Id.lhs

%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[Id]{@Ids@: Value and constructor identifiers}

\begin{code}
-- |
-- #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' represents names that not only have a 'Name.Name' but also a 'TypeRep.Type' and some additional
--   details (a 'IdInfo.IdInfo' and one of 'Var.LocalIdDetails' or 'IdInfo.GlobalIdDetails') that
--   are added, modified and inspected by various compiler passes. These 'Var.Var' names may either 
--   be global or local, see "Var#globalvslocal"
--
-- * 'Var.Var': see "Var#name_types"
module Id (
        -- * The main types
	Id, DictId,

	-- ** Simple construction
	mkGlobalId, mkVanillaGlobal, mkVanillaGlobalWithInfo,
	mkLocalId, mkLocalIdWithInfo, mkExportedLocalId,
	mkSysLocal, mkSysLocalM, mkUserLocal, mkUserLocalM,
	mkTemplateLocals, mkTemplateLocalsNum, mkTemplateLocal,
	mkWorkerId, mkWiredInIdName,

	-- ** Taking an Id apart
	idName, idType, idUnique, idInfo, idDetails,
	isId, idPrimRep,
	recordSelectorFieldLabel,

	-- ** Modifying an Id
	setIdName, setIdUnique, Id.setIdType, 
	setIdExported, setIdNotExported, 
	globaliseId, localiseId, 
	setIdInfo, lazySetIdInfo, modifyIdInfo, maybeModifyIdInfo,
	zapLamIdInfo, zapDemandIdInfo, zapFragileIdInfo, transferPolyIdInfo,
	

	-- ** Predicates on Ids
	isImplicitId, isDeadBinder, isDictId, isStrictId,
	isExportedId, isLocalId, isGlobalId,
	isRecordSelector, isNaughtyRecordSelector,
	isClassOpId_maybe, isDFunId,
	isPrimOpId, isPrimOpId_maybe, 
	isFCallId, isFCallId_maybe,
	isDataConWorkId, isDataConWorkId_maybe, isDataConId_maybe, idDataCon,
        isConLikeId, isBottomingId, idIsFrom,
        isTickBoxOp, isTickBoxOp_maybe,
	hasNoBinding, 

	-- ** Inline pragma stuff
	idInlinePragma, setInlinePragma, modifyInlinePragma,
        idInlineActivation, setInlineActivation, idRuleMatchInfo,

	-- ** One-shot lambdas
	isOneShotBndr, isOneShotLambda, isStateHackType,
	setOneShotLambda, clearOneShotLambda,

	-- ** Reading 'IdInfo' fields
	idArity, 
	idDemandInfo, idDemandInfo_maybe,
	idStrictness, idStrictness_maybe, 
	idUnfolding, realIdUnfolding,
	idSpecialisation, idCoreRules, idHasRules,
	idCafInfo,
	idLBVarInfo,
	idOccInfo,

	-- ** Writing 'IdInfo' fields
	setIdUnfoldingLazily,
	setIdUnfolding,
	setIdArity,
	setIdDemandInfo, 
	setIdStrictness, zapIdStrictness,
	setIdSpecialisation,
	setIdCafInfo,
	setIdOccInfo, zapIdOccInfo,

    ) where

#include "HsVersions.h"

import CoreSyn ( CoreRule, Unfolding( NoUnfolding ) )

import IdInfo
import BasicTypes

-- Imported and re-exported 
import Var( Var, Id, DictId,
            idInfo, idDetails, globaliseId,
            isId, isLocalId, isGlobalId, isExportedId )
import qualified Var

import TyCon
import Type
import TysPrim
import DataCon
import Demand
import Name
import Module
import Class
import PrimOp
import ForeignCall
import Maybes
import SrcLoc
import Outputable
import Unique
import UniqSupply
import FastString
import Util( count )
import StaticFlags

