content: {-
  (c) The University of Glasgow 2006
  (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
  
  
  The @TyCon@ datatype
  -}
  
  {-# LANGUAGE CPP, DeriveDataTypeable #-}
  
  module Eta.Types.TyCon(
          -- * Main TyCon data types
          TyCon, FieldLabel,
  
          AlgTyConRhs(..), visibleDataCons,
          TyConParent(..), isNoParent,
          FamTyConFlav(..), Role(..),
  
          -- ** Constructing TyCons
          mkAlgTyCon,
          mkClassTyCon,
          mkFunTyCon,
          mkPrimTyCon,
          mkKindTyCon,
          mkLiftedPrimTyCon,
          mkTupleTyCon,
          mkSynonymTyCon,
          mkFamilyTyCon,
          mkPromotedDataCon,
          mkPromotedTyCon,
  
          -- ** Predicates on TyCons
          isAlgTyCon,
          isClassTyCon, isFamInstTyCon,
          isFunTyCon,
          isPrimTyCon,
          isTupleTyCon, isUnboxedTupleTyCon, isBoxedTupleTyCon,
          isTypeSynonymTyCon,
          isDecomposableTyCon,
          isPromotedDataCon, isPromotedTyCon, isPromotedTupleTyCon,
          isPromotedDataCon_maybe, isPromotedTyCon_maybe, isLiftedTypeKindTyConName,
          promotableTyCon_maybe, promoteTyCon,
  
          isDataTyCon, isProductTyCon, isDataProductTyCon_maybe,
          isEnumerationTyCon,
          isNewTyCon, isAbstractTyCon,
          isFamilyTyCon, isOpenFamilyTyCon,
          isTypeFamilyTyCon, isDataFamilyTyCon,
          isOpenTypeFamilyTyCon, isClosedSynFamilyTyCon_maybe,
          isBuiltInSynFamTyCon_maybe,
          isUnLiftedTyCon,
          isGadtSyntaxTyCon, isDistinctTyCon, isDistinctAlgRhs,
          isInjectiveTyCon, isGenerativeTyCon, isGenInjAlgRhs,
          isTyConAssoc, tyConAssoc_maybe,
          isRecursiveTyCon,
          isImplicitTyCon,
          -- ETA-specific
          isObjectTyCon,
  
          -- ** Extracting information out of TyCons
          tyConName,
          tyConKind,
          tyConUnique,
          tyConTyVars,
          tyConCType, tyConCType_maybe,
          tyConDataCons, tyConDataCons_maybe,
          tyConSingleDataCon_maybe, tyConSingleAlgDataCon_maybe,
          tyConFamilySize,
          tyConStupidTheta,
          tyConArity,
          tyConRoles,
          tyConParent,
          tyConTuple_maybe, tyConClass_maybe,
          tyConFamInst_maybe, tyConFamInstSig_maybe, tyConFamilyCoercion_maybe,
          synTyConDefn_maybe, synTyConRhs_maybe, famTyConFlav_maybe,
          algTyConRhs,
          newTyConRhs, newTyConEtadArity, newTyConEtadRhs,
          unwrapNewTyCon_maybe, unwrapNewTyConEtad_maybe,
          tupleTyConBoxity, tupleTyConSort, tupleTyConArity,
  
          -- ** Manipulating TyCons
          tcExpandTyCon_maybe, coreExpandTyCon_maybe,
          makeTyConAbstract,
          newTyConCo, newTyConCo_maybe,
          pprPromotionQuote,
  
          -- * Primitive representations of Types
          PrimRep(..), --PrimElemRep(..),
          tyConPrimRep, isVoidRep, isGcPtrRep, isObjectRep, getObjectClass,
          primRepSizeW, --primElemRepSizeB,
  
          -- * Recursion breaking
          RecTcChecker, initRecTc, checkRecTc
  
  ) where
  
  #include "HsVersions.h"
  
  import {-# SOURCE #-} Eta.Types.TypeRep ( Kind, Type, PredType )
  import {-# SOURCE #-} Eta.BasicTypes.DataCon ( DataCon, isVanillaDataCon, dataConTyCon )
  
  import Eta.BasicTypes.Var
  import Eta.Types.Class
  import Eta.BasicTypes.BasicTypes
  import Eta.Main.DynFlags
  import Eta.Prelude.ForeignCall
  import Eta.BasicTypes.Name
  import Eta.BasicTypes.NameSet
  import Eta.Types.CoAxiom
  import Eta.Prelude.PrelNames
  import Eta.Utils.Maybes
  import Eta.Utils.Outputable
  -- import Eta.Main.Constants
  import Eta.Utils.Util
  
  import qualified Data.Data as Data
  import Data.Typeable (Typeable)
  -- TODO: Refactor to FastString?
  import Data.Text (Text)
  
  
  {-
  -----------------------------------------------
          Notes about type families
  -----------------------------------------------
  
  Note [Type synonym families]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  * Type synonym families, also known as "type functions", map directly
    onto the type functions in FC:
  
          type family F a :: *
          type instance F Int = Bool
          ..etc...
  
  * Reply "yes" to isTypeFamilyTyCon, and isFamilyTyCon
  
  * From the user's point of view (F Int) and Bool are simply
    equivalent types.
  
  * A Haskell 98 type synonym is a degenerate form of a type synonym
    family.
  
  * Type functions can't appear in the LHS of a type function:
          type instance F (F Int) = ...   -- BAD!
  
  * Translation of type family decl:
          type family F a :: *
    translates to
      a FamilyTyCon 'F', whose FamTyConFlav is OpenSynFamilyTyCon
  
          type family G a :: * where
            G Int = Bool
            G Bool = Char
            G a = ()
    translates to
      a FamilyTyCon 'G', whose FamTyConFlav is ClosedSynFamilyTyCon, with the
      appropriate CoAxiom representing the equations
  
  * In the future we might want to support
      * injective type families (allow decomposition)
    but we don't at the moment [2013]
  
