{-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}

module GHC.Tc.Instance.Class (
     matchGlobalInst, matchEqualityInst,
     ClsInstResult(..),
     InstanceWhat(..), safeOverlap, instanceReturnsDictCon,
     AssocInstInfo(..), isNotAssociated
  ) where

import GHC.Prelude

import GHC.Driver.DynFlags

import GHC.Core.TyCo.Rep

import GHC.Tc.Utils.Env
import GHC.Tc.Utils.Monad
import GHC.Tc.Utils.TcType
import GHC.Tc.Utils.Instantiate(instDFunType, tcInstType)
import GHC.Tc.Instance.Typeable
import GHC.Tc.Utils.TcMType
import GHC.Tc.Types.Evidence
import GHC.Tc.Types.Origin (InstanceWhat (..), SafeOverlapping)
import GHC.Tc.Instance.Family( tcGetFamInstEnvs, tcInstNewTyCon_maybe, tcLookupDataFamInst )
import GHC.Rename.Env( addUsedGRE, DeprecationWarnings (..) )

import GHC.Builtin.Types
import GHC.Builtin.Types.Prim
import GHC.Builtin.Names

import GHC.Types.FieldLabel
import GHC.Types.Name.Reader
import GHC.Types.SafeHaskell
import GHC.Types.Name   ( Name )
import GHC.Types.Var.Env ( VarEnv )
import GHC.Types.Id
import GHC.Types.Var

import GHC.Core.Predicate
import GHC.Core.Coercion
import GHC.Core.InstEnv
import GHC.Core.Type
import GHC.Core.Make ( mkCharExpr, mkNaturalExpr, mkStringExprFS, mkCoreLams )
import GHC.Core.DataCon
import GHC.Core.TyCon
import GHC.Core.Class

import GHC.Core ( Expr(Var, App, Cast) )

import GHC.Utils.Outputable
import GHC.Utils.Panic
import GHC.Utils.Misc( splitAtList, fstOf3 )
import GHC.Data.FastString

import Language.Haskell.Syntax.Basic (FieldLabelString(..))

{- *******************************************************************
*                                                                    *
              A helper for associated types within
              class instance declarations
*                                                                    *
**********************************************************************-}

-- | Extra information about the parent instance declaration, needed
-- when type-checking associated types. The 'Class' is the enclosing
-- class, the [TyVar] are the /scoped/ type variable of the instance decl.
-- The @VarEnv Type@ maps class variables to their instance types.
data AssocInstInfo
  = NotAssociated
  | InClsInst { AssocInstInfo -> Class
ai_class    :: Class
              , AssocInstInfo -> [DFunId]
ai_tyvars   :: [TyVar]      -- ^ The /scoped/ tyvars of the instance
                                            -- Why scoped?  See bind_me in
                                            -- 'GHC.Tc.Validity.checkConsistentFamInst'
              , AssocInstInfo -> VarEnv PredType
ai_inst_env :: VarEnv Type  -- ^ Maps /class/ tyvars to their instance types
                -- See Note [Matching in the consistent-instantiation check]
    }

isNotAssociated :: AssocInstInfo -> Bool
isNotAssociated :: AssocInstInfo -> Bool
isNotAssociated (NotAssociated {}) = Bool
True
isNotAssociated (InClsInst {})     = Bool
False


{- *******************************************************************
*                                                                    *
                       Class lookup
*                                                                    *
**********************************************************************-}

data ClsInstResult
  = NoInstance   -- Definitely no instance

  | OneInst { ClsInstResult -> [PredType]
cir_new_theta   :: [TcPredType]
            , ClsInstResult -> [EvExpr] -> EvTerm
cir_mk_ev       :: [EvExpr] -> EvTerm
            , ClsInstResult -> Bool
cir_canonical   :: Canonical --   cir_canonical=True => you can specialise on this instance
                                           --   cir_canonical= False => you cannot specialise on this instance
                                           --                           (its OverlapFlag is NonCanonical)
                                           -- See Note [Coherence and specialisation: overview]
            , ClsInstResult -> InstanceWhat
cir_what        :: InstanceWhat }

  | NotSure      -- Multiple matches and/or one or more unifiers

instance Outputable ClsInstResult where
  ppr :: ClsInstResult -> SDoc
ppr ClsInstResult
NoInstance = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"NoInstance"
  ppr ClsInstResult
NotSure    = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"NotSure"
  ppr (OneInst { cir_new_theta :: ClsInstResult -> [PredType]
cir_new_theta = [PredType]
ev
               , cir_what :: ClsInstResult -> InstanceWhat
cir_what = InstanceWhat
what })
    = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"OneInst" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [[PredType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [PredType]
ev, InstanceWhat -> SDoc
forall a. Outputable a => a -> SDoc
ppr InstanceWhat
what]

safeOverlap :: InstanceWhat -> Bool
safeOverlap :: InstanceWhat -> Bool
safeOverlap (TopLevInstance { iw_safe_over :: InstanceWhat -> Bool
iw_safe_over = Bool
so }) = Bool
so
safeOverlap InstanceWhat
_                                      = Bool
True

instanceReturnsDictCon :: InstanceWhat -> Bool
-- See Note [Solved dictionaries] in GHC.Tc.Solver.InertSet
instanceReturnsDictCon :: InstanceWhat -> Bool
instanceReturnsDictCon (TopLevInstance {}) = Bool
True
instanceReturnsDictCon InstanceWhat
BuiltinInstance     = Bool
True
instanceReturnsDictCon BuiltinTypeableInstance {} = Bool
True
instanceReturnsDictCon InstanceWhat
BuiltinEqInstance   = Bool
False
instanceReturnsDictCon InstanceWhat
LocalInstance       = Bool
False

matchGlobalInst :: DynFlags
                -> Bool      -- True <=> caller is the short-cut solver
                             -- See Note [Shortcut solving: overlap]
                -> Class -> [Type] -> TcM ClsInstResult
-- Precondition: Class does not satisfy GHC.Core.Predicate.isEqualityClass
-- (That is handled by a separate code path: see GHC.Tc.Solver.Dict.solveDict,
--  which calls solveEqualityDict for equality classes.)
matchGlobalInst :: DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchGlobalInst DynFlags
dflags Bool
short_cut Class
clas [PredType]
tys
  | Name
cls_name Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== Name
knownNatClassName      = DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchKnownNat    DynFlags
dflags Bool
short_cut Class
clas [PredType]
tys
  | Name
cls_name Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== Name
knownSymbolClassName   = DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchKnownSymbol DynFlags
dflags Bool
short_cut Class
clas [PredType]
tys
  | Name
cls_name Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== Name
knownCharClassName     = DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchKnownChar   DynFlags
dflags Bool
short_cut Class
clas [PredType]
tys
  | Class -> Bool
isCTupleClass Class
clas                 = Class -> [PredType] -> TcM ClsInstResult
matchCTuple                       Class
clas [PredType]
tys
  | Name
cls_name Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== Name
typeableClassName      = Class -> [PredType] -> TcM ClsInstResult
matchTypeable                     Class
clas [PredType]
tys
  | Name
cls_name Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== Name
withDictClassName      = [PredType] -> TcM ClsInstResult
matchWithDict                          [PredType]
tys
  | Name
cls_name Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== Name
hasFieldClassName      = DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchHasField    DynFlags
dflags Bool
short_cut Class
clas [PredType]
tys
  | Name
cls_name Name -> Name -> Bool
forall a. Eq a => a -> a -> Bool
== Name
unsatisfiableClassName = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance -- See (B) in Note [Implementation of Unsatisfiable constraints] in GHC.Tc.Errors
  | Bool
otherwise                          = DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchInstEnv     DynFlags
dflags Bool
short_cut Class
clas [PredType]
tys
  where
    cls_name :: Name
cls_name = Class -> Name
className Class
clas


