TcDerivInfer.hs 39 KB
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{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998


Functions for inferring (and simplifying) the context for derived instances.
-}

{-# LANGUAGE CPP #-}
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{-# LANGUAGE MultiWayIf #-}
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module TcDerivInfer (inferConstraints, simplifyInstanceContexts) where

#include "HsVersions.h"

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import GhcPrelude

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import Bag
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import BasicTypes
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import Class
import DataCon
import ErrUtils
import Inst
import Outputable
import PrelNames
import TcDerivUtils
import TcEnv
import TcGenFunctor
import TcGenGenerics
import TcMType
import TcRnMonad
import TcType
import TyCon
import Type
import TcSimplify
import TcValidity (validDerivPred)
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import TcUnify (buildImplicationFor, checkConstraints)
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import Unify (tcUnifyTy)
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import Util
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import Var
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import VarSet

import Control.Monad
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import Control.Monad.Trans.Class  (lift)
import Control.Monad.Trans.Reader (ask)
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import Data.List
import Data.Maybe

----------------------

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inferConstraints :: DerivSpecMechanism
                 -> DerivM ([ThetaOrigin], [TyVar], [TcType])
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-- inferConstraints figures out the constraints needed for the
-- instance declaration generated by a 'deriving' clause on a
-- data type declaration. It also returns the new in-scope type
-- variables and instance types, in case they were changed due to
-- the presence of functor-like constraints.
-- See Note [Inferring the instance context]

-- e.g. inferConstraints
--        C Int (T [a])    -- Class and inst_tys
--        :RTList a        -- Rep tycon and its arg tys
-- where T [a] ~R :RTList a
--
-- Generate a sufficiently large set of constraints that typechecking the
-- generated method definitions should succeed.   This set will be simplified
-- before being used in the instance declaration
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inferConstraints mechanism
  = do { DerivEnv { denv_tc          = tc
                  , denv_tc_args     = tc_args
                  , denv_cls         = main_cls
                  , denv_cls_tys     = cls_tys } <- ask
       ; let is_anyclass = isDerivSpecAnyClass mechanism
             infer_constraints
               | is_anyclass = inferConstraintsDAC inst_tys
               | otherwise   = inferConstraintsDataConArgs inst_ty inst_tys

             inst_ty  = mkTyConApp tc tc_args
             inst_tys = cls_tys ++ [inst_ty]

             -- Constraints arising from superclasses
             -- See Note [Superclasses of derived instance]
             cls_tvs  = classTyVars main_cls
             sc_constraints = ASSERT2( equalLength cls_tvs inst_tys
                                     , ppr main_cls <+> ppr inst_tys )
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                              [ mkThetaOrigin DerivOrigin TypeLevel [] [] [] $
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                                substTheta cls_subst (classSCTheta main_cls) ]
             cls_subst = ASSERT( equalLength cls_tvs inst_tys )
                         zipTvSubst cls_tvs inst_tys

       ; (inferred_constraints, tvs', inst_tys') <- infer_constraints
       ; lift $ traceTc "inferConstraints" $ vcat
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              [ ppr main_cls <+> ppr inst_tys'
              , ppr inferred_constraints
              ]
       ; return ( sc_constraints ++ inferred_constraints
                , tvs', inst_tys' ) }

-- | Like 'inferConstraints', but used only in the case of deriving strategies
-- where the constraints are inferred by inspecting the fields of each data
-- constructor (i.e., stock- and newtype-deriving).
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inferConstraintsDataConArgs :: TcType -> [TcType]
                            -> DerivM ([ThetaOrigin], [TyVar], [TcType])
inferConstraintsDataConArgs inst_ty inst_tys
  = do DerivEnv { denv_tvs         = tvs
                , denv_rep_tc      = rep_tc
                , denv_rep_tc_args = rep_tc_args
                , denv_cls         = main_cls
                , denv_cls_tys     = cls_tys } <- ask

       let tc_binders = tyConBinders rep_tc
           choose_level bndr
             | isNamedTyConBinder bndr = KindLevel
             | otherwise               = TypeLevel
           t_or_ks = map choose_level tc_binders ++ repeat TypeLevel
              -- want to report *kind* errors when possible