  Note [Data type families]
  ~~~~~~~~~~~~~~~~~~~~~~~~~
  See also Note [Wrappers for data instance tycons] in MkId.lhs
  
  * Data type families are declared thus
          data family T a :: *
          data instance T Int = T1 | T2 Bool
  
    Here T is the "family TyCon".
  
  * Reply "yes" to isDataFamilyTyCon, and isFamilyTyCon
  
  * The user does not see any "equivalent types" as he did with type
    synonym families.  He just sees constructors with types
          T1 :: T Int
          T2 :: Bool -> T Int
  
  * Here's the FC version of the above declarations:
  
          data T a
          data R:TInt = T1 | T2 Bool
          axiom ax_ti : T Int ~ R:TInt
  
    The R:TInt is the "representation TyCons".
    It has an AlgTyConParent of
          FamInstTyCon T [Int] ax_ti
  
  * The axiom ax_ti may be eta-reduced; see
    Note [Eta reduction for data family axioms] in TcInstDcls
  
  * The data contructor T2 has a wrapper (which is what the
    source-level "T2" invokes):
  
          $WT2 :: Bool -> T Int
          $WT2 b = T2 b `cast` sym ax_ti
  
  * A data instance can declare a fully-fledged GADT:
  
          data instance T (a,b) where
            X1 :: T (Int,Bool)
            X2 :: a -> b -> T (a,b)
  
    Here's the FC version of the above declaration:
  
          data R:TPair a where
            X1 :: R:TPair Int Bool
            X2 :: a -> b -> R:TPair a b
          axiom ax_pr :: T (a,b) ~ R:TPair a b
  
          $WX1 :: forall a b. a -> b -> T (a,b)
          $WX1 a b (x::a) (y::b) = X2 a b x y `cast` sym (ax_pr a b)
  
    The R:TPair are the "representation TyCons".
    We have a bit of work to do, to unpick the result types of the
    data instance declaration for T (a,b), to get the result type in the
    representation; e.g.  T (a,b) --> R:TPair a b
  
    The representation TyCon R:TList, has an AlgTyConParent of
  
          FamInstTyCon T [(a,b)] ax_pr
  
  * Notice that T is NOT translated to a FC type function; it just
    becomes a "data type" with no constructors, which can be coerced inot
    into R:TInt, R:TPair by the axioms.  These axioms
    axioms come into play when (and *only* when) you
          - use a data constructor
          - do pattern matching
    Rather like newtype, in fact
  
    As a result
  
    - T behaves just like a data type so far as decomposition is concerned
  
    - (T Int) is not implicitly converted to R:TInt during type inference.
      Indeed the latter type is unknown to the programmer.
  
    - There *is* an instance for (T Int) in the type-family instance
      environment, but it is only used for overlap checking
  
    - It's fine to have T in the LHS of a type function:
      type instance F (T a) = [a]
  
    It was this last point that confused me!  The big thing is that you
    should not think of a data family T as a *type function* at all, not
    even an injective one!  We can't allow even injective type functions
    on the LHS of a type function:
          type family injective G a :: *
          type instance F (G Int) = Bool
    is no good, even if G is injective, because consider
          type instance G Int = Bool
          type instance F Bool = Char
  
    So a data type family is not an injective type function. It's just a
    data type with some axioms that connect it to other data types.
  
  Note [Associated families and their parent class]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  *Associated* families are just like *non-associated* families, except
  that they have a TyConParent of AssocFamilyTyCon, which identifies the
  parent class.
  
  However there is an important sharing relationship between
    * the tyConTyVars of the parent Class
    * the tyConTyvars of the associated TyCon
  
     class C a b where
       data T p a
       type F a q b
  
  Here the 'a' and 'b' are shared with the 'Class'; that is, they have
  the same Unique.
  
  This is important. In an instance declaration we expect
    * all the shared variables to be instantiated the same way
    * the non-shared variables of the associated type should not
      be instantiated at all
  
    instance C [x] (Tree y) where
       data T p [x] = T1 x | T2 p
       type F [x] q (Tree y) = (x,y,q)
  
  Note [TyCon Role signatures]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  Every tycon has a role signature, assigning a role to each of the tyConTyVars
  (or of equal length to the tyConArity, if there are no tyConTyVars). An
  example demonstrates these best: say we have a tycon T, with parameters a at
  nominal, b at representational, and c at phantom. Then, to prove
  representational equality between T a1 b1 c1 and T a2 b2 c2, we need to have
  nominal equality between a1 and a2, representational equality between b1 and
  b2, and nothing in particular (i.e., phantom equality) between c1 and c2. This
  might happen, say, with the following declaration:
  
    data T a b c where
      MkT :: b -> T Int b c
  
  Data and class tycons have their roles inferred (see inferRoles in TcTyDecls),
  as do vanilla synonym tycons. Family tycons have all parameters at role N,
  though it is conceivable that we could relax this restriction. (->)'s and
  tuples' parameters are at role R. Each primitive tycon declares its roles;
  it's worth noting that (~#)'s parameters are at role N. Promoted data
  constructors' type arguments are at role R. All kind arguments are at role
  N.
  
  ************************************************************************
  *                                                                      *
  \subsection{The data type}
  *                                                                      *
  ************************************************************************
  -}
  
  -- | TyCons represent type constructors. Type constructors are introduced by
  -- things such as:
  --
  -- 1) Data declarations: @data Foo = ...@ creates the @Foo@ type constructor of
  --    kind @*@
  --
  -- 2) Type synonyms: @type Foo = ...@ creates the @Foo@ type constructor
  --
  -- 3) Newtypes: @newtype Foo a = MkFoo ...@ creates the @Foo@ type constructor
  --    of kind @* -> *@
  --
  -- 4) Class declarations: @class Foo where@ creates the @Foo@ type constructor
  --    of kind @*@
  --
  -- This data type also encodes a number of primitive, built in type constructors
  -- such as those for function and tuple types.
  
  -- If you edit this type, you may need to update the GHC formalism
  -- See Note [GHC Formalism] in coreSyn/CoreLint.lhs
  data TyCon
    = -- | The function type constructor, @(->)@
      FunTyCon {
          tyConUnique :: Unique,   -- ^ A Unique of this TyCon. Invariant:
                                   -- identical to Unique of Name stored in
                                   -- tyConName field.
  
          tyConName   :: Name,     -- ^ Name of the constructor
  
          tyConKind   :: Kind,     -- ^ Kind of this TyCon (full kind, not just
                                   -- the return kind)
  
          tyConArity  :: Arity     -- ^ Number of arguments this TyCon must
                                   -- receive to be considered saturated
                                   -- (including implicit kind variables)
      }
  
    -- | Algebraic type constructors, which are defined to be those
    -- arising @data@ type and @newtype@ declarations.  All these
    -- constructors are lifted and boxed. See 'AlgTyConRhs' for more
    -- information.
    | AlgTyCon {
          tyConUnique  :: Unique,  -- ^ A Unique of this TyCon. Invariant:
                                   -- identical to Unique of Name stored in
                                   -- tyConName field.
  
          tyConName    :: Name,    -- ^ Name of the constructor
  
          tyConKind    :: Kind,    -- ^ Kind of this TyCon (full kind, not just
                                   -- the return kind)
  
          tyConArity   :: Arity,   -- ^ Number of arguments this TyCon must
                                   -- receive to be considered saturated
                                   -- (including implicit kind variables)
  
          tyConTyVars  :: [TyVar], -- ^ The kind and type variables used in the
                                   -- type constructor.
                                   -- Invariant: length tyvars = arity
                                   -- Precisely, this list scopes over:
                                   --
                                   -- 1. The 'algTcStupidTheta'
                                   -- 2. The cached types in algTyConRhs.NewTyCon
                                   -- 3. The family instance types if present
                                   --
                                   -- Note that it does /not/ scope over the data
                                   -- constructors.
  
          tcRoles      :: [Role],  -- ^ The role for each type variable
                                   -- This list has the same length as tyConTyVars
                                   -- See also Note [TyCon Role signatures]
  
          tyConCType   :: Maybe CType,-- ^ The C type that should be used
                                      -- for this type when using the FFI
                                      -- and CAPI
  
          algTcGadtSyntax  :: Bool,   -- ^ Was the data type declared with GADT
                                      -- syntax?  If so, that doesn't mean it's a
                                      -- true GADT; only that the "where" form
                                      -- was used.  This field is used only to
                                      -- guide pretty-printing
  
          algTcStupidTheta :: [PredType], -- ^ The \"stupid theta\" for the data
                                          -- type (always empty for GADTs).  A
                                          -- \"stupid theta\" is the context to
                                          -- the left of an algebraic type
                                          -- declaration, e.g. @Eq a@ in the
                                          -- declaration @data Eq a => T a ...@.
  
          algTcRhs    :: AlgTyConRhs, -- ^ Contains information about the
                                      -- data constructors of the algebraic type
  
          algTcRec    :: RecFlag,     -- ^ Tells us whether the data type is part
                                      -- of a mutually-recursive group or not
  
          algTcParent :: TyConParent, -- ^ Gives the class or family declaration
                                      -- 'TyCon' for derived 'TyCon's representing
                                      -- class or family instances, respectively.
                                      -- See also 'synTcParent'
  
          tcPromoted  :: Maybe TyCon  -- ^ Promoted TyCon, if any
      }
  
    -- | Represents the infinite family of tuple type constructors,
    --   @()@, @(a,b)@, @(# a, b #)@ etc.
    | TupleTyCon {
          tyConUnique    :: Unique,   -- ^ A Unique of this TyCon. Invariant:
                                      -- identical to Unique of Name stored in
                                      -- tyConName field.
  
          tyConName      :: Name,     -- ^ Name of the constructor
  
          tyConKind      :: Kind,     -- ^ Kind of this TyCon (full kind, not just
                                      -- the return kind)
  
          tyConArity     :: Arity,    -- ^ Number of arguments this TyCon must
                                      -- receive to be considered saturated
                                      -- (including implicit kind variables)
  
          tyConTupleSort :: TupleSort,-- ^ Is this a boxed, unboxed or constraint
                                      -- tuple?
  
          tyConTyVars    :: [TyVar],  -- ^ List of type and kind variables in this
                                      -- TyCon. Includes implicit kind variables.
                                      -- Invariant:
                                      -- length tyConTyVars = tyConArity
  
          dataCon        :: DataCon,  -- ^ Corresponding tuple data constructor
  
          tcPromoted     :: Maybe TyCon
                                      -- ^ Nothing for unboxed tuples
      }
  
    -- | Represents type synonyms
    | SynonymTyCon {
          tyConUnique  :: Unique,  -- ^ A Unique of this TyCon. Invariant:
                                   -- identical to Unique of Name stored in
                                   -- tyConName field.
  
          tyConName    :: Name,    -- ^ Name of the constructor
  
          tyConKind    :: Kind,    -- ^ Kind of this TyCon (full kind, not just
                                   -- the return kind)
  
          tyConArity   :: Arity,   -- ^ Number of arguments this TyCon must
                                   -- receive to be considered saturated
                                   -- (including implicit kind variables)
  
          tyConTyVars  :: [TyVar], -- ^ List of type and kind variables in this
