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{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1998
\section[TyCoRep]{Type and Coercion - friends' interface}

Note [The Type-related module hierarchy]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Class
  CoAxiom
  TyCon    imports Class, CoAxiom
  TyCoRep  imports Class, CoAxiom, TyCon
  TysPrim  imports TyCoRep ( including mkTyConTy )
  Kind     imports TysPrim ( mainly for primitive kinds )
  Type     imports Kind
  Coercion imports Type
-}

-- We expose the relevant stuff from this module via the Type module
{-# OPTIONS_HADDOCK hide #-}
{-# LANGUAGE CPP, DeriveDataTypeable, DeriveFunctor, DeriveFoldable,
             DeriveTraversable, MultiWayIf #-}

module TyCoRep (
        TyThing(..),
        Type(..),
        TyBinder(..),
        TyLit(..),
        KindOrType, Kind,
        PredType, ThetaType,      -- Synonyms
        VisibilityFlag(..),

        -- Coercions
        Coercion(..), LeftOrRight(..),
        UnivCoProvenance(..), CoercionHole(..),

        -- Functions over types
        mkTyConTy, mkTyVarTy, mkTyVarTys,
        mkFunTy, mkFunTys,
        isLiftedTypeKind, isUnliftedTypeKind,
        isCoercionType, isLevityTy, isLevityVar,
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        sameVis,
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        -- Functions over binders
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        binderType, delBinderVar, isInvisibleBinder, isVisibleBinder,
        isNamedBinder, isAnonBinder,
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        -- Functions over coercions
        pickLR,

        -- Pretty-printing
        pprType, pprParendType, pprTypeApp, pprTvBndr, pprTvBndrs,
        pprTyThing, pprTyThingCategory, pprSigmaType,
        pprTheta, pprForAll, pprForAllImplicit, pprUserForAll,
        pprThetaArrowTy, pprClassPred,
        pprKind, pprParendKind, pprTyLit,
        TyPrec(..), maybeParen, pprTcAppCo, pprTcAppTy,
        pprPrefixApp, pprArrowChain, ppr_type,
        pprDataCons,

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        -- * Free variables
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        tyCoVarsOfType, tyCoVarsOfTypeDSet, tyCoVarsOfTypes, tyCoVarsOfTypesDSet,
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        tyCoVarsBndrAcc, tyCoVarsOfTypeAcc, tyCoVarsOfTypeList,
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        tyCoVarsOfTypesAcc, tyCoVarsOfTypesList,
        closeOverKindsDSet, closeOverKindsAcc,
        coVarsOfType, coVarsOfTypes,
        coVarsOfCo, coVarsOfCos,
        tyCoVarsOfCo, tyCoVarsOfCos,
        tyCoVarsOfCoDSet,
        tyCoVarsOfCoAcc, tyCoVarsOfCosAcc,
        tyCoVarsOfCoList, tyCoVarsOfProv,
        closeOverKinds,
        tyCoVarsOfTelescope,

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        -- * Substitutions
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        TCvSubst(..), TvSubstEnv, CvSubstEnv,
        emptyTvSubstEnv, emptyCvSubstEnv, composeTCvSubstEnv, composeTCvSubst,
        emptyTCvSubst, mkEmptyTCvSubst, isEmptyTCvSubst, mkTCvSubst, getTvSubstEnv,
        getCvSubstEnv, getTCvInScope, isInScope, notElemTCvSubst,
        setTvSubstEnv, setCvSubstEnv, zapTCvSubst,
        extendTCvInScope, extendTCvInScopeList, extendTCvInScopeSet,
        extendTCvSubst, extendTCvSubstAndInScope, extendTCvSubstList,
        extendTCvSubstBinder,
        unionTCvSubst, zipTyEnv, zipCoEnv, mkTyCoInScopeSet,
        mkOpenTCvSubst, zipOpenTCvSubst, zipOpenTCvSubstCoVars,
        zipOpenTCvSubstBinders,
        mkTopTCvSubst, zipTopTCvSubst,

        substTelescope,
        substTyWith, substTyWithCoVars, substTysWith, substTysWithCoVars,
        substCoWith,
        substTy,
        substTyWithBinders,
        substTys, substTheta,
        lookupTyVar, substTyVarBndr,
        substCo, substCos, substCoVar, substCoVars, lookupCoVar,
        substCoVarBndr, cloneTyVarBndr, cloneTyVarBndrs,
        substTyVar, substTyVars,
        substForAllCoBndr,
        substTyVarBndrCallback, substForAllCoBndrCallback,
        substCoVarBndrCallback,

        -- * Tidying type related things up for printing
        tidyType,      tidyTypes,
        tidyOpenType,  tidyOpenTypes,
        tidyOpenKind,
        tidyTyCoVarBndr, tidyTyCoVarBndrs, tidyFreeTyCoVars,
        tidyOpenTyCoVar, tidyOpenTyCoVars,
        tidyTyVarOcc,
        tidyTopType,
        tidyKind,
        tidyCo, tidyCos
    ) where

#include "HsVersions.h"

import {-# SOURCE #-} DataCon( dataConTyCon, dataConFullSig
                              , DataCon, eqSpecTyVar )
import {-# SOURCE #-} Type( isPredTy, isCoercionTy, mkAppTy
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                          , partitionInvisibles, coreView )
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   -- Transitively pulls in a LOT of stuff, better to break the loop

import {-# SOURCE #-} Coercion
import {-# SOURCE #-} ConLike ( ConLike(..) )

-- friends:
import Var
import VarEnv
import VarSet
import Name hiding ( varName )
import BasicTypes
import TyCon
import Class
import CoAxiom
import FV

-- others
import PrelNames
import Binary
import Outputable
import DynFlags
import StaticFlags ( opt_PprStyle_Debug )
import FastString
import Pair
import UniqSupply
import ListSetOps
import Util

-- libraries
import qualified Data.Data as Data hiding ( TyCon )
import Data.List
import Data.IORef ( IORef )   -- for CoercionHole

{-
%************************************************************************
%*                                                                      *
\subsection{The data type}
%*                                                                      *
%************************************************************************
-}

-- | The key representation of types within the compiler

-- If you edit this type, you may need to update the GHC formalism
-- See Note [GHC Formalism] in coreSyn/CoreLint.hs
data Type
  -- See Note [Non-trivial definitional equality]
  = TyVarTy Var -- ^ Vanilla type or kind variable (*never* a coercion variable)

  | AppTy         -- See Note [AppTy rep]
        Type
        Type            -- ^ Type application to something other than a 'TyCon'. Parameters:
                        --
                        --  1) Function: must /not/ be a 'TyConApp',
                        --     must be another 'AppTy', or 'TyVarTy'
                        --
                        --  2) Argument type

  | TyConApp      -- See Note [AppTy rep]
        TyCon
        [KindOrType]    -- ^ Application of a 'TyCon', including newtypes /and/ synonyms.
                        -- Invariant: saturated applications of 'FunTyCon' must
                        -- use 'FunTy' and saturated synonyms must use their own
                        -- constructors. However, /unsaturated/ 'FunTyCon's
                        -- do appear as 'TyConApp's.
                        -- Parameters:
                        --
                        -- 1) Type constructor being applied to.
                        --
                        -- 2) Type arguments. Might not have enough type arguments
                        --    here to saturate the constructor.
                        --    Even type synonyms are not necessarily saturated;
                        --    for example unsaturated type synonyms
                        --    can appear as the right hand side of a type synonym.

  | ForAllTy
        TyBinder
        Type            -- ^ A Π type.
                        -- This includes arrow types, constructed with
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                        -- @ForAllTy (Anon ...)@. See also Note [TyBinder].
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  | LitTy TyLit     -- ^ Type literals are similar to type constructors.

  | CastTy
        Type
        Coercion    -- ^ A kind cast. The coercion is always nominal.
                    -- INVARIANT: The cast is never refl.
                    -- INVARIANT: The cast is "pushed down" as far as it
                    -- can go. See Note [Pushing down casts]

  | CoercionTy
        Coercion    -- ^ Injection of a Coercion into a type
                    -- This should only ever be used in the RHS of an AppTy,
                    -- in the list of a TyConApp, when applying a promoted
                    -- GADT data constructor

  deriving (Data.Data, Data.Typeable)


-- NOTE:  Other parts of the code assume that type literals do not contain
-- types or type variables.
data TyLit
  = NumTyLit Integer
  | StrTyLit FastString
  deriving (Eq, Ord, Data.Data, Data.Typeable)

-- | A 'TyBinder' represents an argument to a function. TyBinders can be dependent
-- ('Named') or nondependent ('Anon'). They may also be visible or not.
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-- See also Note [TyBinder]
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data TyBinder
  = Named TyVar VisibilityFlag
  | Anon Type   -- visibility is determined by the type (Constraint vs. *)
    deriving (Data.Typeable, Data.Data)

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-- | Is something required to appear in source Haskell ('Visible'),
-- permitted by request ('Specified') (visible type application), or
-- prohibited entirely from appearing in source Haskell ('Invisible')?
-- Examples in Note [VisibilityFlag]
data VisibilityFlag = Visible | Specified | Invisible
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  deriving (Eq, Data.Typeable, Data.Data)

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-- | Do these denote the same level of visibility? Except that
-- 'Specified' and 'Invisible' are considered the same. Used
-- for printing.
sameVis :: VisibilityFlag -> VisibilityFlag -> Bool
sameVis Visible Visible = True
sameVis Visible _       = False
sameVis _       Visible = False
sameVis _       _       = True