              -- Constraints arising from the arguments of each constructor
           con_arg_constraints
             :: (CtOrigin -> TypeOrKind
                          -> Type
                          -> [([PredOrigin], Maybe TCvSubst)])
             -> ([ThetaOrigin], [TyVar], [TcType])
           con_arg_constraints get_arg_constraints
             = let (predss, mbSubsts) = unzip
                     [ preds_and_mbSubst
                     | data_con <- tyConDataCons rep_tc
                     , (arg_n, arg_t_or_k, arg_ty)
                         <- zip3 [1..] t_or_ks $
                            dataConInstOrigArgTys data_con all_rep_tc_args
                       -- No constraints for unlifted types
                       -- See Note [Deriving and unboxed types]
                     , not (isUnliftedType arg_ty)
                     , let orig = DerivOriginDC data_con arg_n
                     , preds_and_mbSubst
                         <- get_arg_constraints orig arg_t_or_k arg_ty
                     ]
                   preds = concat predss
                   -- If the constraints require a subtype to be of kind
                   -- (* -> *) (which is the case for functor-like
                   -- constraints), then we explicitly unify the subtype's
                   -- kinds with (* -> *).
                   -- See Note [Inferring the instance context]
                   subst        = foldl' composeTCvSubst
                                         emptyTCvSubst (catMaybes mbSubsts)
                   unmapped_tvs = filter (\v -> v `notElemTCvSubst` subst
                                             && not (v `isInScope` subst)) tvs
                   (subst', _)  = mapAccumL substTyVarBndr subst unmapped_tvs
                   preds'       = map (substPredOrigin subst') preds
                   inst_tys'    = substTys subst' inst_tys
                   tvs'         = tyCoVarsOfTypesWellScoped inst_tys'
               in ([mkThetaOriginFromPreds preds'], tvs', inst_tys')

           is_generic  = main_cls `hasKey` genClassKey
           is_generic1 = main_cls `hasKey` gen1ClassKey
           -- is_functor_like: see Note [Inferring the instance context]
           is_functor_like = typeKind inst_ty `tcEqKind` typeToTypeKind
                          || is_generic1

           get_gen1_constraints :: Class -> CtOrigin -> TypeOrKind -> Type
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                                -> [([PredOrigin], Maybe TCvSubst)]
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           get_gen1_constraints functor_cls orig t_or_k ty
              = mk_functor_like_constraints orig t_or_k functor_cls $
                get_gen1_constrained_tys last_tv ty

           get_std_constrained_tys :: CtOrigin -> TypeOrKind -> Type
                                   -> [([PredOrigin], Maybe TCvSubst)]
           get_std_constrained_tys orig t_or_k ty
               | is_functor_like
               = mk_functor_like_constraints orig t_or_k main_cls $
                 deepSubtypesContaining last_tv ty
               | otherwise
               = [( [mk_cls_pred orig t_or_k main_cls ty]
                  , Nothing )]

           mk_functor_like_constraints :: CtOrigin -> TypeOrKind
                                       -> Class -> [Type]
                                       -> [([PredOrigin], Maybe TCvSubst)]
           -- 'cls' is usually main_cls (Functor or Traversable etc), but if
           -- main_cls = Generic1, then 'cls' can be Functor; see
           -- get_gen1_constraints
           --
           -- For each type, generate two constraints,
           -- [cls ty, kind(ty) ~ (*->*)], and a kind substitution that results
           -- from unifying  kind(ty) with * -> *. If the unification is
           -- successful, it will ensure that the resulting instance is well
           -- kinded. If not, the second constraint will result in an error
           -- message which points out the kind mismatch.
           -- See Note [Inferring the instance context]
           mk_functor_like_constraints orig t_or_k cls
              = map $ \ty -> let ki = typeKind ty in
                             ( [ mk_cls_pred orig t_or_k cls ty
                               , mkPredOrigin orig KindLevel
                                   (mkPrimEqPred ki typeToTypeKind) ]
                             , tcUnifyTy ki typeToTypeKind
                             )

           rep_tc_tvs      = tyConTyVars rep_tc
           last_tv         = last rep_tc_tvs
           -- When we first gather up the constraints to solve, most of them
           -- contain rep_tc_tvs, i.e., the type variables from the derived
           -- datatype's type constructor. We don't want these type variables
           -- to appear in the final instance declaration, so we must
           -- substitute each type variable with its counterpart in the derived
           -- instance. rep_tc_args lists each of these counterpart types in
           -- the same order as the type variables.
           all_rep_tc_args
             = rep_tc_args ++ map mkTyVarTy
                                  (drop (length rep_tc_args) rep_tc_tvs)

               -- Stupid constraints
           stupid_constraints
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             = [ mkThetaOrigin DerivOrigin TypeLevel [] [] [] $
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                 substTheta tc_subst (tyConStupidTheta rep_tc) ]
           tc_subst = -- See the comment with all_rep_tc_args for an
                      -- explanation of this assertion
                      ASSERT( equalLength rep_tc_tvs all_rep_tc_args )
                      zipTvSubst rep_tc_tvs all_rep_tc_args

