(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998

\section[InstEnv]{Utilities for typechecking instance declarations}

The bits common to TcInstDcls and TcDeriv.

{-# LANGUAGE CPP, DeriveDataTypeable #-}

module InstEnv (
        DFunId, InstMatch, ClsInstLookupResult,
        OverlapFlag(..), OverlapMode(..), setOverlapModeMaybe,
        ClsInst(..), DFunInstType, pprInstance, pprInstanceHdr, pprInstances,
        instanceHead, instanceSig, mkLocalInstance, mkImportedInstance,
        instanceDFunId, updateClsInstDFun, instanceRoughTcs,
        fuzzyClsInstCmp, orphNamesOfClsInst,

        InstEnvs(..), VisibleOrphanModules, InstEnv,
        emptyInstEnv, extendInstEnv,
        deleteFromInstEnv, deleteDFunFromInstEnv,
        extendInstEnvList, lookupUniqueInstEnv, lookupInstEnv, instEnvElts, instEnvClasses,
        classInstances, instanceBindFun,
        instanceCantMatch, roughMatchTcs,
        isOverlappable, isOverlapping, isIncoherent
    ) where

#include "HsVersions.h"

import GhcPrelude

import TcType -- InstEnv is really part of the type checker,
              -- and depends on TcType in many ways
import CoreSyn ( IsOrphan(..), isOrphan, chooseOrphanAnchor )
import Module
import Class
import Var
import VarSet
import Name
import NameSet
import Unify
import Outputable
import ErrUtils
import BasicTypes
import UniqDFM
import Util
import Id
import Data.Data        ( Data )
import Data.Maybe       ( isJust, isNothing )

*                                                                      *
           ClsInst: the data type for type-class instances
*                                                                      *

-- | A type-class instance. Note that there is some tricky laziness at work
-- here. See Note [ClsInst laziness and the rough-match fields] for more
-- details.
data ClsInst
  = ClsInst {   -- Used for "rough matching"; see
                -- Note [ClsInst laziness and the rough-match fields]
                -- INVARIANT: is_tcs = roughMatchTcs is_tys
               is_cls_nm :: Name        -- ^ Class name
             , is_tcs  :: [Maybe Name]  -- ^ Top of type args

               -- | @is_dfun_name = idName . is_dfun@.
               -- We use 'is_dfun_name' for the visibility check,
               -- 'instIsVisible', which needs to know the 'Module' which the
               -- dictionary is defined in. However, we cannot use the 'Module'
               -- attached to 'is_dfun' since doing so would mean we would
               -- potentially pull in an entire interface file unnecessarily.
               -- This was the cause of #12367.
             , is_dfun_name :: Name

                -- Used for "proper matching"; see Note [Proper-match fields]
             , is_tvs  :: [TyVar]       -- Fresh template tyvars for full match
                                        -- See Note [Template tyvars are fresh]
             , is_cls  :: Class         -- The real class
             , is_tys  :: [Type]        -- Full arg types (mentioning is_tvs)
                -- INVARIANT: is_dfun Id has type
                --      forall is_tvs. (...) => is_cls is_tys
                -- (modulo alpha conversion)

             , is_dfun :: DFunId -- See Note [Haddock assumptions]

             , is_flag :: OverlapFlag   -- See detailed comments with
                                        -- the decl of BasicTypes.OverlapFlag
             , is_orphan :: IsOrphan
  deriving Data

-- | A fuzzy comparison function for class instances, intended for sorting
-- instances before displaying them to the user.
fuzzyClsInstCmp :: ClsInst -> ClsInst -> Ordering
fuzzyClsInstCmp x y =
    stableNameCmp (is_cls_nm x) (is_cls_nm y) `mappend`
    mconcat (map cmp (zip (is_tcs x) (is_tcs y)))
    cmp (Nothing, Nothing) = EQ
    cmp (Nothing, Just _) = LT
    cmp (Just _, Nothing) = GT
    cmp (Just x, Just y) = stableNameCmp x y

isOverlappable, isOverlapping, isIncoherent :: ClsInst -> Bool
isOverlappable i = hasOverlappableFlag (overlapMode (is_flag i))
isOverlapping  i = hasOverlappingFlag  (overlapMode (is_flag i))
isIncoherent   i = hasIncoherentFlag   (overlapMode (is_flag i))

Note [ClsInst laziness and the rough-match fields]
Suppose we load 'instance A.C B.T' from A.hi, but suppose that the type B.T is
otherwise unused in the program. Then it's stupid to load B.hi, the data type
declaration for B.T -- and perhaps further instance declarations!

We avoid this as follows:

* is_cls_nm, is_tcs, is_dfun_name are all Names. We can poke them to our heart's

* Proper-match fields. is_dfun, and its related fields is_tvs, is_cls, is_tys
  contain TyVars, Class, Type, Class etc, and so are all lazy thunks. When we
  poke any of these fields we'll typecheck the DFunId declaration, and hence
  pull in interfaces that it refers to. See Note [Proper-match fields].