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instance Binary VisibilityFlag where
  put_ bh Visible   = putByte bh 0
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  put_ bh Specified = putByte bh 1
  put_ bh Invisible = putByte bh 2
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  get bh = do
    h <- getByte bh
    case h of
      0 -> return Visible
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      1 -> return Specified
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      _ -> return Invisible

type KindOrType = Type -- See Note [Arguments to type constructors]

-- | The key type representing kinds in the compiler.
type Kind = Type

{-
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Note [TyBinder]
~~~~~~~~~~~~~~~
This represents the type of binders -- that is, the type of an argument
to a Pi-type. GHC Core currently supports two different Pi-types:
a non-dependent function, written with ->, and a dependent compile-time-only
polytype, written with forall. Both Pi-types classify terms/types that
take an argument. In other words, if `x` is either a function or a polytype,
`x arg` makes sense (for an appropriate `arg`). It is thus often convenient
to group Pi-types together. This is ForAllTy.

The two constructors for TyBinder sort out the two different possibilities.
`Named` builds a polytype, while `Anon` builds an ordinary function.
(ForAllTy (Anon arg) res used to be called FunTy arg res.)

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Note [The kind invariant]
~~~~~~~~~~~~~~~~~~~~~~~~~
The kinds
   #          UnliftedTypeKind
   OpenKind   super-kind of *, #

can never appear under an arrow or type constructor in a kind; they
can only be at the top level of a kind.  It follows that primitive TyCons,
which have a naughty pseudo-kind
   State# :: * -> #
must always be saturated, so that we can never get a type whose kind
has a UnliftedTypeKind or ArgTypeKind underneath an arrow.

Nor can we abstract over a type variable with any of these kinds.

    k :: = kk | # | ArgKind | (#) | OpenKind
    kk :: = * | kk -> kk | T kk1 ... kkn

So a type variable can only be abstracted kk.

Note [Arguments to type constructors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Because of kind polymorphism, in addition to type application we now
have kind instantiation. We reuse the same notations to do so.

For example:

  Just (* -> *) Maybe
  Right * Nat Zero

are represented by:

  TyConApp (PromotedDataCon Just) [* -> *, Maybe]
  TyConApp (PromotedDataCon Right) [*, Nat, (PromotedDataCon Zero)]

Important note: Nat is used as a *kind* and not as a type. This can be
confusing, since type-level Nat and kind-level Nat are identical. We
use the kind of (PromotedDataCon Right) to know if its arguments are
kinds or types.

This kind instantiation only happens in TyConApp currently.

Note [Pushing down casts]
~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have (a :: k1 -> *), (b :: k1), and (co :: * ~ q).
The type (a b |> co) is `eqType` to ((a |> co') b), where
co' = (->) <k1> co. Thus, to make this visible to functions
that inspect types, we always push down coercions, preferring
the second form. Note that this also applies to TyConApps!

Note [Non-trivial definitional equality]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Is Int |> <*> the same as Int? YES! In order to reduce headaches,
we decide that any reflexive casts in types are just ignored. More
generally, the `eqType` function, which defines Core's type equality
relation, ignores casts and coercion arguments, as long as the
two types have the same kind. This allows us to be a little sloppier
in keeping track of coercions, which is a good thing. It also means
that eqType does not depend on eqCoercion, which is also a good thing.

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Note [VisibilityFlag]
~~~~~~~~~~~~~~~~~~~~~
All named binders are equipped with a visibility flag, which says
whether or not arguments for this binder should be visible (explicit)
in source Haskell. Historically, all named binders (that is, polytype
binders) have been Invisible. But now it's more complicated.

Invisible:
 Argument does not ever appear in source Haskell. With visible type
 application, only GHC-generated polytypes have Invisible binders.
 This exactly corresponds to "generalized" variables from the
 Visible Type Applications paper (ESOP'16).

 Example: f x = x
 `f` will be inferred to have type `forall a. a -> a`, where `a` is
 Invisible. Note that there is no type annotation for `f`.

 Printing: With -fprint-explicit-foralls, Invisible binders are written
 in braces. Otherwise, they are printed like Specified binders.

Specified:
 The argument for this binder may appear in source Haskell only with
 visible type application. Otherwise, it is omitted.

 Example: id :: forall a. a -> a
 `a` is a Specified binder, because you can say `id @Int` in source Haskell.

 Example: const :: a -> b -> a
 Both `a` and `b` are Specified binders, even though they are not bound
 by an explicit forall.

 Printing: a list of Specified binders are put between `forall` and `.`:
 const :: forall a b. a -> b -> a

Visible:
 The argument must be given. Visible binders come up only with TypeInType.

 Example: data Proxy k (a :: k) = P
 The kind of Proxy is forall k -> k -> *, where k is a Visible binder.

 Printing: As in the example above, Visible binders are put between `forall`
 and `->`. This syntax is not parsed (yet), however.

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-------------------------------------
                Note [PredTy]
-}

-- | A type of the form @p@ of kind @Constraint@ represents a value whose type is
-- the Haskell predicate @p@, where a predicate is what occurs before
-- the @=>@ in a Haskell type.
--
-- We use 'PredType' as documentation to mark those types that we guarantee to have
-- this kind.
--
-- It can be expanded into its representation, but:
--
-- * The type checker must treat it as opaque
--
-- * The rest of the compiler treats it as transparent
--
-- Consider these examples:
--
-- > f :: (Eq a) => a -> Int
-- > g :: (?x :: Int -> Int) => a -> Int
-- > h :: (r\l) => {r} => {l::Int | r}
--
-- Here the @Eq a@ and @?x :: Int -> Int@ and @r\l@ are all called \"predicates\"
type PredType = Type

-- | A collection of 'PredType's
type ThetaType = [PredType]

{-
(We don't support TREX records yet, but the setup is designed
to expand to allow them.)

A Haskell qualified type, such as that for f,g,h above, is
represented using
        * a FunTy for the double arrow
        * with a type of kind Constraint as the function argument

The predicate really does turn into a real extra argument to the
function.  If the argument has type (p :: Constraint) then the predicate p is
represented by evidence of type p.

%************************************************************************
%*                                                                      *
            Simple constructors
%*                                                                      *
%************************************************************************

These functions are here so that they can be used by TysPrim,
which in turn is imported by Type
-}

-- named with "Only" to prevent naive use of mkTyVarTy
mkTyVarTy  :: TyVar   -> Type
mkTyVarTy v = ASSERT2( isTyVar v, ppr v <+> dcolon <+> ppr (tyVarKind v) )
                  TyVarTy v

mkTyVarTys :: [TyVar] -> [Type]
mkTyVarTys = map mkTyVarTy -- a common use of mkTyVarTy

infixr 3 `mkFunTy`      -- Associates to the right
-- | Make an arrow type
mkFunTy :: Type -> Type -> Type
mkFunTy arg res
  = ForAllTy (Anon arg) res

-- | Make nested arrow types
mkFunTys :: [Type] -> Type -> Type
mkFunTys tys ty = foldr mkFunTy ty tys

-- | Does this type classify a core Coercion?
isCoercionType :: Type -> Bool
isCoercionType (TyConApp tc tys)
  | (tc `hasKey` eqPrimTyConKey) || (tc `hasKey` eqReprPrimTyConKey)
  , length tys == 4
  = True
isCoercionType _ = False

binderType :: TyBinder -> Type
binderType (Named v _) = varType v
binderType (Anon ty)   = ty

-- | Remove the binder's variable from the set, if the binder has
-- a variable.
delBinderVar :: VarSet -> TyBinder -> VarSet
delBinderVar vars (Named tv _) = vars `delVarSet` tv
delBinderVar vars (Anon {})    = vars

-- | Remove the binder's variable from the set, if the binder has
-- a variable.
delBinderVarFV :: TyBinder -> FV -> FV
delBinderVarFV (Named tv _) vars fv_cand in_scope acc = delFV tv vars fv_cand in_scope acc
delBinderVarFV (Anon {})    vars fv_cand in_scope acc = vars fv_cand in_scope acc

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-- | Does this binder bind an invisible argument?
isInvisibleBinder :: TyBinder -> Bool
isInvisibleBinder (Named _ vis) = vis /= Visible
isInvisibleBinder (Anon ty)     = isPredTy ty

-- | Does this binder bind a visible argument?
isVisibleBinder :: TyBinder -> Bool
isVisibleBinder = not . isInvisibleBinder

isNamedBinder :: TyBinder -> Bool
isNamedBinder (Named {}) = True
isNamedBinder _          = False

isAnonBinder :: TyBinder -> Bool
isAnonBinder (Anon {}) = True
isAnonBinder _         = False

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-- | Create the plain type constructor type which has been applied to no type arguments at all.
mkTyConTy :: TyCon -> Type
mkTyConTy tycon = TyConApp tycon []

{-
Some basic functions, put here to break loops eg with the pretty printer
-}

isLiftedTypeKind :: Kind -> Bool
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isLiftedTypeKind ki | Just ki' <- coreView ki = isLiftedTypeKind ki'
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isLiftedTypeKind (TyConApp tc [TyConApp lev []])
  = tc `hasKey` tYPETyConKey && lev `hasKey` liftedDataConKey
isLiftedTypeKind _                = False

isUnliftedTypeKind :: Kind -> Bool
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isUnliftedTypeKind ki | Just ki' <- coreView ki = isUnliftedTypeKind ki'
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isUnliftedTypeKind (TyConApp tc [TyConApp lev []])
  = tc `hasKey` tYPETyConKey && lev `hasKey` unliftedDataConKey
isUnliftedTypeKind _ = False

-- | Is this the type 'Levity'?
isLevityTy :: Type -> Bool
isLevityTy (TyConApp tc []) = tc `hasKey` levityTyConKey
isLevityTy _                = False

-- | Is a tyvar of type 'Levity'?
isLevityVar :: TyVar -> Bool
isLevityVar = isLevityTy . tyVarKind

{-
%************************************************************************
%*                                                                      *
            Coercions
%*                                                                      *
%************************************************************************
-}

-- | A 'Coercion' is concrete evidence of the equality/convertibility
-- of two types.

-- If you edit this type, you may need to update the GHC formalism
-- See Note [GHC Formalism] in coreSyn/CoreLint.hs
data Coercion
  -- Each constructor has a "role signature", indicating the way roles are
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  -- propagated through coercions.
  --    -  P, N, and R stand for coercions of the given role
  --    -  e stands for a coercion of a specific unknown role
  --           (think "role polymorphism")
  --    -  "e" stands for an explicit role parameter indicating role e.
  --    -   _ stands for a parameter that is not a Role or Coercion.
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  -- These ones mirror the shape of types
  = -- Refl :: "e" -> _ -> e
    Refl Role Type  -- See Note [Refl invariant]
          -- Invariant: applications of (Refl T) to a bunch of identity coercions
          --            always show up as Refl.
          -- For example  (Refl T) (Refl a) (Refl b) shows up as (Refl (T a b)).

          -- Applications of (Refl T) to some coercions, at least one of
          -- which is NOT the identity, show up as TyConAppCo.
          -- (They may not be fully saturated however.)
          -- ConAppCo coercions (like all coercions other than Refl)
          -- are NEVER the identity.

          -- Use (Refl Representational _), not (SubCo (Refl Nominal _))

  -- These ones simply lift the correspondingly-named
  -- Type constructors into Coercions

  -- TyConAppCo :: "e" -> _ -> ?? -> e
  -- See Note [TyConAppCo roles]
  | TyConAppCo Role TyCon [Coercion]    -- lift TyConApp
               -- The TyCon is never a synonym;
               -- we expand synonyms eagerly
               -- But it can be a type function

  | AppCo Coercion Coercion             -- lift AppTy
          -- AppCo :: e -> N -> e

  -- See Note [Forall coercions]
  | ForAllCo TyVar Coercion Coercion
         -- ForAllCo :: _ -> N -> e -> e

  -- These are special
  | CoVarCo CoVar      -- :: _ -> (N or R)
                       -- result role depends on the tycon of the variable's type

    -- AxiomInstCo :: e -> _ -> [N] -> e
  | AxiomInstCo (CoAxiom Branched) BranchIndex [Coercion]
     -- See also [CoAxiom index]
     -- The coercion arguments always *precisely* saturate
     -- arity of (that branch of) the CoAxiom. If there are
     -- any left over, we use AppCo.
     -- See [Coercion axioms applied to coercions]

  | UnivCo UnivCoProvenance Role Type Type
      -- :: _ -> "e" -> _ -> _ -> e

  | SymCo Coercion             -- :: e -> e
  | TransCo Coercion Coercion  -- :: e -> e -> e

    -- The number coercions should match exactly the expectations
    -- of the CoAxiomRule (i.e., the rule is fully saturated).
  | AxiomRuleCo CoAxiomRule [Coercion]

  | NthCo  Int         Coercion     -- Zero-indexed; decomposes (T t0 ... tn)
    -- :: _ -> e -> ?? (inverse of TyConAppCo, see Note [TyConAppCo roles])
    -- Using NthCo on a ForAllCo gives an N coercion always
    -- See Note [NthCo and newtypes]

  | LRCo   LeftOrRight Coercion     -- Decomposes (t_left t_right)
    -- :: _ -> N -> N
  | InstCo Coercion Coercion
    -- :: e -> N -> e
    -- See Note [InstCo roles]

  -- Coherence applies a coercion to the left-hand type of another coercion
  -- See Note [Coherence]
  | CoherenceCo Coercion Coercion
     -- :: e -> N -> e

  -- Extract a kind coercion from a (heterogeneous) type coercion
  -- NB: all kind coercions are Nominal
  | KindCo Coercion
     -- :: e -> N

  | SubCo Coercion                  -- Turns a ~N into a ~R
    -- :: N -> R

  deriving (Data.Data, Data.Typeable)

-- If you edit this type, you may need to update the GHC formalism
-- See Note [GHC Formalism] in coreSyn/CoreLint.hs
data LeftOrRight = CLeft | CRight
                 deriving( Eq, Data.Data, Data.Typeable )

instance Binary LeftOrRight where
   put_ bh CLeft  = putByte bh 0
   put_ bh CRight = putByte bh 1

   get bh = do { h <- getByte bh
               ; case h of
                   0 -> return CLeft
                   _ -> return CRight }

pickLR :: LeftOrRight -> (a,a) -> a
pickLR CLeft  (l,_) = l
pickLR CRight (_,r) = r


{-
Note [Refl invariant]
~~~~~~~~~~~~~~~~~~~~~
Invariant 1:

