Type synonym families may be nullary

[ghc-hetmet.git] / compiler / typecheck / TcTyClsDecls.lhs

%
% (c) The University of Glasgow 2006
% (c) The AQUA Project, Glasgow University, 1996-1998
%

TcTyClsDecls: Typecheck type and class declarations

\begin{code}
module TcTyClsDecls (
        tcTyAndClassDecls, tcFamInstDecl, mkAuxBinds
    ) where

#include "HsVersions.h"

import HsSyn
import HsTypes
import HscTypes
import BuildTyCl
import TcUnify
import TcRnMonad
import TcEnv
import TcTyDecls
import TcClassDcl
import TcHsType
import TcMType
import TcType
import TysWiredIn       ( unitTy )
import Type
import Generics
import Class
import TyCon
import DataCon
import Id
import MkId             ( rEC_SEL_ERROR_ID )
import IdInfo
import Var
import VarSet
import Name
import OccName
import Outputable
import Maybes
import Monad
import Unify
import Util
import SrcLoc
import ListSetOps
import Digraph
import DynFlags
import FastString
import Unique           ( mkBuiltinUnique )
import BasicTypes

import Bag
import Data.List
import Control.Monad    ( mplus )
\end{code}


%************************************************************************
%*                                                                      *
\subsection{Type checking for type and class declarations}
%*                                                                      *
%************************************************************************