-- infixl so you can say (id `set` a `set` b)
infixl 	1 `setIdUnfoldingLazily`,
	  `setIdUnfolding`,
	  `setIdArity`,
	  `setIdOccInfo`,
	  `setIdDemandInfo`,
	  `setIdStrictness`,
	  `setIdSpecialisation`,
	  `setInlinePragma`,
	  `setInlineActivation`,
	  `idCafInfo`
\end{code}

%************************************************************************
%*									*
\subsection{Basic Id manipulation}
%*									*
%************************************************************************

\begin{code}
idName   :: Id -> Name
idName    = Var.varName

idUnique :: Id -> Unique
idUnique  = Var.varUnique

idType   :: Id -> Kind
idType    = Var.varType

idPrimRep :: Id -> PrimRep
idPrimRep id = typePrimRep (idType id)

setIdName :: Id -> Name -> Id
setIdName = Var.setVarName

setIdUnique :: Id -> Unique -> Id
setIdUnique = Var.setVarUnique

-- | Not only does this set the 'Id' 'Type', it also evaluates the type to try and
-- reduce space usage
setIdType :: Id -> Type -> Id
setIdType id ty = seqType ty `seq` Var.setVarType id ty

setIdExported :: Id -> Id
setIdExported = Var.setIdExported

setIdNotExported :: Id -> Id
setIdNotExported = Var.setIdNotExported

localiseId :: Id -> Id
-- Make an with the same unique and type as the 
-- incoming Id, but with an *Internal* Name and *LocalId* flavour
localiseId id 
  | isLocalId id && isInternalName name
  = id
  | otherwise
  = mkLocalIdWithInfo (localiseName name) (idType id) (idInfo id)
  where
    name = idName id

lazySetIdInfo :: Id -> IdInfo -> Id
lazySetIdInfo = Var.lazySetIdInfo

setIdInfo :: Id -> IdInfo -> Id
setIdInfo id info = seqIdInfo info `seq` (lazySetIdInfo id info)
        -- Try to avoid spack leaks by seq'ing

modifyIdInfo :: (IdInfo -> IdInfo) -> Id -> Id
modifyIdInfo fn id = setIdInfo id (fn (idInfo id))

-- maybeModifyIdInfo tries to avoid unnecesary thrashing
maybeModifyIdInfo :: Maybe IdInfo -> Id -> Id
maybeModifyIdInfo (Just new_info) id = lazySetIdInfo id new_info
maybeModifyIdInfo Nothing	  id = id
\end{code}

%************************************************************************
%*									*
\subsection{Simple Id construction}
%*									*
%************************************************************************

Absolutely all Ids are made by mkId.  It is just like Var.mkId,
but in addition it pins free-tyvar-info onto the Id's type, 
where it can easily be found.

Note [Free type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~
At one time we cached the free type variables of the type of an Id
at the root of the type in a TyNote.  The idea was to avoid repeating
the free-type-variable calculation.  But it turned out to slow down
the compiler overall. I don't quite know why; perhaps finding free
type variables of an Id isn't all that common whereas applying a 
substitution (which changes the free type variables) is more common.
Anyway, we removed it in March 2008.

\begin{code}
-- | For an explanation of global vs. local 'Id's, see "Var#globalvslocal"
mkGlobalId :: IdDetails -> Name -> Type -> IdInfo -> Id
mkGlobalId = Var.mkGlobalVar

-- | Make a global 'Id' without any extra information at all
mkVanillaGlobal :: Name -> Type -> Id
mkVanillaGlobal name ty = mkVanillaGlobalWithInfo name ty vanillaIdInfo

-- | Make a global 'Id' with no global information but some generic 'IdInfo'
mkVanillaGlobalWithInfo :: Name -> Type -> IdInfo -> Id
mkVanillaGlobalWithInfo = mkGlobalId VanillaId


-- | For an explanation of global vs. local 'Id's, see "Var#globalvslocal"
mkLocalId :: Name -> Type -> Id
mkLocalId name ty = mkLocalIdWithInfo name ty vanillaIdInfo

mkLocalIdWithInfo :: Name -> Type -> IdInfo -> Id
mkLocalIdWithInfo name ty info = Var.mkLocalVar VanillaId name ty info
	-- Note [Free type variables]

