Simplify.lhs 73.6 KB
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%
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% (c) The AQUA Project, Glasgow University, 1993-1998
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%
\section[Simplify]{The main module of the simplifier}

\begin{code}
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module Simplify ( simplTopBinds, simplExpr ) where
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#include "HsVersions.h"
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import DynFlags	( dopt, DynFlag(Opt_D_dump_inlinings),
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			  SimplifierSwitch(..)
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			)
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import SimplMonad
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import SimplEnv	
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import SimplUtils	( mkCase, mkLam, 
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			  SimplCont(..), DupFlag(..), LetRhsFlag(..), 
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			  mkRhsStop, mkBoringStop,  mkLazyArgStop, pushContArgs,
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			  contResultType, countArgs, contIsDupable, contIsRhsOrArg,
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			  getContArgs, interestingCallContext, interestingArg, isStrictType,
			  preInlineUnconditionally, postInlineUnconditionally, 
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			  interestingArgContext, inlineMode, activeInline, activeRule
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			)
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import Id		( Id, idType, idInfo, idArity, isDataConWorkId, 
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			  idUnfolding, setIdUnfolding, isDeadBinder,
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			  idNewDemandInfo, setIdInfo, 
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			  setIdOccInfo, zapLamIdInfo, setOneShotLambda
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			)
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import MkId		( eRROR_ID )
import Literal		( mkStringLit )
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import IdInfo		( OccInfo(..), isLoopBreaker,
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			  setArityInfo, zapDemandInfo,
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			  setUnfoldingInfo, 
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			  occInfo
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			)
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import NewDemand	( isStrictDmd )
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import TcGadt		( dataConCanMatch )
import DataCon		( DataCon, dataConTyCon, dataConRepStrictness )
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import TyCon		( tyConArity, isAlgTyCon, isNewTyCon, tyConDataCons_maybe )
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import CoreSyn
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import PprCore		( pprParendExpr, pprCoreExpr )
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import CoreUnfold	( mkUnfolding, callSiteInline )
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import CoreUtils	( exprIsDupable, exprIsTrivial, needsCaseBinding,
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			  exprIsConApp_maybe, mkPiTypes, findAlt, 
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			  exprType, exprIsHNF, findDefault, mergeAlts,
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			  exprOkForSpeculation, exprArity, 
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			  mkCoerce, mkSCC, mkInlineMe, applyTypeToArg
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			)
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import Rules		( lookupRule )
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import BasicTypes	( isMarkedStrict )
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import CostCentre	( currentCCS )
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import Type		( TvSubstEnv, isUnLiftedType, seqType, tyConAppArgs, funArgTy,
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			  splitFunTy_maybe, splitFunTy, coreEqType, splitTyConApp_maybe,
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			  isTyVarTy, mkTyVarTys, isFunTy, tcEqType
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			)
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import Coercion         ( Coercion, coercionKind,
                          mkTransCoercion, mkLeftCoercion, mkRightCoercion, 
                          mkSymCoercion, splitCoercionKind_maybe, decomposeCo  )
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import Var		( tyVarKind, mkTyVar )
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import VarEnv		( elemVarEnv, emptyVarEnv )
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import TysPrim		( realWorldStatePrimTy )
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import PrelInfo		( realWorldPrimId )
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import BasicTypes	( TopLevelFlag(..), isTopLevel, 
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			  RecFlag(..), isNonRec
			)
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import OrdList
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import List		( nub )
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import Maybes		( orElse )
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import Outputable
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import Util             ( notNull, filterOut )
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\end{code}


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The guts of the simplifier is in this module, but the driver loop for
the simplifier is in SimplCore.lhs.
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-----------------------------------------
	*** IMPORTANT NOTE ***
-----------------------------------------
The simplifier used to guarantee that the output had no shadowing, but
it does not do so any more.   (Actually, it never did!)  The reason is
documented with simplifyArgs.


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-----------------------------------------
	*** IMPORTANT NOTE ***
-----------------------------------------
Many parts of the simplifier return a bunch of "floats" as well as an
expression. This is wrapped as a datatype SimplUtils.FloatsWith.

All "floats" are let-binds, not case-binds, but some non-rec lets may
be unlifted (with RHS ok-for-speculation).