* Rough-match fields. During instance lookup, we use the is_cls_nm :: Name and
  is_tcs :: [Maybe Name] fields to perform a "rough match", *without* poking
  inside the DFunId. The rough-match fields allow us to say "definitely does not
  match", based only on Names.

  This laziness is very important; see #12367. Try hard to avoid pulling on
  the structured fields unless you really need the instance.

* Another place to watch is InstEnv.instIsVisible, which needs the module to
  which the ClsInst belongs. We can get this from is_dfun_name.

* In is_tcs,
    Nothing  means that this type arg is a type variable

    (Just n) means that this type arg is a
                TyConApp with a type constructor of n.
                This is always a real tycon, never a synonym!
                (Two different synonyms might match, but two
                different real tycons can't.)
                NB: newtypes are not transparent, though!

Note [Template tyvars are fresh]
The is_tvs field of a ClsInst has *completely fresh* tyvars.
That is, they are
  * distinct from any other ClsInst
  * distinct from any tyvars free in predicates that may
    be looked up in the class instance environment
Reason for freshness: we use unification when checking for overlap
etc, and that requires the tyvars to be distinct.

The invariant is checked by the ASSERT in lookupInstEnv'.

Note [Proper-match fields]
The is_tvs, is_cls, is_tys fields are simply cached values, pulled
out (lazily) from the dfun id. They are cached here simply so
that we don't need to decompose the DFunId each time we want
to match it.  The hope is that the rough-match fields mean
that we often never poke the proper-match fields.

However, note that:
 * is_tvs must be a superset of the free vars of is_tys

 * is_tvs, is_tys may be alpha-renamed compared to the ones in
   the dfun Id

Note [Haddock assumptions]
For normal user-written instances, Haddock relies on

 * the SrcSpan of
 * the Name of
 * the is_dfun of
 * an Instance

being equal to

  * the SrcSpan of
  * the instance head type of
  * the InstDecl used to construct the Instance.

instanceDFunId :: ClsInst -> DFunId
instanceDFunId = is_dfun

updateClsInstDFun :: (DFunId -> DFunId) -> ClsInst -> ClsInst
updateClsInstDFun tidy_dfun ispec
  = ispec { is_dfun = tidy_dfun (is_dfun ispec) }

instanceRoughTcs :: ClsInst -> [Maybe Name]
instanceRoughTcs = is_tcs

instance NamedThing ClsInst where
   getName ispec = getName (is_dfun ispec)

instance Outputable ClsInst where
   ppr = pprInstance

pprInstance :: ClsInst -> SDoc
-- Prints the ClsInst as an instance declaration
pprInstance ispec
  = hang (pprInstanceHdr ispec)
       2 (vcat [ text "--" <+> pprDefinedAt (getName ispec)
               , whenPprDebug (ppr (is_dfun ispec)) ])

-- * pprInstanceHdr is used in VStudio to populate the ClassView tree
pprInstanceHdr :: ClsInst -> SDoc
-- Prints the ClsInst as an instance declaration
pprInstanceHdr (ClsInst { is_flag = flag, is_dfun = dfun })
  = text "instance" <+> ppr flag <+> pprSigmaType (idType dfun)

pprInstances :: [ClsInst] -> SDoc
pprInstances ispecs = vcat (map pprInstance ispecs)

instanceHead :: ClsInst -> ([TyVar], Class, [Type])
-- Returns the head, using the fresh tyavs from the ClsInst
instanceHead (ClsInst { is_tvs = tvs, is_tys = tys, is_dfun = dfun })
   = (tvs, cls, tys)
     (_, _, cls, _) = tcSplitDFunTy (idType dfun)

-- | Collects the names of concrete types and type constructors that make
-- up the head of a class instance. For instance, given `class Foo a b`:
-- `instance Foo (Either (Maybe Int) a) Bool` would yield
--      [Either, Maybe, Int, Bool]
-- Used in the implementation of ":info" in GHCi.
-- The 'tcSplitSigmaTy' is because of
--      instance Foo a => Baz T where ...
-- The decl is an orphan if Baz and T are both not locally defined,
--      even if Foo *is* locally defined
orphNamesOfClsInst :: ClsInst -> NameSet
orphNamesOfClsInst (ClsInst { is_cls_nm = cls_nm, is_tys = tys })
  = orphNamesOfTypes tys `unionNameSet` unitNameSet cls_nm

instanceSig :: ClsInst -> ([TyVar], [Type], Class, [Type])
-- Decomposes the DFunId
instanceSig ispec = tcSplitDFunTy (idType (is_dfun ispec))

mkLocalInstance :: DFunId -> OverlapFlag
                -> [TyVar] -> Class -> [Type]
                -> ClsInst
-- Used for local instances, where we can safely pull on the DFunId.
-- Consider using newClsInst instead; this will also warn if
-- the instance is an orphan.
mkLocalInstance dfun oflag tvs cls tys
  = ClsInst { is_flag = oflag, is_dfun = dfun
            , is_tvs = tvs
            , is_dfun_name = dfun_name
            , is_cls = cls, is_cls_nm = cls_name
            , is_tys = tys, is_tcs = roughMatchTcs tys
            , is_orphan = orph
    cls_name = className cls
    dfun_name = idName dfun
    this_mod = ASSERT( isExternalName dfun_name ) nameModule dfun_name
    is_local name = nameIsLocalOrFrom this_mod name

