/ghc-7.0.4/compiler/coreSyn/CoreFVs.lhs

%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
Taken quite directly from the Peyton Jones/Lester paper.

\begin{code}
{-# OPTIONS -fno-warn-incomplete-patterns #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and fix
-- any warnings in the module. See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
-- for details

-- | A module concerned with finding the free variables of an expression.
module CoreFVs (
        -- * Free variables of expressions and binding groups
	exprFreeVars,	-- CoreExpr   -> VarSet	-- Find all locally-defined free Ids or tyvars
	exprFreeIds,	-- CoreExpr   -> IdSet	-- Find all locally-defined free Ids
	exprsFreeVars,	-- [CoreExpr] -> VarSet
	bindFreeVars, 	-- CoreBind   -> VarSet

        -- * Selective free variables of expressions
        InterestingVarFun,
	exprSomeFreeVars, exprsSomeFreeVars,

        -- * Free variables of Rules, Vars and Ids
        varTypeTyVars, varTypeTcTyVars, 
	idUnfoldingVars, idFreeVars, idRuleAndUnfoldingVars,
        idRuleVars, idRuleRhsVars, stableUnfoldingVars,
	ruleRhsFreeVars, rulesFreeVars,
	ruleLhsOrphNames, ruleLhsFreeIds, 

        -- * Core syntax tree annotation with free variables
	CoreExprWithFVs,	-- = AnnExpr Id VarSet
	CoreBindWithFVs,	-- = AnnBind Id VarSet
	freeVars,		-- CoreExpr -> CoreExprWithFVs
	freeVarsOf		-- CoreExprWithFVs -> IdSet
    ) where

#include "HsVersions.h"

import CoreSyn
import Id
import IdInfo
import NameSet
import UniqFM
import Name
import VarSet
import Var
import TcType
import Util
import BasicTypes( Activation )
import Outputable
\end{code}


%************************************************************************
%*									*
\section{Finding the free variables of an expression}
%*									*
%************************************************************************

This function simply finds the free variables of an expression.
So far as type variables are concerned, it only finds tyvars that are

	* free in type arguments, 
	* free in the type of a binder,

but not those that are free in the type of variable occurrence.

\begin{code}
-- | Find all locally-defined free Ids or type variables in an expression
exprFreeVars :: CoreExpr -> VarSet
exprFreeVars = exprSomeFreeVars isLocalVar

-- | Find all locally-defined free Ids in an expression
exprFreeIds :: CoreExpr -> IdSet	-- Find all locally-defined free Ids
exprFreeIds = exprSomeFreeVars isLocalId

-- | Find all locally-defined free Ids or type variables in several expressions
exprsFreeVars :: [CoreExpr] -> VarSet
exprsFreeVars = foldr (unionVarSet . exprFreeVars) emptyVarSet

-- | Find all locally defined free Ids in a binding group
bindFreeVars :: CoreBind -> VarSet
bindFreeVars (NonRec _ r) = exprFreeVars r
bindFreeVars (Rec prs)    = addBndrs (map fst prs) 
				     (foldr (union . rhs_fvs) noVars prs)
				     isLocalVar emptyVarSet

-- | Finds free variables in an expression selected by a predicate
exprSomeFreeVars :: InterestingVarFun 	-- ^ Says which 'Var's are interesting
		 -> CoreExpr
		 -> VarSet
exprSomeFreeVars fv_cand e = expr_fvs e fv_cand emptyVarSet

-- | Finds free variables in several expressions selected by a predicate
exprsSomeFreeVars :: InterestingVarFun 	-- Says which 'Var's are interesting
		  -> [CoreExpr]
		  -> VarSet
exprsSomeFreeVars fv_cand = foldr (unionVarSet . exprSomeFreeVars fv_cand) emptyVarSet