Coercions have the following invariant
     Refl is always lifted as far as possible.

You might think that a consequencs is:
     Every identity coercions has Refl at the root

But that's not quite true because of coercion variables.  Consider
     g         where g :: Int~Int
     Left h    where h :: Maybe Int ~ Maybe Int
etc.  So the consequence is only true of coercions that
have no coercion variables.

Note [Coercion axioms applied to coercions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The reason coercion axioms can be applied to coercions and not just
types is to allow for better optimization.  There are some cases where
we need to be able to "push transitivity inside" an axiom in order to
expose further opportunities for optimization.

For example, suppose we have

  C a : t[a] ~ F a
  g   : b ~ c

and we want to optimize

  sym (C b) ; t[g] ; C c

which has the kind

  F b ~ F c

(stopping through t[b] and t[c] along the way).

We'd like to optimize this to just F g -- but how?  The key is
that we need to allow axioms to be instantiated by *coercions*,
not just by types.  Then we can (in certain cases) push
transitivity inside the axiom instantiations, and then react
opposite-polarity instantiations of the same axiom.  In this
case, e.g., we match t[g] against the LHS of (C c)'s kind, to
obtain the substitution  a |-> g  (note this operation is sort
of the dual of lifting!) and hence end up with

  C g : t[b] ~ F c

which indeed has the same kind as  t[g] ; C c.

Now we have

  sym (C b) ; C g

which can be optimized to F g.

Note [CoAxiom index]
~~~~~~~~~~~~~~~~~~~~
A CoAxiom has 1 or more branches. Each branch has contains a list
of the free type variables in that branch, the LHS type patterns,
and the RHS type for that branch. When we apply an axiom to a list
of coercions, we must choose which branch of the axiom we wish to
use, as the different branches may have different numbers of free
type variables. (The number of type patterns is always the same
among branches, but that doesn't quite concern us here.)

The Int in the AxiomInstCo constructor is the 0-indexed number
of the chosen branch.

Note [Forall coercions]
~~~~~~~~~~~~~~~~~~~~~~~
Constructing coercions between forall-types can be a bit tricky,
because the kinds of the bound tyvars can be different.

The typing rule is:


  kind_co : k1 ~ k2
  tv1:k1 |- co : t1 ~ t2
  -------------------------------------------------------------------
  ForAllCo tv1 kind_co co : all tv1:k1. t1  ~
                            all tv1:k2. (t2[tv1 |-> tv1 |> sym kind_co])

First, the TyVar stored in a ForAllCo is really an optimisation: this field
should be a Name, as its kind is redundant. Thinking of the field as a Name
is helpful in understanding what a ForAllCo means.

The idea is that kind_co gives the two kinds of the tyvar. See how, in the
conclusion, tv1 is assigned kind k1 on the left but kind k2 on the right.

Of course, a type variable can't have different kinds at the same time. So,
we arbitrarily prefer the first kind when using tv1 in the inner coercion
co, which shows that t1 equals t2.

The last wrinkle is that we need to fix the kinds in the conclusion. In
t2, tv1 is assumed to have kind k1, but it has kind k2 in the conclusion of
the rule. So we do a kind-fixing substitution, replacing (tv1:k1) with
(tv1:k2) |> sym kind_co. This substitution is slightly bizarre, because it
mentions the same name with different kinds, but it *is* well-kinded, noting
that `(tv1:k2) |> sym kind_co` has kind k1.

This all really would work storing just a Name in the ForAllCo. But we can't
add Names to, e.g., VarSets, and there generally is just an impedence mismatch
in a bunch of places. So we use tv1. When we need tv2, we can use
setTyVarKind.

