root/compiler/basicTypes/DataCon.lhs

%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1998
%
\section[DataCon]{@DataCon@: Data Constructors}

\begin{code}
{-# OPTIONS -fno-warn-tabs #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

module DataCon (
        -- * Main data types
        DataCon, DataConIds(..),
        ConTag,
        
        -- ** Type construction
        mkDataCon, fIRST_TAG,
        
        -- ** Type deconstruction
        dataConRepType, dataConSig, dataConFullSig,
        dataConName, dataConIdentity, dataConTag, dataConTyCon, 
        dataConOrigTyCon, dataConUserType,
        dataConUnivTyVars, dataConExTyVars, dataConAllTyVars, 
        dataConEqSpec, eqSpecPreds, dataConTheta,
        dataConStupidTheta,  
        dataConInstArgTys, dataConOrigArgTys, dataConOrigResTy,
        dataConInstOrigArgTys, dataConRepArgTys, 
        dataConFieldLabels, dataConFieldType,
        dataConStrictMarks, dataConExStricts,
        dataConSourceArity, dataConRepArity,
        dataConIsInfix,
        dataConWorkId, dataConWrapId, dataConWrapId_maybe, dataConImplicitIds,
        dataConRepStrictness,
        
        -- ** Predicates on DataCons
        isNullarySrcDataCon, isNullaryRepDataCon, isTupleCon, isUnboxedTupleCon,
        isVanillaDataCon, classDataCon, dataConCannotMatch,

        -- * Splitting product types
        splitProductType_maybe, splitProductType, deepSplitProductType,
        deepSplitProductType_maybe,

        -- ** Promotion related functions
        promoteType, isPromotableType, isPromotableTyCon,
        buildPromotedTyCon, buildPromotedDataCon,
    ) where

#include "HsVersions.h"

import Type
import TypeRep( Type(..) )  -- Used in promoteType
import PrelNames( liftedTypeKindTyConKey )
import Kind
import Unify
import Coercion
import TyCon
import Class
import Name
import Var
import Outputable
import Unique
import ListSetOps
import Util
import BasicTypes
import FastString
import Module
import VarEnv

import qualified Data.Data as Data
import qualified Data.Typeable
import Data.Char
import Data.Word
\end{code}


Data constructor representation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the following Haskell data type declaration

        data T = T !Int ![Int]

Using the strictness annotations, GHC will represent this as

        data T = T Int# [Int]

That is, the Int has been unboxed.  Furthermore, the Haskell source construction

        T e1 e2

is translated to

        case e1 of { I# x -> 
        case e2 of { r ->
        T x r }}

That is, the first argument is unboxed, and the second is evaluated.  Finally,
pattern matching is translated too:

        case e of { T a b -> ... }

becomes

        case e of { T a' b -> let a = I# a' in ... }

To keep ourselves sane, we name the different versions of the data constructor
differently, as follows.


Note [Data Constructor Naming]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Each data constructor C has two, and possibly up to four, Names associated with it:

                   OccName   Name space   Name of   Notes
 ---------------------------------------------------------------------------
 The "data con itself"   C     DataName   DataCon   In dom( GlobalRdrEnv )
 The "worker data con"   C     VarName    Id        The worker
 The "wrapper data con"  $WC   VarName    Id        The wrapper
 The "newtype coercion"  :CoT  TcClsName  TyCon
 
EVERY data constructor (incl for newtypes) has the former two (the
data con itself, and its worker.  But only some data constructors have a
wrapper (see Note [The need for a wrapper]).

Each of these three has a distinct Unique.  The "data con itself" name
appears in the output of the renamer, and names the Haskell-source
data constructor.  The type checker translates it into either the wrapper Id
(if it exists) or worker Id (otherwise).

The data con has one or two Ids associated with it:

The "worker Id", is the actual data constructor.
* Every data constructor (newtype or data type) has a worker

* The worker is very like a primop, in that it has no binding.

* For a *data* type, the worker *is* the data constructor;
  it has no unfolding

* For a *newtype*, the worker has a compulsory unfolding which 
  does a cast, e.g.
        newtype T = MkT Int
        The worker for MkT has unfolding
                \\(x:Int). x `cast` sym CoT
  Here CoT is the type constructor, witnessing the FC axiom
        axiom CoT : T = Int

The "wrapper Id", \$WC, goes as follows

* Its type is exactly what it looks like in the source program. 

* It is an ordinary function, and it gets a top-level binding 
  like any other function.

* The wrapper Id isn't generated for a data type if there is
  nothing for the wrapper to do.  That is, if its defn would be
        \$wC = C

Note [The need for a wrapper]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Why might the wrapper have anything to do?  Two reasons:

* Unboxing strict fields (with -funbox-strict-fields)
        data T = MkT !(Int,Int)
        \$wMkT :: (Int,Int) -> T
        \$wMkT (x,y) = MkT x y
  Notice that the worker has two fields where the wapper has 
  just one.  That is, the worker has type
                MkT :: Int -> Int -> T

* Equality constraints for GADTs
        data T a where { MkT :: a -> T [a] }

  The worker gets a type with explicit equality
  constraints, thus:
        MkT :: forall a b. (a=[b]) => b -> T a

  The wrapper has the programmer-specified type:
        \$wMkT :: a -> T [a]
        \$wMkT a x = MkT [a] a [a] x
  The third argument is a coerion
        [a] :: [a]~[a]

INVARIANT: the dictionary constructor for a class
           never has a wrapper.