        -- Compute orphanhood.  See Note [Orphans] in InstEnv
    (cls_tvs, fds) = classTvsFds cls
    arg_names = [filterNameSet is_local (orphNamesOfType ty) | ty <- tys]

    -- See Note [When exactly is an instance decl an orphan?]
    orph | is_local cls_name = NotOrphan (nameOccName cls_name)
         | all notOrphan mb_ns  = ASSERT( not (null mb_ns) ) head mb_ns
         | otherwise         = IsOrphan

    notOrphan NotOrphan{} = True
    notOrphan _ = False

    mb_ns :: [IsOrphan]    -- One for each fundep; a locally-defined name
                           -- that is not in the "determined" arguments
    mb_ns | null fds   = [choose_one arg_names]
          | otherwise  = map do_one fds
    do_one (_ltvs, rtvs) = choose_one [ns | (tv,ns) <- cls_tvs `zip` arg_names
                                            , not (tv `elem` rtvs)]

    choose_one nss = chooseOrphanAnchor (unionNameSets nss)

mkImportedInstance :: Name         -- ^ the name of the class
                   -> [Maybe Name] -- ^ the types which the class was applied to
                   -> Name         -- ^ the 'Name' of the dictionary binding
                   -> DFunId       -- ^ the 'Id' of the dictionary.
                   -> OverlapFlag  -- ^ may this instance overlap?
                   -> IsOrphan     -- ^ is this instance an orphan?
                   -> ClsInst
-- Used for imported instances, where we get the rough-match stuff
-- from the interface file
-- The bound tyvars of the dfun are guaranteed fresh, because
-- the dfun has been typechecked out of the same interface file
mkImportedInstance cls_nm mb_tcs dfun_name dfun oflag orphan
  = ClsInst { is_flag = oflag, is_dfun = dfun
            , is_tvs = tvs, is_tys = tys
            , is_dfun_name = dfun_name
            , is_cls_nm = cls_nm, is_cls = cls, is_tcs = mb_tcs
            , is_orphan = orphan }
    (tvs, _, cls, tys) = tcSplitDFunTy (idType dfun)

Note [When exactly is an instance decl an orphan?]
  (see MkIface.instanceToIfaceInst, which implements this)
Roughly speaking, an instance is an orphan if its head (after the =>)
mentions nothing defined in this module.

Functional dependencies complicate the situation though. Consider

  module M where { class C a b | a -> b }

and suppose we are compiling module X:

  module X where
        import M
        data T = ...
        instance C Int T where ...

This instance is an orphan, because when compiling a third module Y we
might get a constraint (C Int v), and we'd want to improve v to T.  So
we must make sure X's instances are loaded, even if we do not directly
use anything from X.

More precisely, an instance is an orphan iff

  If there are no fundeps, then at least of the names in
  the instance head is locally defined.

  If there are fundeps, then for every fundep, at least one of the
  names free in a *non-determined* part of the instance head is
  defined in this module.

(Note that these conditions hold trivially if the class is locally

*                                                                      *
                InstEnv, ClsInstEnv
*                                                                      *

A @ClsInstEnv@ all the instances of that class.  The @Id@ inside a
ClsInstEnv mapping is the dfun for that instance.

If class C maps to a list containing the item ([a,b], [t1,t2,t3], dfun), then

        forall a b, C t1 t2 t3  can be constructed by dfun

or, to put it another way, we have

        instance (...) => C t1 t2 t3,  witnessed by dfun

Note [InstEnv determinism]
We turn InstEnvs into a list in some places that don't directly affect
the ABI. That happens when we create output for `:info`.
Unfortunately that nondeterminism is nonlocal and it's hard to tell what it
affects without following a chain of functions. It's also easy to accidentally
make that nondeterminism affect the ABI. Furthermore the envs should be
relatively small, so it should be free to use deterministic maps here.
Testing with nofib and validate detected no difference between UniqFM and
UniqDFM. See also Note [Deterministic UniqFM]

type InstEnv = UniqDFM ClsInstEnv      -- Maps Class to instances for that class
  -- See Note [InstEnv determinism]

-- | 'InstEnvs' represents the combination of the global type class instance
-- environment, the local type class instance environment, and the set of
-- transitively reachable orphan modules (according to what modules have been
-- directly imported) used to test orphan instance visibility.
data InstEnvs = InstEnvs {
        ie_global  :: InstEnv,               -- External-package instances
        ie_local   :: InstEnv,               -- Home-package instances
        ie_visible :: VisibleOrphanModules   -- Set of all orphan modules transitively
                                             -- reachable from the module being compiled
                                             -- See Note [Instance lookup and orphan instances]