Note [Coherence]
~~~~~~~~~~~~~~~~
The Coherence typing rule is thus:

  g1 : s ~ t    s : k1    g2 : k1 ~ k2
  ------------------------------------
  CoherenceCo g1 g2 : (s |> g2) ~ t

While this looks (and is) unsymmetric, a combination of other coercion
combinators can make the symmetric version.

For role information, see Note [Roles and kind coercions].

Note [Predicate coercions]
~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
   g :: a~b
How can we coerce between types
   ([c]~a) => [a] -> c
and
   ([c]~b) => [b] -> c
where the equality predicate *itself* differs?

Answer: we simply treat (~) as an ordinary type constructor, so these
types really look like

   ((~) [c] a) -> [a] -> c
   ((~) [c] b) -> [b] -> c

So the coercion between the two is obviously

   ((~) [c] g) -> [g] -> c

Another way to see this to say that we simply collapse predicates to
their representation type (see Type.coreView and Type.predTypeRep).

This collapse is done by mkPredCo; there is no PredCo constructor
in Coercion.  This is important because we need Nth to work on
predicates too:
    Nth 1 ((~) [c] g) = g
See Simplify.simplCoercionF, which generates such selections.

Note [Roles]
~~~~~~~~~~~~
Roles are a solution to the GeneralizedNewtypeDeriving problem, articulated
in Trac #1496. The full story is in docs/core-spec/core-spec.pdf. Also, see
http://ghc.haskell.org/trac/ghc/wiki/RolesImplementation

Here is one way to phrase the problem:

Given:
newtype Age = MkAge Int
type family F x
type instance F Age = Bool
type instance F Int = Char

This compiles down to:
axAge :: Age ~ Int
axF1 :: F Age ~ Bool
axF2 :: F Int ~ Char

Then, we can make:
(sym (axF1) ; F axAge ; axF2) :: Bool ~ Char

Yikes!

The solution is _roles_, as articulated in "Generative Type Abstraction and
Type-level Computation" (POPL 2010), available at
http://www.seas.upenn.edu/~sweirich/papers/popl163af-weirich.pdf

The specification for roles has evolved somewhat since that paper. For the
current full details, see the documentation in docs/core-spec. Here are some
highlights.

We label every equality with a notion of type equivalence, of which there are
three options: Nominal, Representational, and Phantom. A ground type is
nominally equivalent only with itself. A newtype (which is considered a ground
type in Haskell) is representationally equivalent to its representation.
Anything is "phantomly" equivalent to anything else. We use "N", "R", and "P"
to denote the equivalences.

The axioms above would be:
axAge :: Age ~R Int
axF1 :: F Age ~N Bool
axF2 :: F Age ~N Char

Then, because transitivity applies only to coercions proving the same notion
of equivalence, the above construction is impossible.

However, there is still an escape hatch: we know that any two types that are
nominally equivalent are representationally equivalent as well. This is what
the form SubCo proves -- it "demotes" a nominal equivalence into a
representational equivalence. So, it would seem the following is possible:

sub (sym axF1) ; F axAge ; sub axF2 :: Bool ~R Char   -- WRONG

What saves us here is that the arguments to a type function F, lifted into a
coercion, *must* prove nominal equivalence. So, (F axAge) is ill-formed, and
we are safe.

Roles are attached to parameters to TyCons. When lifting a TyCon into a
coercion (through TyConAppCo), we need to ensure that the arguments to the
TyCon respect their roles. For example:

data T a b = MkT a (F b)

If we know that a1 ~R a2, then we know (T a1 b) ~R (T a2 b). But, if we know
that b1 ~R b2, we know nothing about (T a b1) and (T a b2)! This is because
the type function F branches on b's *name*, not representation. So, we say
that 'a' has role Representational and 'b' has role Nominal. The third role,
Phantom, is for parameters not used in the type's definition. Given the
following definition

data Q a = MkQ Int

the Phantom role allows us to say that (Q Bool) ~R (Q Char), because we
can construct the coercion Bool ~P Char (using UnivCo).

See the paper cited above for more examples and information.

Note [TyConAppCo roles]
~~~~~~~~~~~~~~~~~~~~~~~
The TyConAppCo constructor has a role parameter, indicating the role at
which the coercion proves equality. The choice of this parameter affects
the required roles of the arguments of the TyConAppCo. To help explain
it, assume the following definition:

  type instance F Int = Bool   -- Axiom axF : F Int ~N Bool
  newtype Age = MkAge Int      -- Axiom axAge : Age ~R Int
  data Foo a = MkFoo a         -- Role on Foo's parameter is Representational

TyConAppCo Nominal Foo axF : Foo (F Int) ~N Foo Bool
  For (TyConAppCo Nominal) all arguments must have role Nominal. Why?
  So that Foo Age ~N Foo Int does *not* hold.

TyConAppCo Representational Foo (SubCo axF) : Foo (F Int) ~R Foo Bool
TyConAppCo Representational Foo axAge       : Foo Age     ~R Foo Int
  For (TyConAppCo Representational), all arguments must have the roles
  corresponding to the result of tyConRoles on the TyCon. This is the
  whole point of having roles on the TyCon to begin with. So, we can
  have Foo Age ~R Foo Int, if Foo's parameter has role R.

  If a Representational TyConAppCo is over-saturated (which is otherwise fine),
  the spill-over arguments must all be at Nominal. This corresponds to the
  behavior for AppCo.

TyConAppCo Phantom Foo (UnivCo Phantom Int Bool) : Foo Int ~P Foo Bool
  All arguments must have role Phantom. This one isn't strictly
  necessary for soundness, but this choice removes ambiguity.

The rules here dictate the roles of the parameters to mkTyConAppCo
(should be checked by Lint).

Note [NthCo and newtypes]
~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype N a = MkN Int
  type role N representational

This yields axiom

  NTCo:N :: forall a. N a ~R Int

We can then build

  co :: forall a b. N a ~R N b
  co = NTCo:N a ; sym (NTCo:N b)

for any `a` and `b`. Because of the role annotation on N, if we use
NthCo, we'll get out a representational coercion. That is:

  NthCo 0 co :: forall a b. a ~R b

Yikes! Clearly, this is terrible. The solution is simple: forbid
NthCo to be used on newtypes if the internal coercion is representational.

This is not just some corner case discovered by a segfault somewhere;
it was discovered in the proof of soundness of roles and described
in the "Safe Coercions" paper (ICFP '14).

Note [InstCo roles]
~~~~~~~~~~~~~~~~~~~
Here is (essentially) the typing rule for InstCo:

g :: (forall a. t1) ~r (forall a. t2)
w :: s1 ~N s2
------------------------------- InstCo
InstCo g w :: (t1 [a |-> s1]) ~r (t2 [a |-> s2])

Note that the Coercion w *must* be nominal. This is necessary
because the variable a might be used in a "nominal position"
(that is, a place where role inference would require a nominal
role) in t1 or t2. If we allowed w to be representational, we
could get bogus equalities.

A more nuanced treatment might be able to relax this condition
somewhat, by checking if t1 and/or t2 use their bound variables
in nominal ways. If not, having w be representational is OK.

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%************************************************************************
%*                                                                      *
                UnivCoProvenance
%*                                                                      *
%************************************************************************

A UnivCo is a coercion whose proof does not directly express its role
and kind (indeed for some UnivCos, like UnsafeCoerceProv, there /is/
no proof).

The different kinds of UnivCo are described by UnivCoProvenance.  Really
each is entirely separate, but they all share the need to represent their
role and kind, which is done in the UnivCo constructor.

-}

-- | For simplicity, we have just one UnivCo that represents a coercion from
-- some type to some other type, with (in general) no restrictions on the
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-- type. The UnivCoProvenance specifies more exactly what the coercion really
-- is and why a program should (or shouldn't!) trust the coercion.
-- It is reasonable to consider each constructor of 'UnivCoProvenance'
-- as a totally independent coercion form; their only commonality is
-- that they don't tell you what types they coercion between. (That info
-- is in the 'UnivCo' constructor of 'Coercion'.
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data UnivCoProvenance
  = UnsafeCoerceProv   -- ^ From @unsafeCoerce#@. These are unsound.

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  | PhantomProv Coercion -- ^ See Note [Phantom coercions]
983

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  | ProofIrrelProv Coercion  -- ^ From the fact that any two coercions are
                             --   considered equivalent. See Note [ProofIrrelProv]
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  | PluginProv String  -- ^ From a plugin, which asserts that this coercion
                       --   is sound. The string is for the use of the plugin.

  | HoleProv CoercionHole  -- ^ See Note [Coercion holes]
  deriving (Data.Data, Data.Typeable)

instance Outputable UnivCoProvenance where
  ppr UnsafeCoerceProv   = text "(unsafeCoerce#)"
  ppr (PhantomProv _)    = text "(phantom)"
  ppr (ProofIrrelProv _) = text "(proof irrel.)"
  ppr (PluginProv str)   = parens (text "plugin" <+> brackets (text str))
  ppr (HoleProv hole)    = parens (text "hole" <> ppr hole)

-- | A coercion to be filled in by the type-checker. See Note [Coercion holes]
data CoercionHole
  = CoercionHole { chUnique   :: Unique   -- ^ used only for debugging
                 , chCoercion :: IORef (Maybe Coercion)
                 }
  deriving (Data.Typeable)

instance Data.Data CoercionHole where
  -- don't traverse?
  toConstr _   = abstractConstr "CoercionHole"
  gunfold _ _  = error "gunfold"
  dataTypeOf _ = mkNoRepType "CoercionHole"

instance Outputable CoercionHole where
  ppr (CoercionHole u _) = braces (ppr u)