A note about the stupid context
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Data types can have a context:
        
        data (Eq a, Ord b) => T a b = T1 a b | T2 a

and that makes the constructors have a context too
(notice that T2's context is "thinned"):

        T1 :: (Eq a, Ord b) => a -> b -> T a b
        T2 :: (Eq a) => a -> T a b

Furthermore, this context pops up when pattern matching
(though GHC hasn't implemented this, but it is in H98, and
I've fixed GHC so that it now does):

        f (T2 x) = x
gets inferred type
        f :: Eq a => T a b -> a

I say the context is "stupid" because the dictionaries passed
are immediately discarded -- they do nothing and have no benefit.
It's a flaw in the language.

        Up to now [March 2002] I have put this stupid context into the
        type of the "wrapper" constructors functions, T1 and T2, but
        that turned out to be jolly inconvenient for generics, and
        record update, and other functions that build values of type T
        (because they don't have suitable dictionaries available).

        So now I've taken the stupid context out.  I simply deal with
        it separately in the type checker on occurrences of a
        constructor, either in an expression or in a pattern.

        [May 2003: actually I think this decision could evasily be
        reversed now, and probably should be.  Generics could be
        disabled for types with a stupid context; record updates now
        (H98) needs the context too; etc.  It's an unforced change, so
        I'm leaving it for now --- but it does seem odd that the
        wrapper doesn't include the stupid context.]

[July 04] With the advent of generalised data types, it's less obvious
what the "stupid context" is.  Consider
        C :: forall a. Ord a => a -> a -> T (Foo a)
Does the C constructor in Core contain the Ord dictionary?  Yes, it must:

        f :: T b -> Ordering
        f = /\b. \x:T b. 
            case x of
                C a (d:Ord a) (p:a) (q:a) -> compare d p q

Note that (Foo a) might not be an instance of Ord.

%************************************************************************
%*                                                                      *
\subsection{Data constructors}
%*                                                                      *
%************************************************************************

\begin{code}
-- | A data constructor
data DataCon
  = MkData {
        dcName    :: Name,      -- This is the name of the *source data con*
                                -- (see "Note [Data Constructor Naming]" above)
        dcUnique :: Unique,     -- Cached from Name
        dcTag    :: ConTag,     -- ^ Tag, used for ordering 'DataCon's

        -- Running example:
        --
        --      *** As declared by the user
        --  data T a where
        --    MkT :: forall x y. (x~y,Ord x) => x -> y -> T (x,y)

        --      *** As represented internally
        --  data T a where
        --    MkT :: forall a. forall x y. (a~(x,y),x~y,Ord x) => x -> y -> T a
        -- 
        -- The next six fields express the type of the constructor, in pieces
        -- e.g.
        --
        --      dcUnivTyVars  = [a]
        --      dcExTyVars    = [x,y]
        --      dcEqSpec      = [a~(x,y)]
        --      dcOtherTheta  = [x~y, Ord x]    
        --      dcOrigArgTys  = [a,List b]
        --      dcRepTyCon       = T

        dcVanilla :: Bool,      -- True <=> This is a vanilla Haskell 98 data constructor
                                --          Its type is of form
                                --              forall a1..an . t1 -> ... tm -> T a1..an
                                --          No existentials, no coercions, nothing.
                                -- That is: dcExTyVars = dcEqSpec = dcOtherTheta = []
                -- NB 1: newtypes always have a vanilla data con
                -- NB 2: a vanilla constructor can still be declared in GADT-style 
                --       syntax, provided its type looks like the above.
                --       The declaration format is held in the TyCon (algTcGadtSyntax)

        dcUnivTyVars :: [TyVar],        -- Universally-quantified type vars [a,b,c]
                                        -- INVARIANT: length matches arity of the dcRepTyCon
                                        ---           result type of (rep) data con is exactly (T a b c)

        dcExTyVars   :: [TyVar],        -- Existentially-quantified type vars 
                -- In general, the dcUnivTyVars are NOT NECESSARILY THE SAME AS THE TYVARS
                -- FOR THE PARENT TyCon. With GADTs the data con might not even have 
                -- the same number of type variables.
                -- [This is a change (Oct05): previously, vanilla datacons guaranteed to
                --  have the same type variables as their parent TyCon, but that seems ugly.]

        -- INVARIANT: the UnivTyVars and ExTyVars all have distinct OccNames
        -- Reason: less confusing, and easier to generate IfaceSyn

        dcEqSpec :: [(TyVar,Type)],     -- Equalities derived from the result type, 
                                        -- _as written by the programmer_
                -- This field allows us to move conveniently between the two ways
                -- of representing a GADT constructor's type:
                --      MkT :: forall a b. (a ~ [b]) => b -> T a
                --      MkT :: forall b. b -> T [b]
                -- Each equality is of the form (a ~ ty), where 'a' is one of 
                -- the universally quantified type variables
                                        