-- | Set of visible orphan modules, according to what modules have been directly
-- imported.  This is based off of the dep_orphs field, which records
-- transitively reachable orphan modules (modules that define orphan instances).
type VisibleOrphanModules = ModuleSet

newtype ClsInstEnv
  = ClsIE [ClsInst]    -- The instances for a particular class, in any order

instance Outputable ClsInstEnv where
  ppr (ClsIE is) = pprInstances is

--  * The is_tvs are distinct in each ClsInst
--      of a ClsInstEnv (so we can safely unify them)

-- Thus, the @ClassInstEnv@ for @Eq@ might contain the following entry:
--      [a] ===> dfun_Eq_List :: forall a. Eq a => Eq [a]
-- The "a" in the pattern must be one of the forall'd variables in
-- the dfun type.

emptyInstEnv :: InstEnv
emptyInstEnv = emptyUDFM

instEnvElts :: InstEnv -> [ClsInst]
instEnvElts ie = [elt | ClsIE elts <- eltsUDFM ie, elt <- elts]
  -- See Note [InstEnv determinism]

instEnvClasses :: InstEnv -> [Class]
instEnvClasses ie = [is_cls e | ClsIE (e : _) <- eltsUDFM ie]

-- | Test if an instance is visible, by checking that its origin module
-- is in 'VisibleOrphanModules'.
-- See Note [Instance lookup and orphan instances]
instIsVisible :: VisibleOrphanModules -> ClsInst -> Bool
instIsVisible vis_mods ispec
  -- NB: Instances from the interactive package always are visible. We can't
  -- add interactive modules to the set since we keep creating new ones
  -- as a GHCi session progresses.
  = case nameModule_maybe (is_dfun_name ispec) of
      Nothing -> True
      Just mod | isInteractiveModule mod     -> True
               | IsOrphan <- is_orphan ispec -> mod `elemModuleSet` vis_mods
               | otherwise                   -> True

classInstances :: InstEnvs -> Class -> [ClsInst]
classInstances (InstEnvs { ie_global = pkg_ie, ie_local = home_ie, ie_visible = vis_mods }) cls
  = get home_ie ++ get pkg_ie
    get env = case lookupUDFM env cls of
                Just (ClsIE insts) -> filter (instIsVisible vis_mods) insts
                Nothing            -> []

-- | Checks for an exact match of ClsInst in the instance environment.
-- We use this when we do signature checking in TcRnDriver
memberInstEnv :: InstEnv -> ClsInst -> Bool
memberInstEnv inst_env ins_item@(ClsInst { is_cls_nm = cls_nm } ) =
    maybe False (\(ClsIE items) -> any (identicalDFunType ins_item) items)
          (lookupUDFM inst_env cls_nm)
  identicalDFunType cls1 cls2 =
    eqType (varType (is_dfun cls1)) (varType (is_dfun cls2))

extendInstEnvList :: InstEnv -> [ClsInst] -> InstEnv
extendInstEnvList inst_env ispecs = foldl' extendInstEnv inst_env ispecs

extendInstEnv :: InstEnv -> ClsInst -> InstEnv
extendInstEnv inst_env ins_item@(ClsInst { is_cls_nm = cls_nm })
  = addToUDFM_C add inst_env cls_nm (ClsIE [ins_item])
    add (ClsIE cur_insts) _ = ClsIE (ins_item : cur_insts)

deleteFromInstEnv :: InstEnv -> ClsInst -> InstEnv
deleteFromInstEnv inst_env ins_item@(ClsInst { is_cls_nm = cls_nm })
  = adjustUDFM adjust inst_env cls_nm
    adjust (ClsIE items) = ClsIE (filterOut (identicalClsInstHead ins_item) items)

deleteDFunFromInstEnv :: InstEnv -> DFunId -> InstEnv
-- Delete a specific instance fron an InstEnv
deleteDFunFromInstEnv inst_env dfun
  = adjustUDFM adjust inst_env cls
    (_, _, cls, _) = tcSplitDFunTy (idType dfun)
    adjust (ClsIE items) = ClsIE (filterOut same_dfun items)
    same_dfun (ClsInst { is_dfun = dfun' }) = dfun == dfun'

identicalClsInstHead :: ClsInst -> ClsInst -> Bool
-- ^ True when when the instance heads are the same
-- e.g.  both are   Eq [(a,b)]
-- Used for overriding in GHCi
-- Obviously should be insenstive to alpha-renaming
identicalClsInstHead (ClsInst { is_cls_nm = cls_nm1, is_tcs = rough1, is_tys = tys1 })
                     (ClsInst { is_cls_nm = cls_nm2, is_tcs = rough2, is_tys = tys2 })
  =  cls_nm1 == cls_nm2
  && not (instanceCantMatch rough1 rough2)  -- Fast check for no match, uses the "rough match" fields
  && isJust (tcMatchTys tys1 tys2)
  && isJust (tcMatchTys tys2 tys1)

*                                                                      *
        Looking up an instance
*                                                                      *

@lookupInstEnv@ looks up in a @InstEnv@, using a one-way match.  Since
the env is kept ordered, the first match must be the only one.  The
thing we are looking up can have an arbitrary "flexi" part.