Dealing with a group
~~~~~~~~~~~~~~~~~~~~
Consider a mutually-recursive group, binding 
a type constructor T and a class C.

Step 1:         getInitialKind
        Construct a KindEnv by binding T and C to a kind variable 

Step 2:         kcTyClDecl
        In that environment, do a kind check

Step 3: Zonk the kinds

Step 4:         buildTyConOrClass
        Construct an environment binding T to a TyCon and C to a Class.
        a) Their kinds comes from zonking the relevant kind variable
        b) Their arity (for synonyms) comes direct from the decl
        c) The funcional dependencies come from the decl
        d) The rest comes a knot-tied binding of T and C, returned from Step 4
        e) The variances of the tycons in the group is calculated from 
                the knot-tied stuff

Step 5:         tcTyClDecl1
        In this environment, walk over the decls, constructing the TyCons and Classes.
        This uses in a strict way items (a)-(c) above, which is why they must
        be constructed in Step 4. Feed the results back to Step 4.
        For this step, pass the is-recursive flag as the wimp-out flag
        to tcTyClDecl1.
        

Step 6:         Extend environment
        We extend the type environment with bindings not only for the TyCons and Classes,
        but also for their "implicit Ids" like data constructors and class selectors

Step 7:         checkValidTyCl
        For a recursive group only, check all the decls again, just
        to check all the side conditions on validity.  We could not
        do this before because we were in a mutually recursive knot.

Identification of recursive TyCons
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The knot-tying parameters: @rec_details_list@ is an alist mapping @Name@s to
@TyThing@s.

Identifying a TyCon as recursive serves two purposes

1.  Avoid infinite types.  Non-recursive newtypes are treated as
"transparent", like type synonyms, after the type checker.  If we did
this for all newtypes, we'd get infinite types.  So we figure out for
each newtype whether it is "recursive", and add a coercion if so.  In
effect, we are trying to "cut the loops" by identifying a loop-breaker.

2.  Avoid infinite unboxing.  This is nothing to do with newtypes.
Suppose we have
        data T = MkT Int T
        f (MkT x t) = f t
Well, this function diverges, but we don't want the strictness analyser
to diverge.  But the strictness analyser will diverge because it looks
deeper and deeper into the structure of T.   (I believe there are
examples where the function does something sane, and the strictness
analyser still diverges, but I can't see one now.)

Now, concerning (1), the FC2 branch currently adds a coercion for ALL
newtypes.  I did this as an experiment, to try to expose cases in which
the coercions got in the way of optimisations.  If it turns out that we
can indeed always use a coercion, then we don't risk recursive types,
and don't need to figure out what the loop breakers are.

For newtype *families* though, we will always have a coercion, so they
are always loop breakers!  So you can easily adjust the current
algorithm by simply treating all newtype families as loop breakers (and
indeed type families).  I think.

\begin{code}
tcTyAndClassDecls :: ModDetails -> [LTyClDecl Name]
                   -> TcM (TcGblEnv,         -- Input env extended by types and classes 
                                             -- and their implicit Ids,DataCons
                           HsValBinds Name)  -- Renamed bindings for record selectors
-- Fails if there are any errors

tcTyAndClassDecls boot_details allDecls
  = checkNoErrs $       -- The code recovers internally, but if anything gave rise to
                        -- an error we'd better stop now, to avoid a cascade
    do  {       -- Omit instances of type families; they are handled together
                -- with the *heads* of class instances
        ; let decls = filter (not . isFamInstDecl . unLoc) allDecls

                -- First check for cyclic type synonysm or classes
                -- See notes with checkCycleErrs
        ; checkCycleErrs decls
        ; mod <- getModule
        ; traceTc (text "tcTyAndCl" <+> ppr mod)
        ; (syn_tycons, alg_tyclss) <- fixM (\ ~(_rec_syn_tycons, rec_alg_tyclss) ->
          do    { let { -- Seperate ordinary synonyms from all other type and
                        -- class declarations and add all associated type
                        -- declarations from type classes.  The latter is
                        -- required so that the temporary environment for the
                        -- knot includes all associated family declarations.
                      ; (syn_decls, alg_decls) = partition (isSynDecl . unLoc)
                                                   decls
                      ; alg_at_decls           = concatMap addATs alg_decls
                      }
                        -- Extend the global env with the knot-tied results
                        -- for data types and classes
                        -- 
                        -- We must populate the environment with the loop-tied
                        -- T's right away, because the kind checker may "fault
                        -- in" some type  constructors that recursively
                        -- mention T
                ; let gbl_things = mkGlobalThings alg_at_decls rec_alg_tyclss
                ; tcExtendRecEnv gbl_things $ do

                        -- Kind-check the declarations
                { (kc_syn_decls, kc_alg_decls) <- kcTyClDecls syn_decls alg_decls

                ; let { -- Calculate rec-flag
                      ; calc_rec  = calcRecFlags boot_details rec_alg_tyclss
                      ; tc_decl   = addLocM (tcTyClDecl calc_rec) }

                        -- Type-check the type synonyms, and extend the envt
                ; syn_tycons <- tcSynDecls kc_syn_decls
                ; tcExtendGlobalEnv syn_tycons $ do

                        -- Type-check the data types and classes
                { alg_tyclss <- mapM tc_decl kc_alg_decls
                ; return (syn_tycons, concat alg_tyclss)
            }}})
        -- Finished with knot-tying now
        -- Extend the environment with the finished things
        ; tcExtendGlobalEnv (syn_tycons ++ alg_tyclss) $ do

        -- Perform the validity check
        { traceTc (text "ready for validity check")
        ; mapM_ (addLocM checkValidTyCl) decls
        ; traceTc (text "done")
   
        -- Add the implicit things;
        -- we want them in the environment because 
        -- they may be mentioned in interface files
        -- NB: All associated types and their implicit things will be added a
        --     second time here.  This doesn't matter as the definitions are
        --     the same.
        ; let { implicit_things = concatMap implicitTyThings alg_tyclss
              ; aux_binds       = mkAuxBinds alg_tyclss }
        ; traceTc ((text "Adding" <+> ppr alg_tyclss) 
                   $$ (text "and" <+> ppr implicit_things))
        ; env <- tcExtendGlobalEnv implicit_things getGblEnv
        ; return (env, aux_binds) }
    }
  where
    -- Pull associated types out of class declarations, to tie them into the
    -- knot above.  
    -- NB: We put them in the same place in the list as `tcTyClDecl' will
    --     eventually put the matching `TyThing's.  That's crucial; otherwise,
    --     the two argument lists of `mkGlobalThings' don't match up.
    addATs decl@(L _ (ClassDecl {tcdATs = ats})) = decl : ats
    addATs decl                                  = [decl]

mkGlobalThings :: [LTyClDecl Name]      -- The decls
               -> [TyThing]             -- Knot-tied, in 1-1 correspondence with the decls
               -> [(Name,TyThing)]
-- Driven by the Decls, and treating the TyThings lazily
-- make a TypeEnv for the new things
mkGlobalThings decls things
  = map mk_thing (decls `zipLazy` things)
  where
    mk_thing (L _ (ClassDecl {tcdLName = L _ name}), ~(AClass cl))
         = (name, AClass cl)
    mk_thing (L _ decl, ~(ATyCon tc))
         = (tcdName decl, ATyCon tc)
\end{code}


%************************************************************************
%*                                                                      *
               Type checking family instances
%*                                                                      *
%************************************************************************

Family instances are somewhat of a hybrid.  They are processed together with
class instance heads, but can contain data constructors and hence they share a
lot of kinding and type checking code with ordinary algebraic data types (and
GADTs).

\begin{code}
tcFamInstDecl :: LTyClDecl Name -> TcM TyThing
tcFamInstDecl (L loc decl)
  =     -- Prime error recovery, set source location
    setSrcSpan loc                              $
    tcAddDeclCtxt decl                          $
    do { -- type families require -XTypeFamilies and can't be in an
         -- hs-boot file
       ; type_families <- doptM Opt_TypeFamilies
       ; is_boot  <- tcIsHsBoot   -- Are we compiling an hs-boot file?
       ; checkTc type_families $ badFamInstDecl (tcdLName decl)
       ; checkTc (not is_boot) $ badBootFamInstDeclErr

         -- Perform kind and type checking
       ; tc <- tcFamInstDecl1 decl
       ; checkValidTyCon tc     -- Remember to check validity;
                                -- no recursion to worry about here
       ; return (ATyCon tc) }

tcFamInstDecl1 :: TyClDecl Name -> TcM TyCon

  -- "type instance"
tcFamInstDecl1 (decl@TySynonym {tcdLName = L loc tc_name})
  = kcIdxTyPats decl $ \k_tvs k_typats resKind family ->
    do { -- check that the family declaration is for a synonym
         checkTc (isOpenTyCon family) (notFamily family)
       ; checkTc (isSynTyCon family) (wrongKindOfFamily family)

       ; -- (1) kind check the right-hand side of the type equation
       ; k_rhs <- kcCheckLHsType (tcdSynRhs decl) (EK resKind EkUnk)
                  -- ToDo: the ExpKind could be better

         -- we need the exact same number of type parameters as the family
         -- declaration 
       ; let famArity = tyConArity family
       ; checkTc (length k_typats == famArity) $ 
           wrongNumberOfParmsErr famArity

         -- (2) type check type equation
       ; tcTyVarBndrs k_tvs $ \t_tvs -> do {  -- turn kinded into proper tyvars
       ; t_typats <- mapM tcHsKindedType k_typats
       ; t_rhs    <- tcHsKindedType k_rhs

         -- (3) check the well-formedness of the instance
       ; checkValidTypeInst t_typats t_rhs

         -- (4) construct representation tycon
       ; rep_tc_name <- newFamInstTyConName tc_name loc
       ; buildSynTyCon rep_tc_name t_tvs (SynonymTyCon t_rhs) 
                       (typeKind t_rhs) (Just (family, t_typats))
       }}

  -- "newtype instance" and "data instance"
tcFamInstDecl1 (decl@TyData {tcdND = new_or_data, tcdLName = L loc tc_name,
                             tcdCons = cons})
  = kcIdxTyPats decl $ \k_tvs k_typats resKind fam_tycon ->
    do { -- check that the family declaration is for the right kind
         checkTc (isOpenTyCon fam_tycon) (notFamily fam_tycon)
       ; checkTc (isAlgTyCon fam_tycon) (wrongKindOfFamily fam_tycon)

       ; -- (1) kind check the data declaration as usual
       ; k_decl <- kcDataDecl decl k_tvs
       ; let k_ctxt = tcdCtxt k_decl
             k_cons = tcdCons k_decl

         -- result kind must be '*' (otherwise, we have too few patterns)
       ; checkTc (isLiftedTypeKind resKind) $ tooFewParmsErr (tyConArity fam_tycon)

         -- (2) type check indexed data type declaration
       ; tcTyVarBndrs k_tvs $ \t_tvs -> do {  -- turn kinded into proper tyvars
       ; unbox_strict <- doptM Opt_UnboxStrictFields

         -- kind check the type indexes and the context
       ; t_typats     <- mapM tcHsKindedType k_typats
       ; stupid_theta <- tcHsKindedContext k_ctxt

         -- (3) Check that
         --     (a) left-hand side contains no type family applications
         --         (vanilla synonyms are fine, though, and we checked for
         --         foralls earlier)
       ; mapM_ checkTyFamFreeness t_typats

         -- Check that we don't use GADT syntax in H98 world
       ; gadt_ok <- doptM Opt_GADTs
       ; checkTc (gadt_ok || consUseH98Syntax cons) (badGadtDecl tc_name)

         --     (b) a newtype has exactly one constructor
       ; checkTc (new_or_data == DataType || isSingleton k_cons) $
                 newtypeConError tc_name (length k_cons)

         -- (4) construct representation tycon
       ; rep_tc_name <- newFamInstTyConName tc_name loc
       ; let ex_ok = True       -- Existentials ok for type families!
       ; fixM (\ rep_tycon -> do 
             { let orig_res_ty = mkTyConApp fam_tycon t_typats
             ; data_cons <- tcConDecls unbox_strict ex_ok rep_tycon
                                       (t_tvs, orig_res_ty) k_cons
             ; tc_rhs <-
                 case new_or_data of
                   DataType -> return (mkDataTyConRhs data_cons)
                   NewType  -> ASSERT( not (null data_cons) )
                               mkNewTyConRhs rep_tc_name rep_tycon (head data_cons)
             ; buildAlgTyCon rep_tc_name t_tvs stupid_theta tc_rhs Recursive
                             False h98_syntax (Just (fam_tycon, t_typats))
                 -- We always assume that indexed types are recursive.  Why?
                 -- (1) Due to their open nature, we can never be sure that a
                 -- further instance might not introduce a new recursive
                 -- dependency.  (2) They are always valid loop breakers as
                 -- they involve a coercion.
             })
       }}
       where
         h98_syntax = case cons of      -- All constructors have same shape
                        L _ (ConDecl { con_res = ResTyGADT _ }) : _ -> False
                        _ -> True

tcFamInstDecl1 d = pprPanic "tcFamInstDecl1" (ppr d)

-- Kind checking of indexed types
-- -

-- Kind check type patterns and kind annotate the embedded type variables.
--
-- * Here we check that a type instance matches its kind signature, but we do
--   not check whether there is a pattern for each type index; the latter
--   check is only required for type synonym instances.

kcIdxTyPats :: TyClDecl Name
            -> ([LHsTyVarBndr Name] -> [LHsType Name] -> Kind -> TyCon -> TcM a)
               -- ^^kinded tvs         ^^kinded ty pats  ^^res kind
            -> TcM a
kcIdxTyPats decl thing_inside
  = kcHsTyVars (tcdTyVars decl) $ \tvs -> 
    do { let tc_name = tcdLName decl
       ; fam_tycon <- tcLookupLocatedTyCon tc_name
       ; let { (kinds, resKind) = splitKindFunTys (tyConKind fam_tycon)
             ; hs_typats        = fromJust $ tcdTyPats decl }

         -- we may not have more parameters than the kind indicates
       ; checkTc (length kinds >= length hs_typats) $
           tooManyParmsErr (tcdLName decl)

         -- type functions can have a higher-kinded result
       ; let resultKind = mkArrowKinds (drop (length hs_typats) kinds) resKind
       ; typats <- zipWithM kcCheckLHsType hs_typats 
                            [ EK kind (EkArg (ppr tc_name) n) 
                            | (kind,n) <- kinds `zip` [1..]]
       ; thing_inside tvs typats resultKind fam_tycon
       }
\end{code}


%************************************************************************
%*                                                                      *
                Kind checking
%*                                                                      *
%************************************************************************

We need to kind check all types in the mutually recursive group
before we know the kind of the type variables.  For example:

class C a where
   op :: D b => a -> b -> b

class D c where
   bop :: (Monad c) => ...

Here, the kind of the locally-polymorphic type variable "b"
depends on *all the uses of class D*.  For example, the use of
Monad c in bop's type signature means that D must have kind Type->Type.

However type synonyms work differently.  They can have kinds which don't
just involve (->) and *:
        type R = Int#           -- Kind #
        type S a = Array# a     -- Kind * -> #
        type T a b = (# a,b #)  -- Kind * -> * -> (# a,b #)
So we must infer their kinds from their right-hand sides *first* and then
use them, whereas for the mutually recursive data types D we bring into
scope kind bindings D -> k, where k is a kind variable, and do inference.

Type families
~~~~~~~~~~~~~
This treatment of type synonyms only applies to Haskell 98-style synonyms.
General type functions can be recursive, and hence, appear in `alg_decls'.

