root/compiler/basicTypes/Var.lhs

%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section{@Vars@: Variables}

\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

-- |
-- #name_types#
-- GHC uses several kinds of name internally:
--
-- * 'OccName.OccName': see "OccName#name_types"
--
-- * 'RdrName.RdrName': see "RdrName#name_types"
--
-- * 'Name.Name': see "Name#name_types"
--
-- * 'Id.Id': see "Id#name_types"
--
-- * 'Var.Var' is a synonym for the 'Id.Id' type but it may additionally 
--   potentially contain type variables, which have a 'TypeRep.Kind' 
--   rather than a 'TypeRep.Type' and only contain some extra 
--   details during typechecking.
-- 
--   These 'Var.Var' names may either be global or local, see "Var#globalvslocal"
--
-- #globalvslocal#
-- Global 'Id's and 'Var's are those that are imported or correspond 
--    to a data constructor, primitive operation, or record selectors.
-- Local 'Id's and 'Var's are those bound within an expression 
--    (e.g. by a lambda) or at the top level of the module being compiled.

module Var (
        -- * The main data type and synonyms
        Var, CoVar, Id, DictId, DFunId, EvVar, EqVar, EvId, IpId,
        TyVar, TypeVar, KindVar, TKVar,

        -- ** Taking 'Var's apart
        varName, varUnique, varType, 

        -- ** Modifying 'Var's
        setVarName, setVarUnique, setVarType,

        -- ** Constructing, taking apart, modifying 'Id's
        mkGlobalVar, mkLocalVar, mkExportedLocalVar, mkCoVar,
        idInfo, idDetails,
        lazySetIdInfo, setIdDetails, globaliseId,
        setIdExported, setIdNotExported,

        -- ** Predicates
        isId, isTKVar, isTyVar, isTcTyVar,
        isLocalVar, isLocalId,
        isGlobalId, isExportedId,
        mustHaveLocalBinding,

        -- ** Constructing 'TyVar's
        mkTyVar, mkTcTyVar, mkKindVar,

        -- ** Taking 'TyVar's apart
        tyVarName, tyVarKind, tcTyVarDetails, setTcTyVarDetails,

        -- ** Modifying 'TyVar's
        setTyVarName, setTyVarUnique, setTyVarKind, updateTyVarKind,
        updateTyVarKindM

    ) where

#include "HsVersions.h"
#include "Typeable.h"

import {-# SOURCE #-}   TypeRep( Type, Kind, SuperKind )
import {-# SOURCE #-}   TcType( TcTyVarDetails, pprTcTyVarDetails )
import {-# SOURCE #-}   IdInfo( IdDetails, IdInfo, coVarDetails, vanillaIdInfo, pprIdDetails )

import Name hiding (varName)
import Unique
import Util
import FastTypes
import FastString
import Outputable

-- import StaticFlags ( opt_SuppressVarKinds )

import Data.Data
\end{code}


%************************************************************************
%*                                                                      *
                     Synonyms                                                                   
%*                                                                      *
%************************************************************************
-- These synonyms are here and not in Id because otherwise we need a very
-- large number of SOURCE imports of Id.hs :-(

\begin{code}
type Id    = Var       -- A term-level identifier

type TyVar   = Var     -- Type *or* kind variable (historical)

type TKVar   = Var     -- Type *or* kind variable (historical)
type TypeVar = Var     -- Definitely a type variable
type KindVar = Var     -- Definitely a kind variable
                       -- See Note [Kind and type variables]

-- See Note [Evidence: EvIds and CoVars]
type EvId   = Id        -- Term-level evidence: DictId, IpId, or EqVar
type EvVar  = EvId      -- ...historical name for EvId
type DFunId = Id        -- A dictionary function
type DictId = EvId      -- A dictionary variable
type IpId   = EvId      -- A term-level implicit parameter
type EqVar  = EvId      -- Boxed equality evidence

type CoVar = Id         -- See Note [Evidence: EvIds and CoVars]
\end{code}

Note [Evidence: EvIds and CoVars]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* An EvId (evidence Id) is a *boxed*, term-level evidence variable 
  (dictionary, implicit parameter, or equality).

