basicTypes/Var.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    \section{@Vars@: Variables}
    -}

    {-# LANGUAGE CPP, FlexibleContexts, MultiWayIf, FlexibleInstances, DeriveDataTypeable #-}

    -- |
    -- #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 'TyCoRep.Kind'
    --   rather than a 'TyCoRep.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, NcId, DictId, DFunId, EvVar, EqVar, EvId, IpId, JoinId,
            TyVar, TypeVar, KindVar, TKVar, TyCoVar,

            -- * In and Out variants
            InVar,  InCoVar,  InId,  InTyVar,
            OutVar, OutCoVar, OutId, OutTyVar,

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

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

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

            -- ** Predicates
            isId, isTyVar, isTcTyVar,
            isLocalVar, isLocalId, isCoVar, isNonCoVarId, isTyCoVar,
            isGlobalId, isExportedId,
            mustHaveLocalBinding,

            -- * TyVar's
            TyVarBndr(..), ArgFlag(..), TyVarBinder,
            binderVar, binderVars, binderArgFlag, binderKind,
            isVisibleArgFlag, isInvisibleArgFlag, sameVis,

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

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

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

            nonDetCmpVar

        ) where

    #include "HsVersions.h"

    import {-# SOURCE #-}   TyCoRep( Type, Kind, pprKind )
    import {-# SOURCE #-}   TcType( TcTyVarDetails, pprTcTyVarDetails, vanillaSkolemTv )
    import {-# SOURCE #-}   IdInfo( IdDetails, IdInfo, coVarDetails, isCoVarDetails,
                                    vanillaIdInfo, pprIdDetails )

    import Name hiding (varName)
    import Unique ( Uniquable, Unique, getKey, getUnique
                  , mkUniqueGrimily, nonDetCmpUnique )
    import Util
    import Binary
    import DynFlags
    import Outputable

    import Data.Data

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

    -- | Identifier
    type Id    = Var       -- A term-level identifier
                           --  predicate: isId

    -- | Coercion Variable
    type CoVar = Id        -- See Note [Evidence: EvIds and CoVars]
                           --   predicate: isCoVar

    -- |
    type NcId  = Id        -- A term-level (value) variable that is
                           -- /not/ an (unlifted) coercion
                           --    predicate: isNonCoVarId

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

    -- | Type or Kind Variable
    type TKVar   = Var     -- Type *or* kind variable (historical)

    -- | Type Variable
    type TypeVar = Var     -- Definitely a type variable

    -- | Kind Variable
    type KindVar = Var     -- Definitely a kind variable
                           -- See Note [Kind and type variables]

    -- See Note [Evidence: EvIds and CoVars]
    -- | Evidence Identifier
    type EvId   = Id        -- Term-level evidence: DictId, IpId, or EqVar

    -- | Evidence Variable
    type EvVar  = EvId      -- ...historical name for EvId

    -- | Dictionary Function Identifier
    type DFunId = Id        -- A dictionary function

    -- | Dictionary Identifier
    type DictId = EvId      -- A dictionary variable

    -- | Implicit parameter Identifier
    type IpId   = EvId      -- A term-level implicit parameter

    -- | Equality Variable
    type EqVar  = EvId      -- Boxed equality evidence
    type JoinId = Id        -- A join variable

    -- | Type or Coercion Variable
    type TyCoVar = Id       -- Type, *or* coercion variable
                            --   predicate: isTyCoVar

    {- Many passes apply a substitution, and it's very handy to have type
       synonyms to remind us whether or not the subsitution has been applied -}

    type InVar      = Var
    type InTyVar    = TyVar
    type InCoVar    = CoVar
    type InId       = Id
    type OutVar     = Var
    type OutTyVar   = TyVar
    type OutCoVar   = CoVar
    type OutId      = Id

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

    * 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).