           -- Extra Data constraints
           -- The Data class (only) requires that for
           --    instance (...) => Data (T t1 t2)
           -- IF   t1:*, t2:*
           -- THEN (Data t1, Data t2) are among the (...) constraints
           -- Reason: when the IF holds, we generate a method
           --             dataCast2 f = gcast2 f
           --         and we need the Data constraints to typecheck the method
           extra_constraints = [mkThetaOriginFromPreds constrs]
             where
               constrs
                 | main_cls `hasKey` dataClassKey
                 , all (isLiftedTypeKind . typeKind) rep_tc_args
                 = [ mk_cls_pred DerivOrigin t_or_k main_cls ty
                   | (t_or_k, ty) <- zip t_or_ks rep_tc_args]
                 | otherwise
                 = []

           mk_cls_pred orig t_or_k cls ty
                -- Don't forget to apply to cls_tys' too
              = mkPredOrigin orig t_or_k (mkClassPred cls (cls_tys' ++ [ty]))
           cls_tys' | is_generic1 = []
                      -- In the awkward Generic1 case, cls_tys' should be
                      -- empty, since we are applying the class Functor.

                    | otherwise   = cls_tys

       if    -- Generic constraints are easy
          |  is_generic
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           -> return ([], tvs, inst_tys)
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             -- Generic1 needs Functor
             -- See Note [Getting base classes]
          |  is_generic1
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           -> ASSERT( rep_tc_tvs `lengthExceeds` 0 )
              -- Generic1 has a single kind variable
              ASSERT( cls_tys `lengthIs` 1 )
              do { functorClass <- lift $ tcLookupClass functorClassName
                 ; pure $ con_arg_constraints
                        $ get_gen1_constraints functorClass }
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             -- The others are a bit more complicated
          |  otherwise
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           -> -- See the comment with all_rep_tc_args for an explanation of
              -- this assertion
              ASSERT2( equalLength rep_tc_tvs all_rep_tc_args
                     , ppr main_cls <+> ppr rep_tc
                       $$ ppr rep_tc_tvs $$ ppr all_rep_tc_args )
                do { let (arg_constraints, tvs', inst_tys')
                           = con_arg_constraints get_std_constrained_tys
                   ; lift $ traceTc "inferConstraintsDataConArgs" $ vcat
                          [ ppr main_cls <+> ppr inst_tys'
                          , ppr arg_constraints
                          ]
                   ; return ( stupid_constraints ++ extra_constraints
                                                 ++ arg_constraints
                            , tvs', inst_tys') }
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typeToTypeKind :: Kind
typeToTypeKind = liftedTypeKind `mkFunTy` liftedTypeKind

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-- | Like 'inferConstraints', but used only in the case of @DeriveAnyClass@,
-- which gathers its constraints based on the type signatures of the class's
-- methods instead of the types of the data constructor's field.
--
-- See Note [Gathering and simplifying constraints for DeriveAnyClass]
-- for an explanation of how these constraints are used to determine the
-- derived instance context.
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inferConstraintsDAC :: [TcType] -> DerivM ([ThetaOrigin], [TyVar], [TcType])
inferConstraintsDAC inst_tys
  = do { DerivEnv { denv_tvs = tvs
                  , denv_cls = cls } <- ask

       ; let gen_dms = [ (sel_id, dm_ty)
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                       | (sel_id, Just (_, GenericDM dm_ty)) <- classOpItems cls ]
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             cls_tvs = classTyVars cls

             do_one_meth :: (Id, Type) -> TcM ThetaOrigin
               -- (Id,Type) are the selector Id and the generic default method type
               -- NB: the latter is /not/ quantified over the class variables
               -- See Note [Gathering and simplifying constraints for DeriveAnyClass]
             do_one_meth (sel_id, gen_dm_ty)
               = do { let (sel_tvs, _cls_pred, meth_ty)
                                   = tcSplitMethodTy (varType sel_id)
                          meth_ty' = substTyWith sel_tvs inst_tys meth_ty
                          (meth_tvs, meth_theta, meth_tau)
                                   = tcSplitNestedSigmaTys meth_ty'

                          gen_dm_ty' = substTyWith cls_tvs inst_tys gen_dm_ty
                          (dm_tvs, dm_theta, dm_tau)
                                     = tcSplitNestedSigmaTys gen_dm_ty'
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                          tau_eq     = mkPrimEqPred meth_tau dm_tau
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                    ; return (mkThetaOrigin DerivOrigin TypeLevel
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                                meth_tvs dm_tvs meth_theta (tau_eq:dm_theta)) }
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       ; theta_origins <- lift $ mapM do_one_meth gen_dms
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       ; return (theta_origins, tvs, inst_tys) }
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{- Note [Inferring the instance context]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are two sorts of 'deriving':

  * InferTheta: the deriving clause for a data type
      data T a = T1 a deriving( Eq )
    Here we must infer an instance context,
    and generate instance declaration
      instance Eq a => Eq (T a) where ...