* A CoVar (coercion variable) is an *unboxed* term-level evidence variable
  of type (t1 ~# t2).  So it's the unboxed version of an EqVar.

* Only CoVars can occur in Coercions, EqVars appear in TcCoercions.

* DictId, IpId, and EqVar are synonyms when we know what kind of
  evidence we are talking about.  For example, an EqVar has type (t1 ~ t2).

Note [Kind and type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Before kind polymorphism, TyVar were used to mean type variables. Now
they are use to mean kind *or* type variables. KindVar is used when we
know for sure that it is a kind variable. In future, we might want to
go over the whole compiler code to use:
   - TKVar   to mean kind or type variables
   - TypeVar to mean         type variables only
   - KindVar to mean kind         variables


%************************************************************************
%*                                                                      *
\subsection{The main data type declarations}
%*                                                                      *
%************************************************************************


Every @Var@ has a @Unique@, to uniquify it and for fast comparison, a
@Type@, and an @IdInfo@ (non-essential info about it, e.g.,
strictness).  The essential info about different kinds of @Vars@ is
in its @VarDetails@.

\begin{code}
-- | Essentially a typed 'Name', that may also contain some additional information
-- about the 'Var' and it's use sites.
data Var
  = TyVar {  -- Type and kind variables
             -- see Note [Kind and type variables]
        varName    :: !Name,
        realUnique :: FastInt,       -- Key for fast comparison
                                     -- Identical to the Unique in the name,
                                     -- cached here for speed
        varType    :: Kind           -- ^ The type or kind of the 'Var' in question
 }

  | TcTyVar {                           -- Used only during type inference
                                        -- Used for kind variables during 
                                        -- inference, as well
        varName        :: !Name,
        realUnique     :: FastInt,
        varType        :: Kind,
        tc_tv_details  :: TcTyVarDetails }

  | Id {
        varName    :: !Name,
        realUnique :: FastInt,
        varType    :: Type,
        idScope    :: IdScope,
        id_details :: IdDetails,        -- Stable, doesn't change
        id_info    :: IdInfo }          -- Unstable, updated by simplifier
    deriving Typeable

data IdScope    -- See Note [GlobalId/LocalId]
  = GlobalId 
  | LocalId ExportFlag

data ExportFlag 
  = NotExported -- ^ Not exported: may be discarded as dead code.
  | Exported    -- ^ Exported: kept alive
\end{code}