{- ********************************************************************
*                                                                     *
                   Looking in the instance environment
*                                                                     *
***********************************************************************-}


matchInstEnv :: DynFlags -> Bool -> Class -> [Type] -> TcM ClsInstResult
matchInstEnv :: DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchInstEnv DynFlags
dflags Bool
short_cut_solver Class
clas [PredType]
tys
   = do { InstEnvs
instEnvs <- TcM InstEnvs
tcGetInstEnvs
        ; let safeOverlapCheck :: Bool
safeOverlapCheck = DynFlags -> SafeHaskellMode
safeHaskell DynFlags
dflags SafeHaskellMode -> [SafeHaskellMode] -> Bool
forall a. Eq a => a -> [a] -> Bool
forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
`elem` [SafeHaskellMode
Sf_Safe, SafeHaskellMode
Sf_Trustworthy]
              ([InstMatch]
matches, PotentialUnifiers
unify, [InstMatch]
unsafeOverlaps) = Bool
-> InstEnvs
-> Class
-> [PredType]
-> ([InstMatch], PotentialUnifiers, [InstMatch])
lookupInstEnv Bool
True InstEnvs
instEnvs Class
clas [PredType]
tys
              safeHaskFail :: Bool
safeHaskFail = Bool
safeOverlapCheck Bool -> Bool -> Bool
&& Bool -> Bool
not ([InstMatch] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [InstMatch]
unsafeOverlaps)
        ; String -> SDoc -> TcRn ()
traceTc String
"matchInstEnv" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
forall a b. (a -> b) -> a -> b
$
            [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"goal:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Class -> SDoc
forall a. Outputable a => a -> SDoc
ppr Class
clas SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [PredType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [PredType]
tys
                 , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"matches:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstMatch] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstMatch]
matches
                 , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"unify:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PotentialUnifiers -> SDoc
forall a. Outputable a => a -> SDoc
ppr PotentialUnifiers
unify ]
        ; case ([InstMatch]
matches, PotentialUnifiers
unify, Bool
safeHaskFail) of

            -- Nothing matches
            ([], NoUnifiers{}, Bool
_)
                -> do { String -> SDoc -> TcRn ()
traceTc String
"matchClass not matching" (PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
pred SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ InstEnv -> SDoc
forall a. Outputable a => a -> SDoc
ppr (InstEnvs -> InstEnv
ie_local InstEnvs
instEnvs))
                      ; ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance }

            -- A single match (& no safe haskell failure)
            ([(ClsInst
ispec, [DFunInstType]
inst_tys)], NoUnifiers Bool
canonical, Bool
False)
                | Bool
short_cut_solver      -- Called from the short-cut solver
                , ClsInst -> Bool
isOverlappable ClsInst
ispec
                -- If the instance has OVERLAPPABLE or OVERLAPS or INCOHERENT
                -- then don't let the short-cut solver choose it, because a
                -- later instance might overlap it.  #14434 is an example
                -- See Note [Shortcut solving: overlap]
                -> do { String -> SDoc -> TcRn ()
traceTc String
"matchClass: ignoring overlappable" (PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
pred)
                      ; ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NotSure }

                | Bool
otherwise
                -> do { let dfun_id :: DFunId
dfun_id = ClsInst -> DFunId
instanceDFunId ClsInst
ispec
                      ; String -> SDoc -> TcRn ()
traceTc String
"matchClass success" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
forall a b. (a -> b) -> a -> b
$
                        [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"dict" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
pred SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc
parens (if Bool
canonical then String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"canonical" else String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"non-canonical"),
                              String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"witness" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> DFunId -> SDoc
forall a. Outputable a => a -> SDoc
ppr DFunId
dfun_id
                                             SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr (DFunId -> PredType
idType DFunId
dfun_id) ]
                                -- Record that this dfun is needed
                      ; Bool -> Bool -> DFunId -> [DFunInstType] -> TcM ClsInstResult
match_one ([InstMatch] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [InstMatch]
unsafeOverlaps) Bool
canonical DFunId
dfun_id [DFunInstType]
inst_tys }

            -- More than one matches (or Safe Haskell fail!). Defer any
            -- reactions of a multitude until we learn more about the reagent
            ([InstMatch], PotentialUnifiers, Bool)
_   -> do { String -> SDoc -> TcRn ()
traceTc String
"matchClass multiple matches, deferring choice" (SDoc -> TcRn ()) -> SDoc -> TcRn ()
forall a b. (a -> b) -> a -> b
$
                        [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"dict" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr PredType
pred,
                              String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"matches" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstMatch] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstMatch]
matches]
                      ; ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NotSure } }
   where
     pred :: PredType
pred = Class -> [PredType] -> PredType
mkClassPred Class
clas [PredType]
tys

match_one :: SafeOverlapping -> Canonical -> DFunId -> [DFunInstType]
          -> TcM ClsInstResult
match_one :: Bool -> Bool -> DFunId -> [DFunInstType] -> TcM ClsInstResult
match_one Bool
so Bool
canonical DFunId
dfun_id [DFunInstType]
mb_inst_tys
  = do { String -> SDoc -> TcRn ()
traceTc String
"match_one" (DFunId -> SDoc
forall a. Outputable a => a -> SDoc
ppr DFunId
dfun_id SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [DFunInstType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [DFunInstType]
mb_inst_tys)
       ; ([PredType]
tys, [PredType]
theta) <- DFunId -> [DFunInstType] -> TcM ([PredType], [PredType])
instDFunType DFunId
dfun_id [DFunInstType]
mb_inst_tys
       ; String -> SDoc -> TcRn ()
traceTc String
"match_one 2" (DFunId -> SDoc
forall a. Outputable a => a -> SDoc
ppr DFunId
dfun_id SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [PredType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [PredType]
tys SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [PredType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [PredType]
theta)
       ; ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (ClsInstResult -> TcM ClsInstResult)
-> ClsInstResult -> TcM ClsInstResult
forall a b. (a -> b) -> a -> b
$ OneInst { cir_new_theta :: [PredType]
cir_new_theta   = [PredType]
theta
                          , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = DFunId -> [PredType] -> [EvExpr] -> EvTerm
evDFunApp DFunId
dfun_id [PredType]
tys
                          , cir_canonical :: Bool
cir_canonical   = Bool
canonical
                          , cir_what :: InstanceWhat
cir_what        = TopLevInstance { iw_dfun_id :: DFunId
iw_dfun_id = DFunId
dfun_id
                                                             , iw_safe_over :: Bool
iw_safe_over = Bool
so } } }


{- Note [Shortcut solving: overlap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
  instance {-# OVERLAPPABLE #-} C a where ...
and we are typechecking
  f :: C a => a -> a
  f = e  -- Gives rise to [W] C a

We don't want to solve the wanted constraint with the overlappable
instance; rather we want to use the supplied (C a)! That was the whole
point of it being overlappable!  #14434 wwas an example.