{- Note [Phantom coercions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
     data T a = T1 | T2
Then we have
     T s ~R T t
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for any old s,t. The witness for this is (TyConAppCo T Rep co),
where (co :: s ~P t) is a phantom coercion built with PhantomProv.
The role of the UnivCo is always Phantom.  The Coercion stored is the
(nominal) kind coercion between the types
   kind(s) ~N kind (t)
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Note [Coercion holes]
~~~~~~~~~~~~~~~~~~~~~~~~
During typechecking, constraint solving for type classes works by
  - Generate an evidence Id,  d7 :: Num a
  - Wrap it in a Wanted constraint, [W] d7 :: Num a
  - Use the evidence Id where the evidence is needed
  - Solve the constraint later
  - When solved, add an enclosing let-binding  let d7 = .... in ....
    which actually binds d7 to the (Num a) evidence

For equality constraints we use a different strategy.  See Note [The
equality types story] in TysPrim for background on equality constraints.
  - For boxed equality constraints, (t1 ~N t2) and (t1 ~R t2), it's just
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    like type classes above. (Indeed, boxed equality constraints *are* classes.)
  - But for /unboxed/ equality constraints (t1 ~R# t2) and (t1 ~N# t2)
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    we use a different plan

For unboxed equalities:
  - Generate a CoercionHole, a mutable variable just like a unification
    variable
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  - Wrap the CoercionHole in a Wanted constraint; see TcRnTypes.TcEvDest
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  - Use the CoercionHole in a Coercion, via HoleProv
  - Solve the constraint later
  - When solved, fill in the CoercionHole by side effect, instead of
    doing the let-binding thing

The main reason for all this is that there may be no good place to let-bind
the evidence for unboxed equalities:
  - We emit constraints for kind coercions, to be used
    to cast a type's kind. These coercions then must be used in types. Because
    they might appear in a top-level type, there is no place to bind these
   (unlifted) coercions in the usual way.

  - A coercion for (forall a. t1) ~ forall a. t2) will look like
       forall a. (coercion for t1~t2)
    But the coercion for (t1~t2) may mention 'a', and we don't have let-bindings
    within coercions.  We could add them, but coercion holes are easier.

Other notes about HoleCo:

 * INVARIANT: CoercionHole and HoleProv are used only during type checking,
   and should never appear in Core. Just like unification variables; a Type
   can contain a TcTyVar, but only during type checking. If, one day, we
   use type-level information to separate out forms that can appear during
   type-checking vs forms that can appear in core proper, holes in Core will
   be ruled out.

 * The Unique carried with a coercion hole is used solely for debugging.

 * Coercion holes can be compared for equality only like other coercions:
   only by looking at the types coerced.

 * We don't use holes for other evidence because other evidence wants to
   be /shared/. But coercions are entirely erased, so there's little
   benefit to sharing.

Note [ProofIrrelProv]
~~~~~~~~~~~~~~~~~~~~~
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A ProofIrrelProv is a coercion between coercions. For example:
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  data G a where
    MkG :: G Bool

In core, we get

  G :: * -> *
  MkG :: forall (a :: *). (a ~ Bool) -> G a

Now, consider 'MkG -- that is, MkG used in a type -- and suppose we want
a proof that ('MkG co1 a1) ~ ('MkG co2 a2). This will have to be

  TyConAppCo Nominal MkG [co3, co4]
  where
    co3 :: co1 ~ co2
    co4 :: a1 ~ a2

Note that
  co1 :: a1 ~ Bool
  co2 :: a2 ~ Bool

Here,
  co3 = UnivCo (ProofIrrelProv co5) Nominal (CoercionTy co1) (CoercionTy co2)
  where
    co5 :: (a1 ~ Bool) ~ (a2 ~ Bool)
    co5 = TyConAppCo Nominal (~) [<*>, <*>, co4, <Bool>]


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%************************************************************************
%*                                                                      *
                 Free variables of types and coercions
%*                                                                      *
%************************************************************************
-}

-- | Returns free variables of a type, including kind variables as
-- a non-deterministic set. For type synonyms it does /not/ expand the
-- synonym.
tyCoVarsOfType :: Type -> TyCoVarSet
tyCoVarsOfType ty = runFVSet $ tyCoVarsOfTypeAcc ty

-- | `tyVarsOfType` that returns free variables of a type in a deterministic
-- set. For explanation of why using `VarSet` is not deterministic see
-- Note [Deterministic FV] in FV.
tyCoVarsOfTypeDSet :: Type -> DTyCoVarSet
tyCoVarsOfTypeDSet ty = runFVDSet $ tyCoVarsOfTypeAcc ty

-- | `tyVarsOfType` that returns free variables of a type in deterministic
-- order. For explanation of why using `VarSet` is not deterministic see
-- Note [Deterministic FV] in FV.
tyCoVarsOfTypeList :: Type -> [TyCoVar]
tyCoVarsOfTypeList ty = runFVList $ tyCoVarsOfTypeAcc ty

-- | The worker for `tyVarsOfType` and `tyVarsOfTypeList`.
-- The previous implementation used `unionVarSet` which is O(n+m) and can
-- make the function quadratic.
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-- It's exported, so that it can be composed with
-- other functions that compute free variables.
1146
-- See Note [FV naming conventions] in FV.
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--
-- Eta-expanded because that makes it run faster (apparently)
1149
tyCoVarsOfTypeAcc :: Type -> FV
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tyCoVarsOfTypeAcc (TyVarTy v)        a b c = (oneVar v `unionFV` tyCoVarsOfTypeAcc (tyVarKind v)) a b c
tyCoVarsOfTypeAcc (TyConApp _ tys)   a b c = tyCoVarsOfTypesAcc tys a b c
tyCoVarsOfTypeAcc (LitTy {})         a b c = noVars a b c
tyCoVarsOfTypeAcc (AppTy fun arg)    a b c = (tyCoVarsOfTypeAcc fun `unionFV` tyCoVarsOfTypeAcc arg) a b c
tyCoVarsOfTypeAcc (ForAllTy bndr ty) a b c = tyCoVarsBndrAcc bndr (tyCoVarsOfTypeAcc ty)  a b c
tyCoVarsOfTypeAcc (CastTy ty co)     a b c = (tyCoVarsOfTypeAcc ty `unionFV` tyCoVarsOfCoAcc co) a b c
tyCoVarsOfTypeAcc (CoercionTy co)    a b c = tyCoVarsOfCoAcc co a b c

tyCoVarsBndrAcc :: TyBinder -> FV -> FV
-- Free vars of (forall b. <thing with fvs>)
tyCoVarsBndrAcc bndr fvs = delBinderVarFV bndr fvs
                           `unionFV` tyCoVarsOfTypeAcc (binderType bndr)
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-- | Returns free variables of types, including kind variables as
-- a non-deterministic set. For type synonyms it does /not/ expand the
-- synonym.
tyCoVarsOfTypes :: [Type] -> TyCoVarSet
tyCoVarsOfTypes tys = runFVSet $ tyCoVarsOfTypesAcc tys

-- | Returns free variables of types, including kind variables as
-- a deterministic set. For type synonyms it does /not/ expand the
-- synonym.
tyCoVarsOfTypesDSet :: [Type] -> DTyCoVarSet
tyCoVarsOfTypesDSet tys = runFVDSet $ tyCoVarsOfTypesAcc tys

-- | Returns free variables of types, including kind variables as
-- a deterministically ordered list. For type synonyms it does /not/ expand the
-- synonym.
tyCoVarsOfTypesList :: [Type] -> [TyCoVar]
tyCoVarsOfTypesList tys = runFVList $ tyCoVarsOfTypesAcc tys

tyCoVarsOfTypesAcc :: [Type] -> FV
tyCoVarsOfTypesAcc (ty:tys) fv_cand in_scope acc = (tyCoVarsOfTypeAcc ty `unionFV` tyCoVarsOfTypesAcc tys) fv_cand in_scope acc
tyCoVarsOfTypesAcc []       fv_cand in_scope acc = noVars fv_cand in_scope acc

tyCoVarsOfCo :: Coercion -> TyCoVarSet
tyCoVarsOfCo co = runFVSet $ tyCoVarsOfCoAcc co

-- | Get a deterministic set of the vars free in a coercion
tyCoVarsOfCoDSet :: Coercion -> DTyCoVarSet
tyCoVarsOfCoDSet co = runFVDSet $ tyCoVarsOfCoAcc co

tyCoVarsOfCoList :: Coercion -> [TyCoVar]
tyCoVarsOfCoList co = runFVList $ tyCoVarsOfCoAcc co

