not emit generate Derived improvements from Wanteds.
        This saves work in the common case where they aren't needed.
      
      * For improvement we should really do type-class reduction on Derived
        constraints in doTopReactDict.  That entailed changing the GenInst
        constructor a bit; a local and minor change
      
      * Some annoying faffing about with dropping derived constraints;
        see dropDerivedWC, dropDerivedSimples, dropDerivedInsols,
        and their Notes.
      
      * Some substantial refactoring in TcErrors.reportWanteds.
        This work wasn't strictly forced, but I got sucked into it.
        All the changes are in TcErrors.
      
      * Use TcS.unifyTyVar consistently, rather than setWantedTyBind,
        so that unifications are properly tracked.
      
      * Refactoring around solveWantedsTcM, solveWantedsAndDrop.
        They previously guaranteed a zonked result, but it's more
        straightforward for clients to zonk.
      a1275a76
  13. 16 Apr, 2015 1 commit
  14. 14 Apr, 2015 1 commit
    • Simon Peyton Jones's avatar
      Do not allow Typeable on constraints (Trac #9858) · 7b042d5a
      Simon Peyton Jones authored
      The astonishingly-ingenious trio of
      Shachaf Ben-Kiki, Ørjan Johansen and Nathan van Doorn
      managed to persuade GHC 7.10.1 to cough up unsafeCoerce.
      
      That is very bad. This patch fixes it by no allowing Typable
      on Constraint-kinded things.  And that seems right, since
      it is, in effect, a form of impredicative polymorphism,
      which Typeable definitely doesn't support.
      
      We may want to creep back in the direction of allowing
      Typeable on constraints one day, but this is a good
      fix for now, and closes a terrible hole.
      7b042d5a
  15. 10 Apr, 2015 1 commit
  16. 09 Apr, 2015 1 commit
  17. 07 Apr, 2015 1 commit
    • Simon Peyton Jones's avatar
      More aggressive Given/Wanted overlap check · 8b7ceece
      Simon Peyton Jones authored
      This fixes Trac #10195
      
      For some reason we considered untouchability before, but Trac #10195
      shows that this is positively worng.
      
      See Note [Instance and Given overlap] in TcInteract.
      
      Looking at the Git log, it seems that this bug was introduced at the
      same time that we introduced the Given/Wanted overlap check in the first
      place.
      8b7ceece
  18. 23 Mar, 2015 1 commit
    • eir@cis.upenn.edu's avatar
      Do proper depth checking in the flattener to avoid looping. · c1edbdfd
      eir@cis.upenn.edu authored
      This implements (roughly) the plan put forward in comment:14:ticket:7788,
      fixing #7788, #8550, #9554, #10139, and addressing concerns raised in #10079.
      There are some regressions w.r.t. GHC 7.8, but only with pathological type
      families (like F a = F a). This also (hopefully -- don't have a test case)
      fixes #10158. Unsolved problems include #10184 and #10185, which are both
      known deficiencies of the approach used here.
      
      As part of this change, the plumbing around detecting infinite loops has
      changed. Instead of -fcontext-stack and -ftype-function-depth, we now have
      one combined -freduction-depth parameter. Setting it to 0 disbales the
      check, which is now the recommended way to get (terminating) code to
      typecheck in releases. (The number of reduction steps may well change between
      minor GHC releases!)
      
      This commit also introduces a new IntWithInf type in BasicTypes
      that represents an integer+infinity. This type is used in a few
      places throughout the code.
      
      Tests in
        indexed-types/should_fail/T7788
        indexed-types/should_fail/T8550
        indexed-types/should_fail/T9554
        indexed-types/should_compile/T10079
        indexed-types/should_compile/T10139
        typecheck/should_compile/T10184  (expected broken)
        typecheck/should_compile/T10185  (expected broken)
      
      This commit also changes performance testsuite numbers, for the better.
      c1edbdfd
  19. 19 Mar, 2015 2 commits
  20. 07 Mar, 2015 1 commit
    • Iavor S. Diatchki's avatar
      Custom `Typeable` solver, that keeps track of kinds. · b359c886
      Iavor S. Diatchki authored
      Summary:
      This implements the new `Typeable` solver: when GHC sees `Typeable` constraints
      it solves them on the spot.
      
      The current implementation creates `TyCon` representations on the spot.
      
      Pro: No overhead at all in code that does not use `Typeable`
      Cons: Code that uses `Typeable` may create multipe `TyCon` represntations.
      
      We have discussed an implementation where representations of `TyCons` are
      computed once, in the module, where a datatype is declared.  This would
      lead to more code being generated:  for a promotable datatype we need to
      generate `2 + number_of_data_cons` type-constructro representations,
      and we have to do that for all programs, even ones that do not intend to
      use typeable.
      