-- | Predicate on possible free variables: returns @True@ iff the variable is interesting
type InterestingVarFun = Var -> Bool
\end{code}


\begin{code}
type FV = InterestingVarFun 
	-> VarSet		-- In scope
	-> VarSet		-- Free vars

union :: FV -> FV -> FV
union fv1 fv2 fv_cand in_scope = fv1 fv_cand in_scope `unionVarSet` fv2 fv_cand in_scope

noVars :: FV
noVars _ _ = emptyVarSet

--	Comment about obselete code
-- We used to gather the free variables the RULES at a variable occurrence
-- with the following cryptic comment:
--     "At a variable occurrence, add in any free variables of its rule rhss
--     Curiously, we gather the Id's free *type* variables from its binding
--     site, but its free *rule-rhs* variables from its usage sites.  This
--     is a little weird.  The reason is that the former is more efficient,
--     but the latter is more fine grained, and a makes a difference when
--     a variable mentions itself one of its own rule RHSs"
-- Not only is this "weird", but it's also pretty bad because it can make
-- a function seem more recursive than it is.  Suppose
--	f  = ...g...
--	g  = ...
--         RULE g x = ...f...
-- Then f is not mentioned in its own RHS, and needn't be a loop breaker
-- (though g may be).  But if we collect the rule fvs from g's occurrence,
-- it looks as if f mentions itself.  (This bites in the eftInt/eftIntFB
-- code in GHC.Enum.)
-- 
-- Anyway, it seems plain wrong.  The RULE is like an extra RHS for the
-- function, so its free variables belong at the definition site.
--
-- Deleted code looked like
--     foldVarSet add_rule_var var_itself_set (idRuleVars var)
--     add_rule_var var set | keep_it fv_cand in_scope var = extendVarSet set var
--			    | otherwise			   = set
-- 	SLPJ Feb06

oneVar :: Id -> FV
oneVar var fv_cand in_scope
  = ASSERT( isId var ) 
    if keep_it fv_cand in_scope var 
    then unitVarSet var
    else emptyVarSet

someVars :: VarSet -> FV
someVars vars fv_cand in_scope
  = filterVarSet (keep_it fv_cand in_scope) vars

keep_it :: InterestingVarFun -> VarSet -> Var -> Bool
keep_it fv_cand in_scope var
  | var `elemVarSet` in_scope = False
  | fv_cand var		      = True
  | otherwise		      = False


addBndr :: CoreBndr -> FV -> FV
addBndr bndr fv fv_cand in_scope
  = someVars (varTypeTyVars bndr) fv_cand in_scope
	-- Include type varibles in the binder's type
	-- 	(not just Ids; coercion variables too!)
    `unionVarSet`  fv fv_cand (in_scope `extendVarSet` bndr) 

addBndrs :: [CoreBndr] -> FV -> FV
addBndrs bndrs fv = foldr addBndr fv bndrs
\end{code}


\begin{code}
expr_fvs :: CoreExpr -> FV

expr_fvs (Type ty) 	 = someVars (tyVarsOfType ty)
expr_fvs (Var var) 	 = oneVar var
expr_fvs (Lit _)         = noVars
expr_fvs (Note _ expr)   = expr_fvs expr
expr_fvs (App fun arg)   = expr_fvs fun `union` expr_fvs arg
expr_fvs (Lam bndr body) = addBndr bndr (expr_fvs body)
expr_fvs (Cast expr co)  = expr_fvs expr `union` someVars (tyVarsOfType co)

expr_fvs (Case scrut bndr ty alts)
  = expr_fvs scrut `union` someVars (tyVarsOfType ty) `union` addBndr bndr  
      (foldr (union . alt_fvs) noVars alts)
  where
    alt_fvs (_, bndrs, rhs) = addBndrs bndrs (expr_fvs rhs)

expr_fvs (Let (NonRec bndr rhs) body)
  = rhs_fvs (bndr, rhs) `union` addBndr bndr (expr_fvs body)

expr_fvs (Let (Rec pairs) body)
  = addBndrs (map fst pairs) 
	     (foldr (union . rhs_fvs) (expr_fvs body) pairs)

---------
rhs_fvs :: (Id,CoreExpr) -> FV
rhs_fvs (bndr, rhs) = expr_fvs rhs `union` 
                      someVars (bndrRuleAndUnfoldingVars bndr)
	-- Treat any RULES as extra RHSs of the binding

---------
exprs_fvs :: [CoreExpr] -> FV
exprs_fvs exprs = foldr (union . expr_fvs) noVars exprs
\end{code}