                -- The next two fields give the type context of the data constructor
                --      (aside from the GADT constraints, 
                --       which are given by the dcExpSpec)
                -- In GADT form, this is *exactly* what the programmer writes, even if
                -- the context constrains only universally quantified variables
                --      MkT :: forall a b. (a ~ b, Ord b) => a -> T a b
        dcOtherTheta :: ThetaType,  -- The other constraints in the data con's type
                                    -- other than those in the dcEqSpec

        dcStupidTheta :: ThetaType,     -- The context of the data type declaration 
                                        --      data Eq a => T a = ...
                                        -- or, rather, a "thinned" version thereof
                -- "Thinned", because the Report says
                -- to eliminate any constraints that don't mention
                -- tyvars free in the arg types for this constructor
                --
                -- INVARIANT: the free tyvars of dcStupidTheta are a subset of dcUnivTyVars
                -- Reason: dcStupidTeta is gotten by thinning the stupid theta from the tycon
                -- 
                -- "Stupid", because the dictionaries aren't used for anything.  
                -- Indeed, [as of March 02] they are no longer in the type of 
                -- the wrapper Id, because that makes it harder to use the wrap-id 
                -- to rebuild values after record selection or in generics.

        dcOrigArgTys :: [Type],         -- Original argument types
                                        -- (before unboxing and flattening of strict fields)
        dcOrigResTy :: Type,            -- Original result type, as seen by the user
                -- NB: for a data instance, the original user result type may 
                -- differ from the DataCon's representation TyCon.  Example
                --      data instance T [a] where MkT :: a -> T [a]
                -- The OrigResTy is T [a], but the dcRepTyCon might be :T123

        -- Now the strictness annotations and field labels of the constructor
        dcStrictMarks :: [HsBang],
                -- Strictness annotations as decided by the compiler.  
                -- Does *not* include the existential dictionaries
                -- length = dataConSourceArity dataCon

        dcFields  :: [FieldLabel],
                -- Field labels for this constructor, in the
                -- same order as the dcOrigArgTys; 
                -- length = 0 (if not a record) or dataConSourceArity.

        -- Constructor representation
        dcRepArgTys :: [Type],  -- Final, representation argument types, 
                                -- after unboxing and flattening,
                                -- and *including* all existential evidence args

        dcRepStrictness :: [StrictnessMark],
                -- One for each *representation* *value* argument
                -- See also Note [Data-con worker strictness] in MkId.lhs

        -- Result type of constructor is T t1..tn
        dcRepTyCon  :: TyCon,           -- Result tycon, T

        dcRepType   :: Type,    -- Type of the constructor
                                --      forall a x y. (a~(x,y), x~y, Ord x) =>
                                --        x -> y -> T a
                                -- (this is *not* of the constructor wrapper Id:
                                --  see Note [Data con representation] below)
        -- Notice that the existential type parameters come *second*.  
        -- Reason: in a case expression we may find:
        --      case (e :: T t) of
        --        MkT x y co1 co2 (d:Ord x) (v:r) (w:F s) -> ...
        -- It's convenient to apply the rep-type of MkT to 't', to get
        --      forall x y. (t~(x,y), x~y, Ord x) => x -> y -> T t
        -- and use that to check the pattern.  Mind you, this is really only
        -- used in CoreLint.


        -- The curried worker function that corresponds to the constructor:
        -- It doesn't have an unfolding; the code generator saturates these Ids
        -- and allocates a real constructor when it finds one.
        --
        -- An entirely separate wrapper function is built in TcTyDecls
        dcIds :: DataConIds,

        dcInfix :: Bool         -- True <=> declared infix
                                -- Used for Template Haskell and 'deriving' only
                                -- The actual fixity is stored elsewhere
  }
  deriving Data.Typeable.Typeable

-- | Contains the Ids of the data constructor functions
data DataConIds
  = DCIds (Maybe Id) Id         -- Algebraic data types always have a worker, and
                                -- may or may not have a wrapper, depending on whether
                                -- the wrapper does anything.  Newtypes just have a worker

        -- _Neither_ the worker _nor_ the wrapper take the dcStupidTheta dicts as arguments

        -- The wrapper takes dcOrigArgTys as its arguments
        -- The worker takes dcRepArgTys as its arguments
        -- If the worker is absent, dcRepArgTys is the same as dcOrigArgTys

        -- The 'Nothing' case of DCIds is important
        -- Not only is this efficient,
        -- but it also ensures that the wrapper is replaced
        -- by the worker (because it *is* the worker)
        -- even when there are no args. E.g. in
        --              f (:) x
        -- the (:) *is* the worker.
        -- This is really important in rule matching,
        -- (We could match on the wrappers,
        -- but that makes it less likely that rules will match
        -- when we bring bits of unfoldings together.)

-- | Type of the tags associated with each constructor possibility
type ConTag = Int

fIRST_TAG :: ConTag
-- ^ Tags are allocated from here for real constructors
fIRST_TAG =  1
\end{code}

Note [Data con representation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The dcRepType field contains the type of the representation of a contructor
This may differ from the type of the contructor *Id* (built
by MkId.mkDataConId) for two reasons:
        a) the constructor Id may be overloaded, but the dictionary isn't stored
           e.g.    data Eq a => T a = MkT a a

        b) the constructor may store an unboxed version of a strict field.

Here's an example illustrating both:
        data Ord a => T a = MkT Int! a
Here
        T :: Ord a => Int -> a -> T a
but the rep type is
        Trep :: Int# -> a -> T a
Actually, the unboxed part isn't implemented yet!