tyCoVarsOfCoAcc :: Coercion -> FV
-- Extracts type and coercion variables from a coercion
tyCoVarsOfCoAcc (Refl _ ty)         fv_cand in_scope acc = tyCoVarsOfTypeAcc ty fv_cand in_scope acc
tyCoVarsOfCoAcc (TyConAppCo _ _ cos) fv_cand in_scope acc = tyCoVarsOfCosAcc cos fv_cand in_scope acc
tyCoVarsOfCoAcc (AppCo co arg) fv_cand in_scope acc
  = (tyCoVarsOfCoAcc co `unionFV` tyCoVarsOfCoAcc arg) fv_cand in_scope acc
tyCoVarsOfCoAcc (ForAllCo tv kind_co co) fv_cand in_scope acc
  = (delFV tv (tyCoVarsOfCoAcc co) `unionFV` tyCoVarsOfCoAcc kind_co) fv_cand in_scope acc
tyCoVarsOfCoAcc (CoVarCo v) fv_cand in_scope acc
  = (oneVar v `unionFV` tyCoVarsOfTypeAcc (varType v)) fv_cand in_scope acc
tyCoVarsOfCoAcc (AxiomInstCo _ _ cos) fv_cand in_scope acc = tyCoVarsOfCosAcc cos fv_cand in_scope acc
tyCoVarsOfCoAcc (UnivCo p _ t1 t2) fv_cand in_scope acc
  = (tyCoVarsOfProvAcc p `unionFV` tyCoVarsOfTypeAcc t1
                         `unionFV` tyCoVarsOfTypeAcc t2) fv_cand in_scope acc
tyCoVarsOfCoAcc (SymCo co)          fv_cand in_scope acc = tyCoVarsOfCoAcc co fv_cand in_scope acc
tyCoVarsOfCoAcc (TransCo co1 co2)   fv_cand in_scope acc = (tyCoVarsOfCoAcc co1 `unionFV` tyCoVarsOfCoAcc co2) fv_cand in_scope acc
tyCoVarsOfCoAcc (NthCo _ co)        fv_cand in_scope acc = tyCoVarsOfCoAcc co fv_cand in_scope acc
tyCoVarsOfCoAcc (LRCo _ co)         fv_cand in_scope acc = tyCoVarsOfCoAcc co fv_cand in_scope acc
tyCoVarsOfCoAcc (InstCo co arg)     fv_cand in_scope acc = (tyCoVarsOfCoAcc co `unionFV` tyCoVarsOfCoAcc arg) fv_cand in_scope acc
tyCoVarsOfCoAcc (CoherenceCo c1 c2) fv_cand in_scope acc = (tyCoVarsOfCoAcc c1 `unionFV` tyCoVarsOfCoAcc c2) fv_cand in_scope acc
tyCoVarsOfCoAcc (KindCo co)         fv_cand in_scope acc = tyCoVarsOfCoAcc co fv_cand in_scope acc
tyCoVarsOfCoAcc (SubCo co)          fv_cand in_scope acc = tyCoVarsOfCoAcc co fv_cand in_scope acc
tyCoVarsOfCoAcc (AxiomRuleCo _ cs)  fv_cand in_scope acc = tyCoVarsOfCosAcc cs fv_cand in_scope acc

tyCoVarsOfProv :: UnivCoProvenance -> TyCoVarSet
tyCoVarsOfProv prov = runFVSet $ tyCoVarsOfProvAcc prov

tyCoVarsOfProvAcc :: UnivCoProvenance -> FV
tyCoVarsOfProvAcc UnsafeCoerceProv    fv_cand in_scope acc = noVars fv_cand in_scope acc
tyCoVarsOfProvAcc (PhantomProv co)    fv_cand in_scope acc = tyCoVarsOfCoAcc co fv_cand in_scope acc
tyCoVarsOfProvAcc (ProofIrrelProv co) fv_cand in_scope acc = tyCoVarsOfCoAcc co fv_cand in_scope acc
tyCoVarsOfProvAcc (PluginProv _)      fv_cand in_scope acc = noVars fv_cand in_scope acc
tyCoVarsOfProvAcc (HoleProv _)        fv_cand in_scope acc = noVars fv_cand in_scope acc

tyCoVarsOfCos :: [Coercion] -> TyCoVarSet
tyCoVarsOfCos cos = runFVSet $ tyCoVarsOfCosAcc cos

tyCoVarsOfCosAcc :: [Coercion] -> FV
tyCoVarsOfCosAcc []       fv_cand in_scope acc = noVars fv_cand in_scope acc
tyCoVarsOfCosAcc (co:cos) fv_cand in_scope acc = (tyCoVarsOfCoAcc co `unionFV` tyCoVarsOfCosAcc cos) fv_cand in_scope acc

coVarsOfType :: Type -> CoVarSet
coVarsOfType (TyVarTy v)         = coVarsOfType (tyVarKind v)
coVarsOfType (TyConApp _ tys)    = coVarsOfTypes tys
coVarsOfType (LitTy {})          = emptyVarSet
coVarsOfType (AppTy fun arg)     = coVarsOfType fun `unionVarSet` coVarsOfType arg
coVarsOfType (ForAllTy bndr ty)
  = coVarsOfType ty `delBinderVar` bndr
    `unionVarSet` coVarsOfType (binderType bndr)
coVarsOfType (CastTy ty co)      = coVarsOfType ty `unionVarSet` coVarsOfCo co
coVarsOfType (CoercionTy co)     = coVarsOfCo co

coVarsOfTypes :: [Type] -> TyCoVarSet
coVarsOfTypes tys = mapUnionVarSet coVarsOfType tys

coVarsOfCo :: Coercion -> CoVarSet
-- Extract *coercion* variables only.  Tiresome to repeat the code, but easy.
coVarsOfCo (Refl _ ty)         = coVarsOfType ty
coVarsOfCo (TyConAppCo _ _ args) = coVarsOfCos args
coVarsOfCo (AppCo co arg)      = coVarsOfCo co `unionVarSet` coVarsOfCo arg
coVarsOfCo (ForAllCo tv kind_co co)
  = coVarsOfCo co `delVarSet` tv `unionVarSet` coVarsOfCo kind_co
coVarsOfCo (CoVarCo v)         = unitVarSet v `unionVarSet` coVarsOfType (varType v)
coVarsOfCo (AxiomInstCo _ _ args) = coVarsOfCos args
coVarsOfCo (UnivCo p _ t1 t2)  = coVarsOfProv p `unionVarSet` coVarsOfTypes [t1, t2]
coVarsOfCo (SymCo co)          = coVarsOfCo co
coVarsOfCo (TransCo co1 co2)   = coVarsOfCo co1 `unionVarSet` coVarsOfCo co2
coVarsOfCo (NthCo _ co)        = coVarsOfCo co
coVarsOfCo (LRCo _ co)         = coVarsOfCo co
coVarsOfCo (InstCo co arg)     = coVarsOfCo co `unionVarSet` coVarsOfCo arg
coVarsOfCo (CoherenceCo c1 c2) = coVarsOfCos [c1, c2]
coVarsOfCo (KindCo co)         = coVarsOfCo co
coVarsOfCo (SubCo co)          = coVarsOfCo co
coVarsOfCo (AxiomRuleCo _ cs)  = coVarsOfCos cs

coVarsOfProv :: UnivCoProvenance -> CoVarSet
coVarsOfProv UnsafeCoerceProv    = emptyVarSet
coVarsOfProv (PhantomProv co)    = coVarsOfCo co
coVarsOfProv (ProofIrrelProv co) = coVarsOfCo co
coVarsOfProv (PluginProv _)      = emptyVarSet
coVarsOfProv (HoleProv _)        = emptyVarSet

coVarsOfCos :: [Coercion] -> CoVarSet
coVarsOfCos cos = mapUnionVarSet coVarsOfCo cos

-- | Add the kind variables free in the kinds of the tyvars in the given set.
-- Returns a non-deterministic set.
closeOverKinds :: TyVarSet -> TyVarSet
closeOverKinds = runFVSet . closeOverKindsAcc . varSetElems

-- | Given a list of tyvars returns a deterministic FV computation that
-- returns the given tyvars with the kind variables free in the kinds of the
-- given tyvars.
closeOverKindsAcc :: [TyVar] -> FV
closeOverKindsAcc tvs =
  mapUnionFV (tyCoVarsOfTypeAcc . tyVarKind) tvs `unionFV` someVars tvs

-- | Add the kind variables free in the kinds of the tyvars in the given set.
-- Returns a deterministic set.
closeOverKindsDSet :: DTyVarSet -> DTyVarSet
closeOverKindsDSet = runFVDSet . closeOverKindsAcc . dVarSetElems

-- | Gets the free vars of a telescope, scoped over a given free var set.
tyCoVarsOfTelescope :: [Var] -> TyCoVarSet -> TyCoVarSet
tyCoVarsOfTelescope [] fvs = fvs
tyCoVarsOfTelescope (v:vs) fvs = tyCoVarsOfTelescope vs fvs
                                 `delVarSet` v
                                 `unionVarSet` tyCoVarsOfType (varType v)
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{-
%************************************************************************
%*                                                                      *
                        TyThing
%*                                                                      *
%************************************************************************

Despite the fact that DataCon has to be imported via a hi-boot route,
this module seems the right place for TyThing, because it's needed for
funTyCon and all the types in TysPrim.