Note [Instance lookup and orphan instances]
Suppose we are compiling a module M, and we have a zillion packages
loaded, and we are looking up an instance for C (T W).  If we find a
match in module 'X' from package 'p', should be "in scope"; that is,

  is p:X in the transitive closure of modules imported from M?

The difficulty is that the "zillion packages" might include ones loaded
through earlier invocations of the GHC API, or earlier module loads in GHCi.
They might not be in the dependencies of M itself; and if not, the instances
in them should not be visible.  #2182, #8427.

There are two cases:
  * If the instance is *not an orphan*, then module X defines C, T, or W.
    And in order for those types to be involved in typechecking M, it
    must be that X is in the transitive closure of M's imports.  So we
    can use the instance.

  * If the instance *is an orphan*, the above reasoning does not apply.
    So we keep track of the set of orphan modules transitively below M;
    this is the ie_visible field of InstEnvs, of type VisibleOrphanModules.

    If module p:X is in this set, then we can use the instance, otherwise
    we can't.

Note [Rules for instance lookup]
These functions implement the carefully-written rules in the user
manual section on "overlapping instances". At risk of duplication,
here are the rules.  If the rules change, change this text and the
user manual simultaneously.  The link may be this:

The willingness to be overlapped or incoherent is a property of the
instance declaration itself, controlled as follows:

 * An instance is "incoherent"
   if it has an INCOHERENT pragma, or
   if it appears in a module compiled with -XIncoherentInstances.

 * An instance is "overlappable"
   if it has an OVERLAPPABLE or OVERLAPS pragma, or
   if it appears in a module compiled with -XOverlappingInstances, or
   if the instance is incoherent.

 * An instance is "overlapping"
   if it has an OVERLAPPING or OVERLAPS pragma, or
   if it appears in a module compiled with -XOverlappingInstances, or
   if the instance is incoherent.
     compiled with -XOverlappingInstances.

Now suppose that, in some client module, we are searching for an instance
of the target constraint (C ty1 .. tyn). The search works like this.

*  Find all instances `I` that *match* the target constraint; that is, the
   target constraint is a substitution instance of `I`. These instance
   declarations are the *candidates*.

*  Eliminate any candidate `IX` for which both of the following hold:

   -  There is another candidate `IY` that is strictly more specific; that
      is, `IY` is a substitution instance of `IX` but not vice versa.

   -  Either `IX` is *overlappable*, or `IY` is *overlapping*. (This
      "either/or" design, rather than a "both/and" design, allow a
      client to deliberately override an instance from a library,
      without requiring a change to the library.)

-  If exactly one non-incoherent candidate remains, select it. If all
   remaining candidates are incoherent, select an arbitrary one.
   Otherwise the search fails (i.e. when more than one surviving
   candidate is not incoherent).

-  If the selected candidate (from the previous step) is incoherent, the
   search succeeds, returning that candidate.

-  If not, find all instances that *unify* with the target constraint,
   but do not *match* it. Such non-candidate instances might match when
   the target constraint is further instantiated. If all of them are
   incoherent, the search succeeds, returning the selected candidate; if
   not, the search fails.

Notice that these rules are not influenced by flag settings in the
client module, where the instances are *used*. These rules make it
possible for a library author to design a library that relies on
overlapping instances without the client having to know.

Note [Overlapping instances]   (NB: these notes are quite old)
Overlap is permitted, but only in such a way that one can make
a unique choice when looking up.  That is, overlap is only permitted if
one template matches the other, or vice versa.  So this is ok:

  [a]  [Int]

but this is not

  (Int,a)  (b,Int)

If overlap is permitted, the list is kept most specific first, so that
the first lookup is the right choice.

For now we just use association lists.

\subsection{Avoiding a problem with overlapping}

Consider this little program:

     class C a        where c :: a
     class C a => D a where d :: a

     instance C Int where c = 17
     instance D Int where d = 13

     instance C a => C [a] where c = [c]
     instance ({- C [a], -} D a) => D [a] where d = c

     instance C [Int] where c = [37]

     main = print (d :: [Int])

What do you think `main' prints  (assuming we have overlapping instances, and
all that turned on)?  Well, the instance for `D' at type `[a]' is defined to
be `c' at the same type, and we've got an instance of `C' at `[Int]', so the
answer is `[37]', right? (the generic `C [a]' instance shouldn't apply because
the `C [Int]' instance is more specific).

Ghc-4.04 gives `[37]', while ghc-4.06 gives `[17]', so 4.06 is wrong.  That
was easy ;-)  Let's just consult hugs for good measure.  Wait - if I use old
hugs (pre-September99), I get `[17]', and stranger yet, if I use hugs98, it
doesn't even compile!  What's going on!?