%************************************************************************
%*                                                                      *
\subsection{Instances}
%*                                                                      *
%************************************************************************

\begin{code}
instance Eq DataCon where
    a == b = getUnique a == getUnique b
    a /= b = getUnique a /= getUnique b

instance Ord DataCon where
    a <= b = getUnique a <= getUnique b
    a <  b = getUnique a <  getUnique b
    a >= b = getUnique a >= getUnique b
    a >  b = getUnique a > getUnique b
    compare a b = getUnique a `compare` getUnique b

instance Uniquable DataCon where
    getUnique = dcUnique

instance NamedThing DataCon where
    getName = dcName

instance Outputable DataCon where
    ppr con = ppr (dataConName con)

instance Show DataCon where
    showsPrec p con = showsPrecSDoc p (ppr con)

instance Data.Data DataCon where
    -- don't traverse?
    toConstr _   = abstractConstr "DataCon"
    gunfold _ _  = error "gunfold"
    dataTypeOf _ = mkNoRepType "DataCon"
\end{code}


%************************************************************************
%*                                                                      *
\subsection{Construction}
%*                                                                      *
%************************************************************************

\begin{code}
-- | Build a new data constructor
mkDataCon :: Name 
          -> Bool               -- ^ Is the constructor declared infix?
          -> [HsBang]           -- ^ Strictness annotations written in the source file
          -> [FieldLabel]       -- ^ Field labels for the constructor, if it is a record, 
                                --   otherwise empty
          -> [TyVar]            -- ^ Universally quantified type variables
          -> [TyVar]            -- ^ Existentially quantified type variables
          -> [(TyVar,Type)]     -- ^ GADT equalities
          -> ThetaType          -- ^ Theta-type occuring before the arguments proper
          -> [Type]             -- ^ Original argument types
          -> Type               -- ^ Original result type
          -> TyCon              -- ^ Representation type constructor
          -> ThetaType          -- ^ The "stupid theta", context of the data declaration 
                                --   e.g. @data Eq a => T a ...@
          -> DataConIds         -- ^ The Ids of the actual builder functions
          -> DataCon
  -- Can get the tag from the TyCon

mkDataCon name declared_infix
          arg_stricts   -- Must match orig_arg_tys 1-1
          fields
          univ_tvs ex_tvs 
          eq_spec theta
          orig_arg_tys orig_res_ty rep_tycon
          stupid_theta ids
-- Warning: mkDataCon is not a good place to check invariants. 
-- If the programmer writes the wrong result type in the decl, thus:
--      data T a where { MkT :: S }
-- then it's possible that the univ_tvs may hit an assertion failure
-- if you pull on univ_tvs.  This case is checked by checkValidDataCon,
-- so the error is detected properly... it's just that asaertions here
-- are a little dodgy.

  = -- ASSERT( not (any isEqPred theta) )
        -- We don't currently allow any equality predicates on
        -- a data constructor (apart from the GADT ones in eq_spec)
    con
  where
    is_vanilla = null ex_tvs && null eq_spec && null theta
    con = MkData {dcName = name, dcUnique = nameUnique name, 
                  dcVanilla = is_vanilla, dcInfix = declared_infix,
                  dcUnivTyVars = univ_tvs, dcExTyVars = ex_tvs, 
                  dcEqSpec = eq_spec, 
                  dcOtherTheta = theta,
                  dcStupidTheta = stupid_theta, 
                  dcOrigArgTys = orig_arg_tys, dcOrigResTy = orig_res_ty,
                  dcRepTyCon = rep_tycon, 
                  dcRepArgTys = rep_arg_tys,
                  dcStrictMarks = arg_stricts, 
                  dcRepStrictness = rep_arg_stricts,
                  dcFields = fields, dcTag = tag, dcRepType = ty,
                  dcIds = ids }

        -- Strictness marks for source-args
        --      *after unboxing choices*, 
        -- but  *including existential dictionaries*
        -- 
        -- The 'arg_stricts' passed to mkDataCon are simply those for the
        -- source-language arguments.  We add extra ones for the
        -- dictionary arguments right here.
    full_theta   = eqSpecPreds eq_spec ++ theta
    real_arg_tys = full_theta                         ++ orig_arg_tys
    real_stricts = map mk_pred_strict_mark full_theta ++ arg_stricts

        -- Representation arguments and demands
        -- To do: eliminate duplication with MkId
    (rep_arg_stricts, rep_arg_tys) = computeRep real_stricts real_arg_tys

    tag = assoc "mkDataCon" (tyConDataCons rep_tycon `zip` [fIRST_TAG..]) con
    ty  = mkForAllTys univ_tvs $ mkForAllTys ex_tvs $ 
          mkFunTys rep_arg_tys $
          mkTyConApp rep_tycon (mkTyVarTys univ_tvs)

eqSpecPreds :: [(TyVar,Type)] -> ThetaType
eqSpecPreds spec = [ mkEqPred (mkTyVarTy tv) ty | (tv,ty) <- spec ]

mk_pred_strict_mark :: PredType -> HsBang
mk_pred_strict_mark pred 
  | isEqPred pred = HsUnpack    -- Note [Unpack equality predicates]
  | otherwise     = HsNoBang
\end{code}

Note [Unpack equality predicates]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have a GADT with a contructor C :: (a~[b]) => b -> T a
we definitely want that equality predicate *unboxed* so that it
takes no space at all.  This is easily done: just give it
an UNPACK pragma. The rest of the unpack/repack code does the
heavy lifting.  This one line makes every GADT take a word less
space for each equality predicate, so it's pretty important!