  * CheckTheta: standalone deriving
      deriving instance Eq a => Eq (T a)
    Here we only need to fill in the bindings;
    the instance context is user-supplied

For a deriving clause (InferTheta) we must figure out the
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instance context (inferConstraintsDataConArgs). Suppose we are inferring
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the instance context for
    C t1 .. tn (T s1 .. sm)
There are two cases

  * (T s1 .. sm) :: *         (the normal case)
    Then we behave like Eq and guess (C t1 .. tn t)
    for each data constructor arg of type t.  More
    details below.

  * (T s1 .. sm) :: * -> *    (the functor-like case)
    Then we behave like Functor.

In both cases we produce a bunch of un-simplified constraints
and them simplify them in simplifyInstanceContexts; see
Note [Simplifying the instance context].

In the functor-like case, we may need to unify some kind variables with * in
order for the generated instance to be well-kinded. An example from
Trac #10524:

  newtype Compose (f :: k2 -> *) (g :: k1 -> k2) (a :: k1)
    = Compose (f (g a)) deriving Functor

Earlier in the deriving pipeline, GHC unifies the kind of Compose f g
(k1 -> *) with the kind of Functor's argument (* -> *), so k1 := *. But this
alone isn't enough, since k2 wasn't unified with *:

  instance (Functor (f :: k2 -> *), Functor (g :: * -> k2)) =>
    Functor (Compose f g) where ...

The two Functor constraints are ill-kinded. To ensure this doesn't happen, we:

  1. Collect all of a datatype's subtypes which require functor-like
     constraints.
  2. For each subtype, create a substitution by unifying the subtype's kind
     with (* -> *).
  3. Compose all the substitutions into one, then apply that substitution to
     all of the in-scope type variables and the instance types.

Note [Getting base classes]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Functor and Typeable are defined in package 'base', and that is not available
when compiling 'ghc-prim'.  So we must be careful that 'deriving' for stuff in
ghc-prim does not use Functor or Typeable implicitly via these lookups.

Note [Deriving and unboxed types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We have some special hacks to support things like
   data T = MkT Int# deriving ( Show )

Specifically, we use TcGenDeriv.box to box the Int# into an Int
(which we know how to show), and append a '#'. Parenthesis are not required
for unboxed values (`MkT -3#` is a valid expression).

Note [Superclasses of derived instance]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In general, a derived instance decl needs the superclasses of the derived
class too.  So if we have
        data T a = ...deriving( Ord )
then the initial context for Ord (T a) should include Eq (T a).  Often this is
redundant; we'll also generate an Ord constraint for each constructor argument,
and that will probably generate enough constraints to make the Eq (T a) constraint
be satisfied too.  But not always; consider:

 data S a = S
 instance Eq (S a)
 instance Ord (S a)

 data T a = MkT (S a) deriving( Ord )
 instance Num a => Eq (T a)

The derived instance for (Ord (T a)) must have a (Num a) constraint!
Similarly consider:
        data T a = MkT deriving( Data )
Here there *is* no argument field, but we must nevertheless generate
a context for the Data instances:
        instance Typeable a => Data (T a) where ...

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************************************************************************
*                                                                      *
         Finding the fixed point of deriving equations
*                                                                      *
************************************************************************

Note [Simplifying the instance context]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider

        data T a b = C1 (Foo a) (Bar b)
                   | C2 Int (T b a)
                   | C3 (T a a)
                   deriving (Eq)

We want to come up with an instance declaration of the form

        instance (Ping a, Pong b, ...) => Eq (T a b) where
                x == y = ...

It is pretty easy, albeit tedious, to fill in the code "...".  The
trick is to figure out what the context for the instance decl is,
namely Ping, Pong and friends.

Let's call the context reqd for the T instance of class C at types
(a,b, ...)  C (T a b).  Thus:

        Eq (T a b) = (Ping a, Pong b, ...)

Now we can get a (recursive) equation from the data decl.  This part
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is done by inferConstraintsDataConArgs.
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        Eq (T a b) = Eq (Foo a) u Eq (Bar b)    -- From C1
                   u Eq (T b a) u Eq Int        -- From C2
                   u Eq (T a a)                 -- From C3


Foo and Bar may have explicit instances for Eq, in which case we can
just substitute for them.  Alternatively, either or both may have
their Eq instances given by deriving clauses, in which case they
form part of the system of equations.