-- | Create a local 'Id' that is marked as exported. 
-- This prevents things attached to it from being removed as dead code.
mkExportedLocalId :: Name -> Type -> Id
mkExportedLocalId name ty = Var.mkExportedLocalVar VanillaId name ty vanillaIdInfo
	-- Note [Free type variables]


-- | Create a system local 'Id'. These are local 'Id's (see "Var#globalvslocal") 
-- that are created by the compiler out of thin air
mkSysLocal :: FastString -> Unique -> Type -> Id
mkSysLocal fs uniq ty = mkLocalId (mkSystemVarName uniq fs) ty

mkSysLocalM :: MonadUnique m => FastString -> Type -> m Id
mkSysLocalM fs ty = getUniqueM >>= (\uniq -> return (mkSysLocal fs uniq ty))


-- | Create a user local 'Id'. These are local 'Id's (see "Var#globalvslocal") with a name and location that the user might recognize
mkUserLocal :: OccName -> Unique -> Type -> SrcSpan -> Id
mkUserLocal occ uniq ty loc = mkLocalId (mkInternalName uniq occ loc) ty

mkUserLocalM :: MonadUnique m => OccName -> Type -> SrcSpan -> m Id
mkUserLocalM occ ty loc = getUniqueM >>= (\uniq -> return (mkUserLocal occ uniq ty loc))

mkWiredInIdName :: Module -> FastString -> Unique -> Id -> Name
mkWiredInIdName mod fs uniq id
 = mkWiredInName mod (mkOccNameFS varName fs) uniq (AnId id) UserSyntax
\end{code}

Make some local @Ids@ for a template @CoreExpr@.  These have bogus
@Uniques@, but that's OK because the templates are supposed to be
instantiated before use.
 
\begin{code}
-- | Workers get local names. "CoreTidy" will externalise these if necessary
mkWorkerId :: Unique -> Id -> Type -> Id
mkWorkerId uniq unwrkr ty
  = mkLocalId (mkDerivedInternalName mkWorkerOcc uniq (getName unwrkr)) ty

-- | Create a /template local/: a family of system local 'Id's in bijection with @Int@s, typically used in unfoldings
mkTemplateLocal :: Int -> Type -> Id
mkTemplateLocal i ty = mkSysLocal (fsLit "tpl") (mkBuiltinUnique i) ty

-- | Create a template local for a series of types
mkTemplateLocals :: [Type] -> [Id]
mkTemplateLocals = mkTemplateLocalsNum 1

-- | Create a template local for a series of type, but start from a specified template local
mkTemplateLocalsNum :: Int -> [Type] -> [Id]
mkTemplateLocalsNum n tys = zipWith mkTemplateLocal [n..] tys
\end{code}


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

\begin{code}
-- | If the 'Id' is that for a record selector, extract the 'sel_tycon' and label. Panic otherwise
recordSelectorFieldLabel :: Id -> (TyCon, FieldLabel)
recordSelectorFieldLabel id
  = case Var.idDetails id of
        RecSelId { sel_tycon = tycon } -> (tycon, idName id)
        _ -> panic "recordSelectorFieldLabel"

isRecordSelector        :: Id -> Bool
isNaughtyRecordSelector :: Id -> Bool
isPrimOpId              :: Id -> Bool
isFCallId               :: Id -> Bool
isDataConWorkId         :: Id -> Bool
isDFunId                :: Id -> Bool

isClassOpId_maybe       :: Id -> Maybe Class
isPrimOpId_maybe        :: Id -> Maybe PrimOp
isFCallId_maybe         :: Id -> Maybe ForeignCall
isDataConWorkId_maybe   :: Id -> Maybe DataCon

isRecordSelector id = case Var.idDetails id of
                        RecSelId {}  -> True
                        _               -> False