The kind of a type family is solely determinded by its kind signature;
hence, only kind signatures participate in the construction of the initial
kind environment (as constructed by `getInitialKind').  In fact, we ignore
instances of families altogether in the following.  However, we need to
include the kinds of associated families into the construction of the
initial kind environment.  (This is handled by `allDecls').

\begin{code}
kcTyClDecls :: [LTyClDecl Name] -> [Located (TyClDecl Name)]
            -> TcM ([LTyClDecl Name], [Located (TyClDecl Name)])
kcTyClDecls syn_decls alg_decls
  = do  {       -- First extend the kind env with each data type, class, and
                -- indexed type, mapping them to a type variable
          let initialKindDecls = concat [allDecls decl | L _ decl <- alg_decls]
        ; alg_kinds <- mapM getInitialKind initialKindDecls
        ; tcExtendKindEnv alg_kinds $ do

                -- Now kind-check the type synonyms, in dependency order
                -- We do these differently to data type and classes,
                -- because a type synonym can be an unboxed type
                --      type Foo = Int#
                -- and a kind variable can't unify with UnboxedTypeKind
                -- So we infer their kinds in dependency order
        { (kc_syn_decls, syn_kinds) <- kcSynDecls (calcSynCycles syn_decls)
        ; tcExtendKindEnv syn_kinds $  do

                -- Now kind-check the data type, class, and kind signatures,
                -- returning kind-annotated decls; we don't kind-check
                -- instances of indexed types yet, but leave this to
                -- `tcInstDecls1'
        { kc_alg_decls <- mapM (wrapLocM kcTyClDecl)
                            (filter (not . isFamInstDecl . unLoc) alg_decls)

        ; return (kc_syn_decls, kc_alg_decls) }}}
  where
    -- get all declarations relevant for determining the initial kind
    -- environment
    allDecls (decl@ClassDecl {tcdATs = ats}) = decl : [ at 
                                                      | L _ at <- ats
                                                      , isFamilyDecl at]
    allDecls decl | isFamInstDecl decl = []
                  | otherwise          = [decl]

------------------------------------------------------------------------
getInitialKind :: TyClDecl Name -> TcM (Name, TcKind)
-- Only for data type, class, and indexed type declarations
-- Get as much info as possible from the data, class, or indexed type decl,
-- so as to maximise usefulness of error messages
getInitialKind decl
  = do  { arg_kinds <- mapM (mk_arg_kind . unLoc) (tyClDeclTyVars decl)
        ; res_kind  <- mk_res_kind decl
        ; return (tcdName decl, mkArrowKinds arg_kinds res_kind) }
  where
    mk_arg_kind (UserTyVar _)        = newKindVar
    mk_arg_kind (KindedTyVar _ kind) = return kind

    mk_res_kind (TyFamily { tcdKind    = Just kind }) = return kind
    mk_res_kind (TyData   { tcdKindSig = Just kind }) = return kind
        -- On GADT-style declarations we allow a kind signature
        --      data T :: *->* where { ... }
    mk_res_kind _ = return liftedTypeKind


----------------
kcSynDecls :: [SCC (LTyClDecl Name)] 
           -> TcM ([LTyClDecl Name],    -- Kind-annotated decls
                   [(Name,TcKind)])     -- Kind bindings
kcSynDecls []
  = return ([], [])
kcSynDecls (group : groups)
  = do  { (decl,  nk)  <- kcSynDecl group
        ; (decls, nks) <- tcExtendKindEnv [nk] (kcSynDecls groups)
        ; return (decl:decls, nk:nks) }
                        
----------------
kcSynDecl :: SCC (LTyClDecl Name) 
           -> TcM (LTyClDecl Name,      -- Kind-annotated decls
                   (Name,TcKind))       -- Kind bindings
kcSynDecl (AcyclicSCC (L loc decl))
  = tcAddDeclCtxt decl  $
    kcHsTyVars (tcdTyVars decl) (\ k_tvs ->
    do { traceTc (text "kcd1" <+> ppr (unLoc (tcdLName decl)) <+> brackets (ppr (tcdTyVars decl)) 
                        <+> brackets (ppr k_tvs))
       ; (k_rhs, rhs_kind) <- kcLHsType (tcdSynRhs decl)
       ; traceTc (text "kcd2" <+> ppr (unLoc (tcdLName decl)))
       ; let tc_kind = foldr (mkArrowKind . kindedTyVarKind) rhs_kind k_tvs
       ; return (L loc (decl { tcdTyVars = k_tvs, tcdSynRhs = k_rhs }),
                 (unLoc (tcdLName decl), tc_kind)) })

kcSynDecl (CyclicSCC decls)
  = do { recSynErr decls; failM }       -- Fail here to avoid error cascade
                                        -- of out-of-scope tycons

kindedTyVarKind :: LHsTyVarBndr Name -> Kind
kindedTyVarKind (L _ (KindedTyVar _ k)) = k
kindedTyVarKind x = pprPanic "kindedTyVarKind" (ppr x)

------------------------------------------------------------------------
kcTyClDecl :: TyClDecl Name -> TcM (TyClDecl Name)
        -- Not used for type synonyms (see kcSynDecl)

kcTyClDecl decl@(TyData {})
  = ASSERT( not . isFamInstDecl $ decl )   -- must not be a family instance
    kcTyClDeclBody decl $
      kcDataDecl decl

kcTyClDecl decl@(TyFamily {})
  = kcFamilyDecl [] decl      -- the empty list signals a toplevel decl      

kcTyClDecl decl@(ClassDecl {tcdCtxt = ctxt, tcdSigs = sigs, tcdATs = ats})
  = kcTyClDeclBody decl $ \ tvs' ->
    do  { ctxt' <- kcHsContext ctxt     
        ; ats'  <- mapM (wrapLocM (kcFamilyDecl tvs')) ats
        ; sigs' <- mapM (wrapLocM kc_sig) sigs
        ; return (decl {tcdTyVars = tvs', tcdCtxt = ctxt', tcdSigs = sigs',
                        tcdATs = ats'}) }
  where
    kc_sig (TypeSig nm op_ty) = do { op_ty' <- kcHsLiftedSigType op_ty
                                   ; return (TypeSig nm op_ty') }
    kc_sig other_sig          = return other_sig

kcTyClDecl decl@(ForeignType {})
  = return decl

kcTyClDecl (TySynonym {}) = panic "kcTyClDecl TySynonym"

kcTyClDeclBody :: TyClDecl Name
               -> ([LHsTyVarBndr Name] -> TcM a)
               -> TcM a
-- getInitialKind has made a suitably-shaped kind for the type or class
-- Unpack it, and attribute those kinds to the type variables
-- Extend the env with bindings for the tyvars, taken from
-- the kind of the tycon/class.  Give it to the thing inside, and 
-- check the result kind matches
kcTyClDeclBody decl thing_inside
  = tcAddDeclCtxt decl          $
    do  { tc_ty_thing <- tcLookupLocated (tcdLName decl)
        ; let tc_kind    = case tc_ty_thing of
                           AThing k -> k
                           _ -> pprPanic "kcTyClDeclBody" (ppr tc_ty_thing)
              (kinds, _) = splitKindFunTys tc_kind
              hs_tvs     = tcdTyVars decl
              kinded_tvs = ASSERT( length kinds >= length hs_tvs )
                           [ L loc (KindedTyVar (hsTyVarName tv) k)
                           | (L loc tv, k) <- zip hs_tvs kinds]
        ; tcExtendKindEnvTvs kinded_tvs (thing_inside kinded_tvs) }

-- Kind check a data declaration, assuming that we already extended the
-- kind environment with the type variables of the left-hand side (these
-- kinded type variables are also passed as the second parameter).
--
kcDataDecl :: TyClDecl Name -> [LHsTyVarBndr Name] -> TcM (TyClDecl Name)
kcDataDecl decl@(TyData {tcdND = new_or_data, tcdCtxt = ctxt, tcdCons = cons})
           tvs
  = do  { ctxt' <- kcHsContext ctxt     
        ; cons' <- mapM (wrapLocM kc_con_decl) cons
        ; return (decl {tcdTyVars = tvs, tcdCtxt = ctxt', tcdCons = cons'}) }
  where
    -- doc comments are typechecked to Nothing here
    kc_con_decl con_decl@(ConDecl { con_name = name, con_qvars = ex_tvs
                                  , con_cxt = ex_ctxt, con_details = details, con_res = res })
      = addErrCtxt (dataConCtxt name)   $ 
        kcHsTyVars ex_tvs $ \ex_tvs' -> do
        do { ex_ctxt' <- kcHsContext ex_ctxt
           ; details' <- kc_con_details details 
           ; res'     <- case res of
                ResTyH98 -> return ResTyH98
                ResTyGADT ty -> do { ty' <- kcHsSigType ty; return (ResTyGADT ty') }
           ; return (con_decl { con_qvars = ex_tvs', con_cxt = ex_ctxt'
                              , con_details = details', con_res = res' }) }

    kc_con_details (PrefixCon btys) 
        = do { btys' <- mapM kc_larg_ty btys 
             ; return (PrefixCon btys') }
    kc_con_details (InfixCon bty1 bty2) 
        = do { bty1' <- kc_larg_ty bty1
             ; bty2' <- kc_larg_ty bty2
             ; return (InfixCon bty1' bty2') }
    kc_con_details (RecCon fields) 
        = do { fields' <- mapM kc_field fields
             ; return (RecCon fields') }

    kc_field (ConDeclField fld bty d) = do { bty' <- kc_larg_ty bty
                                           ; return (ConDeclField fld bty' d) }

    kc_larg_ty bty = case new_or_data of
                        DataType -> kcHsSigType bty
                        NewType  -> kcHsLiftedSigType bty
        -- Can't allow an unlifted type for newtypes, because we're effectively
        -- going to remove the constructor while coercing it to a lifted type.
        -- And newtypes can't be bang'd
kcDataDecl d _ = pprPanic "kcDataDecl" (ppr d)