Alas even if the instance has no overlap flag, thus
  instance C a where ...
there is nothing to stop it being overlapped. GHC provides no way to
declare an instance as "final" so it can't be overlapped.  But really
only final instances are OK for short-cut solving.  Sigh. #15135
was a puzzling example.
-}


{- ********************************************************************
*                                                                     *
                   Class lookup for CTuples
*                                                                     *
***********************************************************************-}

matchCTuple :: Class -> [Type] -> TcM ClsInstResult
matchCTuple :: Class -> [PredType] -> TcM ClsInstResult
matchCTuple Class
clas [PredType]
tys   -- (isCTupleClass clas) holds
  = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (OneInst { cir_new_theta :: [PredType]
cir_new_theta   = [PredType]
tys
                    , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = [EvExpr] -> EvTerm
tuple_ev
                    , cir_canonical :: Bool
cir_canonical   = Bool
True
                    , cir_what :: InstanceWhat
cir_what        = InstanceWhat
BuiltinInstance })
            -- The dfun *is* the data constructor!
  where
     data_con :: DataCon
data_con = TyCon -> DataCon
tyConSingleDataCon (Class -> TyCon
classTyCon Class
clas)
     tuple_ev :: [EvExpr] -> EvTerm
tuple_ev = DFunId -> [PredType] -> [EvExpr] -> EvTerm
evDFunApp (DataCon -> DFunId
dataConWrapId DataCon
data_con) [PredType]
tys

{- ********************************************************************
*                                                                     *
                   Class lookup for Literals
*                                                                     *
***********************************************************************-}

{-
Note [KnownNat & KnownSymbol and EvLit]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A part of the type-level literals implementation are the classes
"KnownNat" and "KnownSymbol", which provide a "smart" constructor for
defining singleton values.  Here is the key stuff from GHC.TypeNats

  class KnownNat (n :: Nat) where
    natSing :: SNat n

  newtype SNat (n :: Nat) = SNat Natural

Conceptually, this class has infinitely many instances:

  instance KnownNat 0       where natSing = SNat 0
  instance KnownNat 1       where natSing = SNat 1
  instance KnownNat 2       where natSing = SNat 2
  ...

In practice, we solve `KnownNat` predicates in the type-checker (see
`matchKnownNat` in this module) because we can't have infinitely many
instances.  The evidence (aka "dictionary") for `KnownNat` is of the
form `EvLit (EvNum n)`.

We make the following assumptions about dictionaries in GHC:
  1. The "dictionary" for classes with a single method---like `KnownNat`---is
     a newtype for the type of the method, so using a evidence amounts
     to a coercion, and
  2. Newtypes use the same representation as their definition types.

So, the evidence for `KnownNat` is just a value of the representation type,
wrapped in two newtype constructors: one to make it into a `SNat` value,
and another to make it into a `KnownNat` dictionary.

Also note that `natSing` and `SNat` are never actually exposed from the
library---they are just an implementation detail.  Instead, users see
a more convenient function, defined in terms of `natSing`:

  natVal :: KnownNat n => proxy n -> Natural

The reason we don't use this directly in the class is that it is simpler
and more efficient to pass around a Natural rather than an entire function,
especially when the `KnownNat` evidence is packaged up in an existential.

The story for kind `Symbol` is analogous:
  * class KnownSymbol
  * newtype SSymbol
  * Evidence: a Core literal (e.g. mkNaturalExpr)


Note [Fabricating Evidence for Literals in Backpack]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let `T` be a type of kind `Nat`. When solving for a purported instance
of `KnownNat T`, ghc tries to resolve the type `T` to an integer `n`,
in which case the evidence `EvLit (EvNum n)` is generated on the
fly. It might appear that this is sufficient as users cannot define
their own instances of `KnownNat`. However, for backpack module this
would not work (see issue #15379). Consider the signature `Abstract`

> signature Abstract where
>   data T :: Nat
>   instance KnownNat T

and a module `Util` that depends on it:

> module Util where
>  import Abstract
>  printT :: IO ()
>  printT = do print $ natVal (Proxy :: Proxy T)

Clearly, we need to "use" the dictionary associated with `KnownNat T`
in the module `Util`, but it is too early for the compiler to produce
a real dictionary as we still have not fixed what `T` is. Only when we
mixin a concrete module

> module Concrete where
>   type T = 42

do we really get hold of the underlying integer. So the strategy that
we follow is the following

1. If T is indeed available as a type alias for an integer constant,
   generate the dictionary on the fly, failing which

2. Look up the type class environment for the evidence.

Finally actual code gets generate for Util only when a module like
Concrete gets "mixed-in" in place of the signature Abstract. As a
result all things, including the typeclass instances, in Concrete gets
reexported. So `KnownNat` gets resolved the normal way post-Backpack.

A similar generation works for `KnownSymbol` as well

-}

matchKnownNat :: DynFlags
              -> Bool      -- True <=> caller is the short-cut solver
                           -- See Note [Shortcut solving: overlap]
              -> Class -> [Type] -> TcM ClsInstResult
matchKnownNat :: DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchKnownNat DynFlags
dflags Bool
_ Class
clas [PredType
ty]     -- clas = KnownNat
  | Just Integer
n <- PredType -> Maybe Integer
isNumLitTy PredType
ty  = Class -> PredType -> EvExpr -> TcM ClsInstResult
makeLitDict Class
clas PredType
ty (Platform -> Integer -> EvExpr
mkNaturalExpr (DynFlags -> Platform
targetPlatform DynFlags
dflags) Integer
n)
matchKnownNat DynFlags
df Bool
sc Class
clas [PredType]
tys = DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchInstEnv DynFlags
df Bool
sc Class
clas [PredType]
tys
 -- See Note [Fabricating Evidence for Literals in Backpack] for why
 -- this lookup into the instance environment is required.

matchKnownSymbol :: DynFlags
                 -> Bool      -- True <=> caller is the short-cut solver
                              -- See Note [Shortcut solving: overlap]
                 -> Class -> [Type] -> TcM ClsInstResult
matchKnownSymbol :: DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchKnownSymbol DynFlags
_ Bool
_ Class
clas [PredType
ty]  -- clas = KnownSymbol
  | Just FastString
s <- PredType -> Maybe FastString
isStrLitTy PredType
ty = do
        EvExpr
et <- FastString -> IOEnv (Env TcGblEnv TcLclEnv) EvExpr
forall (m :: * -> *). MonadThings m => FastString -> m EvExpr
mkStringExprFS FastString
s
        Class -> PredType -> EvExpr -> TcM ClsInstResult
makeLitDict Class
clas PredType
ty EvExpr
et
matchKnownSymbol DynFlags
df Bool
sc Class
clas [PredType]
tys = DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchInstEnv DynFlags
df Bool
sc Class
clas [PredType]
tys
 -- See Note [Fabricating Evidence for Literals in Backpack] for why
 -- this lookup into the instance environment is required.

matchKnownChar :: DynFlags
                 -> Bool      -- True <=> caller is the short-cut solver
                              -- See Note [Shortcut solving: overlap]
                 -> Class -> [Type] -> TcM ClsInstResult
matchKnownChar :: DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchKnownChar DynFlags
_ Bool
_ Class
clas [PredType
ty]  -- clas = KnownChar
  | Just Char
s <- PredType -> Maybe Char
isCharLitTy PredType
ty = Class -> PredType -> EvExpr -> TcM ClsInstResult
makeLitDict Class
clas PredType
ty (Char -> EvExpr
mkCharExpr Char
s)
matchKnownChar DynFlags
df Bool
sc Class
clas [PredType]
tys = DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchInstEnv DynFlags
df Bool
sc Class
clas [PredType]
tys
 -- See Note [Fabricating Evidence for Literals in Backpack] for why
 -- this lookup into the instance environment is required.