isNaughtyRecordSelector id = case Var.idDetails id of
                        RecSelId { sel_naughty = n } -> n
                        _                               -> False

isClassOpId_maybe id = case Var.idDetails id of
			ClassOpId cls -> Just cls
			_other        -> Nothing

isPrimOpId id = case Var.idDetails id of
                        PrimOpId _ -> True
                        _          -> False

isDFunId id = case Var.idDetails id of
                        DFunId _ -> True
                        _        -> False

isPrimOpId_maybe id = case Var.idDetails id of
                        PrimOpId op -> Just op
                        _           -> Nothing

isFCallId id = case Var.idDetails id of
                        FCallId _ -> True
                        _         -> False

isFCallId_maybe id = case Var.idDetails id of
                        FCallId call -> Just call
                        _            -> Nothing

isDataConWorkId id = case Var.idDetails id of
                        DataConWorkId _ -> True
                        _               -> False

isDataConWorkId_maybe id = case Var.idDetails id of
                        DataConWorkId con -> Just con
                        _                 -> Nothing

isDataConId_maybe :: Id -> Maybe DataCon
isDataConId_maybe id = case Var.idDetails id of
                         DataConWorkId con -> Just con
                         DataConWrapId con -> Just con
                         _                 -> Nothing

idDataCon :: Id -> DataCon
-- ^ Get from either the worker or the wrapper 'Id' to the 'DataCon'. Currently used only in the desugarer.
--
-- INVARIANT: @idDataCon (dataConWrapId d) = d@: remember, 'dataConWrapId' can return either the wrapper or the worker
idDataCon id = isDataConId_maybe id `orElse` pprPanic "idDataCon" (ppr id)


isDictId :: Id -> Bool
isDictId id = isDictTy (idType id)

hasNoBinding :: Id -> Bool
-- ^ Returns @True@ of an 'Id' which may not have a
-- binding, even though it is defined in this module.

-- Data constructor workers used to be things of this kind, but
-- they aren't any more.  Instead, we inject a binding for 
-- them at the CorePrep stage. 
-- EXCEPT: unboxed tuples, which definitely have no binding
hasNoBinding id = case Var.idDetails id of
			PrimOpId _  	 -> True	-- See Note [Primop wrappers]
			FCallId _   	 -> True
			DataConWorkId dc -> isUnboxedTupleCon dc
			_                -> False

isImplicitId :: Id -> Bool
-- ^ 'isImplicitId' tells whether an 'Id's info is implied by other
-- declarations, so we don't need to put its signature in an interface
-- file, even if it's mentioned in some other interface unfolding.
isImplicitId id
  = case Var.idDetails id of
        FCallId {}       -> True
	ClassOpId {}     -> True
        PrimOpId {}      -> True
        DataConWorkId {} -> True
	DataConWrapId {} -> True
		-- These are are implied by their type or class decl;
		-- remember that all type and class decls appear in the interface file.
		-- The dfun id is not an implicit Id; it must *not* be omitted, because 
		-- it carries version info for the instance decl
        _               -> False

idIsFrom :: Module -> Id -> Bool
idIsFrom mod id = nameIsLocalOrFrom mod (idName id)
\end{code}