\begin{code}
-- | The 'Name' of the 'DataCon', giving it a unique, rooted identification
dataConName :: DataCon -> Name
dataConName = dcName

-- | The tag used for ordering 'DataCon's
dataConTag :: DataCon -> ConTag
dataConTag  = dcTag

-- | The type constructor that we are building via this data constructor
dataConTyCon :: DataCon -> TyCon
dataConTyCon = dcRepTyCon

-- | The original type constructor used in the definition of this data
-- constructor.  In case of a data family instance, that will be the family
-- type constructor.
dataConOrigTyCon :: DataCon -> TyCon
dataConOrigTyCon dc 
  | Just (tc, _) <- tyConFamInst_maybe (dcRepTyCon dc) = tc
  | otherwise                                          = dcRepTyCon dc

-- | The representation type of the data constructor, i.e. the sort
-- type that will represent values of this type at runtime
dataConRepType :: DataCon -> Type
dataConRepType = dcRepType

-- | Should the 'DataCon' be presented infix?
dataConIsInfix :: DataCon -> Bool
dataConIsInfix = dcInfix

-- | The universally-quantified type variables of the constructor
dataConUnivTyVars :: DataCon -> [TyVar]
dataConUnivTyVars = dcUnivTyVars

-- | The existentially-quantified type variables of the constructor
dataConExTyVars :: DataCon -> [TyVar]
dataConExTyVars = dcExTyVars

-- | Both the universal and existentiatial type variables of the constructor
dataConAllTyVars :: DataCon -> [TyVar]
dataConAllTyVars (MkData { dcUnivTyVars = univ_tvs, dcExTyVars = ex_tvs })
  = univ_tvs ++ ex_tvs

-- | Equalities derived from the result type of the data constructor, as written
-- by the programmer in any GADT declaration
dataConEqSpec :: DataCon -> [(TyVar,Type)]
dataConEqSpec = dcEqSpec

-- | The *full* constraints on the constructor type
dataConTheta :: DataCon -> ThetaType
dataConTheta (MkData { dcEqSpec = eq_spec, dcOtherTheta = theta }) 
  = eqSpecPreds eq_spec ++ theta

-- | Get the Id of the 'DataCon' worker: a function that is the "actual"
-- constructor and has no top level binding in the program. The type may
-- be different from the obvious one written in the source program. Panics
-- if there is no such 'Id' for this 'DataCon'
dataConWorkId :: DataCon -> Id
dataConWorkId dc = case dcIds dc of
                        DCIds _ wrk_id -> wrk_id

-- | Get the Id of the 'DataCon' wrapper: a function that wraps the "actual"
-- constructor so it has the type visible in the source program: c.f. 'dataConWorkId'.
-- Returns Nothing if there is no wrapper, which occurs for an algebraic data constructor 
-- and also for a newtype (whose constructor is inlined compulsorily)
dataConWrapId_maybe :: DataCon -> Maybe Id
dataConWrapId_maybe dc = case dcIds dc of
                                DCIds mb_wrap _ -> mb_wrap

-- | Returns an Id which looks like the Haskell-source constructor by using
-- the wrapper if it exists (see 'dataConWrapId_maybe') and failing over to
-- the worker (see 'dataConWorkId')
dataConWrapId :: DataCon -> Id
dataConWrapId dc = case dcIds dc of
                        DCIds (Just wrap) _   -> wrap
                        DCIds Nothing     wrk -> wrk        -- worker=wrapper

-- | Find all the 'Id's implicitly brought into scope by the data constructor. Currently,
-- the union of the 'dataConWorkId' and the 'dataConWrapId'
dataConImplicitIds :: DataCon -> [Id]
dataConImplicitIds dc = case dcIds dc of
                          DCIds (Just wrap) work -> [wrap,work]
                          DCIds Nothing     work -> [work]

-- | The labels for the fields of this particular 'DataCon'
dataConFieldLabels :: DataCon -> [FieldLabel]
dataConFieldLabels = dcFields

-- | Extract the type for any given labelled field of the 'DataCon'
dataConFieldType :: DataCon -> FieldLabel -> Type
dataConFieldType con label
  = case lookup label (dcFields con `zip` dcOrigArgTys con) of
      Just ty -> ty
      Nothing -> pprPanic "dataConFieldType" (ppr con <+> ppr label)

-- | The strictness markings decided on by the compiler.  Does not include those for
-- existential dictionaries.  The list is in one-to-one correspondence with the arity of the 'DataCon'
dataConStrictMarks :: DataCon -> [HsBang]
dataConStrictMarks = dcStrictMarks

-- | Strictness of evidence arguments to the wrapper function
dataConExStricts :: DataCon -> [HsBang]
-- Usually empty, so we don't bother to cache this
dataConExStricts dc = map mk_pred_strict_mark (dataConTheta dc)

-- | Source-level arity of the data constructor
dataConSourceArity :: DataCon -> Arity
dataConSourceArity dc = length (dcOrigArgTys dc)

-- | Gives the number of actual fields in the /representation/ of the 
-- data constructor. This may be more than appear in the source code;
-- the extra ones are the existentially quantified dictionaries
dataConRepArity :: DataCon -> Int
dataConRepArity (MkData {dcRepArgTys = arg_tys}) = length arg_tys