Note [ATyCon for classes]
~~~~~~~~~~~~~~~~~~~~~~~~~
Both classes and type constructors are represented in the type environment
as ATyCon.  You can tell the difference, and get to the class, with
   isClassTyCon :: TyCon -> Bool
   tyConClass_maybe :: TyCon -> Maybe Class
The Class and its associated TyCon have the same Name.
-}

-- | A global typecheckable-thing, essentially anything that has a name.
-- Not to be confused with a 'TcTyThing', which is also a typecheckable
-- thing but in the *local* context.  See 'TcEnv' for how to retrieve
-- a 'TyThing' given a 'Name'.
data TyThing
  = AnId     Id
  | AConLike ConLike
  | ATyCon   TyCon       -- TyCons and classes; see Note [ATyCon for classes]
  | ACoAxiom (CoAxiom Branched)
  deriving (Eq, Ord)

instance Outputable TyThing where
  ppr = pprTyThing

pprTyThing :: TyThing -> SDoc
pprTyThing thing = pprTyThingCategory thing <+> quotes (ppr (getName thing))

pprTyThingCategory :: TyThing -> SDoc
pprTyThingCategory (ATyCon tc)
  | isClassTyCon tc = ptext (sLit "Class")
  | otherwise       = ptext (sLit "Type constructor")
pprTyThingCategory (ACoAxiom _) = ptext (sLit "Coercion axiom")
pprTyThingCategory (AnId   _)   = ptext (sLit "Identifier")
pprTyThingCategory (AConLike (RealDataCon _)) = ptext (sLit "Data constructor")
pprTyThingCategory (AConLike (PatSynCon _))  = ptext (sLit "Pattern synonym")


instance NamedThing TyThing where       -- Can't put this with the type
  getName (AnId id)     = getName id    -- decl, because the DataCon instance
  getName (ATyCon tc)   = getName tc    -- isn't visible there
  getName (ACoAxiom cc) = getName cc
  getName (AConLike cl) = getName cl

{-
%************************************************************************
%*                                                                      *
                        Substitutions
      Data type defined here to avoid unnecessary mutual recursion
%*                                                                      *
%************************************************************************
-}

-- | Type & coercion substitution
--
-- #tcvsubst_invariant#
-- The following invariants must hold of a 'TCvSubst':
--
-- 1. The in-scope set is needed /only/ to
-- guide the generation of fresh uniques
--
-- 2. In particular, the /kind/ of the type variables in
-- the in-scope set is not relevant
--
-- 3. The substitution is only applied ONCE! This is because
-- in general such application will not reach a fixed point.
data TCvSubst
  = TCvSubst InScopeSet -- The in-scope type and kind variables
             TvSubstEnv -- Substitutes both type and kind variables
             CvSubstEnv -- Substitutes coercion variables
        -- See Note [Apply Once]
        -- and Note [Extending the TvSubstEnv]
        -- and Note [Substituting types and coercions]

-- | A substitution of 'Type's for 'TyVar's
--                 and 'Kind's for 'KindVar's
type TvSubstEnv = TyVarEnv Type
        -- A TvSubstEnv is used both inside a TCvSubst (with the apply-once
        -- invariant discussed in Note [Apply Once]), and also independently
        -- in the middle of matching, and unification (see Types.Unify)
        -- So you have to look at the context to know if it's idempotent or
        -- apply-once or whatever

-- | A substitution of 'Coercion's for 'CoVar's
type CvSubstEnv = CoVarEnv Coercion

{-
Note [Apply Once]
~~~~~~~~~~~~~~~~~
We use TCvSubsts to instantiate things, and we might instantiate
        forall a b. ty
\with the types
        [a, b], or [b, a].
So the substitution might go [a->b, b->a].  A similar situation arises in Core
when we find a beta redex like
        (/\ a /\ b -> e) b a
Then we also end up with a substitution that permutes type variables. Other
variations happen to; for example [a -> (a, b)].

        ****************************************************
        *** So a TCvSubst must be applied precisely once ***
        ****************************************************

A TCvSubst is not idempotent, but, unlike the non-idempotent substitution
we use during unifications, it must not be repeatedly applied.

Note [Extending the TvSubstEnv]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
See #tcvsubst_invariant# for the invariants that must hold.

This invariant allows a short-cut when the subst envs are empty:
if the TvSubstEnv and CvSubstEnv are empty --- i.e. (isEmptyTCvSubst subst)
holds --- then (substTy subst ty) does nothing.

For example, consider:
        (/\a. /\b:(a~Int). ...b..) Int
We substitute Int for 'a'.  The Unique of 'b' does not change, but
nevertheless we add 'b' to the TvSubstEnv, because b's kind does change

This invariant has several crucial consequences:

* In substTyVarBndr, we need extend the TvSubstEnv
        - if the unique has changed
        - or if the kind has changed

* In substTyVar, we do not need to consult the in-scope set;
  the TvSubstEnv is enough

* In substTy, substTheta, we can short-circuit when the TvSubstEnv is empty

Note [Substituting types and coercions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Types and coercions are mutually recursive, and either may have variables
"belonging" to the other. Thus, every time we wish to substitute in a
type, we may also need to substitute in a coercion, and vice versa.
However, the constructor used to create type variables is distinct from
that of coercion variables, so we carry two VarEnvs in a TCvSubst. Note
that it would be possible to use the CoercionTy constructor to combine
these environments, but that seems like a false economy.

Note that the TvSubstEnv should *never* map a CoVar (built with the Id
constructor) and the CvSubstEnv should *never* map a TyVar. Furthermore,
the range of the TvSubstEnv should *never* include a type headed with
CoercionTy.
-}

emptyTvSubstEnv :: TvSubstEnv
emptyTvSubstEnv = emptyVarEnv

emptyCvSubstEnv :: CvSubstEnv
emptyCvSubstEnv = emptyVarEnv

composeTCvSubstEnv :: InScopeSet
                   -> (TvSubstEnv, CvSubstEnv)
                   -> (TvSubstEnv, CvSubstEnv)
                   -> (TvSubstEnv, CvSubstEnv)
-- ^ @(compose env1 env2)(x)@ is @env1(env2(x))@; i.e. apply @env2@ then @env1@.
-- It assumes that both are idempotent.
-- Typically, @env1@ is the refinement to a base substitution @env2@
composeTCvSubstEnv in_scope (tenv1, cenv1) (tenv2, cenv2)
  = ( tenv1 `plusVarEnv` mapVarEnv (substTy subst1) tenv2
    , cenv1 `plusVarEnv` mapVarEnv (substCo subst1) cenv2 )
        -- First apply env1 to the range of env2
        -- Then combine the two, making sure that env1 loses if
        -- both bind the same variable; that's why env1 is the
        --  *left* argument to plusVarEnv, because the right arg wins
  where
    subst1 = TCvSubst in_scope tenv1 cenv1

-- | Composes two substitutions, applying the second one provided first,
-- like in function composition.
composeTCvSubst :: TCvSubst -> TCvSubst -> TCvSubst
composeTCvSubst (TCvSubst is1 tenv1 cenv1) (TCvSubst is2 tenv2 cenv2)
  = TCvSubst is3 tenv3 cenv3
  where
    is3 = is1 `unionInScope` is2
    (tenv3, cenv3) = composeTCvSubstEnv is3 (tenv1, cenv1) (tenv2, cenv2)

emptyTCvSubst :: TCvSubst
emptyTCvSubst = TCvSubst emptyInScopeSet emptyTvSubstEnv emptyCvSubstEnv

mkEmptyTCvSubst :: InScopeSet -> TCvSubst
mkEmptyTCvSubst is = TCvSubst is emptyTvSubstEnv emptyCvSubstEnv

isEmptyTCvSubst :: TCvSubst -> Bool
         -- See Note [Extending the TvSubstEnv]
isEmptyTCvSubst (TCvSubst _ tenv cenv) = isEmptyVarEnv tenv && isEmptyVarEnv cenv

mkTCvSubst :: InScopeSet -> (TvSubstEnv, CvSubstEnv) -> TCvSubst
mkTCvSubst in_scope (tenv, cenv) = TCvSubst in_scope tenv cenv

getTvSubstEnv :: TCvSubst -> TvSubstEnv
getTvSubstEnv (TCvSubst _ env _) = env

getCvSubstEnv :: TCvSubst -> CvSubstEnv
getCvSubstEnv (TCvSubst _ _ env) = env

getTCvInScope :: TCvSubst -> InScopeSet
getTCvInScope (TCvSubst in_scope _ _) = in_scope

isInScope :: Var -> TCvSubst -> Bool
isInScope v (TCvSubst in_scope _ _) = v `elemInScopeSet` in_scope

notElemTCvSubst :: Var -> TCvSubst -> Bool
notElemTCvSubst v (TCvSubst _ tenv cenv)
  | isTyVar v
  = not (v `elemVarEnv` tenv)
  | otherwise
  = not (v `elemVarEnv` cenv)

setTvSubstEnv :: TCvSubst -> TvSubstEnv -> TCvSubst
setTvSubstEnv (TCvSubst in_scope _ cenv) tenv = TCvSubst in_scope tenv cenv

setCvSubstEnv :: TCvSubst -> CvSubstEnv -> TCvSubst
setCvSubstEnv (TCvSubst in_scope tenv _) cenv = TCvSubst in_scope tenv cenv

zapTCvSubst :: TCvSubst -> TCvSubst
zapTCvSubst (TCvSubst in_scope _ _) = TCvSubst in_scope emptyVarEnv emptyVarEnv

extendTCvInScope :: TCvSubst -> Var -> TCvSubst
extendTCvInScope (TCvSubst in_scope tenv cenv) var
  = TCvSubst (extendInScopeSet in_scope var) tenv cenv

extendTCvInScopeList :: TCvSubst -> [Var] -> TCvSubst
extendTCvInScopeList (TCvSubst in_scope tenv cenv) vars
  = TCvSubst (extendInScopeSetList in_scope vars) tenv cenv

extendTCvInScopeSet :: TCvSubst -> VarSet -> TCvSubst
extendTCvInScopeSet (TCvSubst in_scope tenv cenv) vars
  = TCvSubst (extendInScopeSetSet in_scope vars) tenv cenv

extendSubstEnvs :: (TvSubstEnv, CvSubstEnv) -> Var -> Type
                -> (TvSubstEnv, CvSubstEnv)
extendSubstEnvs (tenv, cenv) v ty
  | isTyVar v
  = ASSERT( not $ isCoercionTy ty )
    (extendVarEnv tenv v ty, cenv)