                                   -- TyCon. Includes implicit kind variables.
                                   -- Invariant: length tyConTyVars = tyConArity
  
          tcRoles      :: [Role],  -- ^ The role for each type variable
                                   -- This list has the same length as tyConTyVars
                                   -- See also Note [TyCon Role signatures]
  
          synTcRhs     :: Type     -- ^ Contains information about the expansion
                                   -- of the synonym
      }
  
    -- | Represents type families
    | FamilyTyCon {
          tyConUnique  :: Unique,  -- ^ A Unique of this TyCon. Invariant:
                                   -- identical to Unique of Name stored in
                                   -- tyConName field.
  
          tyConName    :: Name,    -- ^ Name of the constructor
  
          tyConKind    :: Kind,    -- ^ Kind of this TyCon (full kind, not just
                                   -- the return kind)
  
          tyConArity   :: Arity,   -- ^ Number of arguments this TyCon must
                                   -- receive to be considered saturated
                                   -- (including implicit kind variables)
  
          tyConTyVars  :: [TyVar], -- ^ The kind and type variables used in the
                                   -- type constructor.
                                   -- Invariant: length tyvars = arity
                                   -- Precisely, this list scopes over:
                                   --
                                   -- 1. The 'algTcStupidTheta'
                                   -- 2. The cached types in 'algTyConRhs.NewTyCon'
                                   -- 3. The family instance types if present
                                   --
                                   -- Note that it does /not/ scope over the data
                                   -- constructors.
  
          famTcFlav    :: FamTyConFlav, -- ^ Type family flavour: open, closed,
                                        -- abstract, built-in. See comments for
                                        -- FamTyConFlav
  
          famTcParent  :: TyConParent   -- ^ TyCon of enclosing class for
                                        -- associated type families
  
      }
  
    -- | Primitive types; cannot be defined in Haskell. This includes
    -- the usual suspects (such as @Int#@) as well as foreign-imported
    -- types and kinds
    | PrimTyCon {
          tyConUnique   :: Unique, -- ^ A Unique of this TyCon. Invariant:
                                   -- identical to Unique of Name stored in
                                   -- tyConName field.
  
          tyConName     :: Name,   -- ^ Name of the constructor
  
          tyConKind     :: Kind,   -- ^ Kind of this TyCon (full kind, not just
                                   -- the return kind)
  
          tyConArity    :: Arity,  -- ^ Number of arguments this TyCon must
                                   -- receive to be considered saturated
                                   -- (including implicit kind variables)
  
          tcRoles       :: [Role], -- ^ The role for each type variable
                                   -- This list has the same length as tyConTyVars
                                   -- See also Note [TyCon Role signatures]
  
          primTyConRep  :: PrimRep,-- ^ Many primitive tycons are unboxed, but
                                   -- some are boxed (represented by
                                   -- pointers). This 'PrimRep' holds that
                                   -- information.  Only relevant if tyConKind = *
  
          isUnLifted   :: Bool     -- ^ Most primitive tycons are unlifted (may
                                   -- not contain bottom) but other are lifted,
                                   -- e.g. @RealWorld@
      }
  
    -- | Represents promoted data constructor.
    | PromotedDataCon {          -- See Note [Promoted data constructors]
          tyConUnique :: Unique, -- ^ Same Unique as the data constructor
          tyConName   :: Name,   -- ^ Same Name as the data constructor
          tyConArity  :: Arity,
          tyConKind   :: Kind,   -- ^ Translated type of the data constructor
          tcRoles     :: [Role], -- ^ Roles: N for kind vars, R for type vars
          dataCon     :: DataCon -- ^ Corresponding data constructor
      }
  
    -- | Represents promoted type constructor.
    | PromotedTyCon {
          tyConUnique :: Unique, -- ^ Same Unique as the type constructor
          tyConName   :: Name,   -- ^ Same Name as the type constructor
          tyConArity  :: Arity,  -- ^ n if ty_con :: * -> ... -> *  n times
          tyConKind   :: Kind,   -- ^ Always TysPrim.superKind
          ty_con      :: TyCon   -- ^ Corresponding type constructor
      }
  
    deriving Typeable
  
  -- | Names of the fields in an algebraic record type
  type FieldLabel = Name
  
  -- | Represents right-hand-sides of 'TyCon's for algebraic types
  data AlgTyConRhs
  
      -- | Says that we know nothing about this data type, except that
      -- it's represented by a pointer.  Used when we export a data type
      -- abstractly into an .hi file.
    = AbstractTyCon
        Bool      -- True  <=> It's definitely a distinct data type,
                  --           equal only to itself; ie not a newtype
                  -- False <=> Not sure
                  -- See Note [AbstractTyCon and type equality]
  
      -- | Represents an open type family without a fixed right hand
      -- side.  Additional instances can appear at any time.
      --
      -- These are introduced by either a top level declaration:
      --
      -- > data T a :: *
      --
      -- Or an associated data type declaration, within a class declaration:
      --
      -- > class C a b where
      -- >   data T b :: *
    | DataFamilyTyCon
  
      -- | Information about those 'TyCon's derived from a @data@
      -- declaration. This includes data types with no constructors at
      -- all.
    | DataTyCon {
          data_cons :: [DataCon],
                            -- ^ The data type constructors; can be empty if the
                            --   user declares the type to have no constructors
                            --
                            -- INVARIANT: Kept in order of increasing 'DataCon'
                            -- tag (see the tag assignment in DataCon.mkDataCon)
  
          is_enum :: Bool   -- ^ Cached value: is this an enumeration type?
                            --   See Note [Enumeration types]
      }
  
    -- | Information about those 'TyCon's derived from a @newtype@ declaration
    | NewTyCon {
          data_con :: DataCon,    -- ^ The unique constructor for the @newtype@.
                                  --   It has no existentials
  
          nt_rhs :: Type,         -- ^ Cached value: the argument type of the
                                  -- constructor, which is just the representation
                                  -- type of the 'TyCon' (remember that @newtype@s
                                  -- do not exist at runtime so need a different
                                  -- representation type).
                                  --
                                  -- The free 'TyVar's of this type are the
                                  -- 'tyConTyVars' from the corresponding 'TyCon'
  
          nt_etad_rhs :: ([TyVar], Type),
                          -- ^ Same as the 'nt_rhs', but this time eta-reduced.
                          -- Hence the list of 'TyVar's in this field may be
                          -- shorter than the declared arity of the 'TyCon'.
  
                          -- See Note [Newtype eta]
          nt_co :: CoAxiom Unbranched
                               -- The axiom coercion that creates the @newtype@
                               -- from the representation 'Type'.
  
                               -- See Note [Newtype coercions]
                               -- Invariant: arity = #tvs in nt_etad_rhs;
                               -- See Note [Newtype eta]
                               -- Watch out!  If any newtypes become transparent
                               -- again check Trac #1072.
      }
  
  {-
  Note [AbstractTyCon and type equality]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  TODO
  -}
  
  -- | Extract those 'DataCon's that we are able to learn about.  Note
  -- that visibility in this sense does not correspond to visibility in
  -- the context of any particular user program!
  visibleDataCons :: AlgTyConRhs -> [DataCon]
  visibleDataCons (AbstractTyCon {})            = []
  visibleDataCons DataFamilyTyCon {}            = []
  visibleDataCons (DataTyCon{ data_cons = cs }) = cs
  visibleDataCons (NewTyCon{ data_con = c })    = [c]
  
  -- ^ Both type classes as well as family instances imply implicit
  -- type constructors.  These implicit type constructors refer to their parent
  -- structure (ie, the class or family from which they derive) using a type of
  -- the following form.  We use 'TyConParent' for both algebraic and synonym
  -- types, but the variant 'ClassTyCon' will only be used by algebraic 'TyCon's.
  data TyConParent
    = -- | An ordinary type constructor has no parent.
      NoParentTyCon
  
    -- | Type constructors representing a class dictionary.
    -- See Note [ATyCon for classes] in TypeRep
    | ClassTyCon
          Class           -- INVARIANT: the classTyCon of this Class is the
                          -- current tycon
  
    -- | An *associated* type of a class.
    | AssocFamilyTyCon
          Class           -- The class in whose declaration the family is declared
                          -- See Note [Associated families and their parent class]
  
    -- | Type constructors representing an instance of a *data* family.
    -- Parameters:
    --
    --  1) The type family in question
    --
    --  2) Instance types; free variables are the 'tyConTyVars'
    --  of the current 'TyCon' (not the family one). INVARIANT:
    --  the number of types matches the arity of the family 'TyCon'
    --
    --  3) A 'CoTyCon' identifying the representation
    --  type with the type instance family
    | FamInstTyCon          -- See Note [Data type families]
          (CoAxiom Unbranched)  -- The coercion axiom.
                 -- Generally of kind   T ty1 ty2 ~ R:T a b c
                 -- where T is the family TyCon,
                 -- and R:T is the representation TyCon (ie this one)
                 -- and a,b,c are the tyConTyVars of this TyCon
                 --
                 -- BUT may be eta-reduced; see TcInstDcls
                 --     Note [Eta reduction for data family axioms]
  
            -- Cached fields of the CoAxiom, but adjusted to
            -- use the tyConTyVars of this TyCon
          TyCon   -- The family TyCon
          [Type]  -- Argument types (mentions the tyConTyVars of this TyCon)
                  -- Match in length the tyConTyVars of the family TyCon
  
          -- E.g.  data intance T [a] = ...
          -- gives a representation tycon:
          --      data R:TList a = ...
          --      axiom co a :: T [a] ~ R:TList a
          -- with R:TList's algTcParent = FamInstTyCon T [a] co
  
  instance Outputable TyConParent where
      ppr NoParentTyCon           = text "No parent"
      ppr (ClassTyCon cls)        = text "Class parent" <+> ppr cls
      ppr (AssocFamilyTyCon cls)  =
          text "Class parent (assoc. family)" <+> ppr cls
      ppr (FamInstTyCon _ tc tys) =
          text "Family parent (family instance)" <+> ppr tc <+> sep (map ppr tys)
  
  -- | Checks the invariants of a 'TyConParent' given the appropriate type class
  -- name, if any
  okParent :: Name -> TyConParent -> Bool
  okParent _       NoParentTyCon               = True
  okParent tc_name (AssocFamilyTyCon cls)      = tc_name `elem` map tyConName (classATs cls)
  okParent tc_name (ClassTyCon cls)            = tc_name == tyConName (classTyCon cls)
  okParent _       (FamInstTyCon _ fam_tc tys) = tyConArity fam_tc == length tys
  
  isNoParent :: TyConParent -> Bool
  isNoParent NoParentTyCon = True
  isNoParent _             = False
  
  --------------------
  
  -- | Information pertaining to the expansion of a type synonym (@type@)
  data FamTyConFlav
    = -- | An open type synonym family  e.g. @type family F x y :: * -> *@
       OpenSynFamilyTyCon
  
     -- | A closed type synonym family  e.g.
     -- @type family F x where { F Int = Bool }@
     | ClosedSynFamilyTyCon
         (CoAxiom Branched) -- The one axiom for this family
  
     -- | A closed type synonym family declared in an hs-boot file with
     -- type family F a where ..
     | AbstractClosedSynFamilyTyCon
  
     -- | Built-in type family used by the TypeNats solver
     | BuiltInSynFamTyCon BuiltInSynFamily
  
  {-
  Note [Closed type families]
  ~~~~~~~~~~~~~~~~~~~~~~~~~
  * In an open type family you can add new instances later.  This is the
    usual case.
  