-- | Return whether there are any argument types for this 'DataCon's original source type
isNullarySrcDataCon :: DataCon -> Bool
isNullarySrcDataCon dc = null (dcOrigArgTys dc)

-- | Return whether there are any argument types for this 'DataCon's runtime representation type
isNullaryRepDataCon :: DataCon -> Bool
isNullaryRepDataCon dc = null (dcRepArgTys dc)

dataConRepStrictness :: DataCon -> [StrictnessMark]
-- ^ Give the demands on the arguments of a
-- Core constructor application (Con dc args)
dataConRepStrictness dc = dcRepStrictness dc

-- | The \"signature\" of the 'DataCon' returns, in order:
--
-- 1) The result of 'dataConAllTyVars',
--
-- 2) All the 'ThetaType's relating to the 'DataCon' (coercion, dictionary, implicit
--    parameter - whatever)
--
-- 3) The type arguments to the constructor
--
-- 4) The /original/ result type of the 'DataCon'
dataConSig :: DataCon -> ([TyVar], ThetaType, [Type], Type)
dataConSig (MkData {dcUnivTyVars = univ_tvs, dcExTyVars = ex_tvs, 
                    dcEqSpec = eq_spec, dcOtherTheta  = theta, 
                    dcOrigArgTys = arg_tys, dcOrigResTy = res_ty})
  = (univ_tvs ++ ex_tvs, eqSpecPreds eq_spec ++ theta, arg_tys, res_ty)

-- | The \"full signature\" of the 'DataCon' returns, in order:
--
-- 1) The result of 'dataConUnivTyVars'
--
-- 2) The result of 'dataConExTyVars'
--
-- 3) The result of 'dataConEqSpec'
--
-- 4) The result of 'dataConDictTheta'
--
-- 5) The original argument types to the 'DataCon' (i.e. before 
--    any change of the representation of the type)
--
-- 6) The original result type of the 'DataCon'
dataConFullSig :: DataCon 
               -> ([TyVar], [TyVar], [(TyVar,Type)], ThetaType, [Type], Type)
dataConFullSig (MkData {dcUnivTyVars = univ_tvs, dcExTyVars = ex_tvs, 
                        dcEqSpec = eq_spec, dcOtherTheta = theta,
                        dcOrigArgTys = arg_tys, dcOrigResTy = res_ty})
  = (univ_tvs, ex_tvs, eq_spec, theta, arg_tys, res_ty)

dataConOrigResTy :: DataCon -> Type
dataConOrigResTy dc = dcOrigResTy dc

-- | The \"stupid theta\" of the 'DataCon', such as @data Eq a@ in:
--
-- > data Eq a => T a = ...
dataConStupidTheta :: DataCon -> ThetaType
dataConStupidTheta dc = dcStupidTheta dc

dataConUserType :: DataCon -> Type
-- ^ The user-declared type of the data constructor
-- in the nice-to-read form:
--
-- > T :: forall a b. a -> b -> T [a]
--
-- rather than:
--
-- > T :: forall a c. forall b. (c~[a]) => a -> b -> T c
--
-- NB: If the constructor is part of a data instance, the result type
-- mentions the family tycon, not the internal one.
dataConUserType  (MkData { dcUnivTyVars = univ_tvs, 
                           dcExTyVars = ex_tvs, dcEqSpec = eq_spec,
                           dcOtherTheta = theta, dcOrigArgTys = arg_tys,
                           dcOrigResTy = res_ty })
  = mkForAllTys ((univ_tvs `minusList` map fst eq_spec) ++ ex_tvs) $
    mkFunTys theta $
    mkFunTys arg_tys $
    res_ty

-- | Finds the instantiated types of the arguments required to construct a 'DataCon' representation
-- NB: these INCLUDE any dictionary args
--     but EXCLUDE the data-declaration context, which is discarded
-- It's all post-flattening etc; this is a representation type
dataConInstArgTys :: DataCon    -- ^ A datacon with no existentials or equality constraints
                                -- However, it can have a dcTheta (notably it can be a 
                                -- class dictionary, with superclasses)
                  -> [Type]     -- ^ Instantiated at these types
                  -> [Type]
dataConInstArgTys dc@(MkData {dcRepArgTys = rep_arg_tys, 
                              dcUnivTyVars = univ_tvs, dcEqSpec = eq_spec,
                              dcExTyVars = ex_tvs}) inst_tys
 = ASSERT2 ( length univ_tvs == length inst_tys 
           , ptext (sLit "dataConInstArgTys") <+> ppr dc $$ ppr univ_tvs $$ ppr inst_tys)
   ASSERT2 ( null ex_tvs && null eq_spec, ppr dc )        
   map (substTyWith univ_tvs inst_tys) rep_arg_tys