-----------------------------------------
	ORGANISATION OF FUNCTIONS
-----------------------------------------
simplTopBinds
  - simplify all top-level binders
  - for NonRec, call simplRecOrTopPair
  - for Rec,    call simplRecBind

	
	------------------------------
simplExpr (applied lambda)	==> simplNonRecBind
simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
simplExpr (Let (Rec ...)    ..) ==> simplify binders; simplRecBind

	------------------------------
simplRecBind	[binders already simplfied]
  - use simplRecOrTopPair on each pair in turn

simplRecOrTopPair [binder already simplified]
  Used for: recursive bindings (top level and nested)
	    top-level non-recursive bindings
  Returns: 
  - check for PreInlineUnconditionally
  - simplLazyBind

simplNonRecBind
  Used for: non-top-level non-recursive bindings
	    beta reductions (which amount to the same thing)
  Because it can deal with strict arts, it takes a 
	"thing-inside" and returns an expression

  - check for PreInlineUnconditionally
  - simplify binder, including its IdInfo
  - if strict binding
	simplStrictArg
	mkAtomicArgs
	completeNonRecX
    else
	simplLazyBind
	addFloats

simplNonRecX:	[given a *simplified* RHS, but an *unsimplified* binder]
  Used for: binding case-binder and constr args in a known-constructor case
  - check for PreInLineUnconditionally
  - simplify binder
  - completeNonRecX
 
	------------------------------
simplLazyBind:	[binder already simplified, RHS not]
  Used for: recursive bindings (top level and nested)
	    top-level non-recursive bindings
	    non-top-level, but *lazy* non-recursive bindings
	[must not be strict or unboxed]
  Returns floats + an augmented environment, not an expression
  - substituteIdInfo and add result to in-scope 
	[so that rules are available in rec rhs]
  - simplify rhs
  - mkAtomicArgs
  - float if exposes constructor or PAP
  - completeLazyBind


completeNonRecX:	[binder and rhs both simplified]
  - if the the thing needs case binding (unlifted and not ok-for-spec)
	build a Case
   else
	completeLazyBind
	addFloats

completeLazyBind: 	[given a simplified RHS]
	[used for both rec and non-rec bindings, top level and not]
  - try PostInlineUnconditionally
  - add unfolding [this is the only place we add an unfolding]
  - add arity



Right hand sides and arguments
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In many ways we want to treat 
	(a) the right hand side of a let(rec), and 
	(b) a function argument
in the same way.  But not always!  In particular, we would
like to leave these arguments exactly as they are, so they
will match a RULE more easily.
	
	f (g x, h x)	
	g (+ x)

It's harder to make the rule match if we ANF-ise the constructor,
or eta-expand the PAP:

	f (let { a = g x; b = h x } in (a,b))
	g (\y. + x y)

On the other hand if we see the let-defns

	p = (g x, h x)
	q = + x

then we *do* want to ANF-ise and eta-expand, so that p and q
can be safely inlined.   

Even floating lets out is a bit dubious.  For let RHS's we float lets
out if that exposes a value, so that the value can be inlined more vigorously.
For example

	r = let x = e in (x,x)

Here, if we float the let out we'll expose a nice constructor. We did experiments
that showed this to be a generally good thing.  But it was a bad thing to float
lets out unconditionally, because that meant they got allocated more often.

For function arguments, there's less reason to expose a constructor (it won't
get inlined).  Just possibly it might make a rule match, but I'm pretty skeptical.
So for the moment we don't float lets out of function arguments either.