    * A CoVar is always an un-lifted coercion, of type (t1 ~# t2) or (t1 ~R# 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@.
    -}

    -- | Variable
    --
    -- 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 :: {-# UNPACK #-} !Int,
                                         -- ^ 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     :: {-# UNPACK #-} !Int,
            varType        :: Kind,
            tc_tv_details  :: TcTyVarDetails
      }

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

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

    data ExportFlag   -- See Note [ExportFlag on binders]
      = NotExported   -- ^ Not exported: may be discarded as dead code.
      | Exported      -- ^ Exported: kept alive

    {- Note [ExportFlag on binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    An ExportFlag of "Exported" on a top-level binder says "keep this
    binding alive; do not drop it as dead code".  This transitively
    keeps alive all the other top-level bindings that this binding refers
    to.  This property is persisted all the way down the pipeline, so that
    the binding will be compiled all the way to object code, and its
    symbols will appear in the linker symbol table.

    However, note that this use of "exported" is quite different to the
    export list on a Haskell module.  Setting the ExportFlag on an Id does
    /not/ mean that if you import the module (in Haskell source code) you
    will see this Id.  Of course, things that appear in the export list
    of the source Haskell module do indeed have their ExportFlag set.
    But many other things, such as dictionary functions, are kept alive
    by having their ExportFlag set, even though they are not exported
    in the source-code sense.

    We should probably use a different term for ExportFlag, like
    KeepAlive.

    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
    -}

    instance Outputable Var where
      ppr var = sdocWithDynFlags $ \dflags ->
                getPprStyle $ \ppr_style ->
                if |  debugStyle ppr_style && (not (gopt Opt_SuppressVarKinds dflags))
                     -> parens (ppr (varName var) <+> ppr_debug var ppr_style <+>
                              dcolon <+> pprKind (tyVarKind var))
                   |  otherwise
                     -> ppr (varName var) <> ppr_debug var ppr_style

    ppr_debug :: Var -> PprStyle -> SDoc
    ppr_debug (TyVar {}) sty
      | debugStyle sty = brackets (text "tv")
    ppr_debug (TcTyVar {tc_tv_details = d}) sty
      | dumpStyle sty || debugStyle sty = brackets (pprTcTyVarDetails d)
    ppr_debug (Id { idScope = s, id_details = d }) sty
      | debugStyle sty = brackets (ppr_id_scope s <> pprIdDetails d)
    ppr_debug _ _ = empty

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

    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 = a `nonDetCmpVar` b

    -- | Compare Vars by their Uniques.
    -- This is what Ord Var does, provided here to make it explicit at the
    -- call-site that it can introduce non-determinism.
    -- See Note [Unique Determinism]
    nonDetCmpVar :: Var -> Var -> Ordering
    nonDetCmpVar a b = varUnique a `nonDetCmpUnique` varUnique b

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

    instance HasOccName Var where
      occName = nameOccName . varName

    varUnique :: Var -> Unique
    varUnique var = mkUniqueGrimily (realUnique var)

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

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

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

    updateVarType :: (Type -> Type) -> Id -> Id
    updateVarType f id = id { varType = f (varType id) }

    updateVarTypeM :: Monad m => (Type -> m Type) -> Id -> m Id
    updateVarTypeM f id = do { ty' <- f (varType id)
                             ; return (id { varType = ty' }) }

    {- *********************************************************************
    *                                                                      *
    *                   ArgFlag
    *                                                                      *
    ********************************************************************* -}

    -- | Argument Flag
    --
    -- Is something required to appear in source Haskell ('Required'),
    -- permitted by request ('Specified') (visible type application), or
    -- prohibited entirely from appearing in source Haskell ('Inferred')?
    -- See Note [TyBinders and ArgFlags] in TyCoRep
    data ArgFlag = Required | Specified | Inferred
      deriving (Eq, Data)

    -- | Does this 'ArgFlag' classify an argument that is written in Haskell?
    isVisibleArgFlag :: ArgFlag -> Bool
    isVisibleArgFlag Required = True
    isVisibleArgFlag _        = False

    -- | Does this 'ArgFlag' classify an argument that is not written in Haskell?
    isInvisibleArgFlag :: ArgFlag -> Bool
    isInvisibleArgFlag = not . isVisibleArgFlag

    -- | Do these denote the same level of visibility? 'Required'
    -- arguments are visible, others are not. So this function
    -- equates 'Specified' and 'Inferred'. Used for printing.
    sameVis :: ArgFlag -> ArgFlag -> Bool
    sameVis Required Required = True
    sameVis Required _        = False
    sameVis _        Required = False
    sameVis _        _        = True