-- | Returns just the instantiated /value/ argument types of a 'DataCon',
-- (excluding dictionary args)
dataConInstOrigArgTys 
        :: DataCon      -- Works for any DataCon
        -> [Type]       -- Includes existential tyvar args, but NOT
                        -- equality constraints or dicts
        -> [Type]
-- For vanilla datacons, it's all quite straightforward
-- But for the call in MatchCon, we really do want just the value args
dataConInstOrigArgTys dc@(MkData {dcOrigArgTys = arg_tys,
                                  dcUnivTyVars = univ_tvs, 
                                  dcExTyVars = ex_tvs}) inst_tys
  = ASSERT2( length tyvars == length inst_tys
          , ptext (sLit "dataConInstOrigArgTys") <+> ppr dc $$ ppr tyvars $$ ppr inst_tys )
    map (substTyWith tyvars inst_tys) arg_tys
  where
    tyvars = univ_tvs ++ ex_tvs
\end{code}

\begin{code}
-- | Returns the argument types of the wrapper, excluding all dictionary arguments
-- and without substituting for any type variables
dataConOrigArgTys :: DataCon -> [Type]
dataConOrigArgTys dc = dcOrigArgTys dc

-- | Returns the arg types of the worker, including all dictionaries, after any 
-- flattening has been done and without substituting for any type variables
dataConRepArgTys :: DataCon -> [Type]
dataConRepArgTys dc = dcRepArgTys dc
\end{code}

\begin{code}
-- | The string @package:module.name@ identifying a constructor, which is attached
-- to its info table and used by the GHCi debugger and the heap profiler
dataConIdentity :: DataCon -> [Word8]
-- We want this string to be UTF-8, so we get the bytes directly from the FastStrings.
dataConIdentity dc = bytesFS (packageIdFS (modulePackageId mod)) ++ 
                  fromIntegral (ord ':') : bytesFS (moduleNameFS (moduleName mod)) ++
                  fromIntegral (ord '.') : bytesFS (occNameFS (nameOccName name))
  where name = dataConName dc
        mod  = ASSERT( isExternalName name ) nameModule name
\end{code}

\begin{code}
isTupleCon :: DataCon -> Bool
isTupleCon (MkData {dcRepTyCon = tc}) = isTupleTyCon tc
        
isUnboxedTupleCon :: DataCon -> Bool
isUnboxedTupleCon (MkData {dcRepTyCon = tc}) = isUnboxedTupleTyCon tc

-- | Vanilla 'DataCon's are those that are nice boring Haskell 98 constructors
isVanillaDataCon :: DataCon -> Bool
isVanillaDataCon dc = dcVanilla dc
\end{code}

\begin{code}
classDataCon :: Class -> DataCon
classDataCon clas = case tyConDataCons (classTyCon clas) of
                      (dict_constr:no_more) -> ASSERT( null no_more ) dict_constr 
                      [] -> panic "classDataCon"
\end{code}

\begin{code}
dataConCannotMatch :: [Type] -> DataCon -> Bool
-- Returns True iff the data con *definitely cannot* match a 
--                  scrutinee of type (T tys)
--                  where T is the type constructor for the data con
-- NB: look at *all* equality constraints, not only those
--     in dataConEqSpec; see Trac #5168
dataConCannotMatch tys con
  | null theta        = False   -- Common
  | all isTyVarTy tys = False   -- Also common
  | otherwise
  = typesCantMatch [(Type.substTy subst ty1, Type.substTy subst ty2)
                   | (ty1, ty2) <- concatMap predEqs theta ]
  where
    dc_tvs  = dataConUnivTyVars con
    theta   = dataConTheta con
    subst   = zipTopTvSubst dc_tvs tys

    -- TODO: could gather equalities from superclasses too
    predEqs pred = case classifyPredType pred of
                     EqPred ty1 ty2 -> [(ty1, ty2)]
                     TuplePred ts   -> concatMap predEqs ts
                     _              -> []
\end{code}

%************************************************************************
%*                                                                      *
\subsection{Splitting products}
%*                                                                      *
%************************************************************************

\begin{code}
-- | Extract the type constructor, type argument, data constructor and it's
-- /representation/ argument types from a type if it is a product type.
--
-- Precisely, we return @Just@ for any type that is all of:
--
--  * Concrete (i.e. constructors visible)
--
--  * Single-constructor
--
--  * Not existentially quantified
--
-- Whether the type is a @data@ type or a @newtype@
splitProductType_maybe
        :: Type                         -- ^ A product type, perhaps
        -> Maybe (TyCon,                -- The type constructor
                  [Type],               -- Type args of the tycon
                  DataCon,              -- The data constructor
                  [Type])               -- Its /representation/ arg types

        -- Rejecing existentials is conservative.  Maybe some things
        -- could be made to work with them, but I'm not going to sweat
        -- it through till someone finds it's important.

splitProductType_maybe ty
  = case splitTyConApp_maybe ty of
        Just (tycon,ty_args)
           | isProductTyCon tycon       -- Includes check for non-existential,
                                        -- and for constructors visible
           -> Just (tycon, ty_args, data_con, dataConInstArgTys data_con ty_args)
           where
              data_con = ASSERT( not (null (tyConDataCons tycon)) ) 
                         head (tyConDataCons tycon)
        _other -> Nothing

-- | As 'splitProductType_maybe', but panics if the 'Type' is not a product type
splitProductType :: String -> Type -> (TyCon, [Type], DataCon, [Type])
splitProductType str ty
  = case splitProductType_maybe ty of
        Just stuff -> stuff
        Nothing    -> pprPanic (str ++ ": not a product") (pprType ty)