What hugs complains about is the `D [a]' instance decl.

     ERROR "mj.hs" (line 10): Cannot build superclass instance
     *** Instance            : D [a]
     *** Context supplied    : D a
     *** Required superclass : C [a]

You might wonder what hugs is complaining about.  It's saying that you
need to add `C [a]' to the context of the `D [a]' instance (as appears
in comments).  But there's that `C [a]' instance decl one line above
that says that I can reduce the need for a `C [a]' instance to the
need for a `C a' instance, and in this case, I already have the
necessary `C a' instance (since we have `D a' explicitly in the
context, and `C' is a superclass of `D').

Unfortunately, the above reasoning indicates a premature commitment to the
generic `C [a]' instance.  I.e., it prematurely rules out the more specific
instance `C [Int]'.  This is the mistake that ghc-4.06 makes.  The fix is to
add the context that hugs suggests (uncomment the `C [a]'), effectively
deferring the decision about which instance to use.

Now, interestingly enough, 4.04 has this same bug, but it's covered up
in this case by a little known `optimization' that was disabled in
4.06.  Ghc-4.04 silently inserts any missing superclass context into
an instance declaration.  In this case, it silently inserts the `C
[a]', and everything happens to work out.

(See `basicTypes/MkId:mkDictFunId' for the code in question.  Search for
`Mark Jones', although Mark claims no credit for the `optimization' in
question, and would rather it stopped being called the `Mark Jones
optimization' ;-)

So, what's the fix?  I think hugs has it right.  Here's why.  Let's try
something else out with ghc-4.04.  Let's add the following line:

    d' :: D a => [a]
    d' = c

Everyone raise their hand who thinks that `d :: [Int]' should give a
different answer from `d' :: [Int]'.  Well, in ghc-4.04, it does.  The
`optimization' only applies to instance decls, not to regular
bindings, giving inconsistent behavior.

Old hugs had this same bug.  Here's how we fixed it: like GHC, the
list of instances for a given class is ordered, so that more specific
instances come before more generic ones.  For example, the instance
list for C might contain:
    ..., C Int, ..., C a, ...
When we go to look for a `C Int' instance we'll get that one first.
But what if we go looking for a `C b' (`b' is unconstrained)?  We'll
pass the `C Int' instance, and keep going.  But if `b' is
unconstrained, then we don't know yet if the more specific instance
will eventually apply.  GHC keeps going, and matches on the generic `C
a'.  The fix is to, at each step, check to see if there's a reverse
match, and if so, abort the search.  This prevents hugs from
prematurely chosing a generic instance when a more specific one


BUT NOTE [Nov 2001]: we must actually *unify* not reverse-match in
this test.  Suppose the instance envt had
    ..., forall a b. C a a b, ..., forall a b c. C a b c, ...
(still most specific first)
Now suppose we are looking for (C x y Int), where x and y are unconstrained.
        C x y Int  doesn't match the template {a,b} C a a b
but neither does
        C a a b  match the template {x,y} C x y Int
But still x and y might subsequently be unified so they *do* match.

Simple story: unify, don't match.

type DFunInstType = Maybe Type
        -- Just ty   => Instantiate with this type
        -- Nothing   => Instantiate with any type of this tyvar's kind
        -- See Note [DFunInstType: instantiating types]

type InstMatch = (ClsInst, [DFunInstType])

type ClsInstLookupResult
     = ( [InstMatch]     -- Successful matches
       , [ClsInst]       -- These don't match but do unify
       , [InstMatch] )   -- Unsafe overlapped instances under Safe Haskell
                         -- (see Note [Safe Haskell Overlapping Instances] in
                         -- TcSimplify).

Note [DFunInstType: instantiating types]
A successful match is a ClsInst, together with the types at which
        the dfun_id in the ClsInst should be instantiated
The instantiating types are (Either TyVar Type)s because the dfun
might have some tyvars that *only* appear in arguments
        dfun :: forall a b. C a b, Ord b => D [a]
When we match this against D [ty], we return the instantiating types
        [Just ty, Nothing]
where the 'Nothing' indicates that 'b' can be freely instantiated.
(The caller instantiates it to a flexi type variable, which will
 presumably later become fixed via functional dependencies.)

-- |Look up an instance in the given instance environment. The given class application must match exactly
-- one instance and the match may not contain any flexi type variables.  If the lookup is unsuccessful,
-- yield 'Left errorMessage'.
lookupUniqueInstEnv :: InstEnvs
                    -> Class -> [Type]
                    -> Either MsgDoc (ClsInst, [Type])
lookupUniqueInstEnv instEnv cls tys
  = case lookupInstEnv False instEnv cls tys of
      ([(inst, inst_tys)], _, _)
             | noFlexiVar -> Right (inst, inst_tys')
             | otherwise  -> Left $ text "flexible type variable:" <+>
                                    (ppr $ mkTyConApp (classTyCon cls) tys)
               inst_tys'  = [ty | Just ty <- inst_tys]
               noFlexiVar = all isJust inst_tys
      _other -> Left $ text "instance not found" <+>
                       (ppr $ mkTyConApp (classTyCon cls) tys)