Note [GlobalId/LocalId]
~~~~~~~~~~~~~~~~~~~~~~~
A GlobalId is
  * always a constant (top-level)
  * imported, or data constructor, or primop, or record selector
  * has a Unique that is globally unique across the whole
    GHC invocation (a single invocation may compile multiple modules)
  * never treated as a candidate by the free-variable finder;
        it's a constant!

A LocalId is 
  * bound within an expression (lambda, case, local let(rec))
  * or defined at top level in the module being compiled
  * always treated as a candidate by the free-variable finder

After CoreTidy, top-level LocalIds are turned into GlobalIds

\begin{code}
instance Outputable Var where
  ppr var = ppr (varName var) <+> ifPprDebug (brackets (ppr_debug var))
-- Printing the type on every occurrence is too much!
--            <+> if (not opt_SuppressVarKinds)
--                then ifPprDebug (text "::" <+> ppr (tyVarKind var) <+> text ")")
--                else empty

ppr_debug :: Var -> SDoc
ppr_debug (TyVar {})                           = ptext (sLit "tv")
ppr_debug (TcTyVar {tc_tv_details = d})        = pprTcTyVarDetails d
ppr_debug (Id { idScope = s, id_details = d }) = ppr_id_scope s <> pprIdDetails d

ppr_id_scope :: IdScope -> SDoc
ppr_id_scope GlobalId              = ptext (sLit "gid")
ppr_id_scope (LocalId Exported)    = ptext (sLit "lidx")
ppr_id_scope (LocalId NotExported) = ptext (sLit "lid")

instance Show Var where
  showsPrec p var = showsPrecSDoc p (ppr var)

instance NamedThing Var where
  getName = varName

instance Uniquable Var where
  getUnique = varUnique

instance Eq Var where
    a == b = realUnique a ==# realUnique b

instance Ord Var where
    a <= b = realUnique a <=# realUnique b
    a <  b = realUnique a <#  realUnique b
    a >= b = realUnique a >=# realUnique b
    a >  b = realUnique a >#  realUnique b
    a `compare` b = varUnique a `compare` varUnique b

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


\begin{code}
varUnique :: Var -> Unique
varUnique var = mkUniqueGrimily (iBox (realUnique var))

setVarUnique :: Var -> Unique -> Var
setVarUnique var uniq 
  = var { realUnique = getKeyFastInt uniq, 
          varName = setNameUnique (varName var) uniq }

setVarName :: Var -> Name -> Var
setVarName var new_name
  = var { realUnique = getKeyFastInt (getUnique new_name), 
          varName = new_name }

setVarType :: Id -> Type -> Id
setVarType id ty = id { varType = ty }
\end{code}


%************************************************************************
%*                                                                      *
\subsection{Type and kind variables}
%*                                                                      *
%************************************************************************

\begin{code}
tyVarName :: TyVar -> Name
tyVarName = varName

tyVarKind :: TyVar -> Kind
tyVarKind = varType

setTyVarUnique :: TyVar -> Unique -> TyVar
setTyVarUnique = setVarUnique

setTyVarName :: TyVar -> Name -> TyVar
setTyVarName   = setVarName

setTyVarKind :: TyVar -> Kind -> TyVar
setTyVarKind tv k = tv {varType = k}

updateTyVarKind :: (Kind -> Kind) -> TyVar -> TyVar
updateTyVarKind update tv = tv {varType = update (tyVarKind tv)}

updateTyVarKindM :: (Monad m) => (Kind -> m Kind) -> TyVar -> m TyVar
updateTyVarKindM update tv
  = do { k' <- update (tyVarKind tv)
       ; return $ tv {varType = k'} }
\end{code}

\begin{code}
mkTyVar :: Name -> Kind -> TyVar
mkTyVar name kind = TyVar { varName    = name
                          , realUnique = getKeyFastInt (nameUnique name)
                          , varType  = kind
                        }

mkTcTyVar :: Name -> Kind -> TcTyVarDetails -> TyVar
mkTcTyVar name kind details
  = -- NB: 'kind' may be a coercion kind; cf, 'TcMType.newMetaCoVar'
    TcTyVar {   varName    = name,
                realUnique = getKeyFastInt (nameUnique name),
                varType  = kind,
                tc_tv_details = details
        }

tcTyVarDetails :: TyVar -> TcTyVarDetails
tcTyVarDetails (TcTyVar { tc_tv_details = details }) = details
tcTyVarDetails var = pprPanic "tcTyVarDetails" (ppr var)

setTcTyVarDetails :: TyVar -> TcTyVarDetails -> TyVar
setTcTyVarDetails tv details = tv { tc_tv_details = details }

mkKindVar :: Name -> SuperKind -> KindVar
-- mkKindVar take a SuperKind as argument because we don't have access
-- to superKind here.
mkKindVar name kind = TyVar
  { varName    = name