-- Kind check a family declaration or type family default declaration.
--
kcFamilyDecl :: [LHsTyVarBndr Name]  -- tyvars of enclosing class decl if any
             -> TyClDecl Name -> TcM (TyClDecl Name)
kcFamilyDecl classTvs decl@(TyFamily {tcdKind = kind})
  = kcTyClDeclBody decl $ \tvs' ->
    do { mapM_ unifyClassParmKinds tvs'
       ; return (decl {tcdTyVars = tvs', 
                       tcdKind = kind `mplus` Just liftedTypeKind})
                       -- default result kind is '*'
       }
  where
    unifyClassParmKinds (L _ (KindedTyVar n k))
      | Just classParmKind <- lookup n classTyKinds = unifyKind k classParmKind
      | otherwise                                   = return ()
    unifyClassParmKinds x = pprPanic "kcFamilyDecl/unifyClassParmKinds" (ppr x)
    classTyKinds = [(n, k) | L _ (KindedTyVar n k) <- classTvs]
kcFamilyDecl _ (TySynonym {})              -- type family defaults
  = panic "TcTyClsDecls.kcFamilyDecl: not implemented yet"
kcFamilyDecl _ d = pprPanic "kcFamilyDecl" (ppr d)
\end{code}


%************************************************************************
%*                                                                      *
\subsection{Type checking}
%*                                                                      *
%************************************************************************

\begin{code}
tcSynDecls :: [LTyClDecl Name] -> TcM [TyThing]
tcSynDecls [] = return []
tcSynDecls (decl : decls) 
  = do { syn_tc <- addLocM tcSynDecl decl
       ; syn_tcs <- tcExtendGlobalEnv [syn_tc] (tcSynDecls decls)
       ; return (syn_tc : syn_tcs) }

  -- "type"
tcSynDecl :: TyClDecl Name -> TcM TyThing
tcSynDecl
  (TySynonym {tcdLName = L _ tc_name, tcdTyVars = tvs, tcdSynRhs = rhs_ty})
  = tcTyVarBndrs tvs            $ \ tvs' -> do 
    { traceTc (text "tcd1" <+> ppr tc_name) 
    ; rhs_ty' <- tcHsKindedType rhs_ty
    ; tycon <- buildSynTyCon tc_name tvs' (SynonymTyCon rhs_ty') 
                             (typeKind rhs_ty') Nothing
    ; return (ATyCon tycon) 
    }
tcSynDecl d = pprPanic "tcSynDecl" (ppr d)

--------------------
tcTyClDecl :: (Name -> RecFlag) -> TyClDecl Name -> TcM [TyThing]

tcTyClDecl calc_isrec decl
  = tcAddDeclCtxt decl (tcTyClDecl1 calc_isrec decl)

  -- "type family" declarations
tcTyClDecl1 :: (Name -> RecFlag) -> TyClDecl Name -> TcM [TyThing]
tcTyClDecl1 _calc_isrec 
  (TyFamily {tcdFlavour = TypeFamily, 
             tcdLName = L _ tc_name, tcdTyVars = tvs,
             tcdKind = Just kind}) -- NB: kind at latest added during kind checking
  = tcTyVarBndrs tvs  $ \ tvs' -> do 
  { traceTc (text "type family: " <+> ppr tc_name) 

        -- Check that we don't use families without -XTypeFamilies
  ; idx_tys <- doptM Opt_TypeFamilies
  ; checkTc idx_tys $ badFamInstDecl tc_name

  ; tycon <- buildSynTyCon tc_name tvs' (OpenSynTyCon kind Nothing) kind Nothing
  ; return [ATyCon tycon]
  }

  -- "data family" declaration
tcTyClDecl1 _calc_isrec 
  (TyFamily {tcdFlavour = DataFamily, 
             tcdLName = L _ tc_name, tcdTyVars = tvs, tcdKind = mb_kind})
  = tcTyVarBndrs tvs  $ \ tvs' -> do 
  { traceTc (text "data family: " <+> ppr tc_name) 
  ; extra_tvs <- tcDataKindSig mb_kind
  ; let final_tvs = tvs' ++ extra_tvs    -- we may not need these


        -- Check that we don't use families without -XTypeFamilies
  ; idx_tys <- doptM Opt_TypeFamilies
  ; checkTc idx_tys $ badFamInstDecl tc_name

  ; tycon <- buildAlgTyCon tc_name final_tvs [] 
               mkOpenDataTyConRhs Recursive False True Nothing
  ; return [ATyCon tycon]
  }

  -- "newtype" and "data"
  -- NB: not used for newtype/data instances (whether associated or not)
tcTyClDecl1 calc_isrec
  (TyData {tcdND = new_or_data, tcdCtxt = ctxt, tcdTyVars = tvs,
           tcdLName = L _ tc_name, tcdKindSig = mb_ksig, tcdCons = cons})
  = tcTyVarBndrs tvs    $ \ tvs' -> do 
  { extra_tvs <- tcDataKindSig mb_ksig
  ; let final_tvs = tvs' ++ extra_tvs
  ; stupid_theta <- tcHsKindedContext ctxt
  ; want_generic <- doptM Opt_Generics
  ; unbox_strict <- doptM Opt_UnboxStrictFields
  ; empty_data_decls <- doptM Opt_EmptyDataDecls
  ; kind_signatures <- doptM Opt_KindSignatures
  ; existential_ok <- doptM Opt_ExistentialQuantification
  ; gadt_ok      <- doptM Opt_GADTs
  ; is_boot      <- tcIsHsBoot  -- Are we compiling an hs-boot file?
  ; let ex_ok = existential_ok || gadt_ok       -- Data cons can have existential context

        -- Check that we don't use GADT syntax in H98 world
  ; checkTc (gadt_ok || h98_syntax) (badGadtDecl tc_name)

        -- Check that we don't use kind signatures without Glasgow extensions
  ; checkTc (kind_signatures || isNothing mb_ksig) (badSigTyDecl tc_name)

        -- Check that the stupid theta is empty for a GADT-style declaration
  ; checkTc (null stupid_theta || h98_syntax) (badStupidTheta tc_name)

        -- Check that a newtype has exactly one constructor
        -- Do this before checking for empty data decls, so that
        -- we don't suggest -XEmptyDataDecls for newtypes
  ; checkTc (new_or_data == DataType || isSingleton cons) 
            (newtypeConError tc_name (length cons))

        -- Check that there's at least one condecl,
        -- or else we're reading an hs-boot file, or -XEmptyDataDecls
  ; checkTc (not (null cons) || empty_data_decls || is_boot)
            (emptyConDeclsErr tc_name)
    
  ; tycon <- fixM (\ tycon -> do 
        { let res_ty = mkTyConApp tycon (mkTyVarTys final_tvs)
        ; data_cons <- tcConDecls unbox_strict ex_ok 
                                  tycon (final_tvs, res_ty) cons
        ; tc_rhs <-
            if null cons && is_boot     -- In a hs-boot file, empty cons means
            then return AbstractTyCon   -- "don't know"; hence Abstract
            else case new_or_data of
                   DataType -> return (mkDataTyConRhs data_cons)
                   NewType  -> ASSERT( not (null data_cons) )
                               mkNewTyConRhs tc_name tycon (head data_cons)
        ; buildAlgTyCon tc_name final_tvs stupid_theta tc_rhs is_rec
            (want_generic && canDoGenerics data_cons) h98_syntax Nothing
        })
  ; return [ATyCon tycon]
  }
  where
    is_rec   = calc_isrec tc_name
    h98_syntax = consUseH98Syntax cons