makeLitDict :: Class -> Type -> EvExpr -> TcM ClsInstResult
-- makeLitDict adds a coercion that will convert the literal into a dictionary
-- of the appropriate type.  See Note [KnownNat & KnownSymbol and EvLit]
-- in GHC.Tc.Types.Evidence.  The coercion happens in 2 steps:
--
--     Integer -> SNat n     -- representation of literal to singleton
--     SNat n  -> KnownNat n -- singleton to dictionary
--
--     The process is mirrored for Symbols:
--     String    -> SSymbol n
--     SSymbol n -> KnownSymbol n
makeLitDict :: Class -> PredType -> EvExpr -> TcM ClsInstResult
makeLitDict Class
clas PredType
ty EvExpr
et
    | Just (PredType
_, TcCoercion
co_dict) <- TyCon -> [PredType] -> Maybe (PredType, TcCoercion)
tcInstNewTyCon_maybe (Class -> TyCon
classTyCon Class
clas) [PredType
ty]
          -- co_dict :: KnownNat n ~ SNat n
    , [ DFunId
meth ]   <- Class -> [DFunId]
classMethods Class
clas
    , Just TyCon
tcRep <- PredType -> Maybe TyCon
tyConAppTyCon_maybe (DFunId -> PredType
classMethodTy DFunId
meth)
                    -- If the method type is forall n. KnownNat n => SNat n
                    -- then tcRep is SNat
    , Just (PredType
_, TcCoercion
co_rep) <- TyCon -> [PredType] -> Maybe (PredType, TcCoercion)
tcInstNewTyCon_maybe TyCon
tcRep [PredType
ty]
          -- SNat n ~ Integer
    , let ev_tm :: EvTerm
ev_tm = EvExpr -> TcCoercion -> EvTerm
mkEvCast EvExpr
et (TcCoercion -> TcCoercion
mkSymCo (TcCoercion -> TcCoercion -> TcCoercion
mkTransCo TcCoercion
co_dict TcCoercion
co_rep))
    = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (ClsInstResult -> TcM ClsInstResult)
-> ClsInstResult -> TcM ClsInstResult
forall a b. (a -> b) -> a -> b
$ OneInst { cir_new_theta :: [PredType]
cir_new_theta   = []
                       , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = \[EvExpr]
_ -> EvTerm
ev_tm
                       , cir_canonical :: Bool
cir_canonical   = Bool
True
                       , cir_what :: InstanceWhat
cir_what        = InstanceWhat
BuiltinInstance }

    | Bool
otherwise
    = String -> SDoc -> TcM ClsInstResult
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"makeLitDict" (SDoc -> TcM ClsInstResult) -> SDoc -> TcM ClsInstResult
forall a b. (a -> b) -> a -> b
$
      String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Unexpected evidence for" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Name -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Class -> Name
className Class
clas)
      SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat ((DFunId -> SDoc) -> [DFunId] -> [SDoc]
forall a b. (a -> b) -> [a] -> [b]
map (PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr (PredType -> SDoc) -> (DFunId -> PredType) -> DFunId -> SDoc
forall b c a. (b -> c) -> (a -> b) -> a -> c
. DFunId -> PredType
idType) (Class -> [DFunId]
classMethods Class
clas))

{- ********************************************************************
*                                                                     *
                   Class lookup for WithDict
*                                                                     *
***********************************************************************-}

-- See Note [withDict]
matchWithDict :: [Type] -> TcM ClsInstResult
matchWithDict :: [PredType] -> TcM ClsInstResult
matchWithDict [PredType
cls, PredType
mty]
    -- Check that cls is a class constraint `C t_1 ... t_n`, where
    -- `dict_tc = C` and `dict_args = t_1 ... t_n`.
  | Just (TyCon
dict_tc, [PredType]
dict_args) <- HasCallStack => PredType -> Maybe (TyCon, [PredType])
PredType -> Maybe (TyCon, [PredType])
tcSplitTyConApp_maybe PredType
cls
    -- Check that C is a class of the form
    -- `class C a_1 ... a_n where op :: meth_ty`
    -- and in that case let
    -- co :: C t1 ..tn ~R# inst_meth_ty
  , Just (PredType
inst_meth_ty, TcCoercion
co) <- TyCon -> [PredType] -> Maybe (PredType, TcCoercion)
tcInstNewTyCon_maybe TyCon
dict_tc [PredType]
dict_args
  = do { DFunId
sv <- FastString
-> PredType -> PredType -> IOEnv (Env TcGblEnv TcLclEnv) DFunId
forall (m :: * -> *).
MonadUnique m =>
FastString -> PredType -> PredType -> m DFunId
mkSysLocalM (String -> FastString
fsLit String
"withDict_s") PredType
ManyTy PredType
mty
       ; DFunId
k  <- FastString
-> PredType -> PredType -> IOEnv (Env TcGblEnv TcLclEnv) DFunId
forall (m :: * -> *).
MonadUnique m =>
FastString -> PredType -> PredType -> m DFunId
mkSysLocalM (String -> FastString
fsLit String
"withDict_k") PredType
ManyTy (HasDebugCallStack => PredType -> PredType -> PredType
PredType -> PredType -> PredType
mkInvisFunTy PredType
cls PredType
openAlphaTy)

       -- Given co2 : mty ~N# inst_meth_ty, construct the method of
       -- the WithDict dictionary:
       --
       --   \@(r :: RuntimeRep) @(a :: TYPE r) (sv :: mty) (k :: cls => a) ->
       --     k (sv |> (sub co ; sym co2))
       ; let evWithDict :: TcCoercion -> EvExpr
evWithDict TcCoercion
co2 =
               [DFunId] -> EvExpr -> EvExpr
mkCoreLams [ DFunId
runtimeRep1TyVar, DFunId
openAlphaTyVar, DFunId
sv, DFunId
k ] (EvExpr -> EvExpr) -> EvExpr -> EvExpr
forall a b. (a -> b) -> a -> b
$
                 DFunId -> EvExpr
forall b. DFunId -> Expr b
Var DFunId
k
                   EvExpr -> EvExpr -> EvExpr
forall b. Expr b -> Expr b -> Expr b
`App`
                 (DFunId -> EvExpr
forall b. DFunId -> Expr b
Var DFunId
sv EvExpr -> TcCoercion -> EvExpr
forall b. Expr b -> TcCoercion -> Expr b
`Cast` TcCoercion -> TcCoercion -> TcCoercion
mkTransCo (HasDebugCallStack => TcCoercion -> TcCoercion
TcCoercion -> TcCoercion
mkSubCo TcCoercion
co2) (TcCoercion -> TcCoercion
mkSymCo TcCoercion
co))

       ; TyCon
tc <- Name -> TcM TyCon
tcLookupTyCon Name
withDictClassName
       ; let Just DataCon
withdict_data_con
                 = TyCon -> Maybe DataCon
tyConSingleDataCon_maybe TyCon
tc    -- "Data constructor"
                                                  -- for WithDict
             mk_ev :: [EvExpr] -> EvTerm
mk_ev [EvExpr
c] = DataCon -> [PredType] -> [EvExpr] -> EvTerm
evDataConApp DataCon
withdict_data_con
                            [PredType
cls, PredType
mty] [TcCoercion -> EvExpr
evWithDict (EvTerm -> TcCoercion
evTermCoercion (EvExpr -> EvTerm
EvExpr EvExpr
c))]
             mk_ev [EvExpr]
e   = String -> SDoc -> EvTerm
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"matchWithDict" ([EvExpr] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [EvExpr]
e)

       ; ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (ClsInstResult -> TcM ClsInstResult)
-> ClsInstResult -> TcM ClsInstResult
forall a b. (a -> b) -> a -> b
$ OneInst { cir_new_theta :: [PredType]
cir_new_theta   = [PredType -> PredType -> PredType
mkPrimEqPred PredType
mty PredType
inst_meth_ty]
                          , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = [EvExpr] -> EvTerm
mk_ev
                          , cir_canonical :: Bool
cir_canonical   = Bool
False -- See (WD6) in Note [withDict]
                          , cir_what :: InstanceWhat
cir_what        = InstanceWhat
BuiltinInstance }
       }

matchWithDict [PredType]
_
  = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance

{-
Note [withDict]
~~~~~~~~~~~~~~~
The class `WithDict` is defined as:

    class WithDict cls meth where
        withDict :: forall {rr :: RuntimeRep} (r :: TYPE rr). meth -> (cls => r) -> r

This class is special, like `Typeable`: GHC automatically solves
for instances of `WithDict`, users cannot write their own.

It is used to implement a primitive that we cannot define in Haskell
but we can write in Core.

`WithDict` is used to create dictionaries for classes with a single method.
Consider a class like this:

   class C a where
     f :: T a

We can use `withDict` to cast values of type `T a` into dictionaries for `C a`.
To do this, we can define a function like this in the library:

  withT :: T a -> (C a => b) -> b
  withT t k = withDict @(C a) @(T a) t k

Here:

* The `cls` in `withDict` is instantiated to `C a`.

* The `meth` in `withDict` is instantiated to `T a`.
  The definition of `T` itself is irrelevant, only that `C a` is a class
  with a single method of type `T a`.

* The `r` in `withDict` is instantiated to `b`.

For any single-method class C:
   class C a1 .. an where op :: meth_ty

The solver will solve the constraint `WithDict (C t1 .. tn) mty`
as if the following instance declaration existed:

instance (mty ~# inst_meth_ty) => WithDict (C t1..tn) mty where
  withDict = \@{rr} @(r :: TYPE rr) (sv :: mty) (k :: C t1..tn => r) ->
    k (sv |> (sub co2 ; sym co))

That is, it matches on the first (constraint) argument of C; if C is
a single-method class, the instance "fires" and emits an equality
constraint `mty ~ inst_meth_ty`, where `inst_meth_ty` is `meth_ty[ti/ai]`.
The coercion `co2` witnesses the equality `mty ~ inst_meth_ty`.

The coercion `co` is a newtype coercion that coerces from `C t1 ... tn`
to `inst_meth_ty`.
This coercion is guaranteed to exist by virtue of the fact that
C is a class with exactly one method and no superclasses, so it
is treated like a newtype when compiled to Core.

The condition that `C` is a single-method class is implemented in the
guards of matchWithDict's definition.
If the conditions are not held, the rewriting will not fire,
and we'll report an unsolved constraint.

Some further observations about `withDict`:

(WD1) The `cls` in the type of withDict must be explicitly instantiated with
      visible type application, as invoking `withDict` would be ambiguous
      otherwise.

      For examples of how `withDict` is used in the `base` library, see `withSNat`
      in GHC.TypeNats, as well as `withSChar` and `withSSymbol` in GHC.TypeLits.

(WD2) The `r` is representation-polymorphic, to support things like
      `withTypeable` in `Data.Typeable.Internal`.

(WD3) As an alternative to `withDict`, one could define functions like `withT`
      above in terms of `unsafeCoerce`. This is more error-prone, however.

(WD4) In order to define things like `withKnownNat` below:

        withKnownNat :: SNat n -> (KnownNat n => r) -> r

      `withDict` needs to be instantiated with `Any`, like so:

        withKnownNat = withDict @(KnownNat Any) @(SNat Any) @r

      The use of `Any` is explained in Note [NOINLINE withSomeSNat] in
      base:GHC.TypeNats.

(WD5) In earlier implementations, `withDict` was implemented as an identifier
      with special handling during either constant-folding or desugaring.
      The current approach is more robust: previously, the type of `withDict`
      did not have a type-class constraint and was overly polymorphic.
      See #19915.

(WD6) In fact, we desugar `withDict @cls @mty @{rr} @r` to

         \@(r :: RuntimeRep) @(a :: TYPE r) (sv :: mty) (k :: cls => a) ->
           k (sv |> (sub co2 ; sym co)))

      That is, we cast the method using a coercion, and apply k to
      it. Moreover, we mark the evidence as non-canonical, resulting in
      the use of the 'nospec' magicId (see Note [nospecId magic] in
      GHC.Types.Id.Make) to ensure that the typeclass specialiser
      doesn't incorrectly common-up distinct evidence terms. This is
      super important! Suppose we have calls

          withDict A k
          withDict B k

      where k1, k2 :: C T -> blah.  If we desugared withDict naively, we'd get

          k (A |> co1)
          k (B |> co2)

      and the Specialiser would assume that those arguments (of type `C T`) are
      the same. It would then specialise `k` for that type, and then call the same,
      specialised function from both call sites.  #21575 is a concrete case in point.

      To avoid this, we need to stop the typeclass specialiser from seeing this
      structure, by using nospec. This function is inlined only in CorePrep; crucially
      this means that it still appears in interface files, so that the desugaring of
      withDict remains opaque to the typeclass specialiser across modules.
      This means the specialiser will always see instead:

          nospec @(cls => a) k (A |> co1)
          nospec @(cls => a) k (B |> co2)

      Why does this work? Recall that nospec is not an overloaded function;
      it has the type

        nospec :: forall a. a -> a

      This means that there is nothing for the specialiser to do with function calls
      such as

        nospec @(cls => a) k (A |> co)

      as the specialiser only looks at calls of the form `f dict` for an
      overloaded function `f` (e.g. with a type such as `f :: Eq a => ...`).

      See test-case T21575b.

-}

{- ********************************************************************
*                                                                     *
                   Class lookup for Typeable
*                                                                     *
***********************************************************************-}

-- | Assumes that we've checked that this is the 'Typeable' class,
-- and it was applied to the correct argument.
matchTypeable :: Class -> [Type] -> TcM ClsInstResult
matchTypeable :: Class -> [PredType] -> TcM ClsInstResult
matchTypeable Class
clas [PredType
k,PredType
t]  -- clas = Typeable
  -- Forall types: see Note [No Typeable for polytypes or qualified types]
  | PredType -> Bool
isForAllTy PredType
k = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance

  -- Functions; but only with a visible argment
  | Just (FunTyFlag
af,PredType
mult,PredType
arg,PredType
ret) <- PredType -> Maybe (FunTyFlag, PredType, PredType, PredType)
splitFunTy_maybe PredType
t
  = if FunTyFlag -> Bool
isVisibleFunArg FunTyFlag
af
    then Class
-> PredType
-> PredType
-> PredType
-> PredType
-> TcM ClsInstResult
doFunTy Class
clas PredType
t PredType
mult PredType
arg PredType
ret
    else ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance
      -- 'else' case: qualified types like (Num a => blah) are not typeable
      -- see Note [No Typeable for polytypes or qualified types]

  -- Now cases that do work
  | PredType
k PredType -> PredType -> Bool
`eqType` PredType
naturalTy      = Name -> PredType -> TcM ClsInstResult
doTyLit Name
knownNatClassName         PredType
t
  | PredType
k PredType -> PredType -> Bool
`eqType` PredType
typeSymbolKind = Name -> PredType -> TcM ClsInstResult
doTyLit Name
knownSymbolClassName      PredType
t
  | PredType
k PredType -> PredType -> Bool
`eqType` PredType
charTy         = Name -> PredType -> TcM ClsInstResult