Note [Primop wrappers]
~~~~~~~~~~~~~~~~~~~~~~
Currently hasNoBinding claims that PrimOpIds don't have a curried
function definition.  But actually they do, in GHC.PrimopWrappers,
which is auto-generated from prelude/primops.txt.pp.  So actually, hasNoBinding
could return 'False' for PrimOpIds.

But we'd need to add something in CoreToStg to swizzle any unsaturated
applications of GHC.Prim.plusInt# to GHC.PrimopWrappers.plusInt#.

Nota Bene: GHC.PrimopWrappers is needed *regardless*, because it's
used by GHCi, which does not implement primops direct at all.



\begin{code}
isDeadBinder :: Id -> Bool
isDeadBinder bndr | isId bndr = isDeadOcc (idOccInfo bndr)
		  | otherwise = False	-- TyVars count as not dead
\end{code}

\begin{code}
isTickBoxOp :: Id -> Bool
isTickBoxOp id = 
  case Var.idDetails id of
    TickBoxOpId _    -> True
    _                -> False

isTickBoxOp_maybe :: Id -> Maybe TickBoxOp
isTickBoxOp_maybe id = 
  case Var.idDetails id of
    TickBoxOpId tick -> Just tick
    _                -> Nothing
\end{code}

%************************************************************************
%*									*
\subsection{IdInfo stuff}
%*									*
%************************************************************************

\begin{code}
	---------------------------------
	-- ARITY
idArity :: Id -> Arity
idArity id = arityInfo (idInfo id)

setIdArity :: Id -> Arity -> Id
setIdArity id arity = modifyIdInfo (`setArityInfo` arity) id

-- | Returns true if an application to n args would diverge
isBottomingId :: Id -> Bool
isBottomingId id = isBottomingSig (idStrictness id)

idStrictness_maybe :: Id -> Maybe StrictSig
idStrictness :: Id -> StrictSig

idStrictness_maybe id = strictnessInfo (idInfo id)
idStrictness       id = idStrictness_maybe id `orElse` topSig

setIdStrictness :: Id -> StrictSig -> Id
setIdStrictness id sig = modifyIdInfo (`setStrictnessInfo` Just sig) id

zapIdStrictness :: Id -> Id
zapIdStrictness id = modifyIdInfo (`setStrictnessInfo` Nothing) id

-- | This predicate says whether the 'Id' has a strict demand placed on it or
-- has a type such that it can always be evaluated strictly (e.g., an
-- unlifted type, but see the comment for 'isStrictType').  We need to
-- check separately whether the 'Id' has a so-called \"strict type\" because if
-- the demand for the given @id@ hasn't been computed yet but @id@ has a strict
-- type, we still want @isStrictId id@ to be @True@.
isStrictId :: Id -> Bool
isStrictId id
  = ASSERT2( isId id, text "isStrictId: not an id: " <+> ppr id )
           (isStrictDmd (idDemandInfo id)) || 
           (isStrictType (idType id))

	---------------------------------
	-- UNFOLDING
idUnfolding :: Id -> Unfolding
-- Do not expose the unfolding of a loop breaker!
idUnfolding id 
  | isNonRuleLoopBreaker (occInfo info) = NoUnfolding
  | otherwise                           = unfoldingInfo info
  where
    info = idInfo id

realIdUnfolding :: Id -> Unfolding
-- Expose the unfolding if there is one, including for loop breakers
realIdUnfolding id = unfoldingInfo (idInfo id)

setIdUnfoldingLazily :: Id -> Unfolding -> Id
setIdUnfoldingLazily id unfolding = modifyIdInfo (`setUnfoldingInfoLazily` unfolding) id

setIdUnfolding :: Id -> Unfolding -> Id
setIdUnfolding id unfolding = modifyIdInfo (`setUnfoldingInfo` unfolding) id

idDemandInfo_maybe :: Id -> Maybe Demand
idDemandInfo       :: Id -> Demand

idDemandInfo_maybe id = demandInfo (idInfo id)
idDemandInfo       id = demandInfo (idInfo id) `orElse` topDmd

setIdDemandInfo :: Id -> Demand -> Id
setIdDemandInfo id dmd = modifyIdInfo (`setDemandInfo` Just dmd) id

	---------------------------------
	-- SPECIALISATION

-- See Note [Specialisations and RULES in IdInfo] in IdInfo.lhs

idSpecialisation :: Id -> SpecInfo
idSpecialisation id = specInfo (idInfo id)

idCoreRules :: Id -> [CoreRule]
idCoreRules id = specInfoRules (idSpecialisation id)

idHasRules :: Id -> Bool
idHasRules id = not (isEmptySpecInfo (idSpecialisation id))

setIdSpecialisation :: Id -> SpecInfo -> Id
setIdSpecialisation id spec_info = modifyIdInfo (`setSpecInfo` spec_info) id

	---------------------------------
	-- CAF INFO
idCafInfo :: Id -> CafInfo
idCafInfo id = cafInfo (idInfo id)

setIdCafInfo :: Id -> CafInfo -> Id
setIdCafInfo id caf_info = modifyIdInfo (`setCafInfo` caf_info) id