-- | As 'splitProductType_maybe', but in turn instantiates the 'TyCon' returned
-- and hence recursively tries to unpack it as far as it able to
deepSplitProductType_maybe :: Type -> Maybe (TyCon, [Type], DataCon, [Type])
deepSplitProductType_maybe ty
  = do { (res@(tycon, tycon_args, _, _)) <- splitProductType_maybe ty
       ; let {result 
             | Just (ty', _co) <- instNewTyCon_maybe tycon tycon_args
             , not (isRecursiveTyCon tycon)
             = deepSplitProductType_maybe ty'   -- Ignore the coercion?
             | isNewTyCon tycon = Nothing  -- cannot unbox through recursive
                                           -- newtypes nor through families
             | otherwise = Just res}
       ; result
       }

-- | As 'deepSplitProductType_maybe', but panics if the 'Type' is not a product type
deepSplitProductType :: String -> Type -> (TyCon, [Type], DataCon, [Type])
deepSplitProductType str ty 
  = case deepSplitProductType_maybe ty of
      Just stuff -> stuff
      Nothing -> pprPanic (str ++ ": not a product") (pprType ty)

-- | Compute the representation type strictness and type suitable for a 'DataCon'
computeRep :: [HsBang]                  -- ^ Original argument strictness
           -> [Type]                    -- ^ Original argument types
           -> ([StrictnessMark],        -- Representation arg strictness
               [Type])                  -- And type

computeRep stricts tys
  = unzip $ concat $ zipWithEqual "computeRep" unbox stricts tys
  where
    unbox HsNoBang       ty = [(NotMarkedStrict, ty)]
    unbox HsStrict       ty = [(MarkedStrict,    ty)]
    unbox HsNoUnpack     ty = [(MarkedStrict,    ty)]
    unbox HsUnpackFailed ty = [(MarkedStrict,    ty)]
    unbox HsUnpack ty = zipEqual "computeRep" (dataConRepStrictness arg_dc) arg_tys
                      where
                        (_tycon, _tycon_args, arg_dc, arg_tys) 
                           = deepSplitProductType "unbox_strict_arg_ty" ty
\end{code}


%************************************************************************
%*                                                                      *
        Promoting of data types to the kind level
%*                                                                      *
%************************************************************************

These two 'buildPromoted..' functions are here because
 * They belong together
 * 'buildPromotedTyCon' is used by promoteType
 * 'buildPromotedTyCon' depends on DataCon stuff

\begin{code}
buildPromotedTyCon :: TyCon -> TyCon
buildPromotedTyCon tc
  = mkPromotedTyCon tc (promoteKind (tyConKind tc))

buildPromotedDataCon :: DataCon -> TyCon
buildPromotedDataCon dc 
  = ASSERT ( isPromotableType ty )
    mkPromotedDataCon dc (getName dc) (getUnique dc) kind arity
  where 
    ty    = dataConUserType dc
    kind  = promoteType ty
    arity = dataConSourceArity dc
\end{code}

Note [Promoting a Type to a Kind]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppsoe we have a data constructor D
     D :: forall (a:*). Maybe a -> T a
We promote this to be a type constructor 'D:
     'D :: forall (k:BOX). 'Maybe k -> 'T k

The transformation from type to kind is done by promoteType

  * Convert forall (a:*) to forall (k:BOX), and substitute

  * Ensure all foralls are at the top (no higher rank stuff)

  * Ensure that all type constructors mentioned (Maybe and T
    in the example) are promotable; that is, they have kind 
          * -> ... -> * -> *

\begin{code}
isPromotableType :: Type -> Bool
isPromotableType ty
  = all (isLiftedTypeKind . tyVarKind) tvs
    && go rho
  where
    (tvs, rho) = splitForAllTys ty
    go (TyConApp tc tys) | Just n <- isPromotableTyCon tc
                         = tys `lengthIs` n && all go tys
    go (FunTy arg res)   = go arg && go res
    go (TyVarTy tvar)    = tvar `elem` tvs
    go _                 = False

-- If tc's kind is [ *^n -> * ] returns [ Just n ], else returns [ Nothing ]
isPromotableTyCon :: TyCon -> Maybe Int
isPromotableTyCon tc
  | all isLiftedTypeKind (res:args) = Just $ length args
  | otherwise                       = Nothing
  where
    (args, res) = splitKindFunTys (tyConKind tc)

-- | Promotes a type to a kind. 
-- Assumes the argument satisfies 'isPromotableType'
promoteType :: Type -> Kind
promoteType ty
  = mkForAllTys kvs (go rho)
  where
    (tvs, rho) = splitForAllTys ty
    kvs = [ mkKindVar (tyVarName tv) superKind | tv <- tvs ]
    env = zipVarEnv tvs kvs

    go (TyConApp tc tys) = mkTyConApp (buildPromotedTyCon tc) (map go tys)
    go (FunTy arg res)   = mkArrowKind (go arg) (go res)
    go (TyVarTy tv)      | Just kv <- lookupVarEnv env tv 
                         = TyVarTy kv
    go _ = panic "promoteType"  -- Argument did not satisfy isPromotableType

promoteKind :: Kind -> SuperKind
-- Promote the kind of a type constructor
-- from (* -> * -> *) to (BOX -> BOX -> BOX) 
promoteKind (TyConApp tc []) 
  | tc `hasKey` liftedTypeKindTyConKey = superKind
promoteKind (FunTy arg res) = FunTy (promoteKind arg) (promoteKind res)
promoteKind k = pprPanic "promoteKind" (ppr k)
\end{code}