doTyLit Name
knownCharClassName        PredType
t

  -- TyCon applied to its kind args
  -- No special treatment of Type and Constraint; they get distinct TypeReps
  -- see wrinkle (W4) of Note [Type and Constraint are not apart]
  --     in GHC.Builtin.Types.Prim.
  | Just (TyCon
tc, [PredType]
ks) <- HasDebugCallStack => PredType -> Maybe (TyCon, [PredType])
PredType -> Maybe (TyCon, [PredType])
splitTyConApp_maybe PredType
t -- See Note [Typeable (T a b c)]
  , TyCon -> [PredType] -> Bool
onlyNamedBndrsApplied TyCon
tc [PredType]
ks            = Class -> PredType -> TyCon -> [PredType] -> TcM ClsInstResult
doTyConApp Class
clas PredType
t TyCon
tc [PredType]
ks

  | Just (PredType
f,PredType
kt)   <- PredType -> Maybe (PredType, PredType)
splitAppTy_maybe PredType
t    = Class -> PredType -> PredType -> PredType -> TcM ClsInstResult
doTyApp    Class
clas PredType
t PredType
f PredType
kt

matchTypeable Class
_ [PredType]
_ = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance

-- | Representation for a type @ty@ of the form @arg -> ret@.
doFunTy :: Class -> Type -> Mult -> Type -> Type -> TcM ClsInstResult
doFunTy :: Class
-> PredType
-> PredType
-> PredType
-> PredType
-> TcM ClsInstResult
doFunTy Class
clas PredType
ty PredType
mult PredType
arg_ty PredType
ret_ty
  = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (ClsInstResult -> TcM ClsInstResult)
-> ClsInstResult -> TcM ClsInstResult
forall a b. (a -> b) -> a -> b
$ OneInst { cir_new_theta :: [PredType]
cir_new_theta   = [PredType]
preds
                     , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = [EvExpr] -> EvTerm
mk_ev
                     , cir_canonical :: Bool
cir_canonical   = Bool
True
                     , cir_what :: InstanceWhat
cir_what        = InstanceWhat
BuiltinInstance }
  where
    preds :: [PredType]
preds = (PredType -> PredType) -> [PredType] -> [PredType]
forall a b. (a -> b) -> [a] -> [b]
map (Class -> PredType -> PredType
mk_typeable_pred Class
clas) [PredType
mult, PredType
arg_ty, PredType
ret_ty]
    mk_ev :: [EvExpr] -> EvTerm
mk_ev [EvExpr
mult_ev, EvExpr
arg_ev, EvExpr
ret_ev] = PredType -> EvTypeable -> EvTerm
evTypeable PredType
ty (EvTypeable -> EvTerm) -> EvTypeable -> EvTerm
forall a b. (a -> b) -> a -> b
$
                        EvTerm -> EvTerm -> EvTerm -> EvTypeable
EvTypeableTrFun (EvExpr -> EvTerm
EvExpr EvExpr
mult_ev) (EvExpr -> EvTerm
EvExpr EvExpr
arg_ev) (EvExpr -> EvTerm
EvExpr EvExpr
ret_ev)
    mk_ev [EvExpr]
_ = String -> EvTerm
forall a. HasCallStack => String -> a
panic String
"GHC.Tc.Instance.Class.doFunTy"


-- | Representation for type constructor applied to some kinds.
-- 'onlyNamedBndrsApplied' has ensured that this application results in a type
-- of monomorphic kind (e.g. all kind variables have been instantiated).
doTyConApp :: Class -> Type -> TyCon -> [Kind] -> TcM ClsInstResult
doTyConApp :: Class -> PredType -> TyCon -> [PredType] -> TcM ClsInstResult
doTyConApp Class
clas PredType
ty TyCon
tc [PredType]
kind_args
  | TyCon -> Bool
tyConIsTypeable TyCon
tc
  = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (ClsInstResult -> TcM ClsInstResult)
-> ClsInstResult -> TcM ClsInstResult
forall a b. (a -> b) -> a -> b
$ OneInst { cir_new_theta :: [PredType]
cir_new_theta   = (PredType -> PredType) -> [PredType] -> [PredType]
forall a b. (a -> b) -> [a] -> [b]
map (Class -> PredType -> PredType
mk_typeable_pred Class
clas) [PredType]
kind_args
                     , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = [EvExpr] -> EvTerm
mk_ev
                     , cir_canonical :: Bool
cir_canonical   = Bool
True
                     , cir_what :: InstanceWhat
cir_what        = TyCon -> InstanceWhat
BuiltinTypeableInstance TyCon
tc }
  | Bool
otherwise
  = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance
  where
    mk_ev :: [EvExpr] -> EvTerm
mk_ev [EvExpr]
kinds = PredType -> EvTypeable -> EvTerm
evTypeable PredType
ty (EvTypeable -> EvTerm) -> EvTypeable -> EvTerm
forall a b. (a -> b) -> a -> b
$ TyCon -> [EvTerm] -> EvTypeable
EvTypeableTyCon TyCon
tc ((EvExpr -> EvTerm) -> [EvExpr] -> [EvTerm]
forall a b. (a -> b) -> [a] -> [b]
map EvExpr -> EvTerm
EvExpr [EvExpr]
kinds)

-- | Representation for TyCon applications of a concrete kind. We just use the
-- kind itself, but first we must make sure that we've instantiated all kind-
-- polymorphism, but no more.
onlyNamedBndrsApplied :: TyCon -> [KindOrType] -> Bool
onlyNamedBndrsApplied :: TyCon -> [PredType] -> Bool
onlyNamedBndrsApplied TyCon
tc [PredType]
ks
 = (TyConBinder -> Bool) -> [TyConBinder] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all TyConBinder -> Bool
isNamedTyConBinder [TyConBinder]
used_bndrs Bool -> Bool -> Bool
&&
   Bool -> Bool
not ((TyConBinder -> Bool) -> [TyConBinder] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
any TyConBinder -> Bool
isNamedTyConBinder [TyConBinder]
leftover_bndrs)
 where
   bndrs :: [TyConBinder]
bndrs                        = TyCon -> [TyConBinder]
tyConBinders TyCon
tc
   ([TyConBinder]
used_bndrs, [TyConBinder]
leftover_bndrs) = [PredType] -> [TyConBinder] -> ([TyConBinder], [TyConBinder])
forall b a. [b] -> [a] -> ([a], [a])
splitAtList [PredType]
ks [TyConBinder]
bndrs

doTyApp :: Class -> Type -> Type -> KindOrType -> TcM ClsInstResult
-- Representation for an application of a type to a type-or-kind.
--  This may happen when the type expression starts with a type variable.
--  Example (ignoring kind parameter):
--    Typeable (f Int Char)                      -->
--    (Typeable (f Int), Typeable Char)          -->
--    (Typeable f, Typeable Int, Typeable Char)  --> (after some simp. steps)
--    Typeable f
doTyApp :: Class -> PredType -> PredType -> PredType -> TcM ClsInstResult
doTyApp Class
clas PredType
ty PredType
f PredType
tk
  | PredType -> Bool
isForAllTy (HasDebugCallStack => PredType -> PredType
PredType -> PredType
typeKind PredType
f)
  = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance -- We can't solve until we know the ctr.
  | Bool
otherwise
  = ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (ClsInstResult -> TcM ClsInstResult)
-> ClsInstResult -> TcM ClsInstResult
forall a b. (a -> b) -> a -> b
$ OneInst { cir_new_theta :: [PredType]
cir_new_theta   = (PredType -> PredType) -> [PredType] -> [PredType]
forall a b. (a -> b) -> [a] -> [b]
map (Class -> PredType -> PredType
mk_typeable_pred Class
clas) [PredType
f, PredType
tk]
                     , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = [EvExpr] -> EvTerm
mk_ev
                     , cir_canonical :: Bool
cir_canonical   = Bool
True
                     , cir_what :: InstanceWhat
cir_what        = InstanceWhat
BuiltinInstance }
  where
    mk_ev :: [EvExpr] -> EvTerm
mk_ev [EvExpr
t1,EvExpr
t2] = PredType -> EvTypeable -> EvTerm
evTypeable PredType
ty (EvTypeable -> EvTerm) -> EvTypeable -> EvTerm
forall a b. (a -> b) -> a -> b
$ EvTerm -> EvTerm -> EvTypeable
EvTypeableTyApp (EvExpr -> EvTerm
EvExpr EvExpr
t1) (EvExpr -> EvTerm
EvExpr EvExpr
t2)
    mk_ev [EvExpr]
_ = String -> EvTerm
forall a. HasCallStack => String -> a
panic String
"doTyApp"


-- Emit a `Typeable` constraint for the given type.
mk_typeable_pred :: Class -> Type -> PredType
mk_typeable_pred :: Class -> PredType -> PredType