  * In a closed type family you can only put equations where the family
    is defined.
  
  
  Note [Promoted data constructors]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  A data constructor can be promoted to become a type constructor,
  via the PromotedTyCon alternative in TyCon.
  
  * Only data constructors with
       (a) no kind polymorphism
       (b) no constraints in its type (eg GADTs)
    are promoted.  Existentials are ok; see Trac #7347.
  
  * The TyCon promoted from a DataCon has the *same* Name and Unique as
    the DataCon.  Eg. If the data constructor Data.Maybe.Just(unique 78,
    say) is promoted to a TyCon whose name is Data.Maybe.Just(unique 78)
  
  * The *kind* of a promoted DataCon may be polymorphic.  Example:
      type of DataCon           Just :: forall (a:*). a -> Maybe a
      kind of (promoted) tycon  Just :: forall (a:box). a -> Maybe a
    The kind is not identical to the type, because of the */box
    kind signature on the forall'd variable; so the tyConKind field of
    PromotedTyCon is not identical to the dataConUserType of the
    DataCon.  But it's the same modulo changing the variable kinds,
    done by DataCon.promoteType.
  
  * Small note: We promote the *user* type of the DataCon.  Eg
       data T = MkT {-# UNPACK #-} !(Bool, Bool)
    The promoted kind is
       MkT :: (Bool,Bool) -> T
    *not*
       MkT :: Bool -> Bool -> T
  
  Note [Enumeration types]
  ~~~~~~~~~~~~~~~~~~~~~~~~
  We define datatypes with no constructors to *not* be
  enumerations; this fixes trac #2578,  Otherwise we
  end up generating an empty table for
    <mod>_<type>_closure_tbl
  which is used by tagToEnum# to map Int# to constructors
  in an enumeration. The empty table apparently upset
  the linker.
  
  Moreover, all the data constructor must be enumerations, meaning
  they have type  (forall abc. T a b c).  GADTs are not enumerations.
  For example consider
      data T a where
        T1 :: T Int
        T2 :: T Bool
        T3 :: T a
  What would [T1 ..] be?  [T1,T3] :: T Int? Easiest thing is to exclude them.
  See Trac #4528.
  
  Note [Newtype coercions]
  ~~~~~~~~~~~~~~~~~~~~~~~~
  The NewTyCon field nt_co is a CoAxiom which is used for coercing from
  the representation type of the newtype, to the newtype itself. For
  example,
  
     newtype T a = MkT (a -> a)
  
  the NewTyCon for T will contain nt_co = CoT where CoT t : T t ~ t -> t.
  