	---------------------------------
	-- Occcurrence INFO
idOccInfo :: Id -> OccInfo
idOccInfo id = occInfo (idInfo id)

setIdOccInfo :: Id -> OccInfo -> Id
setIdOccInfo id occ_info = modifyIdInfo (`setOccInfo` occ_info) id

zapIdOccInfo :: Id -> Id
zapIdOccInfo b = b `setIdOccInfo` NoOccInfo
\end{code}


	---------------------------------
	-- INLINING
The inline pragma tells us to be very keen to inline this Id, but it's still
OK not to if optimisation is switched off.

\begin{code}
idInlinePragma :: Id -> InlinePragma
idInlinePragma id = inlinePragInfo (idInfo id)

setInlinePragma :: Id -> InlinePragma -> Id
setInlinePragma id prag = modifyIdInfo (`setInlinePragInfo` prag) id

modifyInlinePragma :: Id -> (InlinePragma -> InlinePragma) -> Id
modifyInlinePragma id fn = modifyIdInfo (\info -> info `setInlinePragInfo` (fn (inlinePragInfo info))) id

idInlineActivation :: Id -> Activation
idInlineActivation id = inlinePragmaActivation (idInlinePragma id)

setInlineActivation :: Id -> Activation -> Id
setInlineActivation id act = modifyInlinePragma id (\prag -> setInlinePragmaActivation prag act)

idRuleMatchInfo :: Id -> RuleMatchInfo
idRuleMatchInfo id = inlinePragmaRuleMatchInfo (idInlinePragma id)

isConLikeId :: Id -> Bool
isConLikeId id = isDataConWorkId id || isConLike (idRuleMatchInfo id)
\end{code}


	---------------------------------
	-- ONE-SHOT LAMBDAS
\begin{code}
idLBVarInfo :: Id -> LBVarInfo
idLBVarInfo id = lbvarInfo (idInfo id)

-- | Returns whether the lambda associated with the 'Id' is certainly applied at most once
-- OR we are applying the \"state hack\" which makes it appear as if theis is the case for
-- lambdas used in @IO@. You should prefer using this over 'isOneShotLambda'
isOneShotBndr :: Id -> Bool
-- This one is the "business end", called externally.
-- Its main purpose is to encapsulate the Horrible State Hack
isOneShotBndr id = isOneShotLambda id || isStateHackType (idType id)

-- | Should we apply the state hack to values of this 'Type'?
isStateHackType :: Type -> Bool
isStateHackType ty
  | opt_NoStateHack 
  = False
  | otherwise
  = case splitTyConApp_maybe ty of
	Just (tycon,_) -> tycon == statePrimTyCon
        _              -> False
	-- This is a gross hack.  It claims that 
	-- every function over realWorldStatePrimTy is a one-shot
	-- function.  This is pretty true in practice, and makes a big
	-- difference.  For example, consider
	--	a `thenST` \ r -> ...E...
	-- The early full laziness pass, if it doesn't know that r is one-shot
	-- will pull out E (let's say it doesn't mention r) to give
	--	let lvl = E in a `thenST` \ r -> ...lvl...
	-- When `thenST` gets inlined, we end up with
	--	let lvl = E in \s -> case a s of (r, s') -> ...lvl...
	-- and we don't re-inline E.
	--
	-- It would be better to spot that r was one-shot to start with, but
	-- I don't want to rely on that.
	--
	-- Another good example is in fill_in in PrelPack.lhs.  We should be able to
	-- spot that fill_in has arity 2 (and when Keith is done, we will) but we can't yet.


-- | Returns whether the lambda associated with the 'Id' is certainly applied at most once.
-- You probably want to use 'isOneShotBndr' instead
isOneShotLambda :: Id -> Bool
isOneShotLambda id = case idLBVarInfo id of
                       IsOneShotLambda  -> True
                       NoLBVarInfo      -> False

setOneShotLambda :: Id -> Id
setOneShotLambda id = modifyIdInfo (`setLBVarInfo` IsOneShotLambda) id

clearOneShotLambda :: Id -> Id
clearOneShotLambda id 
  | isOneShotLambda id = modifyIdInfo (`setLBVarInfo` NoLBVarInfo) id
  | otherwise	       = id			

-- The OneShotLambda functions simply fiddle with the IdInfo flag
-- But watch out: this may change the type of something else
--	f = \x -> e
-- If we change the one-shot-ness of x, f's type changes
\end{code}

\begin{code}
zapInfo :: (IdInfo -> Maybe IdInfo) -> Id -> Id
zapInfo zapper id = maybeModifyIdInfo (zapper (idInfo id)) id

zapLamIdInfo :: Id -> Id
zapLamIdInfo = zapInfo zapLamInfo

zapDemandIdInfo :: Id -> Id
zapDemandIdInfo = zapInfo zapDemandInfo

zapFragileIdInfo :: Id -> Id
zapFragileIdInfo = zapInfo zapFragileInfo 
\end{code}

Note [transferPolyIdInfo]
~~~~~~~~~~~~~~~~~~~~~~~~~
This transfer is used in two places: 
	FloatOut (long-distance let-floating)
	SimplUtils.abstractFloats (short-distance let-floating)