mk_typeable_pred Class
clas PredType
ty = Class -> [PredType] -> PredType
mkClassPred Class
clas [ HasDebugCallStack => PredType -> PredType
PredType -> PredType
typeKind PredType
ty, PredType
ty ]

  -- Typeable is implied by KnownNat/KnownSymbol. In the case of a type literal
  -- we generate a sub-goal for the appropriate class.
  -- See Note [Typeable for Nat and Symbol]
doTyLit :: Name -> Type -> TcM ClsInstResult
doTyLit :: Name -> PredType -> TcM ClsInstResult
doTyLit Name
kc PredType
t = do { Class
kc_clas <- Name -> TcM Class
tcLookupClass Name
kc
                  ; let kc_pred :: PredType
kc_pred    = Class -> [PredType] -> PredType
mkClassPred Class
kc_clas [ PredType
t ]
                        mk_ev :: [EvExpr] -> EvTerm
mk_ev [EvExpr
ev] = PredType -> EvTypeable -> EvTerm
evTypeable PredType
t (EvTypeable -> EvTerm) -> EvTypeable -> EvTerm
forall a b. (a -> b) -> a -> b
$ EvTerm -> EvTypeable
EvTypeableTyLit (EvExpr -> EvTerm
EvExpr EvExpr
ev)
                        mk_ev [EvExpr]
_    = String -> EvTerm
forall a. HasCallStack => String -> a
panic String
"doTyLit"
                  ; ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return (OneInst { cir_new_theta :: [PredType]
cir_new_theta   = [PredType
kc_pred]
                                    , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = [EvExpr] -> EvTerm
mk_ev
                                    , cir_canonical :: Bool
cir_canonical   = Bool
True
                                    , cir_what :: InstanceWhat
cir_what        = InstanceWhat
BuiltinInstance }) }

{- Note [Typeable (T a b c)]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For type applications we always decompose using binary application,
via doTyApp (building a TrApp), until we get to a *kind* instantiation
(building a TrTyCon).  We detect a pure kind instantiation using
`onlyNamedBndrsApplied`.

Example: Proxy :: forall k. k -> *

  To solve Typeable (Proxy @(* -> *) Maybe) we

  - First decompose with doTyApp (onlyNamedBndrsApplied is False)
    to get (Typeable (Proxy @(* -> *))) and Typeable Maybe.
    This step returns a TrApp.

  - Then solve (Typeable (Proxy @(* -> *))) with doTyConApp
    (onlyNamedBndrsApplied is True).
    This step returns a TrTyCon

  So the TypeRep we build is
    TrApp (TrTyCon ("Proxy" @(*->*))) (TrTyCon "Maybe")

Notice also that TYPE and CONSTRAINT are distinct so, in effect, we
allow (Typeable TYPE) and (Typeable CONSTRAINT), giving disinct TypeReps.
This is very important: we may want to get a TypeRep for a kind like
   Type -> Constraint

If we attempt to short-cut by solving it all at once, via
doTyConApp

(this note is sadly truncated FIXME)


Note [No Typeable for polytypes or qualified types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We do not support impredicative typeable, such as
   Typeable (forall a. a->a)
   Typeable (Eq a => a -> a)
   Typeable (() => Int)
   Typeable (((),()) => Int)

See #9858.  For forall's the case is clear: we simply don't have
a TypeRep for them.  For qualified but not polymorphic types, like
(Eq a => a -> a), things are murkier.  But:

 * We don't need a TypeRep for these things.  TypeReps are for
   monotypes only.

 * Perhaps we could treat `=>` as another type constructor for `Typeable`
   purposes, and thus support things like `Eq Int => Int`, however,
   at the current state of affairs this would be an odd exception as
   no other class works with impredicative types.
   For now we leave it off, until we have a better story for impredicativity.


Note [Typeable for Nat and Symbol]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We have special Typeable instances for Nat and Symbol.  Roughly we
have this instance, implemented here by doTyLit:
      instance KnownNat n => Typeable (n :: Nat) where
         typeRep = typeNatTypeRep @n
where
   Data.Typeable.Internal.typeNatTypeRep :: KnownNat a => TypeRep a

Ultimately typeNatTypeRep uses 'natSing' from KnownNat to get a
runtime value 'n'; it turns it into a string with 'show' and uses
that to whiz up a TypeRep TyCon for 'n', with mkTypeLitTyCon.
See #10348.

Because of this rule it's inadvisable (see #15322) to have a constraint
    f :: (Typeable (n :: Nat)) => blah
in a function signature; it gives rise to overlap problems just as
if you'd written
    f :: Eq [a] => blah
-}

{- ********************************************************************
*                                                                     *
                   Class lookup for lifted equality
*                                                                     *
***********************************************************************-}

-- See also Note [The equality types story] in GHC.Builtin.Types.Prim
matchEqualityInst :: Class -> [Type] -> (DataCon, Role, Type, Type)
-- Precondition: `cls` satisfies GHC.Core.Predicate.isEqualityClass
-- See Note [Solving equality classes] in GHC.Tc.Solver.Dict
matchEqualityInst :: Class -> [PredType] -> (DataCon, Role, PredType, PredType)
matchEqualityInst Class
cls [PredType]
args
  | Class
cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
eqTyConKey  -- Solves (t1 ~ t2)
  , [PredType
_,PredType
t1,PredType
t2] <- [PredType]
args
  = (DataCon
eqDataCon, Role
Nominal, PredType
t1, PredType
t2)

  | Class
cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
heqTyConKey -- Solves (t1 ~~ t2)
  , [PredType
_,PredType
_,PredType
t1,PredType
t2] <- [PredType]
args
  = (DataCon
heqDataCon,  Role
Nominal, PredType
t1, PredType
t2)

  | Class
cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
coercibleTyConKey  -- Solves (Coercible t1 t2)
  , [PredType
_, PredType
t1, PredType
t2] <- [PredType]
args
  = (DataCon
coercibleDataCon, Role
Representational, PredType
t1, PredType
t2)

  | Bool
otherwise  -- Does not satisfy the precondition
  = String -> SDoc -> (DataCon, Role, PredType, PredType)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"matchEqualityInst" (PredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Class -> [PredType] -> PredType
mkClassPred Class
cls [PredType]
args))


{- ********************************************************************
*                                                                     *
              Class lookup for overloaded record fields
*                                                                     *
***********************************************************************-}

{-
Note [HasField instances]
~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

    data T y = MkT { foo :: [y] }

and `foo` is in scope.  Then GHC will automatically solve a constraint like

    HasField "foo" (T Int) b

by emitting a new wanted

    T alpha -> [alpha] ~# T Int -> b

and building a HasField dictionary out of the selector function `foo`,
appropriately cast.

The HasField class is defined (in GHC.Records) thus:

    class HasField (x :: k) r a | x r -> a where
      getField :: r -> a

Since this is a one-method class, it is represented as a newtype.
Hence we can solve `HasField "foo" (T Int) b` by taking an expression
of type `T Int -> b` and casting it using the newtype coercion.
Note that

    foo :: forall y . T y -> [y]

so the expression we construct is

    foo @alpha |> co

where