      I added code to emit warning whenevar `deriving Typeable` is encountered---
      the idea being that this is not needed anymore, and shold be fixed.
      
      Also, we allow `instance Typeable T` in .hs-boot files, but they result
      in a warning, and are ignored.  This last one was to avoid breaking exisitng
      code, and should become an error, eventually.
      
      Test Plan:
      1. GHC can compile itself.
      2. I compiled a number of large libraries, including `lens`.
          - I had to make some small changes:
            `unordered-containers` uses internals of `TypeReps`, so I had to do a 1 line fix
          - `lens` needed one instance changed, due to a poly-kinded `Typeble` instance
      
      3. I also run some code that uses `syb` to traverse a largish datastrucutre.
      I didn't notice any signifiant performance difference between the 7.8.3 version,
      and this implementation.
      
      Reviewers: simonpj, simonmar, austin, hvr
      
      Reviewed By: austin, hvr
      
      Subscribers: thomie
      
      Differential Revision: https://phabricator.haskell.org/D652
      
      GHC Trac Issues: #9858
      b359c886
  21. 20 Feb, 2015 1 commit
  22. 10 Feb, 2015 1 commit
  23. 27 Jan, 2015 1 commit
  24. 20 Jan, 2015 1 commit
  25. 19 Jan, 2015 1 commit
    • Eric Seidel's avatar
      Expose source locations via Implicit Parameters of type GHC.Location.Location · c024af13
      Eric Seidel authored
      Summary:
      IPs with this type will always be solved for the current source
      location. If another IP of the same type is in scope, the two locations will be
      appended, creating a call-stack. The Location type is kept abstract so users
      cannot create them, but a Location can be turned into a list of SrcLocs, which
      correspond to individual locations in a program. Each SrcLoc contains a
      package/module/file name and start/end lines and columns.
      
      The only thing missing from the SrcLoc in my opinion is the name of the
      top-level definition it inhabits. I suspect that would also be useful, but it's
      not clear to me how to extract the current top-level binder from within the
      constraint solver. (Surely I'm just missing something here?)
      
      I made the (perhaps controversial) decision to have GHC completely ignore
      the names of Location IPs, meaning that in the following code:
      
          bar :: (?myloc :: Location) => String
          bar = foo
      
          foo :: (?loc :: Location) => String
          foo = show ?loc
      
      if I call `bar`, the resulting call-stack will include locations for
      
      1. the use of `?loc` inside `foo`,
      2. `foo`s call-site inside `bar`, and
      3. `bar`s call-site, wherever that may be.
      
      This makes Location IPs very special indeed, and I'm happy to change it if the
      dissonance is too great.
      
      I've also left out any changes to base to make use of Location IPs, since there
      were some concerns about a snowball effect. I think it would be reasonable to
      mark this as an experimental feature for now (it is!), and defer using it in
      base until we have more experience with it. It is, after all, quite easy to
      define your own version of `error`, `undefined`, etc. that use Location IPs.
      
      Test Plan: validate, new test-case is testsuite/tests/typecheck/should_run/IPLocation.hs
      
      Reviewers: austin, hvr, simonpj
      
      Reviewed By: simonpj
      
      Subscribers: simonmar, rodlogic, carter, thomie
      
      Differential Revision: https://phabricator.haskell.org/D578
      
      GHC Trac Issues: #9049
      c024af13
  26. 09 Jan, 2015 1 commit
    • Simon Peyton Jones's avatar
      A little tidying up in the flattener · 3d449110
      Simon Peyton Jones authored
      Particularly, flatten_many was exported, but the caller was not doing
      runFlatten.  Moreover it was always used at nominal role.
      
      This patch makes the API clearer, and more robust
      3d449110
  27. 06 Jan, 2015 1 commit
    • Simon Peyton Jones's avatar
      Major patch to add -fwarn-redundant-constraints · 32973bf3
      Simon Peyton Jones authored
      The idea was promted by Trac #9939, but it was Christmas, so I did
      some recreational programming that went much further.
      
      The idea is to warn when a constraint in a user-supplied context is
      redundant.  Everything is described in detail in
        Note [Tracking redundant constraints]
      in TcSimplify.
      
      Main changes:
      
       * The new ic_status field in an implication, of type ImplicStatus.
         It replaces ic_insol, and includes information about redundant
         constraints.
      
       * New function TcSimplify.setImplicationStatus sets the ic_status.
      