  , realUnique = getKeyFastInt (nameUnique name)
  , varType    = kind }

\end{code}

%************************************************************************
%*                                                                      *
\subsection{Ids}
%*                                                                      *
%************************************************************************

\begin{code}
idInfo :: Id -> IdInfo
idInfo (Id { id_info = info }) = info
idInfo other                   = pprPanic "idInfo" (ppr other)

idDetails :: Id -> IdDetails
idDetails (Id { id_details = details }) = details
idDetails other                         = pprPanic "idDetails" (ppr other)

-- The next three have a 'Var' suffix even though they always build
-- Ids, becuase Id.lhs uses 'mkGlobalId' etc with different types
mkGlobalVar :: IdDetails -> Name -> Type -> IdInfo -> Id
mkGlobalVar details name ty info
  = mk_id name ty GlobalId details info

mkLocalVar :: IdDetails -> Name -> Type -> IdInfo -> Id
mkLocalVar details name ty info
  = mk_id name ty (LocalId NotExported) details  info

mkCoVar :: Name -> Type -> CoVar
-- Coercion variables have no IdInfo
mkCoVar name ty = mk_id name ty (LocalId NotExported) coVarDetails vanillaIdInfo

-- | Exported 'Var's will not be removed as dead code
mkExportedLocalVar :: IdDetails -> Name -> Type -> IdInfo -> Id
mkExportedLocalVar details name ty info 
  = mk_id name ty (LocalId Exported) details info

mk_id :: Name -> Type -> IdScope -> IdDetails -> IdInfo -> Id
mk_id name ty scope details info
  = Id { varName    = name, 
         realUnique = getKeyFastInt (nameUnique name),
         varType    = ty,       
         idScope    = scope,
         id_details = details,
         id_info    = info }

-------------------
lazySetIdInfo :: Id -> IdInfo -> Var
lazySetIdInfo id info = id { id_info = info }

setIdDetails :: Id -> IdDetails -> Id
setIdDetails id details = id { id_details = details }

globaliseId :: Id -> Id
-- ^ If it's a local, make it global
globaliseId id = id { idScope = GlobalId }

setIdExported :: Id -> Id
-- ^ Exports the given local 'Id'. Can also be called on global 'Id's, such as data constructors
-- and class operations, which are born as global 'Id's and automatically exported
setIdExported id@(Id { idScope = LocalId {} }) = id { idScope = LocalId Exported }
setIdExported id@(Id { idScope = GlobalId })   = id
setIdExported tv                               = pprPanic "setIdExported" (ppr tv)

setIdNotExported :: Id -> Id
-- ^ We can only do this to LocalIds
setIdNotExported id = ASSERT( isLocalId id ) 
                      id { idScope = LocalId NotExported }
\end{code}

%************************************************************************
%*                                                                      *
\subsection{Predicates over variables}
%*                                                                      *
%************************************************************************

\begin{code}
isTyVar :: Var -> Bool
isTyVar = isTKVar     -- Historical

isTKVar :: Var -> Bool  -- True of both type and kind variables
isTKVar (TyVar {})   = True
isTKVar (TcTyVar {}) = True
isTKVar _            = False

isTcTyVar :: Var -> Bool
isTcTyVar (TcTyVar {}) = True
isTcTyVar _            = False

isId :: Var -> Bool
isId (Id {}) = True
isId _       = False

isLocalId :: Var -> Bool
isLocalId (Id { idScope = LocalId _ }) = True
isLocalId _                            = False

-- | 'isLocalVar' returns @True@ for type variables as well as local 'Id's
-- These are the variables that we need to pay attention to when finding free
-- variables, or doing dependency analysis.
isLocalVar :: Var -> Bool
isLocalVar v = not (isGlobalId v)

isGlobalId :: Var -> Bool
isGlobalId (Id { idScope = GlobalId }) = True
isGlobalId _                           = False

-- | 'mustHaveLocalBinding' returns @True@ of 'Id's and 'TyVar's
-- that must have a binding in this module.  The converse
-- is not quite right: there are some global 'Id's that must have
-- bindings, such as record selectors.  But that doesn't matter,
-- because it's only used for assertions
mustHaveLocalBinding        :: Var -> Bool
mustHaveLocalBinding var = isLocalVar var

-- | 'isExportedIdVar' means \"don't throw this away\"
isExportedId :: Var -> Bool
isExportedId (Id { idScope = GlobalId })        = True
isExportedId (Id { idScope = LocalId Exported}) = True
isExportedId _ = False
\end{code}