tcTyClDecl1 calc_isrec 
  (ClassDecl {tcdLName = L _ class_name, tcdTyVars = tvs, 
              tcdCtxt = ctxt, tcdMeths = meths,
              tcdFDs = fundeps, tcdSigs = sigs, tcdATs = ats} )
  = tcTyVarBndrs tvs            $ \ tvs' -> do 
  { ctxt' <- tcHsKindedContext ctxt
  ; fds' <- mapM (addLocM tc_fundep) fundeps
  ; atss <- mapM (addLocM (tcTyClDecl1 (const Recursive))) ats
            -- NB: 'ats' only contains "type family" and "data family"
            --     declarations as well as type family defaults
  ; let ats' = map (setAssocFamilyPermutation tvs') (concat atss)
  ; sig_stuff <- tcClassSigs class_name sigs meths
  ; clas <- fixM (\ clas ->
                let     -- This little knot is just so we can get
                        -- hold of the name of the class TyCon, which we
                        -- need to look up its recursiveness
                    tycon_name = tyConName (classTyCon clas)
                    tc_isrec = calc_isrec tycon_name
                in
                buildClass False {- Must include unfoldings for selectors -}
                           class_name tvs' ctxt' fds' ats'
                           sig_stuff tc_isrec)
  ; return (AClass clas : ats')
      -- NB: Order is important due to the call to `mkGlobalThings' when
      --     tying the the type and class declaration type checking knot.
  }
  where
    tc_fundep (tvs1, tvs2) = do { tvs1' <- mapM tcLookupTyVar tvs1 ;
                                ; tvs2' <- mapM tcLookupTyVar tvs2 ;
                                ; return (tvs1', tvs2') }

tcTyClDecl1 _
  (ForeignType {tcdLName = L _ tc_name, tcdExtName = tc_ext_name})
  = return [ATyCon (mkForeignTyCon tc_name tc_ext_name liftedTypeKind 0)]

tcTyClDecl1 _ d = pprPanic "tcTyClDecl1" (ppr d)

-----------------------------------
tcConDecls :: Bool -> Bool -> TyCon -> ([TyVar], Type)
           -> [LConDecl Name] -> TcM [DataCon]
tcConDecls unbox ex_ok rep_tycon res_tmpl cons
  = mapM (addLocM (tcConDecl unbox ex_ok rep_tycon res_tmpl)) cons

tcConDecl :: Bool               -- True <=> -funbox-strict_fields
          -> Bool               -- True <=> -XExistentialQuantificaton or -XGADTs
          -> TyCon              -- Representation tycon
          -> ([TyVar], Type)    -- Return type template (with its template tyvars)
          -> ConDecl Name 
          -> TcM DataCon

tcConDecl unbox_strict existential_ok rep_tycon res_tmpl        -- Data types
          (ConDecl {con_name =name, con_qvars = tvs, con_cxt = ctxt
                   , con_details = details, con_res = res_ty })
  = addErrCtxt (dataConCtxt name)       $ 
    tcTyVarBndrs tvs                    $ \ tvs' -> do 
    { ctxt' <- tcHsKindedContext ctxt
    ; checkTc (existential_ok || (null tvs && null (unLoc ctxt)))
              (badExistential name)
    ; (univ_tvs, ex_tvs, eq_preds, res_ty') <- tcResultType res_tmpl tvs' res_ty
    ; let 
        tc_datacon is_infix field_lbls btys
          = do { (arg_tys, stricts) <- mapAndUnzipM (tcConArg unbox_strict) btys
               ; buildDataCon (unLoc name) is_infix
                    stricts field_lbls
                    univ_tvs ex_tvs eq_preds ctxt' arg_tys
                    res_ty' rep_tycon }
                -- NB:  we put data_tc, the type constructor gotten from the
                --      constructor type signature into the data constructor;
                --      that way checkValidDataCon can complain if it's wrong.

    ; case details of
        PrefixCon btys     -> tc_datacon False [] btys
        InfixCon bty1 bty2 -> tc_datacon True  [] [bty1,bty2]
        RecCon fields      -> tc_datacon False field_names btys
                           where
                              field_names = map (unLoc . cd_fld_name) fields
                              btys        = map cd_fld_type fields
    }

-- Example
--   data instance T (b,c) where 
--      TI :: forall e. e -> T (e,e)
--
-- The representation tycon looks like this:
--   data :R7T b c where 
--      TI :: forall b1 c1. (b1 ~ c1) => b1 -> :R7T b1 c1
-- In this case orig_res_ty = T (e,e)

tcResultType :: ([TyVar], Type) -- Template for result type; e.g.
                                -- data instance T [a] b c = ...  
                                --      gives template ([a,b,c], T [a] b c)
             -> [TyVar]         -- where MkT :: forall x y z. ...
             -> ResType Name
             -> TcM ([TyVar],           -- Universal
                     [TyVar],           -- Existential (distinct OccNames from univs)
                     [(TyVar,Type)],    -- Equality predicates
                     Type)              -- Typechecked return type
        -- We don't check that the TyCon given in the ResTy is
        -- the same as the parent tycon, becuase we are in the middle
        -- of a recursive knot; so it's postponed until checkValidDataCon

tcResultType (tmpl_tvs, res_ty) dc_tvs ResTyH98
  = return (tmpl_tvs, dc_tvs, [], res_ty)
        -- In H98 syntax the dc_tvs are the existential ones
        --      data T a b c = forall d e. MkT ...
        -- The {a,b,c} are tc_tvs, and {d,e} are dc_tvs

tcResultType (tmpl_tvs, res_tmpl) dc_tvs (ResTyGADT res_ty)
        -- E.g.  data T [a] b c where
        --         MkT :: forall x y z. T [(x,y)] z z
        -- Then we generate
        --      Univ tyvars     Eq-spec
        --          a              a~(x,y)
        --          b              b~z
        --          z              
        -- Existentials are the leftover type vars: [x,y]
        -- So we return ([a,b,z], [x,y], [a~(x,y),b~z], T [(x,y)] z z)
  = do  { res_ty' <- tcHsKindedType res_ty
        ; let Just subst = tcMatchTy (mkVarSet tmpl_tvs) res_tmpl res_ty'

                -- /Lazily/ figure out the univ_tvs etc
                -- Each univ_tv is either a dc_tv or a tmpl_tv
              (univ_tvs, eq_spec) = foldr choose ([], []) tidy_tmpl_tvs
              choose tmpl (univs, eqs)
                | Just ty <- lookupTyVar subst tmpl 
                = case tcGetTyVar_maybe ty of
                    Just tv | not (tv `elem` univs)
                            -> (tv:univs,   eqs)
                    _other  -> (tmpl:univs, (tmpl,ty):eqs)
                | otherwise = pprPanic "tcResultType" (ppr res_ty)
              ex_tvs = dc_tvs `minusList` univ_tvs

        ; return (univ_tvs, ex_tvs, eq_spec, res_ty') }
  where
        -- NB: tmpl_tvs and dc_tvs are distinct, but
        -- we want them to be *visibly* distinct, both for
        -- interface files and general confusion.  So rename
        -- the tc_tvs, since they are not used yet (no 
        -- consequential renaming needed)
    (_, tidy_tmpl_tvs) = mapAccumL tidy_one init_occ_env tmpl_tvs
    init_occ_env       = initTidyOccEnv (map getOccName dc_tvs)
    tidy_one env tv    = (env', setTyVarName tv (tidyNameOcc name occ'))
              where
                 name = tyVarName tv
                 (env', occ') = tidyOccName env (getOccName name) 

consUseH98Syntax :: [LConDecl a] -> Bool
consUseH98Syntax (L _ (ConDecl { con_res = ResTyGADT _ }) : _) = False
consUseH98Syntax _                                             = True
                 -- All constructors have same shape

-------------------
tcConArg :: Bool                -- True <=> -funbox-strict_fields
           -> LHsType Name
           -> TcM (TcType, StrictnessMark)
tcConArg unbox_strict bty
  = do  { arg_ty <- tcHsBangType bty
        ; let bang = getBangStrictness bty
        ; return (arg_ty, chooseBoxingStrategy unbox_strict arg_ty bang) }

-- We attempt to unbox/unpack a strict field when either:
--   (i)  The field is marked '!!', or
--   (ii) The field is marked '!', and the -funbox-strict-fields flag is on.
--
-- We have turned off unboxing of newtypes because coercions make unboxing 
-- and reboxing more complicated
chooseBoxingStrategy :: Bool -> TcType -> HsBang -> StrictnessMark
chooseBoxingStrategy unbox_strict_fields arg_ty bang
  = case bang of
        HsNoBang                                    -> NotMarkedStrict
        HsStrict | unbox_strict_fields 
                   && can_unbox arg_ty              -> MarkedUnboxed
        HsUnbox  | can_unbox arg_ty                 -> MarkedUnboxed
        _                                           -> MarkedStrict
  where
    -- we can unbox if the type is a chain of newtypes with a product tycon
    -- at the end
    can_unbox arg_ty = case splitTyConApp_maybe arg_ty of
                   Nothing                      -> False
                   Just (arg_tycon, tycon_args) -> 
                       not (isRecursiveTyCon arg_tycon) &&      -- Note [Recusive unboxing]
                       isProductTyCon arg_tycon &&
                       (if isNewTyCon arg_tycon then 
                            can_unbox (newTyConInstRhs arg_tycon tycon_args)
                        else True)
\end{code}