       * TcSigInfo has sig_report_redundant field to say whenther a
         redundant constraint should be reported; and similarly
         the FunSigCtxt constructor of UserTypeCtxt
      
       * EvBinds has a field eb_is_given, to record whether it is a given
         or wanted binding. Some consequential chagnes to creating an evidence
         binding (so that we record whether it is given or wanted).
      
       * AbsBinds field abs_ev_binds is now a *list* of TcEvBiinds;
         see Note [Typechecking plan for instance declarations] in
         TcInstDcls
      
       * Some significant changes to the type checking of instance
         declarations; Note [Typechecking plan for instance declarations]
         in TcInstDcls.
      
       * I found that TcErrors.relevantBindings was failing to zonk the
         origin of the constraint it was looking at, and hence failing to
         find some relevant bindings.  Easy to fix, and orthogonal to
         everything else, but hard to disentangle.
      
      Some minor refactorig:
      
       * TcMType.newSimpleWanteds moves to Inst, renamed as newWanteds
      
       * TcClassDcl and TcInstDcls now have their own code for typechecking
         a method body, rather than sharing a single function. The shared
         function (ws TcClassDcl.tcInstanceMethodBody) didn't have much code
         and the differences were growing confusing.
      
       * Add new function TcRnMonad.pushLevelAndCaptureConstraints, and
         use it
      
       * Add new function Bag.catBagMaybes, and use it in TcSimplify
      32973bf3
  28. 31 Dec, 2014 1 commit
  29. 30 Dec, 2014 1 commit
  30. 23 Dec, 2014 1 commit
    • Simon Peyton Jones's avatar
      Eliminate so-called "silent superclass parameters" · a6f0f5ab
      Simon Peyton Jones authored
      The purpose of silent superclass parameters was to solve the
      awkward problem of superclass dictinaries being bound to bottom.
      See THE PROBLEM in Note [Recursive superclasses] in TcInstDcls
      
      Although the silent-superclass idea worked,
      
        * It had non-local consequences, and had effects even in Haddock,
          where we had to discard silent parameters before displaying
          instance declarations
      
        * It had unexpected peformance costs, shown up by Trac #3064 and its
          test case.  In monad-transformer code, when constructing a Monad
          dictionary you had to pass an Applicative dictionary; and to
          construct that you neede a Functor dictionary. Yet these extra
          dictionaries were often never used.  (All this got much worse when
          we added Applicative as a superclass of Monad.) Test T3064
          compiled *far* faster after silent superclasses were eliminated.
      
        * It introduced new bugs.  For example SilentParametersOverlapping,
          T5051, and T7862, all failed to compile because of instance overlap
          directly because of the silent-superclass trick.
      
      So this patch takes a new approach, which I worked out with Dimitrios
      in the closing hours before Christmas.  It is described in detail
      in THE PROBLEM in Note [Recursive superclasses] in TcInstDcls.
      
      Seems to work great!
      
      Quite a bit of knock-on effect
      
       * The main implementation work is in tcSuperClasses in TcInstDcls
         Everything else is fall-out
      
       * IdInfo.DFunId no longer needs its n-silent argument
         * Ditto IDFunId in IfaceSyn
         * Hence interface file format changes
      
       * Now that DFunIds do not have silent superclass parameters, printing
         out instance declarations is simpler. There is tiny knock-on effect
         in Haddock, so that submodule is updated
      
       * I realised that when computing the "size of a dictionary type"
         in TcValidity.sizePred, we should be rather conservative about
         type functions, which can arbitrarily increase the size of a type.
         Hence the new datatype TypeSize, which has a TSBig constructor for
         "arbitrarily big".
      
       * instDFunType moves from TcSMonad to Inst
      
       * Interestingly, CmmNode and CmmExpr both now need a non-silent
         (Ord r) in a couple of instance declarations. These were previously
         silent but must now be explicit.
      
       * Quite a bit of wibbling in error messages
      a6f0f5ab
  31. 12 Dec, 2014 2 commits
    • eir@cis.upenn.edu's avatar
      Flat constraint --> Simple constraint · 8a0de692
      eir@cis.upenn.edu authored
      8a0de692
    • eir@cis.upenn.edu's avatar
      Rewrite `Coercible` solver · 0cc47eb9
      eir@cis.upenn.edu authored
      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.
      
      This commit includes some refactoring as discussed on D546.
      It also removes the ability of Deriveds to rewrite Deriveds.
      
      This fixes bugs #9117 and #8984.
      
      Reviewers: simonpj, austin, nomeata
      
      Subscribers: carter, thomie
      
      Differential Revision: https://phabricator.haskell.org/D546
      
      GHC Trac Issues: #9117, #8984
      0cc47eb9
  32. 10 Dec, 2014 3 commits