Consider the short-distance let-floating:

   f = /\a. let g = rhs in ...

Then if we float thus

   g' = /\a. rhs
   f = /\a. ...[g' a/g]....

we *do not* want to lose g's
  * strictness information
  * arity 
  * inline pragma (though that is bit more debatable)
  * occurrence info

Mostly this is just an optimisation, but it's *vital* to
transfer the occurrence info.  Consider
   
   NonRec { f = /\a. let Rec { g* = ..g.. } in ... }

where the '*' means 'LoopBreaker'.  Then if we float we must get

   Rec { g'* = /\a. ...(g' a)... }
   NonRec { f = /\a. ...[g' a/g]....}

where g' is also marked as LoopBreaker.  If not, terrible things
can happen if we re-simplify the binding (and the Simplifier does
sometimes simplify a term twice); see Trac #4345.

It's not so simple to retain
  * worker info
  * rules
so we simply discard those.  Sooner or later this may bite us.

If we abstract wrt one or more *value* binders, we must modify the 
arity and strictness info before transferring it.  E.g. 
      f = \x. e
-->
      g' = \y. \x. e
      + substitute (g' y) for g
Notice that g' has an arity one more than the original g

\begin{code}
transferPolyIdInfo :: Id	-- Original Id
		   -> [Var]	-- Abstract wrt these variables
		   -> Id	-- New Id
		   -> Id
transferPolyIdInfo old_id abstract_wrt new_id
  = modifyIdInfo transfer new_id
  where
    arity_increase = count isId abstract_wrt	-- Arity increases by the
    		     	   			-- number of value binders

    old_info 	    = idInfo old_id
    old_arity       = arityInfo old_info
    old_inline_prag = inlinePragInfo old_info
    old_occ_info    = occInfo old_info
    new_arity       = old_arity + arity_increase
    old_strictness  = strictnessInfo old_info
    new_strictness  = fmap (increaseStrictSigArity arity_increase) old_strictness

    transfer new_info = new_info `setStrictnessInfo` new_strictness
			         `setArityInfo` new_arity
 			         `setInlinePragInfo` old_inline_prag
				 `setOccInfo` old_occ_info
\end{code}