Eta expansion
~~~~~~~~~~~~~~
For eta expansion, we want to catch things like

	case e of (a,b) -> \x -> case a of (p,q) -> \y -> r

If the \x was on the RHS of a let, we'd eta expand to bring the two
lambdas together.  And in general that's a good thing to do.  Perhaps
we should eta expand wherever we find a (value) lambda?  Then the eta
expansion at a let RHS can concentrate solely on the PAP case.
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%************************************************************************
%*									*
\subsection{Bindings}
%*									*
%************************************************************************

\begin{code}
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simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
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simplTopBinds env binds
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  = 	-- Put all the top-level binders into scope at the start
	-- so that if a transformation rule has unexpectedly brought
	-- anything into scope, then we don't get a complaint about that.
	-- It's rather as if the top-level binders were imported.
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    simplRecBndrs env (bindersOfBinds binds)	`thenSmpl` \ (env, bndrs') -> 
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    simpl_binds env binds bndrs'		`thenSmpl` \ (floats, _) ->
    freeTick SimplifierDone			`thenSmpl_`
    returnSmpl (floatBinds floats)
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  where
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	-- We need to track the zapped top-level binders, because
	-- they should have their fragile IdInfo zapped (notably occurrence info)
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	-- That's why we run down binds and bndrs' simultaneously.
    simpl_binds :: SimplEnv -> [InBind] -> [OutId] -> SimplM (FloatsWith ())
    simpl_binds env []		 bs = ASSERT( null bs ) returnSmpl (emptyFloats env, ())
    simpl_binds env (bind:binds) bs = simpl_bind env bind bs 		`thenSmpl` \ (floats,env) ->
				      addFloats env floats		$ \env -> 
				      simpl_binds env binds (drop_bs bind bs)

    drop_bs (NonRec _ _) (_ : bs) = bs
    drop_bs (Rec prs)    bs	  = drop (length prs) bs

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    simpl_bind env bind bs 
      = getDOptsSmpl				`thenSmpl` \ dflags ->
        if dopt Opt_D_dump_inlinings dflags then
	   pprTrace "SimplBind" (ppr (bindersOf bind)) $ simpl_bind1 env bind bs
	else
	   simpl_bind1 env bind bs

    simpl_bind1 env (NonRec b r) (b':_) = simplRecOrTopPair env TopLevel b b' r
    simpl_bind1 env (Rec pairs)  bs'    = simplRecBind      env TopLevel pairs bs'
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\end{code}


%************************************************************************
%*									*
\subsection{simplNonRec}
%*									*
%************************************************************************

simplNonRecBind is used for
  * non-top-level non-recursive lets in expressions
  * beta reduction

It takes 
  * An unsimplified (binder, rhs) pair
  * The env for the RHS.  It may not be the same as the
	current env because the bind might occur via (\x.E) arg

It uses the CPS form because the binding might be strict, in which
case we might discard the continuation:
	let x* = error "foo" in (...x...)

It needs to turn unlifted bindings into a @case@.  They can arise
from, say: 	(\x -> e) (4# + 3#)

\begin{code}
simplNonRecBind :: SimplEnv
		-> InId 				-- Binder
	  	-> InExpr -> SimplEnv			-- Arg, with its subst-env
	  	-> OutType				-- Type of thing computed by the context
	  	-> (SimplEnv -> SimplM FloatsWithExpr)	-- The body
	  	-> SimplM FloatsWithExpr
#ifdef DEBUG
simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
  | isTyVar bndr
  = pprPanic "simplNonRecBind" (ppr bndr <+> ppr rhs)
#endif

simplNonRecBind env bndr rhs rhs_se cont_ty thing_inside
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  = simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside

simplNonRecBind' env bndr rhs rhs_se cont_ty thing_inside
  | preInlineUnconditionally env NotTopLevel bndr rhs
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  = tick (PreInlineUnconditionally bndr)		`thenSmpl_`
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    thing_inside (extendIdSubst env bndr (mkContEx rhs_se rhs))
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  | isStrictDmd (idNewDemandInfo bndr) || isStrictType bndr_ty	-- A strict let
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  =  	-- Don't use simplBinder because that doesn't keep 
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	-- fragile occurrence info in the substitution
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    simplNonRecBndr env bndr					`thenSmpl` \ (env, bndr1) ->
    simplStrictArg AnRhs env rhs rhs_se (idType bndr1) cont_ty	$ \ env1 rhs1 ->
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	-- Now complete the binding and simplify the body
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    let
	(env2,bndr2) = addLetIdInfo env1 bndr bndr1
    in
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    completeNonRecX env2 True {- strict -} bndr bndr2 rhs1 thing_inside
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  | otherwise							-- Normal, lazy case
  =  	-- Don't use simplBinder because that doesn't keep 
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	-- fragile occurrence info in the substitution
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    simplNonRecBndr env bndr				`thenSmpl` \ (env, bndr') ->