  In the case that the right hand side is a type application
  ending with the same type variables as the left hand side, we
  "eta-contract" the coercion.  So if we had
  
     newtype S a = MkT [a]
  
  then we would generate the arity 0 axiom CoS : S ~ [].  The
  primary reason we do this is to make newtype deriving cleaner.
  
  In the paper we'd write
          axiom CoT : (forall t. T t) ~ (forall t. [t])
  and then when we used CoT at a particular type, s, we'd say
          CoT @ s
  which encodes as (TyConApp instCoercionTyCon [TyConApp CoT [], s])
  
  Note [Newtype eta]
  ~~~~~~~~~~~~~~~~~~
  Consider
          newtype Parser a = MkParser (IO a) deriving Monad
  Are these two types equal (to Core)?
          Monad Parser
          Monad IO
  which we need to make the derived instance for Monad Parser.
  
  Well, yes.  But to see that easily we eta-reduce the RHS type of
  Parser, in this case to ([], Froogle), so that even unsaturated applications
  of Parser will work right.  This eta reduction is done when the type
  constructor is built, and cached in NewTyCon.  The cached field is
  only used in coreExpandTyCon_maybe.
  
  Here's an example that I think showed up in practice
  Source code:
          newtype T a = MkT [a]
          newtype Foo m = MkFoo (forall a. m a -> Int)
  
          w1 :: Foo []
          w1 = ...
  
          w2 :: Foo T
          w2 = MkFoo (\(MkT x) -> case w1 of MkFoo f -> f x)
  
  After desugaring, and discarding the data constructors for the newtypes,
  we get:
          w2 :: Foo T
          w2 = w1
  And now Lint complains unless Foo T == Foo [], and that requires T==[]
  
  This point carries over to the newtype coercion, because we need to
  say
          w2 = w1 `cast` Foo CoT
  
  so the coercion tycon CoT must have
          kind:    T ~ []
   and    arity:   0
  
  ************************************************************************
  *                                                                      *
  \subsection{PrimRep}
  *                                                                      *
  ************************************************************************
  
  Note [rep swamp]
  
  GHC has a rich selection of types that represent "primitive types" of
  one kind or another.  Each of them makes a different set of
  distinctions, and mostly the differences are for good reasons,
  although it's probably true that we could merge some of these.
  
  Roughly in order of "includes more information":
  
   - A Width (cmm/CmmType) is simply a binary value with the specified
     number of bits.  It may represent a signed or unsigned integer, a
     floating-point value, or an address.
  
      data Width = W8 | W16 | W32 | W64 | W80 | W128
  
   - Size, which is used in the native code generator, is Width +
     floating point information.
  
     data Size = II8 | II16 | II32 | II64 | FF32 | FF64 | FF80
  
     it is necessary because e.g. the instruction to move a 64-bit float
     on x86 (movsd) is different from the instruction to move a 64-bit
     integer (movq), so the mov instruction is parameterised by Size.
  
   - CmmType wraps Width with more information: GC ptr, float, or
     other value.
  
      data CmmType = CmmType CmmCat Width
  
      data CmmCat     -- "Category" (not exported)
         = GcPtrCat   -- GC pointer
         | BitsCat    -- Non-pointer
         | FloatCat   -- Float
  
     It is important to have GcPtr information in Cmm, since we generate
     info tables containing pointerhood for the GC from this.  As for
     why we have float (and not signed/unsigned) here, see Note [Signed
     vs unsigned].
  
   - ArgRep makes only the distinctions necessary for the call and
     return conventions of the STG machine.  It is essentially CmmType
     + void.
  
   - PrimRep makes a few more distinctions than ArgRep: it divides
     non-GC-pointers into signed/unsigned and addresses, information
     that is necessary for passing these values to foreign functions.
  
  There's another tension here: whether the type encodes its size in
  bytes, or whether its size depends on the machine word size.  Width
  and CmmType have the size built-in, whereas ArgRep and PrimRep do not.
  
  This means to turn an ArgRep/PrimRep into a CmmType requires DynFlags.
  
  On the other hand, CmmType includes some "nonsense" values, such as
  CmmType GcPtrCat W32 on a 64-bit machine.
  -}
  
  -- | A 'PrimRep' is an abstraction of a type.  It contains information that
  -- the code generator needs in order to pass arguments, return results,
  -- and store values of this type.
  data PrimRep
    = VoidRep
    | PtrRep
    | IntRep        -- ^ Signed, word-sized value
    | WordRep       -- ^ Unsigned, word-sized value
    | Int64Rep      -- ^ Signed, 64 bit value (with 32-bit words only)
    | Word64Rep     -- ^ Unsigned, 64 bit value (with 32-bit words only)
    | AddrRep       -- ^ A pointer, but /not/ to a Haskell value (use 'PtrRep')
    | FloatRep
    | DoubleRep
    -- ETA-specific
    | BoolRep
    | CharRep
    | ByteRep
    | ShortRep
    | ObjectRep Text
    | ArrayRep  PrimRep
  --  | VecRep Int PrimElemRep  -- ^ A vector
    deriving( Eq, Show )
  
  -- data PrimElemRep
  --   = Int8ElemRep
  --   | Int16ElemRep
  --   | Int32ElemRep
  --   | Int64ElemRep
  --   | Word8ElemRep
  --   | Word16ElemRep
  --   | Word32ElemRep
  --   | Word64ElemRep
  --   | FloatElemRep
  --   | DoubleElemRep
  --    deriving( Eq, Show )
  
  instance Outputable PrimRep where
    ppr r = text (show r)
  
  -- instance Outputable PrimElemRep where
  --   ppr r = text (show r)
  
  isVoidRep :: PrimRep -> Bool
  isVoidRep VoidRep = True
  isVoidRep _       = False
  
  isGcPtrRep :: PrimRep -> Bool
  isGcPtrRep PtrRep = True
  isGcPtrRep _      = False
  
  isObjectRep :: PrimRep -> Bool
  isObjectRep (ObjectRep _) = True
  isObjectRep (ArrayRep  _) = True
  isObjectRep _             = False
  
  getObjectClass :: PrimRep -> Text
  getObjectClass (ObjectRep t)   = t
  getObjectClass (ArrayRep  _) = error $ "getObjectClass: Array type"
    -- arrayWrap . objectWrap $ getObjectClass rep
  getObjectClass rep             = error $ "getObjectClass: " ++ show rep
  
  -- | Find the size of a 'PrimRep', in words
  primRepSizeW :: DynFlags -> PrimRep -> Int
  primRepSizeW _      IntRep        = 1
  primRepSizeW _      WordRep       = 1
  primRepSizeW _      Int64Rep      = 2
  primRepSizeW _      Word64Rep     = 2
  primRepSizeW _      FloatRep      = 1    -- NB. might not take a full word
  primRepSizeW _      DoubleRep     = 2
  primRepSizeW _      AddrRep       = 1
  primRepSizeW _      PtrRep        = 1
  primRepSizeW _      VoidRep       = 0
  primRepSizeW _      BoolRep       = 1
  primRepSizeW _      CharRep       = 1
  primRepSizeW _      ByteRep       = 1
  primRepSizeW _      ShortRep      = 1
  primRepSizeW _      (ObjectRep _) = 1
  primRepSizeW _ _ = error $ "primRepSizeW: bad primRepSizeW"
  -- primRepSizeW dflags (VecRep len rep) = len * primElemRepSizeB rep `quot` 4
  
  -- primElemRepSizeB :: PrimElemRep -> Int
  -- primElemRepSizeB Int8ElemRep   = 1
  -- primElemRepSizeB Int16ElemRep  = 2
  -- primElemRepSizeB Int32ElemRep  = 4
  -- primElemRepSizeB Int64ElemRep  = 8
  -- primElemRepSizeB Word8ElemRep  = 1
  -- primElemRepSizeB Word16ElemRep = 2
  -- primElemRepSizeB Word32ElemRep = 4
  -- primElemRepSizeB Word64ElemRep = 8
  -- primElemRepSizeB FloatElemRep  = 4
  -- primElemRepSizeB DoubleElemRep = 8
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection{TyCon Construction}
  *                                                                      *
  ************************************************************************
  
  Note: the TyCon constructors all take a Kind as one argument, even though
  they could, in principle, work out their Kind from their other arguments.
  But to do so they need functions from Types, and that makes a nasty
  module mutual-recursion.  And they aren't called from many places.
  So we compromise, and move their Kind calculation to the call site.
  -}
  
  -- | Given the name of the function type constructor and it's kind, create the
  -- corresponding 'TyCon'. It is reccomended to use 'TypeRep.funTyCon' if you want
  -- this functionality
  mkFunTyCon :: Name -> Kind -> TyCon
  mkFunTyCon name kind
    = FunTyCon {
          tyConUnique = nameUnique name,
          tyConName   = name,
          tyConKind   = kind,
          tyConArity  = 2
      }
  
  -- | This is the making of an algebraic 'TyCon'. Notably, you have to
  -- pass in the generic (in the -XGenerics sense) information about the
  -- type constructor - you can get hold of it easily (see Generics
  -- module)
  mkAlgTyCon :: Name
             -> Kind              -- ^ Kind of the resulting 'TyCon'
             -> [TyVar]           -- ^ 'TyVar's scoped over: see 'tyConTyVars'.
                                  --   Arity is inferred from the length of this
                                  --   list
             -> [Role]            -- ^ The roles for each TyVar
             -> Maybe CType       -- ^ The C type this type corresponds to
                                  --   when using the CAPI FFI
             -> [PredType]        -- ^ Stupid theta: see 'algTcStupidTheta'
             -> AlgTyConRhs       -- ^ Information about dat aconstructors
             -> TyConParent
             -> RecFlag           -- ^ Is the 'TyCon' recursive?
             -> Bool              -- ^ Was the 'TyCon' declared with GADT syntax?
             -> Maybe TyCon       -- ^ Promoted version
             -> TyCon
  mkAlgTyCon name kind tyvars roles cType stupid rhs parent is_rec gadt_syn prom_tc
    = AlgTyCon {
          tyConName        = name,
          tyConUnique      = nameUnique name,
          tyConKind        = kind,
          tyConArity       = length tyvars,
          tyConTyVars      = tyvars,
          tcRoles          = roles,
          tyConCType       = cType,
          algTcStupidTheta = stupid,
          algTcRhs         = rhs,
          algTcParent      = ASSERT2( okParent name parent, ppr name $$ ppr parent ) parent,
          algTcRec         = is_rec,
          algTcGadtSyntax  = gadt_syn,
          tcPromoted       = prom_tc
      }
  
  -- | Simpler specialization of 'mkAlgTyCon' for classes
  mkClassTyCon :: Name -> Kind -> [TyVar] -> [Role] -> AlgTyConRhs -> Class
               -> RecFlag -> TyCon
  mkClassTyCon name kind tyvars roles rhs clas is_rec
    = mkAlgTyCon name kind tyvars roles Nothing [] rhs (ClassTyCon clas)
                 is_rec False
                 Nothing    -- Class TyCons are not pormoted
  
  mkTupleTyCon :: Name
               -> Kind    -- ^ Kind of the resulting 'TyCon'
               -> Arity   -- ^ Arity of the tuple
               -> [TyVar] -- ^ 'TyVar's scoped over: see 'tyConTyVars'
               -> DataCon
               -> TupleSort    -- ^ Whether the tuple is boxed or unboxed
               -> Maybe TyCon  -- ^ Promoted version
               -> TyCon
  mkTupleTyCon name kind arity tyvars con sort prom_tc
    = TupleTyCon {
          tyConUnique = nameUnique name,
          tyConName = name,
          tyConKind = kind,
          tyConArity = arity,
          tyConTupleSort = sort,