lookupInstEnv' :: InstEnv          -- InstEnv to look in
               -> VisibleOrphanModules   -- But filter against this
               -> Class -> [Type]  -- What we are looking for
               -> ([InstMatch],    -- Successful matches
                   [ClsInst])      -- These don't match but do unify
                                   -- (no incoherent ones in here)
-- The second component of the result pair happens when we look up
--      Foo [a]
-- in an InstEnv that has entries for
--      Foo [Int]
--      Foo [b]
-- Then which we choose would depend on the way in which 'a'
-- is instantiated.  So we report that Foo [b] is a match (mapping b->a)
-- but Foo [Int] is a unifier.  This gives the caller a better chance of
-- giving a suitable error message

lookupInstEnv' ie vis_mods cls tys
  = lookup ie
    rough_tcs  = roughMatchTcs tys
    all_tvs    = all isNothing rough_tcs

    lookup env = case lookupUDFM env cls of
                   Nothing -> ([],[])   -- No instances for this class
                   Just (ClsIE insts) -> find [] [] insts

    find ms us [] = (ms, us)
    find ms us (item@(ClsInst { is_tcs = mb_tcs, is_tvs = tpl_tvs
                              , is_tys = tpl_tys }) : rest)
      | not (instIsVisible vis_mods item)
      = find ms us rest  -- See Note [Instance lookup and orphan instances]

        -- Fast check for no match, uses the "rough match" fields
      | instanceCantMatch rough_tcs mb_tcs
      = find ms us rest

      | Just subst <- tcMatchTys tpl_tys tys
      = find ((item, map (lookupTyVar subst) tpl_tvs) : ms) us rest

        -- Does not match, so next check whether the things unify
        -- See Note [Overlapping instances]
        -- Ignore ones that are incoherent: Note [Incoherent instances]
      | isIncoherent item
      = find ms us rest

      | otherwise
      = ASSERT2( tyCoVarsOfTypes tys `disjointVarSet` tpl_tv_set,
                 (ppr cls <+> ppr tys <+> ppr all_tvs) $$
                 (ppr tpl_tvs <+> ppr tpl_tys)
                -- Unification will break badly if the variables overlap
                -- They shouldn't because we allocate separate uniques for them
                -- See Note [Template tyvars are fresh]
        case tcUnifyTys instanceBindFun tpl_tys tys of
            Just _   -> find ms (item:us) rest
            Nothing  -> find ms us        rest
        tpl_tv_set = mkVarSet tpl_tvs

-- This is the common way to call this function.
lookupInstEnv :: Bool              -- Check Safe Haskell overlap restrictions
              -> InstEnvs          -- External and home package inst-env
              -> Class -> [Type]   -- What we are looking for
              -> ClsInstLookupResult
-- ^ See Note [Rules for instance lookup]
-- ^ See Note [Safe Haskell Overlapping Instances] in TcSimplify
-- ^ See Note [Safe Haskell Overlapping Instances Implementation] in TcSimplify
lookupInstEnv check_overlap_safe
              (InstEnvs { ie_global = pkg_ie
                        , ie_local = home_ie
                        , ie_visible = vis_mods })
  = -- pprTrace "lookupInstEnv" (ppr cls <+> ppr tys $$ ppr home_ie) $
    (final_matches, final_unifs, unsafe_overlapped)
    (home_matches, home_unifs) = lookupInstEnv' home_ie vis_mods cls tys
    (pkg_matches,  pkg_unifs)  = lookupInstEnv' pkg_ie  vis_mods cls tys
    all_matches = home_matches ++ pkg_matches
    all_unifs   = home_unifs   ++ pkg_unifs
    final_matches = foldr insert_overlapping [] all_matches
        -- Even if the unifs is non-empty (an error situation)
        -- we still prune the matches, so that the error message isn't
        -- misleading (complaining of multiple matches when some should be
        -- overlapped away)

       = case final_matches of
           [match] -> check_safe match
           _       -> []

    -- If the selected match is incoherent, discard all unifiers
    final_unifs = case final_matches of
                    (m:_) | isIncoherent (fst m) -> []
                    _                            -> all_unifs

    -- NOTE [Safe Haskell isSafeOverlap]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    -- We restrict code compiled in 'Safe' mode from overriding code
    -- compiled in any other mode. The rationale is that code compiled
    -- in 'Safe' mode is code that is untrusted by the ghc user. So
    -- we shouldn't let that code change the behaviour of code the
    -- user didn't compile in 'Safe' mode since that's the code they
    -- trust. So 'Safe' instances can only overlap instances from the
    -- same module. A same instance origin policy for safe compiled
    -- instances.
    check_safe (inst,_)
        = case check_overlap_safe && unsafeTopInstance inst of
                -- make sure it only overlaps instances from the same module
                True -> go [] all_matches
                -- most specific is from a trusted location.
                False -> []
            go bad [] = bad
            go bad (i@(x,_):unchecked) =
                if inSameMod x || isOverlappable x
                    then go bad unchecked
                    else go (i:bad) unchecked

            inSameMod b =
                let na = getName $ getName inst
                    la = isInternalName na
                    nb = getName $ getName b
                    lb = isInternalName nb
                in (la && lb) || (nameModule na == nameModule nb)