Now all we need do is simplify and solve the equations, iterating to
find the least fixpoint.  This is done by simplifyInstanceConstraints.
Notice that the order of the arguments can
switch around, as here in the recursive calls to T.

Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.

We start with:

        Eq (T a b) = {}         -- The empty set

Next iteration:
        Eq (T a b) = Eq (Foo a) u Eq (Bar b)    -- From C1
                   u Eq (T b a) u Eq Int        -- From C2
                   u Eq (T a a)                 -- From C3

        After simplification:
                   = Eq a u Ping b u {} u {} u {}
                   = Eq a u Ping b

Next iteration:

        Eq (T a b) = Eq (Foo a) u Eq (Bar b)    -- From C1
                   u Eq (T b a) u Eq Int        -- From C2
                   u Eq (T a a)                 -- From C3

        After simplification:
                   = Eq a u Ping b
                   u (Eq b u Ping a)
                   u (Eq a u Ping a)

                   = Eq a u Ping b u Eq b u Ping a

The next iteration gives the same result, so this is the fixpoint.  We
need to make a canonical form of the RHS to ensure convergence.  We do
this by simplifying the RHS to a form in which

        - the classes constrain only tyvars
        - the list is sorted by tyvar (major key) and then class (minor key)
        - no duplicates, of course

Note [Deterministic simplifyInstanceContexts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Canonicalisation uses nonDetCmpType which is nondeterministic. Sorting
with nonDetCmpType puts the returned lists in a nondeterministic order.
If we were to return them, we'd get class constraints in
nondeterministic order.

Consider:

  data ADT a b = Z a b deriving Eq

The generated code could be either:

  instance (Eq a, Eq b) => Eq (Z a b) where

Or:

  instance (Eq b, Eq a) => Eq (Z a b) where

To prevent the order from being nondeterministic we only
canonicalize when comparing and return them in the same order as
simplifyDeriv returned them.
See also Note [nonDetCmpType nondeterminism]
-}


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simplifyInstanceContexts :: [DerivSpec [ThetaOrigin]]
                         -> TcM [DerivSpec ThetaType]
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-- Used only for deriving clauses (InferTheta)
-- not for standalone deriving
-- See Note [Simplifying the instance context]

simplifyInstanceContexts [] = return []

simplifyInstanceContexts infer_specs
  = do  { traceTc "simplifyInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
        ; iterate_deriv 1 initial_solutions }
  where
    ------------------------------------------------------------------
        -- The initial solutions for the equations claim that each
        -- instance has an empty context; this solution is certainly
        -- in canonical form.
    initial_solutions :: [ThetaType]
    initial_solutions = [ [] | _ <- infer_specs ]

    ------------------------------------------------------------------
        -- iterate_deriv calculates the next batch of solutions,
        -- compares it with the current one; finishes if they are the
        -- same, otherwise recurses with the new solutions.
        -- It fails if any iteration fails
    iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec ThetaType]
    iterate_deriv n current_solns
      | n > 20  -- Looks as if we are in an infinite loop
                -- This can happen if we have -XUndecidableInstances
                -- (See TcSimplify.tcSimplifyDeriv.)
      = pprPanic "solveDerivEqns: probable loop"
                 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
      | otherwise
      = do {      -- Extend the inst info from the explicit instance decls
                  -- with the current set of solutions, and simplify each RHS
             inst_specs <- zipWithM newDerivClsInst current_solns infer_specs
           ; new_solns <- checkNoErrs $
                          extendLocalInstEnv inst_specs $
                          mapM gen_soln infer_specs

           ; if (current_solns `eqSolution` new_solns) then
                return [ spec { ds_theta = soln }
                       | (spec, soln) <- zip infer_specs current_solns ]
             else
                iterate_deriv (n+1) new_solns }

    eqSolution a b = eqListBy (eqListBy eqType) (canSolution a) (canSolution b)
       -- Canonicalise for comparison
       -- See Note [Deterministic simplifyInstanceContexts]
    canSolution = map (sortBy nonDetCmpType)
    ------------------------------------------------------------------
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    gen_soln :: DerivSpec [ThetaOrigin] -> TcM ThetaType
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    gen_soln (DS { ds_loc = loc, ds_tvs = tyvars
                 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
      = setSrcSpan loc  $
        addErrCtxt (derivInstCtxt the_pred) $
        do { theta <- simplifyDeriv the_pred tyvars deriv_rhs
                -- checkValidInstance tyvars theta clas inst_tys
                -- Not necessary; see Note [Exotic derived instance contexts]

           ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
                -- Claim: the result instance declaration is guaranteed valid
                -- Hence no need to call:
                --   checkValidInstance tyvars theta clas inst_tys
           ; return theta }
      where
        the_pred = mkClassPred clas inst_tys

derivInstCtxt :: PredType -> MsgDoc
derivInstCtxt pred
  = text "When deriving the instance for" <+> parens (ppr pred)

{-
***********************************************************************************
*                                                                                 *
*            Simplify derived constraints
*                                                                                 *
***********************************************************************************
-}

-- | Given @instance (wanted) => C inst_ty@, simplify 'wanted' as much
-- as possible. Fail if not possible.
simplifyDeriv :: PredType -- ^ @C inst_ty@, head of the instance we are
                          -- deriving.  Only used for SkolemInfo.
              -> [TyVar]  -- ^ The tyvars bound by @inst_ty@.
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              -> [ThetaOrigin] -- ^ Given and wanted constraints
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              -> TcM ThetaType -- ^ Needed constraints (after simplification),
                               -- i.e. @['PredType']@.
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simplifyDeriv pred tvs thetas
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  = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize
                -- The constraint solving machinery
                -- expects *TcTyVars* not TyVars.
                -- We use *non-overlappable* (vanilla) skolems
                -- See Note [Overlap and deriving]

       ; let skol_set  = mkVarSet tvs_skols
             skol_info = DerivSkol pred
             doc = text "deriving" <+> parens (ppr pred)

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             mk_given_ev :: PredType -> TcM EvVar
             mk_given_ev given =
               let given_pred = substTy skol_subst given
               in newEvVar given_pred

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             mk_wanted_cts :: [TyVar] -> [PredOrigin] -> TcM [CtEvidence]
             mk_wanted_cts metas_to_be wanteds
               = do -- We instantiate metas_to_be with fresh meta type
                    -- variables. Currently, these can only be type variables
                    -- quantified in generic default type signatures.
                    -- See Note [Gathering and simplifying constraints for
                    -- DeriveAnyClass]
                    (meta_subst, _meta_tvs) <- newMetaTyVars metas_to_be
                    let wanted_subst = skol_subst `unionTCvSubst` meta_subst
                        mk_wanted_ct (PredOrigin wanted o t_or_k)
                          = newWanted o (Just t_or_k) $
                            substTyUnchecked wanted_subst wanted
                    mapM mk_wanted_ct wanteds
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             -- Create the implications we need to solve. For stock and newtype
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             -- deriving, these implication constraints will be simple class
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             -- constraints like (C a, Ord b).
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             -- But with DeriveAnyClass, we make an implication constraint.
             -- See Note [Gathering and simplifying constraints for DeriveAnyClass]
             mk_wanteds :: ThetaOrigin -> TcM WantedConstraints
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             mk_wanteds (ThetaOrigin { to_anyclass_skols  = ac_skols
                                     , to_anyclass_metas  = ac_metas
                                     , to_anyclass_givens = ac_givens
                                     , to_wanted_origins  = wanteds })
               = do { ac_given_evs <- mapM mk_given_ev ac_givens
                    ; (_, wanteds)
                        <- captureConstraints $
                           checkConstraints skol_info ac_skols ac_given_evs $
                           do { cts <- mk_wanted_cts ac_metas wanteds
                              ; emitSimples $ listToCts
                                            $ map mkNonCanonical cts }
                    ; pure wanteds }
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       -- See [STEP DAC BUILD]
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       -- Generate the implication constraints constraints to solve with the
       -- skolemized variables
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       ; wanteds <- mapM mk_wanteds thetas
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       ; traceTc "simplifyDeriv inputs" $
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         vcat [ pprTyVars tvs $$ ppr thetas $$ ppr wanteds, doc ]

       -- See [STEP DAC SOLVE]
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       -- Simplify the constraints
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       ; solved_implics <- runTcSDeriveds $ solveWantedsAndDrop
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                                          $ unionsWC wanteds
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       -- It's not yet zonked!  Obviously zonk it before peering at it
       ; solved_implics <- zonkWC solved_implics
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       -- See [STEP DAC HOIST]
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       -- Split the resulting constraints into bad and good constraints,
       -- building an @unsolved :: WantedConstraints@ representing all
       -- the constraints we can't just shunt to the predicates.
       -- See Note [Exotic derived instance contexts]
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       ; let residual_simple = approximateWC True solved_implics
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             (bad, good) = partitionBagWith get_good residual_simple

             get_good :: Ct -> Either Ct PredType
             get_good ct | validDerivPred skol_set p
                         , isWantedCt ct
                         = Right p
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                          -- TODO: This is wrong
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                          -- NB re 'isWantedCt': residual_wanted may contain
                          -- unsolved CtDerived and we stick them into the
                          -- bad set so that reportUnsolved may decide what
                          -- to do with them
                         | otherwise
                         = Left ct
                           where p = ctPred ct