          tyConTyVars = tyvars,
          dataCon = con,
          tcPromoted = prom_tc
      }
  
  -- | Create an unlifted primitive 'TyCon', such as @Int#@
  mkPrimTyCon :: Name  -> Kind -> [Role] -> PrimRep -> TyCon
  mkPrimTyCon name kind roles rep
    = mkPrimTyCon' name kind roles rep True
  
  -- | Kind constructors
  mkKindTyCon :: Name -> Kind -> TyCon
  mkKindTyCon name kind
    = mkPrimTyCon' name kind [] VoidRep True
  
  -- | Create a lifted primitive 'TyCon' such as @RealWorld@
  mkLiftedPrimTyCon :: Name  -> Kind -> [Role] -> PrimRep -> TyCon
  mkLiftedPrimTyCon name kind roles rep
    = mkPrimTyCon' name kind roles rep False
  
  mkPrimTyCon' :: Name  -> Kind -> [Role] -> PrimRep -> Bool -> TyCon
  mkPrimTyCon' name kind roles rep is_unlifted
    = PrimTyCon {
          tyConName    = name,
          tyConUnique  = nameUnique name,
          tyConKind    = kind,
          tyConArity   = length roles,
          tcRoles      = roles,
          primTyConRep = rep,
          isUnLifted   = is_unlifted
      }
  
  -- | Create a type synonym 'TyCon'
  mkSynonymTyCon :: Name -> Kind -> [TyVar] -> [Role] -> Type -> TyCon
  mkSynonymTyCon name kind tyvars roles rhs
    = SynonymTyCon {
          tyConName   = name,
          tyConUnique = nameUnique name,
          tyConKind   = kind,
          tyConArity  = length tyvars,
          tyConTyVars = tyvars,
          tcRoles     = roles,
          synTcRhs    = rhs
      }
  
  -- | Create a type family 'TyCon'
  mkFamilyTyCon:: Name -> Kind -> [TyVar] -> FamTyConFlav -> TyConParent
               -> TyCon
  mkFamilyTyCon name kind tyvars flav parent
    = FamilyTyCon
        { tyConUnique = nameUnique name
        , tyConName   = name
        , tyConKind   = kind
        , tyConArity  = length tyvars
        , tyConTyVars = tyvars
        , famTcFlav   = flav
        , famTcParent = parent
        }
  
  
  -- | Create a promoted data constructor 'TyCon'
  -- Somewhat dodgily, we give it the same Name
  -- as the data constructor itself; when we pretty-print
  -- the TyCon we add a quote; see the Outputable TyCon instance
  mkPromotedDataCon :: DataCon -> Name -> Unique -> Kind -> [Role] -> TyCon
  mkPromotedDataCon con name unique kind roles
    = PromotedDataCon {
          tyConName   = name,
          tyConUnique = unique,
          tyConArity  = arity,
          tcRoles     = roles,
          tyConKind   = kind,
          dataCon     = con
    }
    where
      arity = length roles
  
  -- | Create a promoted type constructor 'TyCon'
  -- Somewhat dodgily, we give it the same Name
  -- as the type constructor itself
  mkPromotedTyCon :: TyCon -> Kind -> TyCon
  mkPromotedTyCon tc kind
    = PromotedTyCon {
          tyConName   = getName tc,
          tyConUnique = getUnique tc,
          tyConArity  = tyConArity tc,
          tyConKind   = kind,
          ty_con      = tc
    }
  
  isFunTyCon :: TyCon -> Bool
  isFunTyCon (FunTyCon {}) = True
  isFunTyCon _             = False
  
  -- | Test if the 'TyCon' is algebraic but abstract (invisible data constructors)
  isAbstractTyCon :: TyCon -> Bool
  isAbstractTyCon (AlgTyCon { algTcRhs = AbstractTyCon {} }) = True
  isAbstractTyCon _ = False
  
  -- | Make an algebraic 'TyCon' abstract. Panics if the supplied 'TyCon' is not
  -- algebraic
  makeTyConAbstract :: TyCon -> TyCon
  makeTyConAbstract tc@(AlgTyCon { algTcRhs = rhs })
    = tc { algTcRhs = AbstractTyCon (isDistinctAlgRhs rhs) }
  makeTyConAbstract tc = pprPanic "makeTyConAbstract" (ppr tc)
  
  -- | Does this 'TyCon' represent something that cannot be defined in Haskell?
  isPrimTyCon :: TyCon -> Bool
  isPrimTyCon (PrimTyCon {}) = True
  isPrimTyCon _              = False
  
  -- | Is this 'TyCon' unlifted (i.e. cannot contain bottom)? Note that this can
  -- only be true for primitive and unboxed-tuple 'TyCon's
  isUnLiftedTyCon :: TyCon -> Bool
  isUnLiftedTyCon (PrimTyCon  {isUnLifted = is_unlifted}) = is_unlifted
  isUnLiftedTyCon (TupleTyCon {tyConTupleSort = sort})
      = not (isBoxed (tupleSortBoxity sort))
  isUnLiftedTyCon _                                       = False
  
  -- | Returns @True@ if the supplied 'TyCon' resulted from either a
  -- @data@ or @newtype@ declaration
  isAlgTyCon :: TyCon -> Bool
  isAlgTyCon (AlgTyCon {})   = True
  isAlgTyCon (TupleTyCon {}) = True
  isAlgTyCon _               = False
  
  isDataTyCon :: TyCon -> Bool
  -- ^ Returns @True@ for data types that are /definitely/ represented by
  -- heap-allocated constructors.  These are scrutinised by Core-level
  -- @case@ expressions, and they get info tables allocated for them.
  --
  -- Generally, the function will be true for all @data@ types and false
  -- for @newtype@s, unboxed tuples and type family 'TyCon's. But it is
  -- not guaranteed to return @True@ in all cases that it could.
  --
  -- NB: for a data type family, only the /instance/ 'TyCon's
  --     get an info table.  The family declaration 'TyCon' does not
  isDataTyCon (AlgTyCon {algTcRhs = rhs})
    = case rhs of
          DataTyCon {}       -> True
          NewTyCon {}        -> False
          DataFamilyTyCon {} -> False
          AbstractTyCon {}   -> False      -- We don't know, so return False
  isDataTyCon (TupleTyCon {tyConTupleSort = sort}) = isBoxed (tupleSortBoxity sort)
  isDataTyCon _ = False
  
  -- | 'isInjectiveTyCon' is true of 'TyCon's for which this property holds
  -- (where X is the role passed in):
  --   If (T a1 b1 c1) ~X (T a2 b2 c2), then (a1 ~X1 a2), (b1 ~X2 b2), and (c1 ~X3 c2)
  -- (where X1, X2, and X3, are the roles given by tyConRolesX tc X)
  -- See also Note [Decomposing equalities] in TcCanonical
  isInjectiveTyCon :: TyCon -> Role -> Bool
  isInjectiveTyCon _                             Phantom          = False
  isInjectiveTyCon (FunTyCon {})                 _                = True
  isInjectiveTyCon (AlgTyCon {})                 Nominal          = True
  isInjectiveTyCon (AlgTyCon {algTcRhs = rhs})   Representational
    = isGenInjAlgRhs rhs
  isInjectiveTyCon (TupleTyCon {})               _                = True
  isInjectiveTyCon (SynonymTyCon {})             _                = False
  isInjectiveTyCon (FamilyTyCon {})              _                = False
  isInjectiveTyCon (PrimTyCon {})                _                = True
  isInjectiveTyCon (PromotedDataCon {})          _                = True
  isInjectiveTyCon (PromotedTyCon {ty_con = tc}) r
    = isInjectiveTyCon tc r
  
  -- | 'isGenerativeTyCon' is true of 'TyCon's for which this property holds
  -- (where X is the role passed in):
  --   If (T tys ~X t), then (t's head ~X T).
  -- See also Note [Decomposing equalities] in TcCanonical
  isGenerativeTyCon :: TyCon -> Role -> Bool
  isGenerativeTyCon = isInjectiveTyCon
    -- as it happens, generativity and injectivity coincide, but there's
    -- no a priori reason this must be the case
  
  -- | Is this an 'AlgTyConRhs' of a 'TyCon' that is generative and injective
  -- with respect to representational equality?
  isGenInjAlgRhs :: AlgTyConRhs -> Bool
  isGenInjAlgRhs (DataTyCon {})           = True
  isGenInjAlgRhs (DataFamilyTyCon {})     = False
  isGenInjAlgRhs (AbstractTyCon distinct) = distinct
  isGenInjAlgRhs (NewTyCon {})            = False
  
  -- | 'isDistinctTyCon' is true of 'TyCon's that are equal only to
  -- themselves, even via coercions (except for unsafeCoerce).
  -- This excludes newtypes, type functions, type synonyms.
  -- It relates directly to the FC consistency story:
  --     If the axioms are consistent,
  --     and  co : S tys ~ T tys, and S,T are "distinct" TyCons,
  --     then S=T.
  -- Cf Note [Pruning dead case alternatives] in Unify
  isDistinctTyCon :: TyCon -> Bool
  isDistinctTyCon (AlgTyCon {algTcRhs = rhs}) = isDistinctAlgRhs rhs
  isDistinctTyCon (FunTyCon {})               = True
  isDistinctTyCon (TupleTyCon {})             = True
  isDistinctTyCon (PrimTyCon {})              = True
  isDistinctTyCon (PromotedDataCon {})        = True
  isDistinctTyCon _                           = False
  
  isDistinctAlgRhs :: AlgTyConRhs -> Bool
  isDistinctAlgRhs (DataTyCon {})           = True
  isDistinctAlgRhs (DataFamilyTyCon {})     = False
  isDistinctAlgRhs (AbstractTyCon distinct) = distinct
  isDistinctAlgRhs (NewTyCon {})            = False
  
  -- | Is this 'TyCon' that for a @newtype@
  isNewTyCon :: TyCon -> Bool
  isNewTyCon (AlgTyCon {algTcRhs = NewTyCon {}}) = True
  isNewTyCon _                                   = False
  
  -- | Take a 'TyCon' apart into the 'TyVar's it scopes over, the 'Type' it expands
  -- into, and (possibly) a coercion from the representation type to the @newtype@.
  -- Returns @Nothing@ if this is not possible.
  unwrapNewTyCon_maybe :: TyCon -> Maybe ([TyVar], Type, CoAxiom Unbranched)
  unwrapNewTyCon_maybe (AlgTyCon { tyConTyVars = tvs,
                                   algTcRhs = NewTyCon { nt_co = co,
                                                         nt_rhs = rhs }})
                             = Just (tvs, rhs, co)
  unwrapNewTyCon_maybe _     = Nothing
  
  unwrapNewTyConEtad_maybe :: TyCon -> Maybe ([TyVar], Type, CoAxiom Unbranched)
  unwrapNewTyConEtad_maybe (AlgTyCon { algTcRhs = NewTyCon { nt_co = co,
                                                             nt_etad_rhs = (tvs,rhs) }})
                             = Just (tvs, rhs, co)
  unwrapNewTyConEtad_maybe _ = Nothing
  
  isProductTyCon :: TyCon -> Bool
  -- True of datatypes or newtypes that have
  --   one, vanilla, data constructor
  isProductTyCon tc@(AlgTyCon {}) = case algTcRhs tc of
                                      DataTyCon{ data_cons = [data_con] }
                                                  -> isVanillaDataCon data_con
                                      NewTyCon {} -> True
                                      _           -> False
  isProductTyCon (TupleTyCon {})  = True
  isProductTyCon _                = False
  
  
  isDataProductTyCon_maybe :: TyCon -> Maybe DataCon
  -- True of datatypes (not newtypes) with
  --   one, vanilla, data constructor
  isDataProductTyCon_maybe (AlgTyCon { algTcRhs = DataTyCon { data_cons = cons } })
    | [con] <- cons         -- Singleton
    , isVanillaDataCon con  -- Vanilla
    = Just con
  isDataProductTyCon_maybe (TupleTyCon { dataCon = con })
    = Just con
  isDataProductTyCon_maybe _ = Nothing
  
  -- | Is this a 'TyCon' representing a regular H98 type synonym (@type@)?
  isTypeSynonymTyCon :: TyCon -> Bool
  isTypeSynonymTyCon (SynonymTyCon {}) = True
  isTypeSynonymTyCon _                 = False
  
  
  -- As for newtypes, it is in some contexts important to distinguish between
  -- closed synonyms and synonym families, as synonym families have no unique
  -- right hand side to which a synonym family application can expand.
  --
  
  isDecomposableTyCon :: TyCon -> Bool
  -- True iff we can decompose (T a b c) into ((T a b) c)
  --   I.e. is it injective?
  -- Specifically NOT true of synonyms (open and otherwise)
  -- Ultimately we may have injective associated types
  -- in which case this test will become more interesting
  --
  -- It'd be unusual to call isDecomposableTyCon on a regular H98
  -- type synonym, because you should probably have expanded it first