%************************************************************************
%*									*
\section{Free names}
%*									*
%************************************************************************

\begin{code}
-- | ruleLhsOrphNames is used when deciding whether 
-- a rule is an orphan.  In particular, suppose that T is defined in this 
-- module; we want to avoid declaring that a rule like:
-- 
-- > fromIntegral T = fromIntegral_T
--
-- is an orphan. Of course it isn't, and declaring it an orphan would
-- make the whole module an orphan module, which is bad.
ruleLhsOrphNames :: CoreRule -> NameSet
ruleLhsOrphNames (BuiltinRule { ru_fn = fn }) = unitNameSet fn
ruleLhsOrphNames (Rule { ru_fn = fn, ru_args = tpl_args })
  = addOneToNameSet (exprsOrphNames tpl_args) fn
		-- No need to delete bndrs, because
		-- exprsOrphNames finds only External names

-- | Finds the free /external/ names of an expression, notably
-- including the names of type constructors (which of course do not show
-- up in 'exprFreeVars').
exprOrphNames :: CoreExpr -> NameSet
-- There's no need to delete local binders, because they will all
-- be /internal/ names.
exprOrphNames e
  = go e
  where
    go (Var v) 
      | isExternalName n    = unitNameSet n
      | otherwise	    = emptyNameSet
      where n = idName v
    go (Lit _) 	   	    = emptyNameSet
    go (Type ty)   	    = orphNamesOfType ty	-- Don't need free tyvars
    go (App e1 e2) 	    = go e1 `unionNameSets` go e2
    go (Lam v e)   	    = go e `delFromNameSet` idName v
    go (Note _ e)           = go e
    go (Cast e co)          = go e `unionNameSets` orphNamesOfType co
    go (Let (NonRec _ r) e) = go e `unionNameSets` go r
    go (Let (Rec prs) e)    = exprsOrphNames (map snd prs) `unionNameSets` go e
    go (Case e _ ty as)     = go e `unionNameSets` orphNamesOfType ty
                              `unionNameSets` unionManyNameSets (map go_alt as)

    go_alt (_,_,r) = go r

-- | Finds the free /external/ names of several expressions: see 'exprOrphNames' for details
exprsOrphNames :: [CoreExpr] -> NameSet
exprsOrphNames es = foldr (unionNameSets . exprOrphNames) emptyNameSet es
\end{code}

%************************************************************************
%*									*
\section[freevars-everywhere]{Attaching free variables to every sub-expression}
%*									*
%************************************************************************

\begin{code}
-- | Those variables free in the right hand side of a rule
ruleRhsFreeVars :: CoreRule -> VarSet
ruleRhsFreeVars (BuiltinRule {}) = noFVs
ruleRhsFreeVars (Rule { ru_fn = fn, ru_bndrs = bndrs, ru_rhs = rhs })
  = delFromUFM fvs fn	 -- Note [Rule free var hack]
  where
    fvs = addBndrs bndrs (expr_fvs rhs) isLocalVar emptyVarSet

-- | Those variables free in the both the left right hand sides of a rule
ruleFreeVars :: CoreRule -> VarSet
ruleFreeVars (BuiltinRule {}) = noFVs
ruleFreeVars (Rule { ru_fn = fn, ru_bndrs = bndrs, ru_rhs = rhs, ru_args = args })
  = delFromUFM fvs fn	-- Note [Rule free var hack]
  where
    fvs = addBndrs bndrs (exprs_fvs (rhs:args)) isLocalVar emptyVarSet

idRuleRhsVars :: (Activation -> Bool) -> Id -> VarSet
-- Just the variables free on the *rhs* of a rule
idRuleRhsVars is_active id 
  = foldr (unionVarSet . get_fvs) emptyVarSet (idCoreRules id)
  where
    get_fvs (Rule { ru_fn = fn, ru_bndrs = bndrs
                  , ru_rhs = rhs, ru_act = act })
      | is_active act
      	    -- See Note [Finding rule RHS free vars] in OccAnal.lhs
      = delFromUFM fvs fn	 -- Note [Rule free var hack]
      where
        fvs = addBndrs bndrs (expr_fvs rhs) isLocalVar emptyVarSet
    get_fvs _ = noFVs

-- | Those variables free in the right hand side of several rules
rulesFreeVars :: [CoreRule] -> VarSet
rulesFreeVars rules = foldr (unionVarSet . ruleFreeVars) emptyVarSet rules

ruleLhsFreeIds :: CoreRule -> VarSet
-- ^ This finds all locally-defined free Ids on the left hand side of a rule
ruleLhsFreeIds (BuiltinRule {}) = noFVs
ruleLhsFreeIds (Rule { ru_bndrs = bndrs, ru_args = args })
  = addBndrs bndrs (exprs_fvs args) isLocalId emptyVarSet
\end{code}