    -- NB: v might *not* be a proper covar, because it might be lifted.
    -- This happens in tcCoercionToCoercion
  | CoercionTy co <- ty
  = (tenv, extendVarEnv cenv v co)
  | otherwise
  = pprPanic "extendSubstEnvs" (ppr v <+> ptext (sLit "|->") <+> ppr ty)

extendTCvSubst :: TCvSubst -> Var -> Type -> TCvSubst
extendTCvSubst (TCvSubst in_scope tenv cenv) tv ty
  = TCvSubst in_scope tenv' cenv'
  where (tenv', cenv') = extendSubstEnvs (tenv, cenv) tv ty

extendTCvSubstAndInScope :: TCvSubst -> TyCoVar -> Type -> TCvSubst
-- Also extends the in-scope set
extendTCvSubstAndInScope (TCvSubst in_scope tenv cenv) tv ty
  = TCvSubst (in_scope `extendInScopeSetSet` tyCoVarsOfType ty)
             tenv' cenv'
  where (tenv', cenv') = extendSubstEnvs (tenv, cenv) tv ty

extendTCvSubstList :: TCvSubst -> [Var] -> [Type] -> TCvSubst
extendTCvSubstList subst tvs tys
  = foldl2 extendTCvSubst subst tvs tys

extendTCvSubstBinder :: TCvSubst -> TyBinder -> Type -> TCvSubst
extendTCvSubstBinder env (Anon {})    _  = env
extendTCvSubstBinder env (Named tv _) ty = extendTCvSubst env tv ty

unionTCvSubst :: TCvSubst -> TCvSubst -> TCvSubst
-- Works when the ranges are disjoint
unionTCvSubst (TCvSubst in_scope1 tenv1 cenv1) (TCvSubst in_scope2 tenv2 cenv2)
  = ASSERT( not (tenv1 `intersectsVarEnv` tenv2)
         && not (cenv1 `intersectsVarEnv` cenv2) )
    TCvSubst (in_scope1 `unionInScope` in_scope2)
             (tenv1     `plusVarEnv`   tenv2)
             (cenv1     `plusVarEnv`   cenv2)

-- mkOpenTCvSubst and zipOpenTCvSubst generate the in-scope set from
-- the types given; but it's just a thunk so with a bit of luck
-- it'll never be evaluated

-- Note [Generating the in-scope set for a substitution]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- If we want to substitute [a -> ty1, b -> ty2] I used to
-- think it was enough to generate an in-scope set that includes
-- fv(ty1,ty2).  But that's not enough; we really should also take the
-- free vars of the type we are substituting into!  Example:
--      (forall b. (a,b,x)) [a -> List b]
-- Then if we use the in-scope set {b}, there is a danger we will rename
-- the forall'd variable to 'x' by mistake, getting this:
--      (forall x. (List b, x, x))
-- Urk!  This means looking at all the calls to mkOpenTCvSubst....


-- | Generates an in-scope set from the free variables in a list of types
-- and a list of coercions
mkTyCoInScopeSet :: [Type] -> [Coercion] -> InScopeSet
mkTyCoInScopeSet tys cos
  = mkInScopeSet (tyCoVarsOfTypes tys `unionVarSet` tyCoVarsOfCos cos)

-- | Generates the in-scope set for the 'TCvSubst' from the types in the incoming
-- environment, hence "open"
mkOpenTCvSubst :: TvSubstEnv -> CvSubstEnv -> TCvSubst
mkOpenTCvSubst tenv cenv
  = TCvSubst (mkTyCoInScopeSet (varEnvElts tenv) (varEnvElts cenv)) tenv cenv

-- | Generates the in-scope set for the 'TCvSubst' from the types in the incoming
-- environment, hence "open". No CoVars, please!
zipOpenTCvSubst :: [TyVar] -> [Type] -> TCvSubst
zipOpenTCvSubst tyvars tys
  | debugIsOn && (length tyvars /= length tys)
  = pprTrace "zipOpenTCvSubst" (ppr tyvars $$ ppr tys) emptyTCvSubst
  | otherwise
  = TCvSubst (mkInScopeSet (tyCoVarsOfTypes tys)) tenv emptyCvSubstEnv
  where tenv = zipTyEnv tyvars tys

-- | Generates the in-scope set for the 'TCvSubst' from the types in the incoming
-- environment, hence "open".
zipOpenTCvSubstCoVars :: [CoVar] -> [Coercion] -> TCvSubst
zipOpenTCvSubstCoVars cvs cos
  | debugIsOn && (length cvs /= length cos)
  = pprTrace "zipOpenTCvSubstCoVars" (ppr cvs $$ ppr cos) emptyTCvSubst
  | otherwise
  = TCvSubst (mkInScopeSet (tyCoVarsOfCos cos)) emptyTvSubstEnv cenv
  where cenv = zipCoEnv cvs cos


-- | Create an open TCvSubst combining the binders and types provided.
-- NB: It is OK if the lists are of different lengths.
zipOpenTCvSubstBinders :: [TyBinder] -> [Type] -> TCvSubst
zipOpenTCvSubstBinders bndrs tys
  = TCvSubst is tenv emptyCvSubstEnv
  where
    is = mkInScopeSet (tyCoVarsOfTypes tys)
    (tvs, tys') = unzip [ (tv, ty) | (Named tv _, ty) <- zip bndrs tys ]
    tenv = zipTyEnv tvs tys'

-- | Called when doing top-level substitutions. Here we expect that the
-- free vars of the range of the substitution will be empty.
mkTopTCvSubst :: [(TyCoVar, Type)] -> TCvSubst
mkTopTCvSubst prs = TCvSubst emptyInScopeSet tenv cenv
  where (tenv, cenv) = foldl extend (emptyTvSubstEnv, emptyCvSubstEnv) prs
        extend envs (v, ty) = extendSubstEnvs envs v ty

-- | Makes a subst with an empty in-scope-set. No CoVars, please!
zipTopTCvSubst :: [TyVar] -> [Type] -> TCvSubst
zipTopTCvSubst tyvars tys
  | debugIsOn && (length tyvars /= length tys)
  = pprTrace "zipTopTCvSubst" (ppr tyvars $$ ppr tys) emptyTCvSubst
  | otherwise
  = TCvSubst emptyInScopeSet tenv emptyCvSubstEnv
  where tenv = zipTyEnv tyvars tys

zipTyEnv :: [TyVar] -> [Type] -> TvSubstEnv
zipTyEnv tyvars tys
  = ASSERT( all (not . isCoercionTy) tys )
    mkVarEnv (zipEqual "zipTyEnv" tyvars tys)
        -- There used to be a special case for when
        --      ty == TyVarTy tv
        -- (a not-uncommon case) in which case the substitution was dropped.
        -- But the type-tidier changes the print-name of a type variable without
        -- changing the unique, and that led to a bug.   Why?  Pre-tidying, we had
        -- a type {Foo t}, where Foo is a one-method class.  So Foo is really a newtype.
        -- And it happened that t was the type variable of the class.  Post-tiding,
        -- it got turned into {Foo t2}.  The ext-core printer expanded this using
        -- sourceTypeRep, but that said "Oh, t == t2" because they have the same unique,
        -- and so generated a rep type mentioning t not t2.
        --
        -- Simplest fix is to nuke the "optimisation"

zipCoEnv :: [CoVar] -> [Coercion] -> CvSubstEnv
zipCoEnv cvs cos = mkVarEnv (zipEqual "zipCoEnv" cvs cos)

instance Outputable TCvSubst where
  ppr (TCvSubst ins tenv cenv)
    = brackets $ sep[ ptext (sLit "TCvSubst"),
                      nest 2 (ptext (sLit "In scope:") <+> ppr ins),
                      nest 2 (ptext (sLit "Type env:") <+> ppr tenv),
                      nest 2 (ptext (sLit "Co env:") <+> ppr cenv) ]

{-
%************************************************************************
%*                                                                      *
                Performing type or kind substitutions
%*                                                                      *
%************************************************************************

Note [Sym and ForAllCo]
~~~~~~~~~~~~~~~~~~~~~~~
In OptCoercion, we try to push "sym" out to the leaves of a coercion. But,
how do we push sym into a ForAllCo? It's a little ugly.

Here is the typing rule:

h : k1 ~# k2
(tv : k1) |- g : ty1 ~# ty2
----------------------------
ForAllCo tv h g : (ForAllTy (tv : k1) ty1) ~#
                  (ForAllTy (tv : k2) (ty2[tv |-> tv |> sym h]))

Here is what we want:

ForAllCo tv h' g' : (ForAllTy (tv : k2) (ty2[tv |-> tv |> sym h])) ~#
                    (ForAllTy (tv : k1) ty1)


Because the kinds of the type variables to the right of the colon are the kinds
coerced by h', we know (h' : k2 ~# k1). Thus, (h' = sym h).

Now, we can rewrite ty1 to be (ty1[tv |-> tv |> sym h' |> h']). We thus want

ForAllCo tv h' g' :
  (ForAllTy (tv : k2) (ty2[tv |-> tv |> h'])) ~#
  (ForAllTy (tv : k1) (ty1[tv |-> tv |> h'][tv |-> tv |> sym h']))

We thus see that we want

g' : ty2[tv |-> tv |> h'] ~# ty1[tv |-> tv |> h']

and thus g' = sym (g[tv |-> tv |> h']).

Putting it all together, we get this:

sym (ForAllCo tv h g)
==>
ForAllCo tv (sym h) (sym g[tv |-> tv |> sym h])

-}

-- | Create a substitution from tyvars to types, but later types may depend
-- on earlier ones. Return the substed types and the built substitution.
substTelescope :: [TyCoVar] -> [Type] -> ([Type], TCvSubst)
substTelescope = go_subst emptyTCvSubst
  where
    go_subst :: TCvSubst -> [TyCoVar] -> [Type] -> ([Type], TCvSubst)
    go_subst subst [] [] = ([], subst)
    go_subst subst (tv:tvs