  -- But regardless, it's not decomposable
  isDecomposableTyCon (SynonymTyCon {}) = False
  isDecomposableTyCon (FamilyTyCon  {}) = False
  isDecomposableTyCon _other            = True
  
  -- | Is this an algebraic 'TyCon' declared with the GADT syntax?
  isGadtSyntaxTyCon :: TyCon -> Bool
  isGadtSyntaxTyCon (AlgTyCon { algTcGadtSyntax = res }) = res
  isGadtSyntaxTyCon _                                    = False
  
  -- | Is this an algebraic 'TyCon' which is just an enumeration of values?
  isEnumerationTyCon :: TyCon -> Bool
  -- See Note [Enumeration types] in TyCon
  isEnumerationTyCon (AlgTyCon {algTcRhs = DataTyCon { is_enum = res }}) = res
  isEnumerationTyCon (TupleTyCon {tyConArity = arity}) = arity == 0
  isEnumerationTyCon _                                                   = False
  
  -- | Is this a 'TyCon', synonym or otherwise, that defines a family?
  isFamilyTyCon :: TyCon -> Bool
  isFamilyTyCon (FamilyTyCon {})                           = True
  isFamilyTyCon (AlgTyCon {algTcRhs = DataFamilyTyCon {}}) = True
  isFamilyTyCon _                                          = False
  
  -- | Is this a 'TyCon', synonym or otherwise, that defines a family with
  -- instances?
  isOpenFamilyTyCon :: TyCon -> Bool
  isOpenFamilyTyCon (FamilyTyCon {famTcFlav = OpenSynFamilyTyCon }) = True
  isOpenFamilyTyCon (AlgTyCon    {algTcRhs  = DataFamilyTyCon    }) = True
  isOpenFamilyTyCon _                                               = False
  
  -- | Is this a synonym 'TyCon' that can have may have further instances appear?
  isTypeFamilyTyCon :: TyCon -> Bool
  isTypeFamilyTyCon (FamilyTyCon {}) = True
  isTypeFamilyTyCon _                = False
  
  isOpenTypeFamilyTyCon :: TyCon -> Bool
  isOpenTypeFamilyTyCon (FamilyTyCon {famTcFlav = OpenSynFamilyTyCon }) = True
  isOpenTypeFamilyTyCon _                                               = False
  
  -- leave out abstract closed families here
  isClosedSynFamilyTyCon_maybe :: TyCon -> Maybe (CoAxiom Branched)
  isClosedSynFamilyTyCon_maybe
    (FamilyTyCon {famTcFlav = ClosedSynFamilyTyCon ax}) = Just ax
  isClosedSynFamilyTyCon_maybe _                        = Nothing
  
  isBuiltInSynFamTyCon_maybe :: TyCon -> Maybe BuiltInSynFamily
  isBuiltInSynFamTyCon_maybe
    (FamilyTyCon {famTcFlav = BuiltInSynFamTyCon ops }) = Just ops
  isBuiltInSynFamTyCon_maybe _                          = Nothing
  
  -- | Is this a synonym 'TyCon' that can have may have further instances appear?
  isDataFamilyTyCon :: TyCon -> Bool
  isDataFamilyTyCon (AlgTyCon {algTcRhs = DataFamilyTyCon {}}) = True
  isDataFamilyTyCon _ = False
  
  -- | Are we able to extract informationa 'TyVar' to class argument list
  -- mappping from a given 'TyCon'?
  isTyConAssoc :: TyCon -> Bool
  isTyConAssoc tc = isJust (tyConAssoc_maybe tc)
  
  tyConAssoc_maybe :: TyCon -> Maybe Class
  tyConAssoc_maybe tc = case tyConParent tc of
                          AssocFamilyTyCon cls -> Just cls
                          _                    -> Nothing
  
  -- The unit tycon didn't used to be classed as a tuple tycon
  -- but I thought that was silly so I've undone it
  -- If it can't be for some reason, it should be a AlgTyCon
  isTupleTyCon :: TyCon -> Bool
  -- ^ Does this 'TyCon' represent a tuple?
  --
  -- NB: when compiling @Data.Tuple@, the tycons won't reply @True@ to
  -- 'isTupleTyCon', because they are built as 'AlgTyCons'.  However they
  -- get spat into the interface file as tuple tycons, so I don't think
  -- it matters.
  isTupleTyCon (TupleTyCon {}) = True
  isTupleTyCon _               = False
  
  -- | Is this the 'TyCon' for an unboxed tuple?
  isUnboxedTupleTyCon :: TyCon -> Bool
  isUnboxedTupleTyCon (TupleTyCon {tyConTupleSort = sort}) =
      not (isBoxed (tupleSortBoxity sort))
  isUnboxedTupleTyCon _ = False
  
  -- | Is this the 'TyCon' for a /promoted/ tuple?
  isPromotedTupleTyCon :: TyCon -> Bool
  isPromotedTupleTyCon tyCon
    | Just dataCon <- isPromotedDataCon_maybe tyCon
    , isTupleTyCon (dataConTyCon dataCon) = True
    | otherwise = False
  
  -- | Is this the 'TyCon' for a boxed tuple?
  isBoxedTupleTyCon :: TyCon -> Bool
  isBoxedTupleTyCon (TupleTyCon {tyConTupleSort = sort}) = isBoxed (tupleSortBoxity sort)
  isBoxedTupleTyCon _  = False
  
  -- | Extract the boxity of the given 'TyCon', if it is a 'TupleTyCon'.
  -- Panics otherwise
  tupleTyConBoxity :: TyCon -> Boxity
  tupleTyConBoxity tc = tupleSortBoxity (tyConTupleSort tc)
  
  -- | Extract the 'TupleSort' of the given 'TyCon', if it is a 'TupleTyCon'.
  -- Panics otherwise
  tupleTyConSort :: TyCon -> TupleSort
  tupleTyConSort tc = tyConTupleSort tc
  
  -- | Extract the arity of the given 'TyCon', if it is a 'TupleTyCon'.
  -- Panics otherwise
  tupleTyConArity :: TyCon -> Arity
  tupleTyConArity tc = tyConArity tc
  
  -- | Is this a recursive 'TyCon'?
  isRecursiveTyCon :: TyCon -> Bool
  isRecursiveTyCon (AlgTyCon {algTcRec = Recursive}) = True
  isRecursiveTyCon _                                 = False
  
  promotableTyCon_maybe :: TyCon -> Maybe TyCon
  promotableTyCon_maybe (AlgTyCon { tcPromoted = prom })   = prom
  promotableTyCon_maybe (TupleTyCon { tcPromoted = prom }) = prom
  promotableTyCon_maybe _                                  = Nothing
  
  promoteTyCon :: TyCon -> TyCon
  promoteTyCon tc = case promotableTyCon_maybe tc of
                      Just prom_tc -> prom_tc
                      Nothing      -> pprPanic "promoteTyCon" (ppr tc)
  
  -- | Is this a PromotedTyCon?
  isPromotedTyCon :: TyCon -> Bool
  isPromotedTyCon (PromotedTyCon {}) = True
  isPromotedTyCon _                  = False
  
  -- | Retrieves the promoted TyCon if this is a PromotedTyCon;
  isPromotedTyCon_maybe :: TyCon -> Maybe TyCon
  isPromotedTyCon_maybe (PromotedTyCon { ty_con = tc }) = Just tc
  isPromotedTyCon_maybe _ = Nothing
  
  -- | Is this a PromotedDataCon?
  isPromotedDataCon :: TyCon -> Bool
  isPromotedDataCon (PromotedDataCon {}) = True
  isPromotedDataCon _                    = False
  
  -- | Retrieves the promoted DataCon if this is a PromotedDataCon;
  isPromotedDataCon_maybe :: TyCon -> Maybe DataCon
  isPromotedDataCon_maybe (PromotedDataCon { dataCon = dc }) = Just dc
  isPromotedDataCon_maybe _ = Nothing
  
  isLiftedTypeKindTyConName :: Name -> Bool
  isLiftedTypeKindTyConName n = n `hasKey` liftedTypeKindTyConKey
  
  -- | Identifies implicit tycons that, in particular, do not go into interface
  -- files (because they are implicitly reconstructed when the interface is
  -- read).
  --
  -- Note that:
  --
  -- * Associated families are implicit, as they are re-constructed from
  --   the class declaration in which they reside, and
  --
  -- * Family instances are /not/ implicit as they represent the instance body
  --   (similar to a @dfun@ does that for a class instance).
  isImplicitTyCon :: TyCon -> Bool
  isImplicitTyCon (FunTyCon {})        = True
  isImplicitTyCon (TupleTyCon {})      = True
  isImplicitTyCon (PrimTyCon {})       = True
  isImplicitTyCon (PromotedDataCon {}) = True
  isImplicitTyCon (PromotedTyCon {})   = True
  isImplicitTyCon (AlgTyCon { algTcParent = AssocFamilyTyCon {} })    = True
  isImplicitTyCon (AlgTyCon {})                                       = False
  isImplicitTyCon (FamilyTyCon { famTcParent = AssocFamilyTyCon {} }) = True
  isImplicitTyCon (FamilyTyCon {})                                    = False
  isImplicitTyCon (SynonymTyCon {})                                   = False
  
  tyConCType_maybe :: TyCon -> Maybe CType
  tyConCType_maybe tc@(AlgTyCon {}) = tyConCType tc
  tyConCType_maybe _ = Nothing
  
  isObjectTyCon :: TyCon -> Bool
  isObjectTyCon = (`hasKey` jobjectPrimTyConKey)
  
  {-
  -----------------------------------------------
  --      Expand type-constructor applications
  -----------------------------------------------
  -}
  
  tcExpandTyCon_maybe, coreExpandTyCon_maybe
          :: TyCon
          -> [tyco]                 -- ^ Arguments to 'TyCon'
          -> Maybe ([(TyVar,tyco)],
                    Type,
                    [tyco])         -- ^ Returns a 'TyVar' substitution, the body
                                    -- type of the synonym (not yet substituted)
                                    -- and any arguments remaining from the
                                    -- application
  
  -- ^ Used to create the view the /typechecker/ has on 'TyCon's.
  -- We expand (closed) synonyms only, cf. 'coreExpandTyCon_maybe'
  tcExpandTyCon_maybe (SynonymTyCon { tyConTyVars = tvs
                                    , synTcRhs    = rhs }) tys
     = expand tvs rhs tys
  tcExpandTyCon_maybe _ _ = Nothing
  
  ---------------
  
  -- ^ Used to create the view /Core/ has on 'TyCon's. We expand
  -- not only closed synonyms like 'tcExpandTyCon_maybe',
  -- but also non-recursive @newtype@s
  coreExpandTyCon_maybe tycon tys = tcExpandTyCon_maybe tycon tys
  
  
  ----------------
  expand  :: [TyVar] -> Type                 -- Template
          -> [a]                             -- Args
          -> Maybe ([(TyVar,a)], Type, [a])  -- Expansion
  expand tvs rhs tys
    = case n_tvs `compare` length tys of
          LT -> Just (tvs `zip` tys, rhs, drop n_tvs tys)
          EQ -> Just (tvs `zip` tys, rhs, [])
          GT -> Nothing
     where
       n_tvs = length tvs
  
  -- | As 'tyConDataCons_maybe', but returns the empty list of constructors if no
  -- constructors could be found
  tyConDataCons :: TyCon -> [DataCon]
  -- It's convenient for tyConDataCons to return the
  -- empty list for type synonyms etc
  tyConDataCons tycon = tyConDataCons_maybe tycon `orElse` []
  
  -- | Determine the 'DataCon's originating from the given 'TyCon', if the 'TyCon'
  -- is the sort that can have any constructors (note: this does not include
  -- abstract algebraic types)
  tyConDataCons_maybe :: TyCon -> Maybe [DataCon]
  tyConDataCons_maybe (AlgTyCon {algTcRhs = DataTyCon { data_cons = cons }})
      = Just cons
  tyConDataCons_maybe (AlgTyCon {algTcRhs = NewTyCon { data_con = con }})
      = Just [con]
  tyConDataCons_maybe (TupleTyCon {dataCon = con})
      = Just [con]
  tyConDataCons_maybe _
      = Nothing
  
  -- | Determine the number of value constructors a 'TyCon' has. Panics if the
  -- 'TyCon' is not algebraic or a tuple
  tyConFamilySize  :: TyCon -> Int
  tyConFamilySize (AlgTyCon   {algTcRhs = DataTyCon {data_cons = cons}}) =
    length cons
  tyConFamilySize (AlgTyCon   {algTcRhs = NewTyCon {}})        = 1
  tyConFamilySize (AlgTyCon   {algTcRhs = DataFamilyTyCon {}}) = 0
  tyConFamilySize (TupleTyCon {})                              = 1
  tyConFamilySize other = pprPanic "tyConFamilySize:" (ppr other)
  
  -- | Extract an 'AlgTyConRhs' with information about data constructors from an
  -- algebraic or tuple 'TyCon'. Panics for any other sort of 'TyCon'
  algTyConRhs :: TyCon -> AlgTyConRhs
  algTyConRhs (AlgTyCon {algTcRhs = rhs}) = rhs
  algTyConRhs (TupleTyCon {dataCon = con, tyConArity = arity})
      = DataTyCon { data_cons = [con], is_enum = arity == 0 }
  algTyConRhs other = pprPanic "algTyConRhs" (ppr other)
  
  -- | Get the list of roles for the type parameters of a TyCon
  tyConRoles :: TyCon -> [Role]
  -- See also Note [TyCon Role signatures]
  tyConRoles tc
    = case tc of