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    simplLazyBind env NotTopLevel NonRecursive
		  bndr bndr' rhs rhs_se 		`thenSmpl` \ (floats, env) ->
    addFloats env floats thing_inside
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  where
    bndr_ty = idType bndr
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\end{code}

A specialised variant of simplNonRec used when the RHS is already simplified, notably
in knownCon.  It uses case-binding where necessary.

\begin{code}
simplNonRecX :: SimplEnv
	     -> InId 		-- Old binder
	     -> OutExpr		-- Simplified RHS
	     -> (SimplEnv -> SimplM FloatsWithExpr)
	     -> SimplM FloatsWithExpr

simplNonRecX env bndr new_rhs thing_inside
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  = do	{ (env, bndr') <- simplBinder env bndr
	; completeNonRecX env False {- Non-strict; pessimistic -} 
		          bndr bndr' new_rhs thing_inside }


completeNonRecX :: SimplEnv
		-> Bool			-- Strict binding
	        -> InId 		-- Old binder
		-> OutId		-- New binder
	     	-> OutExpr		-- Simplified RHS
	     	-> (SimplEnv -> SimplM FloatsWithExpr)
	     	-> SimplM FloatsWithExpr

completeNonRecX env is_strict old_bndr new_bndr new_rhs thing_inside
  | needsCaseBinding (idType new_bndr) new_rhs
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	-- Make this test *before* the preInlineUnconditionally
	-- Consider 	case I# (quotInt# x y) of 
	--		  I# v -> let w = J# v in ...
	-- If we gaily inline (quotInt# x y) for v, we end up building an
	-- extra thunk:
	--		  let w = J# (quotInt# x y) in ...
	-- because quotInt# can fail.
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  = do	{ (floats, body) <- thing_inside env
	; let body' = wrapFloats floats body
	; return (emptyFloats env, Case new_rhs new_bndr (exprType body) 
					[(DEFAULT, [], body')]) }
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  | otherwise
  = 	-- Make the arguments atomic if necessary, 
	-- adding suitable bindings
    -- pprTrace "completeNonRecX" (ppr new_bndr <+> ppr new_rhs) $
    mkAtomicArgsE env is_strict new_rhs		$ \ env new_rhs ->
    completeLazyBind env NotTopLevel
		     old_bndr new_bndr new_rhs	`thenSmpl` \ (floats, env) ->
    addFloats env floats thing_inside

{- No, no, no!  Do not try preInlineUnconditionally in completeNonRecX
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   Doing so risks exponential behaviour, because new_rhs has been simplified once already
   In the cases described by the folowing commment, postInlineUnconditionally will 
   catch many of the relevant cases.
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  	-- This happens; for example, the case_bndr during case of
	-- known constructor:  case (a,b) of x { (p,q) -> ... }
	-- Here x isn't mentioned in the RHS, so we don't want to
	-- create the (dead) let-binding  let x = (a,b) in ...
	--
	-- Similarly, single occurrences can be inlined vigourously
	-- e.g.  case (f x, g y) of (a,b) -> ....
	-- If a,b occur once we can avoid constructing the let binding for them.
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  | preInlineUnconditionally env NotTopLevel bndr new_rhs
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  = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
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  -- NB: completeLazyBind uses postInlineUnconditionally; no need to do that here
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-}
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\end{code}


%************************************************************************
%*									*
\subsection{Lazy bindings}
%*									*
%************************************************************************

simplRecBind is used for
	* recursive bindings only

\begin{code}
simplRecBind :: SimplEnv -> TopLevelFlag
	     -> [(InId, InExpr)] -> [OutId]
	     -> SimplM (FloatsWith SimplEnv)
simplRecBind env top_lvl pairs bndrs'
  = go env pairs bndrs'		`thenSmpl` \ (floats, env) ->
    returnSmpl (flattenFloats floats, env)
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  where
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    go env [] _ = returnSmpl (emptyFloats env, env)
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    go env ((bndr, rhs) : pairs) (bndr' : bndrs')
	= simplRecOrTopPair env top_lvl bndr bndr' rhs 	`thenSmpl` \ (floats, env) ->
	  addFloats env floats (\env -> go env pairs bndrs')
\end{code}


simplRecOrTopPair is used for
	* recursive bindings (whether top level or not)
	* top-level non-recursive bindings

It assumes the binder has already been simplified, but not its IdInfo.

\begin{code}
simplRecOrTopPair :: SimplEnv
	     	  -> TopLevelFlag
	     	  -> InId -> OutId		-- Binder, both pre-and post simpl
	     	  -> InExpr 			-- The RHS and its environment
	     	  -> SimplM (FloatsWith SimplEnv)

simplRecOrTopPair env top_lvl bndr bndr' rhs
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  | preInlineUnconditionally env top_lvl bndr rhs  	-- Check for unconditional inline
  = tick (PreInlineUnconditionally bndr)		`thenSmpl_`
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    returnSmpl (emptyFloats env, extendIdSubst env bndr (mkContEx env rhs))
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  | otherwise
  = simplLazyBind env top_lvl Recursive bndr bndr' rhs env
	-- May not actually be recursive, but it doesn't matter
\end{code}


simplLazyBind is used for
	* recursive bindings (whether top level or not)
	* top-level non-recursive bindings
	* non-top-level *lazy* non-recursive bindings