Note [Recursive unboxing]
~~~~~~~~~~~~~~~~~~~~~~~~~
Be careful not to try to unbox this!
        data T = MkT !T Int
But it's the *argument* type that matters. This is fine:
        data S = MkS S !Int
because Int is non-recursive.


%************************************************************************
%*                                                                      *
                Validity checking
%*                                                                      *
%************************************************************************

Validity checking is done once the mutually-recursive knot has been
tied, so we can look at things freely.

\begin{code}
checkCycleErrs :: [LTyClDecl Name] -> TcM ()
checkCycleErrs tyclss
  | null cls_cycles
  = return ()
  | otherwise
  = do  { mapM_ recClsErr cls_cycles
        ; failM }       -- Give up now, because later checkValidTyCl
                        -- will loop if the synonym is recursive
  where
    cls_cycles = calcClassCycles tyclss

checkValidTyCl :: TyClDecl Name -> TcM ()
-- We do the validity check over declarations, rather than TyThings
-- only so that we can add a nice context with tcAddDeclCtxt
checkValidTyCl decl
  = tcAddDeclCtxt decl $
    do  { thing <- tcLookupLocatedGlobal (tcdLName decl)
        ; traceTc (text "Validity of" <+> ppr thing)    
        ; case thing of
            ATyCon tc -> checkValidTyCon tc
            AClass cl -> checkValidClass cl 
            _ -> panic "checkValidTyCl"
        ; traceTc (text "Done validity of" <+> ppr thing)       
        }

-------------------------
-- For data types declared with record syntax, we require
-- that each constructor that has a field 'f' 
--      (a) has the same result type
--      (b) has the same type for 'f'
-- module alpha conversion of the quantified type variables
-- of the constructor.
--
-- Note that we allow existentials to match becuase the
-- fields can never meet. E.g
--      data T where
--        T1 { f1 :: b, f2 :: a, f3 ::Int } :: T
--        T2 { f1 :: c, f2 :: c, f3 ::Int } :: T  
-- Here we do not complain about f1,f2 because they are existential

checkValidTyCon :: TyCon -> TcM ()
checkValidTyCon tc 
  | isSynTyCon tc 
  = case synTyConRhs tc of
      OpenSynTyCon _ _ -> return ()
      SynonymTyCon ty  -> checkValidType syn_ctxt ty
  | otherwise
  = do  -- Check the context on the data decl
    checkValidTheta (DataTyCtxt name) (tyConStupidTheta tc)
        
        -- Check arg types of data constructors
    mapM_ (checkValidDataCon tc) data_cons

        -- Check that fields with the same name share a type
    mapM_ check_fields groups

  where
    syn_ctxt  = TySynCtxt name
    name      = tyConName tc
    data_cons = tyConDataCons tc

    groups = equivClasses cmp_fld (concatMap get_fields data_cons)
    cmp_fld (f1,_) (f2,_) = f1 `compare` f2
    get_fields con = dataConFieldLabels con `zip` repeat con
        -- dataConFieldLabels may return the empty list, which is fine

    -- See Note [GADT record selectors] in MkId.lhs
    -- We must check (a) that the named field has the same 
    --                   type in each constructor
    --               (b) that those constructors have the same result type
    --
    -- However, the constructors may have differently named type variable
    -- and (worse) we don't know how the correspond to each other.  E.g.
    --     C1 :: forall a b. { f :: a, g :: b } -> T a b
    --     C2 :: forall d c. { f :: c, g :: c } -> T c d
    -- 
    -- So what we do is to ust Unify.tcMatchTys to compare the first candidate's
    -- result type against other candidates' types BOTH WAYS ROUND.
    -- If they magically agrees, take the substitution and
    -- apply them to the latter ones, and see if they match perfectly.
    check_fields ((label, con1) : other_fields)
        -- These fields all have the same name, but are from
        -- different constructors in the data type
        = recoverM (return ()) $ mapM_ checkOne other_fields
                -- Check that all the fields in the group have the same type
                -- NB: this check assumes that all the constructors of a given
                -- data type use the same type variables
        where
        (tvs1, _, _, res1) = dataConSig con1
        ts1 = mkVarSet tvs1
        fty1 = dataConFieldType con1 label

        checkOne (_, con2)    -- Do it bothways to ensure they are structurally identical
            = do { checkFieldCompat label con1 con2 ts1 res1 res2 fty1 fty2
                 ; checkFieldCompat label con2 con1 ts2 res2 res1 fty2 fty1 }
            where        
                (tvs2, _, _, res2) = dataConSig con2
                ts2 = mkVarSet tvs2
                fty2 = dataConFieldType con2 label
    check_fields [] = panic "checkValidTyCon/check_fields []"

checkFieldCompat :: Name -> DataCon -> DataCon -> TyVarSet
                 -> Type -> Type -> Type -> Type -> TcM ()
checkFieldCompat fld con1 con2 tvs1 res1 res2 fty1 fty2
  = do  { checkTc (isJust mb_subst1) (resultTypeMisMatch fld con1 con2)
        ; checkTc (isJust mb_subst2) (fieldTypeMisMatch fld con1 con2) }
  where
    mb_subst1 = tcMatchTy tvs1 res1 res2
    mb_subst2 = tcMatchTyX tvs1 (expectJust "checkFieldCompat" mb_subst1) fty1 fty2

-------------------------------
checkValidDataCon :: TyCon -> DataCon -> TcM ()
checkValidDataCon tc con
  = setSrcSpan (srcLocSpan (getSrcLoc con))     $
    addErrCtxt (dataConCtxt con)                $ 
    do  { traceTc (ptext (sLit "Validity of data con") <+> ppr con)
        ; let tc_tvs = tyConTyVars tc
              res_ty_tmpl = mkFamilyTyConApp tc (mkTyVarTys tc_tvs)
              actual_res_ty = dataConOrigResTy con
        ; checkTc (isJust (tcMatchTy (mkVarSet tc_tvs)
                                res_ty_tmpl
                                actual_res_ty))
                  (badDataConTyCon con res_ty_tmpl actual_res_ty)
        ; checkValidMonoType (dataConOrigResTy con)
                -- Disallow MkT :: T (forall a. a->a)
                -- Reason: it's really the argument of an equality constraint
        ; checkValidType ctxt (dataConUserType con)
        ; when (isNewTyCon tc) (checkNewDataCon con)
    }
  where
    ctxt = ConArgCtxt (dataConName con) 

-------------------------------
checkNewDataCon :: DataCon -> TcM ()
-- Checks for the data constructor of a newtype
checkNewDataCon con
  = do  { checkTc (isSingleton arg_tys) (newtypeFieldErr con (length arg_tys))
                -- One argument
        ; checkTc (null eq_spec) (newtypePredError con)
                -- Return type is (T a b c)
        ; checkTc (null ex_tvs && null eq_theta && null dict_theta) (newtypeExError con)
                -- No existentials
        ; checkTc (not (any isMarkedStrict (dataConStrictMarks con))) 
                  (newtypeStrictError con)
                -- No strictness
    }
  where
    (_univ_tvs, ex_tvs, eq_spec, eq_theta, dict_theta, arg_tys, _res_ty) = dataConFullSig con

-------------------------------
checkValidClass :: Class -> TcM ()
checkValidClass cls
  = do  { constrained_class_methods <- doptM Opt_ConstrainedClassMethods
        ; multi_param_type_classes <- doptM Opt_MultiParamTypeClasses
        ; fundep_classes <- doptM Opt_FunctionalDependencies

        -- Check that the class is unary, unless GlaExs
        ; checkTc (notNull tyvars) (nullaryClassErr cls)
        ; checkTc (multi_param_type_classes || unary) (classArityErr cls)
        ; checkTc (fundep_classes || null fundeps) (classFunDepsErr cls)

        -- Check the super-classes
        ; checkValidTheta (ClassSCCtxt (className cls)) theta

        -- Check the class operations
        ; mapM_ (check_op constrained_class_methods) op_stuff

        -- Check that if the class has generic methods, then the
        -- class has only one parameter.  We can't do generic
        -- multi-parameter type classes!
        ; checkTc (unary || no_generics) (genericMultiParamErr cls)
        }
  where
    (tyvars, fundeps, theta, _, _, op_stuff) = classExtraBigSig cls
    unary       = isSingleton tyvars
    no_generics = null [() | (_, GenDefMeth) <- op_stuff]

    check_op constrained_class_methods (sel_id, dm) 
      = addErrCtxt (classOpCtxt sel_id tau) $ do