      { FunTyCon {}                         -> const_role Representational
      ; AlgTyCon { tcRoles = roles }        -> roles
      ; TupleTyCon {}                       -> const_role Representational
      ; SynonymTyCon { tcRoles = roles }    -> roles
      ; FamilyTyCon {}                      -> const_role Nominal
      ; PrimTyCon { tcRoles = roles }       -> roles
      ; PromotedDataCon { tcRoles = roles } -> roles
      ; PromotedTyCon {}                    -> const_role Nominal
      }
    where
      const_role r = replicate (tyConArity tc) r
  
  -- | Extract the bound type variables and type expansion of a type synonym
  -- 'TyCon'. Panics if the 'TyCon' is not a synonym
  newTyConRhs :: TyCon -> ([TyVar], Type)
  newTyConRhs (AlgTyCon {tyConTyVars = tvs, algTcRhs = NewTyCon { nt_rhs = rhs }})
      = (tvs, rhs)
  newTyConRhs tycon = pprPanic "newTyConRhs" (ppr tycon)
  
  -- | The number of type parameters that need to be passed to a newtype to
  -- resolve it. May be less than in the definition if it can be eta-contracted.
  newTyConEtadArity :: TyCon -> Int
  newTyConEtadArity (AlgTyCon {algTcRhs = NewTyCon { nt_etad_rhs = tvs_rhs }})
          = length (fst tvs_rhs)
  newTyConEtadArity tycon = pprPanic "newTyConEtadArity" (ppr tycon)
  
  -- | Extract the bound type variables and type expansion of an eta-contracted
  -- type synonym 'TyCon'.  Panics if the 'TyCon' is not a synonym
  newTyConEtadRhs :: TyCon -> ([TyVar], Type)
  newTyConEtadRhs (AlgTyCon {algTcRhs = NewTyCon { nt_etad_rhs = tvs_rhs }}) = tvs_rhs
  newTyConEtadRhs tycon = pprPanic "newTyConEtadRhs" (ppr tycon)
  
  -- | Extracts the @newtype@ coercion from such a 'TyCon', which can be used to
  -- construct something with the @newtype@s type from its representation type
  -- (right hand side). If the supplied 'TyCon' is not a @newtype@, returns
  -- @Nothing@
  newTyConCo_maybe :: TyCon -> Maybe (CoAxiom Unbranched)
  newTyConCo_maybe (AlgTyCon {algTcRhs = NewTyCon { nt_co = co }}) = Just co
  newTyConCo_maybe _                                               = Nothing
  
  newTyConCo :: TyCon -> CoAxiom Unbranched
  newTyConCo tc = case newTyConCo_maybe tc of
                   Just co -> co
                   Nothing -> pprPanic "newTyConCo" (ppr tc)
  
  -- | Find the primitive representation of a 'TyCon'
  tyConPrimRep :: TyCon -> PrimRep
  tyConPrimRep (PrimTyCon {primTyConRep = rep}) = rep
  tyConPrimRep tc = ASSERT(not (isUnboxedTupleTyCon tc)) PtrRep
  
  -- | Find the \"stupid theta\" of the 'TyCon'. A \"stupid theta\" is the context
  -- to the left of an algebraic type declaration, e.g. @Eq a@ in the declaration
  -- @data Eq a => T a ...@
  tyConStupidTheta :: TyCon -> [PredType]
  tyConStupidTheta (AlgTyCon {algTcStupidTheta = stupid}) = stupid
  tyConStupidTheta (TupleTyCon {})                        = []
  tyConStupidTheta tycon = pprPanic "tyConStupidTheta" (ppr tycon)
  
  -- | Extract the 'TyVar's bound by a vanilla type synonym
  -- and the corresponding (unsubstituted) right hand side.
  synTyConDefn_maybe :: TyCon -> Maybe ([TyVar], Type)
  synTyConDefn_maybe (SynonymTyCon {tyConTyVars = tyvars, synTcRhs = ty})
    = Just (tyvars, ty)
  synTyConDefn_maybe _ = Nothing
  
  -- | Extract the information pertaining to the right hand side of a type synonym
  -- (@type@) declaration.
  synTyConRhs_maybe :: TyCon -> Maybe Type
  synTyConRhs_maybe (SynonymTyCon {synTcRhs = rhs}) = Just rhs
  synTyConRhs_maybe _                               = Nothing
  
  -- | Extract the flavour of a type family (with all the extra information that
  -- it carries)
  famTyConFlav_maybe :: TyCon -> Maybe FamTyConFlav
  famTyConFlav_maybe (FamilyTyCon {famTcFlav = flav}) = Just flav
  famTyConFlav_maybe _                                = Nothing
  
  -- | If the given 'TyCon' has a /single/ data constructor, i.e. it is a @data@
  -- type with one alternative, a tuple type or a @newtype@ then that constructor
  -- is returned. If the 'TyCon' has more than one constructor, or represents a
  -- primitive or function type constructor then @Nothing@ is returned. In any
  -- other case, the function panics
  tyConSingleDataCon_maybe :: TyCon -> Maybe DataCon
  tyConSingleDataCon_maybe (TupleTyCon {dataCon = c})
      = Just c
  tyConSingleDataCon_maybe (AlgTyCon {algTcRhs = DataTyCon { data_cons = [c] }})
      = Just c
  tyConSingleDataCon_maybe (AlgTyCon {algTcRhs = NewTyCon { data_con = c }})
      = Just c
  tyConSingleDataCon_maybe _
      = Nothing
  
  tyConSingleAlgDataCon_maybe :: TyCon -> Maybe DataCon
  -- Returns (Just con) for single-constructor *algebraic* data types
  -- *not* newtypes
  tyConSingleAlgDataCon_maybe (TupleTyCon {dataCon = c})
      = Just c
  tyConSingleAlgDataCon_maybe (AlgTyCon {algTcRhs = DataTyCon { data_cons= [c] }})
      = Just c
  tyConSingleAlgDataCon_maybe _
      = Nothing
  
  -- | Is this 'TyCon' that for a class instance?
  isClassTyCon :: TyCon -> Bool
  isClassTyCon (AlgTyCon {algTcParent = ClassTyCon _}) = True
  isClassTyCon _                                       = False
  
  -- | If this 'TyCon' is that for a class instance, return the class it is for.
  -- Otherwise returns @Nothing@
  tyConClass_maybe :: TyCon -> Maybe Class
  tyConClass_maybe (AlgTyCon {algTcParent = ClassTyCon clas}) = Just clas
  tyConClass_maybe _                                          = Nothing
  
  tyConTuple_maybe :: TyCon -> Maybe TupleSort
  tyConTuple_maybe (TupleTyCon {tyConTupleSort = sort}) = Just sort
  tyConTuple_maybe _                                    = Nothing
  
  ----------------------------------------------------------------------------
  tyConParent :: TyCon -> TyConParent
  tyConParent (AlgTyCon    {algTcParent = parent}) = parent
  tyConParent (FamilyTyCon {famTcParent = parent}) = parent
  tyConParent _                                    = NoParentTyCon
  
  ----------------------------------------------------------------------------
  -- | Is this 'TyCon' that for a data family instance?
  isFamInstTyCon :: TyCon -> Bool
  isFamInstTyCon tc = case tyConParent tc of
                        FamInstTyCon {} -> True
                        _               -> False
  
  tyConFamInstSig_maybe :: TyCon -> Maybe (TyCon, [Type], CoAxiom Unbranched)
  tyConFamInstSig_maybe tc
    = case tyConParent tc of
        FamInstTyCon ax f ts -> Just (f, ts, ax)
        _                    -> Nothing
  
  -- | If this 'TyCon' is that of a family instance, return the family in question
  -- and the instance types. Otherwise, return @Nothing@
  tyConFamInst_maybe :: TyCon -> Maybe (TyCon, [Type])
  tyConFamInst_maybe tc
    = case tyConParent tc of
        FamInstTyCon _ f ts -> Just (f, ts)
        _                   -> Nothing
  
  -- | If this 'TyCon' is that of a family instance, return a 'TyCon' which
  -- represents a coercion identifying the representation type with the type
  -- instance family.  Otherwise, return @Nothing@
  tyConFamilyCoercion_maybe :: TyCon -> Maybe (CoAxiom Unbranched)
  tyConFamilyCoercion_maybe tc
    = case tyConParent tc of
        FamInstTyCon co _ _ -> Just co
        _                   -> Nothing
  
  {-
  ************************************************************************
  *                                                                      *
  \subsection[TyCon-instances]{Instance declarations for @TyCon@}
  *                                                                      *
  ************************************************************************
  
  @TyCon@s are compared by comparing their @Unique@s.
  
  The strictness analyser needs @Ord@. It is a lexicographic order with
  the property @(a<=b) || (b<=a)@.
  -}
  
  instance Eq TyCon where
      a == b = case (a `compare` b) of { EQ -> True;   _ -> False }
      a /= b = case (a `compare` b) of { EQ -> False;  _ -> True  }
  
  instance Ord TyCon where
      a <= b = case (a `compare` b) of { LT -> True;  EQ -> True;  GT -> False }
      a <  b = case (a `compare` b) of { LT -> True;  EQ -> False; GT -> False }
      a >= b = case (a `compare` b) of { LT -> False; EQ -> True;  GT -> True  }
      a >  b = case (a `compare` b) of { LT -> False; EQ -> False; GT -> True  }
      compare a b = getUnique a `compare` getUnique b
  
  instance Uniquable TyCon where
      getUnique tc = tyConUnique tc
  
  instance Outputable TyCon where
    -- At the moment a promoted TyCon has the same Name as its
    -- corresponding TyCon, so we add the quote to distinguish it here
    ppr tc = pprPromotionQuote tc <> ppr (tyConName tc)
  
  pprPromotionQuote :: TyCon -> SDoc
  pprPromotionQuote (PromotedDataCon {}) = char '\''   -- Quote promoted DataCons
                                                       -- in types
  pprPromotionQuote (PromotedTyCon {})   = ifPprDebug (char '\'')
  pprPromotionQuote _                    = empty -- However, we don't quote TyCons
                                                 -- in kinds e.g.
                                                 -- type family T a :: Bool -> *
                                                 -- cf Trac #5952.
                                                 -- Except with -dppr-debug
  
  instance NamedThing TyCon where
      getName = tyConName
  
  instance Data.Data TyCon where
      -- don't traverse?
      toConstr _   = abstractConstr "TyCon"
      gunfold _ _  = error "gunfold"
      dataTypeOf _ = mkNoRepType "TyCon"
  
  {-
  ************************************************************************
  *                                                                      *
             Walking over recursive TyCons
  *                                                                      *
  ************************************************************************
  
  Note [Expanding newtypes and products]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  When expanding a type to expose a data-type constructor, we need to be
  careful about newtypes, lest we fall into an infinite loop. Here are
  the key examples:
  
    newtype Id  x = MkId x
    newtype Fix f = MkFix (f (Fix f))
    newtype T     = MkT (T -> T)
  
    Type           Expansion
   --------------------------
    T              T -> T
    Fix Maybe      Maybe (Fix Maybe)
    Id (Id Int)    Int
    Fix Id         NO NO NO
  
  Notice that we can expand T, even though it's recursive.
  And we can expand Id (Id Int), even though the Id shows up
  twice at the outer level.
  
  So, when expanding, we keep track of when we've seen a recursive
  newtype at outermost level; and bale out if we see it again.
  
  We sometimes want to do the same for product types, so that the
  strictness analyser doesn't unbox infinitely deeply.
  
  The function that manages this is checkRecTc.
  -}
  
  newtype RecTcChecker = RC NameSet
  
  initRecTc :: RecTcChecker
  initRecTc = RC emptyNameSet
  
  checkRecTc :: RecTcChecker -> TyCon -> Maybe RecTcChecker
  -- Nothing      => Recursion detected
  -- Just rec_tcs => Keep going
  checkRecTc (RC rec_nts) tc
    | not (isRecursiveTyCon tc)     = Just (RC rec_nts)
    | tc_name `elemNameSet` rec_nts = Nothing
    | otherwise                     = Just (RC (extendNameSet rec_nts tc_name))
    where
      tc_name = tyConName tc
  