Note [Rule free var hack]
~~~~~~~~~~~~~~~~~~~~~~~~~
Don't include the Id in its own rhs free-var set.
Otherwise the occurrence analyser makes bindings recursive
that shoudn't be.  E.g.
	RULE:  f (f x y) z  ==>  f x (f y z)

Also since rule_fn is a Name, not a Var, we have to use the grungy delUFM.

%************************************************************************
%*									*
\section[freevars-everywhere]{Attaching free variables to every sub-expression}
%*									*
%************************************************************************

The free variable pass annotates every node in the expression with its
NON-GLOBAL free variables and type variables.

\begin{code}
-- | Every node in a binding group annotated with its 
-- (non-global) free variables, both Ids and TyVars
type CoreBindWithFVs = AnnBind Id VarSet
-- | Every node in an expression annotated with its 
-- (non-global) free variables, both Ids and TyVars
type CoreExprWithFVs = AnnExpr Id VarSet

freeVarsOf :: CoreExprWithFVs -> IdSet
-- ^ Inverse function to 'freeVars'
freeVarsOf (free_vars, _) = free_vars

noFVs :: VarSet
noFVs    = emptyVarSet

aFreeVar :: Var -> VarSet
aFreeVar = unitVarSet

unionFVs :: VarSet -> VarSet -> VarSet
unionFVs = unionVarSet

delBindersFV :: [Var] -> VarSet -> VarSet
delBindersFV bs fvs = foldr delBinderFV fvs bs

delBinderFV :: Var -> VarSet -> VarSet
-- This way round, so we can do it multiple times using foldr

-- (b `delBinderFV` s) removes the binder b from the free variable set s,
-- but *adds* to s
--
--	the free variables of b's type
--
-- This is really important for some lambdas:
-- 	In (\x::a -> x) the only mention of "a" is in the binder.
--
-- Also in
--	let x::a = b in ...
-- we should really note that "a" is free in this expression.
-- It'll be pinned inside the /\a by the binding for b, but
-- it seems cleaner to make sure that a is in the free-var set 
-- when it is mentioned.
--
-- This also shows up in recursive bindings.  Consider:
--	/\a -> letrec x::a = x in E
-- Now, there are no explicit free type variables in the RHS of x,
-- but nevertheless "a" is free in its definition.  So we add in
-- the free tyvars of the types of the binders, and include these in the
-- free vars of the group, attached to the top level of each RHS.
--
-- This actually happened in the defn of errorIO in IOBase.lhs:
--	errorIO (ST io) = case (errorIO# io) of
--	    		    _ -> bottom
--			  where
--			    bottom = bottom -- Never evaluated

delBinderFV b s = (s `delVarSet` b) `unionFVs` varTypeTyVars b
	-- Include coercion variables too!

varTypeTyVars :: Var -> TyVarSet
-- Find the type variables free in the type of the variable
-- Remember, coercion variables can mention type variables...
varTypeTyVars var
  | isLocalId var || isCoVar var = tyVarsOfType (idType var)
  | otherwise = emptyVarSet	-- Global Ids and non-coercion TyVars

varTypeTcTyVars :: Var -> TyVarSet
-- Find the type variables free in the type of the variable
-- Remember, coercion variables can mention type variables...
varTypeTcTyVars var
  | isLocalId var || isCoVar var = tcTyVarsOfType (idType var)
  | otherwise = emptyVarSet	-- Global Ids and non-coercion TyVars

idFreeVars :: Id -> VarSet
-- Type variables, rule variables, and inline variables
idFreeVars id = ASSERT( isId id) 
		varTypeTyVars id `unionVarSet`
	        idRuleAndUnfoldingVars id

bndrRuleAndUnfoldingVars ::Var -> VarSet
-- A 'let' can bind a type variable, and idRuleVars assumes 
-- it's seeing an Id. This function tests first.
bndrRuleAndUnfoldingVars v | isTyCoVar v = emptyVarSet
	                   | otherwise = idRuleAndUnfoldingVars v