        { checkValidTheta SigmaCtxt (tail theta)
                -- The 'tail' removes the initial (C a) from the
                -- class itself, leaving just the method type

        ; traceTc (text "class op type" <+> ppr op_ty <+> ppr tau)
        ; checkValidType (FunSigCtxt op_name) tau

                -- Check that the type mentions at least one of
                -- the class type variables...or at least one reachable
                -- from one of the class variables.  Example: tc223
                --   class Error e => Game b mv e | b -> mv e where
                --      newBoard :: MonadState b m => m ()
                -- Here, MonadState has a fundep m->b, so newBoard is fine
        ; let grown_tyvars = growThetaTyVars theta (mkVarSet tyvars)
        ; checkTc (tyVarsOfType tau `intersectsVarSet` grown_tyvars)
                  (noClassTyVarErr cls sel_id)

                -- Check that for a generic method, the type of 
                -- the method is sufficiently simple
        ; checkTc (dm /= GenDefMeth || validGenericMethodType tau)
                  (badGenericMethodType op_name op_ty)
        }
        where
          op_name = idName sel_id
          op_ty   = idType sel_id
          (_,theta1,tau1) = tcSplitSigmaTy op_ty
          (_,theta2,tau2)  = tcSplitSigmaTy tau1
          (theta,tau) | constrained_class_methods = (theta1 ++ theta2, tau2)
                      | otherwise = (theta1, mkPhiTy (tail theta1) tau1)
                -- Ugh!  The function might have a type like
                --      op :: forall a. C a => forall b. (Eq b, Eq a) => tau2
                -- With -XConstrainedClassMethods, we want to allow this, even though the inner 
                -- forall has an (Eq a) constraint.  Whereas in general, each constraint 
                -- in the context of a for-all must mention at least one quantified
                -- type variable.  What a mess!
\end{code}


%************************************************************************
%*                                                                      *
                Building record selectors
%*                                                                      *
%************************************************************************

\begin{code}
mkAuxBinds :: [TyThing] -> HsValBinds Name
-- NB We produce *un-typechecked* bindings, rather like 'deriving'
--    This makes life easier, because the later type checking will add
--    all necessary type abstractions and applications
mkAuxBinds ty_things
  = ValBindsOut [(NonRecursive, b) | b <- binds] sigs
  where
    (sigs, binds) = unzip rec_sels
    rec_sels = map mkRecSelBind [ (tc,fld) 
                                | ATyCon tc <- ty_things 
                                , fld <- tyConFields tc ]

mkRecSelBind :: (TyCon, FieldLabel) -> (LSig Name, LHsBinds Name)
mkRecSelBind (tycon, sel_name)
  = (L loc (IdSig sel_id), unitBag (L loc sel_bind))
  where
    loc         = getSrcSpan tycon    
    sel_id      = Var.mkLocalVar rec_details sel_name sel_ty vanillaIdInfo
    rec_details = RecSelId { sel_tycon = tycon, sel_naughty = is_naughty }

    -- Find a representative constructor, con1
    all_cons     = tyConDataCons tycon 
    cons_w_field = [ con | con <- all_cons
                   , sel_name `elem` dataConFieldLabels con ] 
    con1 = ASSERT( not (null cons_w_field) ) head cons_w_field

    -- Selector type; Note [Polymorphic selectors]
    field_ty   = dataConFieldType con1 sel_name
    data_ty    = dataConOrigResTy con1
    data_tvs   = tyVarsOfType data_ty
    is_naughty = not (tyVarsOfType field_ty `subVarSet` data_tvs)  
    (field_tvs, field_theta, field_tau) = tcSplitSigmaTy field_ty
    sel_ty | is_naughty = unitTy  -- See Note [Naughty record selectors]
           | otherwise  = mkForAllTys (varSetElems data_tvs ++ field_tvs) $ 
                          mkPhiTy (dataConStupidTheta con1) $   -- Urgh!
                          mkPhiTy field_theta               $   -- Urgh!
                          mkFunTy data_ty field_tau

    -- Make the binding: sel (C2 { fld = x }) = x
    --                   sel (C7 { fld = x }) = x
    --    where cons_w_field = [C2,C7]
    sel_bind | is_naughty = mkFunBind sel_lname [mkSimpleMatch [] unit_rhs]
             | otherwise  = mkFunBind sel_lname (map mk_match cons_w_field ++ deflt)
    mk_match con = mkSimpleMatch [L loc (mk_sel_pat con)] 
                                 (L loc (HsVar field_var))
    mk_sel_pat con = ConPatIn (L loc (getName con)) (RecCon rec_fields)
    rec_fields = HsRecFields { rec_flds = [rec_field], rec_dotdot = Nothing }
    rec_field  = HsRecField { hsRecFieldId = sel_lname
                            , hsRecFieldArg = nlVarPat field_var
                            , hsRecPun = False }
    sel_lname = L loc sel_name
    field_var = mkInternalName (mkBuiltinUnique 1) (getOccName sel_name) loc

    -- Add catch-all default case unless the case is exhaustive
    -- We do this explicitly so that we get a nice error message that
    -- mentions this particular record selector
    deflt | length cons_w_field == length all_cons = []
          | otherwise = [mkSimpleMatch [nlWildPat] 
                            (nlHsApp (nlHsVar (getName rEC_SEL_ERROR_ID))
                                     (nlHsLit msg_lit))]

    unit_rhs = L loc $ ExplicitTuple [] Boxed
    msg_lit = HsStringPrim $ mkFastString $ 
              occNameString (getOccName sel_name)

---------------
tyConFields :: TyCon -> [FieldLabel]
tyConFields tc 
  | isAlgTyCon tc = nub (concatMap dataConFieldLabels (tyConDataCons tc))
  | otherwise     = []
\end{code}

Note [Polymorphic selectors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When a record has a polymorphic field, we pull the foralls out to the front.
   data T = MkT { f :: forall a. [a] -> a }
Then f :: forall a. T -> [a] -> a
NOT  f :: T -> forall a. [a] -> a

This is horrid.  It's only needed in deeply obscure cases, which I hate.
The only case I know is test tc163, which is worth looking at.  It's far
from clear that this test should succeed at all!

Note [Naughty record selectors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A "naughty" field is one for which we can't define a record 
selector, because an existential type variable would escape.  For example:
        data T = forall a. MkT { x,y::a }
We obviously can't define       
        x (MkT v _) = v
Nevertheless we *do* put a RecSelId into the type environment
so that if the user tries to use 'x' as a selector we can bleat
helpfully, rather than saying unhelpfully that 'x' is not in scope.
Hence the sel_naughty flag, to identify record selectors that don't really exist.

In general, a field is "naughty" if its type mentions a type variable that
isn't in the result type of the constructor.  Note that this *allows*
GADT record selectors (Note [GADT record selectors]) whose types may look 
like     sel :: T [a] -> a

For naughty selectors we make a dummy binding 
   sel = ()
for naughty selectors, so that the later type-check will add them to the
environment, and they'll be exported.  The function is never called, because
the tyepchecker spots the sel_naughty field.

Note [GADT record selectors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For GADTs, we require that all constructors with a common field 'f' have the same
result type (modulo alpha conversion).  [Checked in TcTyClsDecls.checkValidTyCon]
E.g. 
        data T where
          T1 { f :: Maybe a } :: T [a]
          T2 { f :: Maybe a, y :: b  } :: T [a]

and now the selector takes that result type as its argument:
   f :: forall a. T [a] -> Maybe a

Details: the "real" types of T1,T2 are:
   T1 :: forall r a.   (r~[a]) => a -> T r
   T2 :: forall r a b. (r~[a]) => a -> b -> T r

So the selector loooks like this:
   f :: forall a. T [a] -> Maybe a
   f (a:*) (t:T [a])
     = case t of
         T1 c   (g:[a]~[c]) (v:Maybe c)       -> v `cast` Maybe (right (sym g))
         T2 c d (g:[a]~[c]) (v:Maybe c) (w:d) -> v `cast` Maybe (right (sym g))

Note the forall'd tyvars of the selector are just the free tyvars
of the result type; there may be other tyvars in the constructor's
type (e.g. 'b' in T2).

Note the need for casts in the result!