       ; traceTc "simplifyDeriv outputs" $
         vcat [ ppr tvs_skols, ppr residual_simple, ppr good, ppr bad ]

       -- Return the good unsolved constraints (unskolemizing on the way out.)
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       ; let min_theta = mkMinimalBySCs id (bagToList good)
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             -- An important property of mkMinimalBySCs (used above) is that in
             -- addition to removing constraints that are made redundant by
             -- superclass relationships, it also removes _duplicate_
             -- constraints.
             -- See Note [Gathering and simplifying constraints for
             --           DeriveAnyClass]
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             subst_skol = zipTvSubst tvs_skols $ mkTyVarTys tvs
                          -- The reverse substitution (sigh)
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       -- See [STEP DAC RESIDUAL]
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       ; min_theta_vars <- mapM newEvVar min_theta
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       ; tc_lvl <- getTcLevel
       ; (leftover_implic, _)
           <- buildImplicationFor (pushTcLevel tc_lvl) skol_info tvs_skols
                                  min_theta_vars solved_implics
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       -- This call to simplifyTop is purely for error reporting
       -- See Note [Error reporting for deriving clauses]
       -- See also Note [Exotic derived instance contexts], which are caught
       -- in this line of code.
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       ; simplifyTopImplic leftover_implic
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       ; return (substTheta subst_skol min_theta) }

{-
Note [Overlap and deriving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider some overlapping instances:
  data Show a => Show [a] where ..
  data Show [Char] where ...

Now a data type with deriving:
  data T a = MkT [a] deriving( Show )

We want to get the derived instance
  instance Show [a] => Show (T a) where...
and NOT
  instance Show a => Show (T a) where...
so that the (Show (T Char)) instance does the Right Thing

It's very like the situation when we're inferring the type
of a function
   f x = show [x]
and we want to infer
   f :: Show [a] => a -> String

BOTTOM LINE: use vanilla, non-overlappable skolems when inferring
             the context for the derived instance.
             Hence tcInstSkolTyVars not tcInstSuperSkolTyVars

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Note [Gathering and simplifying constraints for DeriveAnyClass]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DeriveAnyClass works quite differently from stock and newtype deriving in
the way it gathers and simplifies constraints to be used in a derived
instance's context. Stock and newtype deriving gather constraints by looking
at the data constructors of the data type for which we are deriving an
instance. But DeriveAnyClass doesn't need to know about a data type's
definition at all!

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To see why, consider this example of DeriveAnyClass:
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  class Foo a where
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    bar :: forall b. Ix b => a -> b -> String
    default bar :: (Show a, Ix c) => a -> c -> String
    bar x y = show x ++ show (range (y,y))
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    baz :: Eq a => a -> a -> Bool
    default baz :: (Ord a, Show a) => a -> a -> Bool
    baz x y = compare x y == EQ

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Because 'bar' and 'baz' have default signatures, this generates a top-level
definition for these generic default methods
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  $gdm_bar :: forall a. Foo a
           => forall c. (Show a, Ix c)
           => a -> c -> String
  $gdm_bar x y = show x ++ show (range (y,y))
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(and similarly for baz).  Now consider a 'deriving' clause
  data Maybe s = ... deriving Foo
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This derives an instance of the form:
  instance (CX) => Foo (Maybe s) where
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    bar = $gdm_bar
    baz = $gdm_baz

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Now it is GHC's job to fill in a suitable instance context (CX).  If
GHC were typechecking the binding
   bar = $gdm bar
it would
   * skolemise the expected type of bar
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   * instantiate the type of $gdm_bar with meta-type variables
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   * build an implication constraint

[STEP DAC BUILD]
So that's what we do.  We build the constraint (call it C1)
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   forall b. Ix b => (Show (Maybe s), Ix cc,
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                      Maybe s -> b -> String
                          ~ Maybe s -> cc -> String)
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Here, the 'b' comes from the quantified type variable in the expected type
of bar (i.e., 'to_anyclass_skols' in 'ThetaOrigin'). The 'cc' is a unification
variable that comes from instantiating the quantified type variable 'c' in
$gdm_bar's type (i.e., 'to_anyclass_metas' in 'ThetaOrigin).

The (Ix b) constraint comes from the context of bar's type
(i.e., 'to_wanted_givens' in 'ThetaOrigin'). The (Show (Maybe s)) and (Ix cc)
constraints come from the context of $gdm_bar's type
(i.e., 'to_anyclass_givens' in 'ThetaOrigin').