    co :: (T alpha -> [alpha]) ~# HasField "foo" (T Int) b

is built from

    co1 :: (T alpha -> [alpha]) ~# (T Int -> b)

which is the new wanted, and

    co2 :: (T Int -> b) ~# HasField "foo" (T Int) b

which can be derived from the newtype coercion.

If `foo` is not in scope, or has a higher-rank or existentially
quantified type, then the constraint is not solved automatically, but
may be solved by a user-supplied HasField instance.  Similarly, if we
encounter a HasField constraint where the field is not a literal
string, or does not belong to the type, then we fall back on the
normal constraint solver behaviour.


Note [Unused name reporting and HasField]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When a HasField constraint is solved by the type-checker, we must record a use
of the corresponding field name, as otherwise it might be reported as unused.
See #19213.  We need to call keepAlive to add the name to the tcg_keep set,
which accumulates names used by the constraint solver, as described by
Note [Tracking unused binding and imports] in GHC.Tc.Types.

We need to call addUsedGRE as well because there may be a deprecation warning on
the field, which will be reported by addUsedGRE.  But calling addUsedGRE without
keepAlive is not enough, because the field might be defined locally, and
addUsedGRE extends tcg_used_gres with imported GREs only.
-}

-- See Note [HasField instances]
matchHasField :: DynFlags -> Bool -> Class -> [Type] -> TcM ClsInstResult
matchHasField :: DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchHasField DynFlags
dflags Bool
short_cut Class
clas [PredType]
tys
  = do { FamInstEnvs
fam_inst_envs <- TcM FamInstEnvs
tcGetFamInstEnvs
       ; GlobalRdrEnv
rdr_env       <- TcRn GlobalRdrEnv
getGlobalRdrEnv
       ; case [PredType]
tys of
           -- We are matching HasField {k} x r a...
           [PredType
_k_ty, PredType
x_ty, PredType
r_ty, PredType
a_ty]
               -- x should be a literal string
             | Just FastString
x <- PredType -> Maybe FastString
isStrLitTy PredType
x_ty
               -- r should be an applied type constructor
             , Just (TyCon
tc, [PredType]
args) <- HasCallStack => PredType -> Maybe (TyCon, [PredType])
PredType -> Maybe (TyCon, [PredType])
tcSplitTyConApp_maybe PredType
r_ty
               -- use representation tycon (if data family); it has the fields
             , let r_tc :: TyCon
r_tc = (TyCon, [PredType], TcCoercion) -> TyCon
forall a b c. (a, b, c) -> a
fstOf3 (FamInstEnvs
-> TyCon -> [PredType] -> (TyCon, [PredType], TcCoercion)
tcLookupDataFamInst FamInstEnvs
fam_inst_envs TyCon
tc [PredType]
args)
               -- x should be a field of r
             , Just FieldLabel
fl <- FieldLabelString -> TyCon -> Maybe FieldLabel
lookupTyConFieldLabel (FastString -> FieldLabelString
FieldLabelString FastString
x) TyCon
r_tc
               -- the field selector should be in scope
             , Just FieldGlobalRdrElt
gre <- GlobalRdrEnv -> FieldLabel -> Maybe FieldGlobalRdrElt
lookupGRE_FieldLabel GlobalRdrEnv
rdr_env FieldLabel
fl

             -> do { DFunId
sel_id <- Name -> IOEnv (Env TcGblEnv TcLclEnv) DFunId
tcLookupId (FieldLabel -> Name
flSelector FieldLabel
fl)
                   ; ([(Name, DFunId)]
tv_prs, [PredType]
preds, PredType
sel_ty) <- ([DFunId] -> TcM (Subst, [DFunId]))
-> DFunId -> TcM ([(Name, DFunId)], [PredType], PredType)
tcInstType [DFunId] -> TcM (Subst, [DFunId])
newMetaTyVars DFunId
sel_id

                         -- The first new wanted constraint equates the actual
                         -- type of the selector with the type (r -> a) within
                         -- the HasField x r a dictionary.  The preds will
                         -- typically be empty, but if the datatype has a
                         -- "stupid theta" then we have to include it here.
                   ; let theta :: [PredType]
theta = PredType -> PredType -> PredType
mkPrimEqPred PredType
sel_ty (HasDebugCallStack => PredType -> PredType -> PredType
PredType -> PredType -> PredType
mkVisFunTyMany PredType
r_ty PredType
a_ty) PredType -> [PredType] -> [PredType]
forall a. a -> [a] -> [a]
: [PredType]
preds

                         -- Use the equality proof to cast the selector Id to
                         -- type (r -> a), then use the newtype coercion to cast
                         -- it to a HasField dictionary.
                         mk_ev :: [EvExpr] -> EvTerm
mk_ev (EvExpr
ev1:[EvExpr]
evs) = DFunId -> [PredType] -> [EvExpr] -> EvExpr
evSelector DFunId
sel_id [PredType]
tvs [EvExpr]
evs EvExpr -> TcCoercion -> EvTerm
`evCast` TcCoercion
co
                           where
                             co :: TcCoercion
co = HasDebugCallStack => TcCoercion -> TcCoercion
TcCoercion -> TcCoercion
mkSubCo (EvTerm -> TcCoercion
evTermCoercion (EvExpr -> EvTerm
EvExpr EvExpr
ev1))
                                      TcCoercion -> TcCoercion -> TcCoercion
`mkTransCo` TcCoercion -> TcCoercion
mkSymCo TcCoercion
co2
                         mk_ev [] = String -> EvTerm
forall a. HasCallStack => String -> a
panic String
"matchHasField.mk_ev"

                         Just (PredType
_, TcCoercion
co2) = TyCon -> [PredType] -> Maybe (PredType, TcCoercion)
tcInstNewTyCon_maybe (Class -> TyCon
classTyCon Class
clas)
                                                              [PredType]
tys

                         tvs :: [PredType]
tvs = [DFunId] -> [PredType]
mkTyVarTys (((Name, DFunId) -> DFunId) -> [(Name, DFunId)] -> [DFunId]
forall a b. (a -> b) -> [a] -> [b]
map (Name, DFunId) -> DFunId
forall a b. (a, b) -> b
snd [(Name, DFunId)]
tv_prs)

                     -- The selector must not be "naughty" (i.e. the field
                     -- cannot have an existentially quantified type), and
                     -- it must not be higher-rank.
                   ; if Bool -> Bool
not (DFunId -> Bool
isNaughtyRecordSelector DFunId
sel_id) Bool -> Bool -> Bool
&& PredType -> Bool
isTauTy PredType
sel_ty
                     then do { -- See Note [Unused name reporting and HasField]
                               DeprecationWarnings -> FieldGlobalRdrElt -> TcRn ()
addUsedGRE DeprecationWarnings
AllDeprecationWarnings FieldGlobalRdrElt
gre
                             ; Name -> TcRn ()
keepAlive (FieldGlobalRdrElt -> Name
forall info. GlobalRdrEltX info -> Name
greName FieldGlobalRdrElt
gre)
                             ; ClsInstResult -> TcM ClsInstResult
forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
forall (m :: * -> *) a. Monad m => a -> m a
return OneInst { cir_new_theta :: [PredType]
cir_new_theta   = [PredType]
theta
                                              , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev       = [EvExpr] -> EvTerm
mk_ev
                                              , cir_canonical :: Bool
cir_canonical   = Bool
True
                                              , cir_what :: InstanceWhat
cir_what        = InstanceWhat
BuiltinInstance } }
                     else DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchInstEnv DynFlags
dflags Bool
short_cut Class
clas [PredType]
tys }

           [PredType]
_ -> DynFlags -> Bool -> Class -> [PredType] -> TcM ClsInstResult
matchInstEnv DynFlags
dflags Bool
short_cut Class
clas [PredType]
tys }