Note [Selector running example]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's OK to combine GADTs and type families.  Here's a running example:

        data instance T [a] where 
          T1 { fld :: b } :: T [Maybe b]

The representation type looks like this
        data :R7T a where
          T1 { fld :: b } :: :R7T (Maybe b)

and there's coercion from the family type to the representation type
        :CoR7T a :: T [a] ~ :R7T a

The selector we want for fld looks like this:

        fld :: forall b. T [Maybe b] -> b
        fld = /\b. \(d::T [Maybe b]).
              case d `cast` :CoR7T (Maybe b) of 
                T1 (x::b) -> x

The scrutinee of the case has type :R7T (Maybe b), which can be
gotten by appying the eq_spec to the univ_tvs of the data con.

%************************************************************************
%*                                                                      *
                Error messages
%*                                                                      *
%************************************************************************

\begin{code}
resultTypeMisMatch :: Name -> DataCon -> DataCon -> SDoc
resultTypeMisMatch field_name con1 con2
  = vcat [sep [ptext (sLit "Constructors") <+> ppr con1 <+> ptext (sLit "and") <+> ppr con2, 
                ptext (sLit "have a common field") <+> quotes (ppr field_name) <> comma],
          nest 2 $ ptext (sLit "but have different result types")]

fieldTypeMisMatch :: Name -> DataCon -> DataCon -> SDoc
fieldTypeMisMatch field_name con1 con2
  = sep [ptext (sLit "Constructors") <+> ppr con1 <+> ptext (sLit "and") <+> ppr con2, 
         ptext (sLit "give different types for field"), quotes (ppr field_name)]

dataConCtxt :: Outputable a => a -> SDoc
dataConCtxt con = ptext (sLit "In the definition of data constructor") <+> quotes (ppr con)

classOpCtxt :: Var -> Type -> SDoc
classOpCtxt sel_id tau = sep [ptext (sLit "When checking the class method:"),
                              nest 2 (ppr sel_id <+> dcolon <+> ppr tau)]

nullaryClassErr :: Class -> SDoc
nullaryClassErr cls
  = ptext (sLit "No parameters for class")  <+> quotes (ppr cls)

classArityErr :: Class -> SDoc
classArityErr cls
  = vcat [ptext (sLit "Too many parameters for class") <+> quotes (ppr cls),
          parens (ptext (sLit "Use -XMultiParamTypeClasses to allow multi-parameter classes"))]

classFunDepsErr :: Class -> SDoc
classFunDepsErr cls
  = vcat [ptext (sLit "Fundeps in class") <+> quotes (ppr cls),
          parens (ptext (sLit "Use -XFunctionalDependencies to allow fundeps"))]

noClassTyVarErr :: Class -> Var -> SDoc
noClassTyVarErr clas op
  = sep [ptext (sLit "The class method") <+> quotes (ppr op),
         ptext (sLit "mentions none of the type variables of the class") <+> 
                ppr clas <+> hsep (map ppr (classTyVars clas))]

genericMultiParamErr :: Class -> SDoc
genericMultiParamErr clas
  = ptext (sLit "The multi-parameter class") <+> quotes (ppr clas) <+> 
    ptext (sLit "cannot have generic methods")

badGenericMethodType :: Name -> Kind -> SDoc
badGenericMethodType op op_ty
  = hang (ptext (sLit "Generic method type is too complex"))
       4 (vcat [ppr op <+> dcolon <+> ppr op_ty,
                ptext (sLit "You can only use type variables, arrows, lists, and tuples")])

recSynErr :: [LTyClDecl Name] -> TcRn ()
recSynErr syn_decls
  = setSrcSpan (getLoc (head sorted_decls)) $
    addErr (sep [ptext (sLit "Cycle in type synonym declarations:"),
                 nest 2 (vcat (map ppr_decl sorted_decls))])
  where
    sorted_decls = sortLocated syn_decls
    ppr_decl (L loc decl) = ppr loc <> colon <+> ppr decl

recClsErr :: [Located (TyClDecl Name)] -> TcRn ()
recClsErr cls_decls
  = setSrcSpan (getLoc (head sorted_decls)) $
    addErr (sep [ptext (sLit "Cycle in class declarations (via superclasses):"),
                 nest 2 (vcat (map ppr_decl sorted_decls))])
  where
    sorted_decls = sortLocated cls_decls
    ppr_decl (L loc decl) = ppr loc <> colon <+> ppr (decl { tcdSigs = [] })

sortLocated :: [Located a] -> [Located a]
sortLocated things = sortLe le things
  where
    le (L l1 _) (L l2 _) = l1 <= l2

badDataConTyCon :: DataCon -> Type -> Type -> SDoc
badDataConTyCon data_con res_ty_tmpl actual_res_ty
  = hang (ptext (sLit "Data constructor") <+> quotes (ppr data_con) <+>
                ptext (sLit "returns type") <+> quotes (ppr actual_res_ty))
       2 (ptext (sLit "instead of an instance of its parent type") <+> quotes (ppr res_ty_tmpl))

badGadtDecl :: Name -> SDoc
badGadtDecl tc_name
  = vcat [ ptext (sLit "Illegal generalised algebraic data declaration for") <+> quotes (ppr tc_name)
         , nest 2 (parens $ ptext (sLit "Use -XGADTs to allow GADTs")) ]

badExistential :: Located Name -> SDoc
badExistential con_name
  = hang (ptext (sLit "Data constructor") <+> quotes (ppr con_name) <+>
                ptext (sLit "has existential type variables, or a context"))
       2 (parens $ ptext (sLit "Use -XExistentialQuantification or -XGADTs to allow this"))

badStupidTheta :: Name -> SDoc
badStupidTheta tc_name
  = ptext (sLit "A data type declared in GADT style cannot have a context:") <+> quotes (ppr tc_name)

newtypeConError :: Name -> Int -> SDoc
newtypeConError tycon n
  = sep [ptext (sLit "A newtype must have exactly one constructor,"),
         nest 2 $ ptext (sLit "but") <+> quotes (ppr tycon) <+> ptext (sLit "has") <+> speakN n ]

newtypeExError :: DataCon -> SDoc
newtypeExError con
  = sep [ptext (sLit "A newtype constructor cannot have an existential context,"),
         nest 2 $ ptext (sLit "but") <+> quotes (ppr con) <+> ptext (sLit "does")]

newtypeStrictError :: DataCon -> SDoc
newtypeStrictError con
  = sep [ptext (sLit "A newtype constructor cannot have a strictness annotation,"),
         nest 2 $ ptext (sLit "but") <+> quotes (ppr con) <+> ptext (sLit "does")]

newtypePredError :: DataCon -> SDoc
newtypePredError con
  = sep [ptext (sLit "A newtype constructor must have a return type of form T a1 ... an"),
         nest 2 $ ptext (sLit "but") <+> quotes (ppr con) <+> ptext (sLit "does not")]

newtypeFieldErr :: DataCon -> Int -> SDoc
newtypeFieldErr con_name n_flds
  = sep [ptext (sLit "The constructor of a newtype must have exactly one field"), 
         nest 2 $ ptext (sLit "but") <+> quotes (ppr con_name) <+> ptext (sLit "has") <+> speakN n_flds]

badSigTyDecl :: Name -> SDoc
badSigTyDecl tc_name
  = vcat [ ptext (sLit "Illegal kind signature") <+>
           quotes (ppr tc_name)
         , nest 2 (parens $ ptext (sLit "Use -XKindSignatures to allow kind signatures")) ]

badFamInstDecl :: Outputable a => a -> SDoc
badFamInstDecl tc_name
  = vcat [ ptext (sLit "Illegal family instance for") <+>
           quotes (ppr tc_name)
         , nest 2 (parens $ ptext (sLit "Use -XTypeFamilies to allow indexed type families")) ]

tooManyParmsErr :: Located Name -> SDoc
tooManyParmsErr tc_name
  = ptext (sLit "Family instance has too many parameters:") <+> 
    quotes (ppr tc_name)

tooFewParmsErr :: Arity -> SDoc
tooFewParmsErr arity
  = ptext (sLit "Family instance has too few parameters; expected") <+> 
    ppr arity

wrongNumberOfParmsErr :: Arity -> SDoc
wrongNumberOfParmsErr exp_arity
  = ptext (sLit "Number of parameters must match family declaration; expected")
    <+> ppr exp_arity

badBootFamInstDeclErr :: SDoc
badBootFamInstDeclErr
  = ptext (sLit "Illegal family instance in hs-boot file")

notFamily :: TyCon -> SDoc
notFamily tycon
  = vcat [ ptext (sLit "Illegal family instance for") <+> quotes (ppr tycon)
         , nest 2 $ parens (ppr tycon <+> ptext (sLit "is not an indexed type family"))]
  
wrongKindOfFamily :: TyCon -> SDoc
wrongKindOfFamily family
  = ptext (sLit "Wrong category of family instance; declaration was for a")
    <+> kindOfFamily
  where
    kindOfFamily | isSynTyCon family = ptext (sLit "type synonym")
                 | isAlgTyCon family = ptext (sLit "data type")
                 | otherwise = pprPanic "wrongKindOfFamily" (ppr family)

emptyConDeclsErr :: Name -> SDoc
emptyConDeclsErr tycon
  = sep [quotes (ppr tycon) <+> ptext (sLit "has no constructors"),
         nest 2 $ ptext (sLit "(-XEmptyDataDecls permits this)")]
\end{code}