    -- We consider the most specific instance unsafe when it both:
    --   (1) Comes from a module compiled as `Safe`
    --   (2) Is an orphan instance, OR, an instance for a MPTC
    unsafeTopInstance inst = isSafeOverlap (is_flag inst) &&
        (isOrphan (is_orphan inst) || classArity (is_cls inst) > 1)

insert_overlapping :: InstMatch -> [InstMatch] -> [InstMatch]
-- ^ Add a new solution, knocking out strictly less specific ones
-- See Note [Rules for instance lookup]
insert_overlapping new_item [] = [new_item]
insert_overlapping new_item@(new_inst,_) (old_item@(old_inst,_) : old_items)
  | new_beats_old        -- New strictly overrides old
  , not old_beats_new
  , new_inst `can_override` old_inst
  = insert_overlapping new_item old_items

  | old_beats_new        -- Old strictly overrides new
  , not new_beats_old
  , old_inst `can_override` new_inst
  = old_item : old_items

  -- Discard incoherent instances; see Note [Incoherent instances]
  | isIncoherent old_inst      -- Old is incoherent; discard it
  = insert_overlapping new_item old_items
  | isIncoherent new_inst      -- New is incoherent; discard it
  = old_item : old_items

  -- Equal or incomparable, and neither is incoherent; keep both
  | otherwise
  = old_item : insert_overlapping new_item old_items

    new_beats_old = new_inst `more_specific_than` old_inst
    old_beats_new = old_inst `more_specific_than` new_inst

    -- `instB` can be instantiated to match `instA`
    -- or the two are equal
    instA `more_specific_than` instB
      = isJust (tcMatchTys (is_tys instB) (is_tys instA))

    instA `can_override` instB
       = isOverlapping instA || isOverlappable instB
       -- Overlap permitted if either the more specific instance
       -- is marked as overlapping, or the more general one is
       -- marked as overlappable.
       -- Latest change described in: #9242.
       -- Previous change: #3877, Dec 10.

Note [Incoherent instances]
For some classes, the choice of a particular instance does not matter, any one
is good. E.g. consider

        class D a b where { opD :: a -> b -> String }
        instance D Int b where ...
        instance D a Int where ...

        g (x::Int) = opD x x  -- Wanted: D Int Int

For such classes this should work (without having to add an "instance D Int
Int", and using -XOverlappingInstances, which would then work). This is what
-XIncoherentInstances is for: Telling GHC "I don't care which instance you use;
if you can use one, use it."

Should this logic only work when *all* candidates have the incoherent flag, or
even when all but one have it? The right choice is the latter, which can be
justified by comparing the behaviour with how -XIncoherentInstances worked when
it was only about the unify-check (note [Overlapping instances]):

        class C a b c where foo :: (a,b,c)
        instance C [a] b Int
        instance [incoherent] [Int] b c
        instance [incoherent] C a Int c
Thanks to the incoherent flags,
        [Wanted]  C [a] b Int
works: Only instance one matches, the others just unify, but are marked

So I can write
        (foo :: ([a],b,Int)) :: ([Int], Int, Int).
but if that works then I really want to be able to write
        foo :: ([Int], Int, Int)
as well. Now all three instances from above match. None is more specific than
another, so none is ruled out by the normal overlapping rules. One of them is
not incoherent, but we still want this to compile. Hence the

The implementation is in insert_overlapping, where we remove matching
incoherent instances as long as there are others.

*                                                                      *
        Binding decisions
*                                                                      *

instanceBindFun :: TyCoVar -> BindFlag
instanceBindFun tv | isOverlappableTyVar tv = Skolem
                   | otherwise              = BindMe
   -- Note [Binding when looking up instances]

Note [Binding when looking up instances]
When looking up in the instance environment, or family-instance environment,
we are careful about multiple matches, as described above in
Note [Overlapping instances]

The key_tys can contain skolem constants, and we can guarantee that those
are never going to be instantiated to anything, so we should not involve
them in the unification test.  Example:
        class Foo a where { op :: a -> Int }
        instance Foo a => Foo [a]       -- NB overlap
        instance Foo [Int]              -- NB overlap
        data T = forall a. Foo a => MkT a
        f :: T -> Int
        f (MkT x) = op [x,x]
The op [x,x] means we need (Foo [a]).  Without the filterVarSet we'd
complain, saying that the choice of instance depended on the instantiation
of 'a'; but of course it isn't *going* to be instantiated.

We do this only for isOverlappableTyVar skolems.  For example we reject
        g :: forall a => [a] -> Int
        g x = op x
on the grounds that the correct instance depends on the instantiation of 'a'