    {- *********************************************************************
    *                                                                      *
    *                   TyVarBndr, TyVarBinder
    *                                                                      *
    ********************************************************************* -}

    -- Type Variable Binder
    --
    -- TyVarBndr is polymorphic in both tyvar and visiblity fields:
    --   * tyvar can be TyVar or IfaceTv
    --   * argf  can be ArgFlag or TyConBndrVis
    data TyVarBndr tyvar argf = TvBndr tyvar argf
      deriving( Data )

    -- | Type Variable Binder
    --
    -- A 'TyVarBinder' is the binder of a ForAllTy
    -- It's convenient to define this synonym here rather its natural
    -- home in TyCoRep, because it's used in DataCon.hs-boot
    type TyVarBinder = TyVarBndr TyVar ArgFlag

    binderVar :: TyVarBndr tv argf -> tv
    binderVar (TvBndr v _) = v

    binderVars :: [TyVarBndr tv argf] -> [tv]
    binderVars tvbs = map binderVar tvbs

    binderArgFlag :: TyVarBndr tv argf -> argf
    binderArgFlag (TvBndr _ argf) = argf

    binderKind :: TyVarBndr TyVar argf -> Kind
    binderKind (TvBndr tv _) = tyVarKind tv

    {-
    ************************************************************************
    *                                                                      *
    *                 Type and kind variables                              *
    *                                                                      *
    ************************************************************************
    -}

    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'} }

    mkTyVar :: Name -> Kind -> TyVar
    mkTyVar name kind = TyVar { varName    = name
                              , realUnique = getKey (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 = getKey (nameUnique name),
                    varType  = kind,
                    tc_tv_details = details
            }

    tcTyVarDetails :: TyVar -> TcTyVarDetails
    -- See Note [TcTyVars in the typechecker] in TcType
    tcTyVarDetails (TcTyVar { tc_tv_details = details }) = details
    tcTyVarDetails (TyVar {})                            = vanillaSkolemTv
    tcTyVarDetails var = pprPanic "tcTyVarDetails" (ppr var <+> dcolon <+> pprKind (tyVarKind var))

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

    -------------------------------------
    instance Outputable tv => Outputable (TyVarBndr tv ArgFlag) where
      ppr (TvBndr v Required)  = ppr v
      ppr (TvBndr v Specified) = char '@' <> ppr v
      ppr (TvBndr v Inferred)  = braces (ppr v)

    instance Outputable ArgFlag where
      ppr Required  = text "[req]"
      ppr Specified = text "[spec]"
      ppr Inferred  = text "[infrd]"

    instance (Binary tv, Binary vis) => Binary (TyVarBndr tv vis) where
      put_ bh (TvBndr tv vis) = do { put_ bh tv; put_ bh vis }

      get bh = do { tv <- get bh; vis <- get bh; return (TvBndr tv vis) }

    instance Binary ArgFlag where
      put_ bh Required  = putByte bh 0
      put_ bh Specified = putByte bh 1
      put_ bh Inferred  = putByte bh 2

      get bh = do
        h <- getByte bh
        case h of
          0 -> return Required
          1 -> return Specified
          _ -> return Inferred

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

    idInfo :: HasDebugCallStack => 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, because Id.hs 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 = getKey (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 }

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

    isTyVar :: Var -> Bool        -- True of both TyVar and TcTyVar
    isTyVar (TyVar {})   = True
    isTyVar (TcTyVar {}) = True
    isTyVar _            = False

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

    isTyCoVar :: Var -> Bool
    isTyCoVar v = isTyVar v || isCoVar v

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

    isCoVar :: Var -> Bool
    -- A coercion variable
    isCoVar (Id { id_details = details }) = isCoVarDetails details
    isCoVar _                             = False

    isNonCoVarId :: Var -> Bool
    -- A term variable (Id) that is /not/ a coercion variable
    isNonCoVarId (Id { id_details = details }) = not (isCoVarDetails details)
    isNonCoVarId _                             = 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