idRuleAndUnfoldingVars :: Id -> VarSet
idRuleAndUnfoldingVars id = ASSERT( isId id) 
			    idRuleVars id    `unionVarSet` 
			    idUnfoldingVars id

idRuleVars ::Id -> VarSet  -- Does *not* include CoreUnfolding vars
idRuleVars id = ASSERT( isId id) specInfoFreeVars (idSpecialisation id)

idUnfoldingVars :: Id -> VarSet
-- Produce free vars for an unfolding, but NOT for an ordinary
-- (non-inline) unfolding, since it is a dup of the rhs
-- and we'll get exponential behaviour if we look at both unf and rhs!
-- But do look at the *real* unfolding, even for loop breakers, else
-- we might get out-of-scope variables
idUnfoldingVars id = stableUnfoldingVars (realIdUnfolding id)

stableUnfoldingVars :: Unfolding -> VarSet
stableUnfoldingVars (CoreUnfolding { uf_tmpl = rhs, uf_src = src })
  | isStableSource src                       = exprFreeVars rhs
stableUnfoldingVars (DFunUnfolding _ _ args) = exprsFreeVars (dfunArgExprs args)
stableUnfoldingVars _                        = emptyVarSet
\end{code}


%************************************************************************
%*									*
\subsection{Free variables (and types)}
%*									*
%************************************************************************

\begin{code}
freeVars :: CoreExpr -> CoreExprWithFVs
-- ^ Annotate a 'CoreExpr' with its (non-global) free type and value variables at every tree node
freeVars (Var v)
  = (fvs, AnnVar v)
  where
	-- ToDo: insert motivating example for why we *need*
	-- to include the idSpecVars in the FV list.
	--	Actually [June 98] I don't think it's necessary
	-- fvs = fvs_v `unionVarSet` idSpecVars v

    fvs | isLocalVar v = aFreeVar v
	| otherwise    = noFVs

freeVars (Lit lit) = (noFVs, AnnLit lit)
freeVars (Lam b body)
  = (b `delBinderFV` freeVarsOf body', AnnLam b body')
  where
    body' = freeVars body

freeVars (App fun arg)
  = (freeVarsOf fun2 `unionFVs` freeVarsOf arg2, AnnApp fun2 arg2)
  where
    fun2 = freeVars fun
    arg2 = freeVars arg

freeVars (Case scrut bndr ty alts)
  = ((bndr `delBinderFV` alts_fvs) `unionFVs` freeVarsOf scrut2 `unionFVs` tyVarsOfType ty,
     AnnCase scrut2 bndr ty alts2)
  where
    scrut2 = freeVars scrut

    (alts_fvs_s, alts2) = mapAndUnzip fv_alt alts
    alts_fvs 		= foldr1 unionFVs alts_fvs_s

    fv_alt (con,args,rhs) = (delBindersFV args (freeVarsOf rhs2),
			     (con, args, rhs2))
		    	  where
			     rhs2 = freeVars rhs

freeVars (Let (NonRec binder rhs) body)
  = (freeVarsOf rhs2 
       `unionFVs` body_fvs 
       `unionFVs` bndrRuleAndUnfoldingVars binder,
		-- Remember any rules; cf rhs_fvs above
     AnnLet (AnnNonRec binder rhs2) body2)
  where
    rhs2     = freeVars rhs
    body2    = freeVars body
    body_fvs = binder `delBinderFV` freeVarsOf body2

freeVars (Let (Rec binds) body)
  = (delBindersFV binders all_fvs,
     AnnLet (AnnRec (binders `zip` rhss2)) body2)
  where
    (binders, rhss) = unzip binds

    rhss2     = map freeVars rhss
    rhs_body_fvs = foldr (unionFVs . freeVarsOf) body_fvs rhss2
    all_fvs      = foldr (unionFVs . idRuleAndUnfoldingVars) rhs_body_fvs binders
	-- The "delBinderFV" happens after adding the idSpecVars,
	-- since the latter may add some of the binders as fvs

    body2     = freeVars body
    body_fvs  = freeVarsOf body2


freeVars (Cast expr co)
  = (freeVarsOf expr2 `unionFVs` cfvs, AnnCast expr2 co)
  where
    expr2 = freeVars expr
    cfvs  = tyVarsOfType co

freeVars (Note other_note expr)
  = (freeVarsOf expr2, AnnNote other_note expr2)
  where
    expr2 = freeVars expr

freeVars (Type ty) = (tyVarsOfType ty, AnnType ty)
\end{code}