The equality constraint (Maybe s -> b -> String) ~ (Maybe s -> cc -> String)
comes from marrying up the instantiated type of $gdm_bar with the specified
type of bar. Notice that the type variables from the instance, 's' in this
case, are global to this constraint.

Note that it is vital that we instantiate the `c` in $gdm_bar's type with a new
unification variable for each iteration of simplifyDeriv. If we re-use the same
unification variable across multiple iterations, then bad things can happen,
such as Trac #14933.
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Similarly for 'baz', givng the constraint C2
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   forall. Eq (Maybe s) => (Ord a, Show a,
                            Maybe s -> Maybe s -> Bool
                                ~ Maybe s -> Maybe s -> Bool)
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In this case baz has no local quantification, so the implication
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constraint has no local skolems and there are no unification
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variables.
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[STEP DAC SOLVE]
We can combine these two implication constraints into a single
constraint (C1, C2), and simplify, unifying cc:=b, to get:
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   forall b. Ix b => Show a
   /\
   forall. Eq (Maybe s) => (Ord a, Show a)
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[STEP DAC HOIST]
Let's call that (C1', C2').  Now we need to hoist the unsolved
constraints out of the implications to become our candidate for
(CX). That is done by approximateWC, which will return:
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  (Show a, Ord a, Show a)

Now we can use mkMinimalBySCs to remove superclasses and duplicates, giving

  (Show a, Ord a)

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And that's what GHC uses for CX.

[STEP DAC RESIDUAL]
In this case we have solved all the leftover constraints, but what if
we don't?  Simple!  We just form the final residual constraint

   forall s. CX => (C1',C2')

and simplify that. In simple cases it'll succeed easily, because CX
literally contains the constraints in C1', C2', but if there is anything
more complicated it will be reported in a civilised way.
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Note [Error reporting for deriving clauses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A suprisingly tricky aspect of deriving to get right is reporting sensible
error messages. In particular, if simplifyDeriv reaches a constraint that it
cannot solve, which might include:

1. Insoluble constraints
2. "Exotic" constraints (See Note [Exotic derived instance contexts])

Then we report an error immediately in simplifyDeriv.

Another possible choice is to punt and let another part of the typechecker
(e.g., simplifyInstanceContexts) catch the errors. But this tends to lead
to worse error messages, so we do it directly in simplifyDeriv.

simplifyDeriv checks for errors in a clever way. If the deriving machinery
infers the context (Foo a)--that is, if this instance is to be generated:

  instance Foo a => ...

Then we form an implication of the form:

  forall a. Foo a => <residual_wanted_constraints>

And pass it to the simplifier. If the context (Foo a) is enough to discharge
all the constraints in <residual_wanted_constraints>, then everything is
hunky-dory. But if <residual_wanted_constraints> contains, say, an insoluble
constraint, then (Foo a) won't be able to solve it, causing GHC to error.

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Note [Exotic derived instance contexts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a 'derived' instance declaration, we *infer* the context.  It's a
bit unclear what rules we should apply for this; the Haskell report is
silent.  Obviously, constraints like (Eq a) are fine, but what about
        data T f a = MkT (f a) deriving( Eq )
where we'd get an Eq (f a) constraint.  That's probably fine too.

One could go further: consider
        data T a b c = MkT (Foo a b c) deriving( Eq )
        instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)

Notice that this instance (just) satisfies the Paterson termination
conditions.  Then we *could* derive an instance decl like this:

        instance (C Int a, Eq b, Eq c) => Eq (T a b c)
even though there is no instance for (C Int a), because there just
*might* be an instance for, say, (C Int Bool) at a site where we
need the equality instance for T's.

However, this seems pretty exotic, and it's quite tricky to allow
this, and yet give sensible error messages in the (much more common)
case where we really want that instance decl for C.

So for now we simply require that the derived instance context
should have only type-variable constraints.

Here is another example:
        data Fix f = In (f (Fix f)) deriving( Eq )
Here, if we are prepared to allow -XUndecidableInstances we
could derive the instance
        instance Eq (f (Fix f)) => Eq (Fix f)
but this is so delicate that I don't think it should happen inside
'deriving'. If you want this, write it yourself!

NB: if you want to lift this condition, make sure you still meet the
termination conditions!  If not, the deriving mechanism generates
larger and larger constraints.  Example:
  data Succ a = S a
  data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show

Note the lack of a Show instance for Succ.  First we'll generate
  instance (Show (Succ a), Show a) => Show (Seq a)
and then
  instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
and so on.  Instead we want to complain of no instance for (Show (Succ a)).

The bottom line
~~~~~~~~~~~~~~~
Allow constraints which consist only of type variables, with no repeats.
-}