[Thus it deals with the lazy cases from simplNonRecBind, and all cases
from SimplRecOrTopBind]

Nota bene:
    1. It assumes that the binder is *already* simplified, 
       and is in scope, but not its IdInfo

    2. It assumes that the binder type is lifted.

    3. It does not check for pre-inline-unconditionallly;
       that should have been done already.

\begin{code}
simplLazyBind :: SimplEnv
	      -> TopLevelFlag -> RecFlag
	      -> InId -> OutId		-- Binder, both pre-and post simpl
	      -> InExpr -> SimplEnv 	-- The RHS and its environment
	      -> SimplM (FloatsWith SimplEnv)

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simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
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  = let	
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	(env1,bndr2)      = addLetIdInfo env bndr bndr1
	rhs_env      	  = setInScope rhs_se env1
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 	is_top_level	  = isTopLevel top_lvl
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	ok_float_unlifted = not is_top_level && isNonRec is_rec
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	rhs_cont	  = mkRhsStop (idType bndr2)
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    in
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  	-- Simplify the RHS; note the mkRhsStop, which tells 
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	-- the simplifier that this is the RHS of a let.
    simplExprF rhs_env rhs rhs_cont		`thenSmpl` \ (floats, rhs1) ->

	-- If any of the floats can't be floated, give up now
	-- (The allLifted predicate says True for empty floats.)
    if (not ok_float_unlifted && not (allLifted floats)) then
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	completeLazyBind env1 top_lvl bndr bndr2
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			 (wrapFloats floats rhs1)
    else	

	-- ANF-ise a constructor or PAP rhs
    mkAtomicArgs False {- Not strict -} 
		 ok_float_unlifted rhs1 		`thenSmpl` \ (aux_binds, rhs2) ->

	-- If the result is a PAP, float the floats out, else wrap them
	-- By this time it's already been ANF-ised (if necessary)
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    if isEmptyFloats floats && isNilOL aux_binds then	-- Shortcut a common case
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	completeLazyBind env1 top_lvl bndr bndr2 rhs2
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    else if is_top_level || exprIsTrivial rhs2 || exprIsHNF rhs2 then
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	-- 	WARNING: long dodgy argument coming up
	--	WANTED: a better way to do this
	--		
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	-- We can't use "exprIsCheap" instead of exprIsHNF, 
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	-- because that causes a strictness bug.
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	--     	   x = let y* = E in case (scc y) of { T -> F; F -> T}
	-- The case expression is 'cheap', but it's wrong to transform to
	-- 	   y* = E; x = case (scc y) of {...}
 	-- Either we must be careful not to float demanded non-values, or
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	-- we must use exprIsHNF for the test, which ensures that the
	-- thing is non-strict.  So exprIsHNF => bindings are non-strict
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	-- I think.  The WARN below tests for this.
	--
	-- We use exprIsTrivial here because we want to reveal lone variables.  
	-- E.g.  let { x = letrec { y = E } in y } in ...
	-- Here we definitely want to float the y=E defn. 
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	-- exprIsHNF definitely isn't right for that.
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	--
	-- Again, the floated binding can't be strict; if it's recursive it'll
	-- be non-strict; if it's non-recursive it'd be inlined.
	--
	-- Note [SCC-and-exprIsTrivial]
	-- If we have
	--	y = let { x* = E } in scc "foo" x
	-- then we do *not* want to float out the x binding, because
	-- it's strict!  Fortunately, exprIsTrivial replies False to
	-- (scc "foo" x).
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		-- There's a subtlety here.  There may be a binding (x* = e) in the
		-- floats, where the '*' means 'will be demanded'.  So is it safe
		-- to float it out?  Answer no, but it won't matter because
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		-- we only float if (a) arg' is a WHNF, or (b) it's going to top level
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		-- and so there can't be any 'will be demanded' bindings in the floats.
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		-- Hence the warning
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        WARN( not (is_top_level || not (any demanded_float (floatBinds floats))), 
	      ppr (filter demanded_float (floatBinds floats)) )
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	tick LetFloatFromLet			`thenSmpl_` (
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	addFloats env1 floats			$ \ env2 ->
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	addAtomicBinds env2 (fromOL aux_binds)	$ \ env3 ->
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	completeLazyBind env3 top_lvl bndr bndr2 rhs2)
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    else
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	completeLazyBind env1 top_lvl bndr bndr2 (wrapFloats floats rhs1)
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#ifdef DEBUG
demanded_float (NonRec b r) = isStrictDmd (idNewDemandInfo b) && not (isUnLiftedType (idType b))
		-- Unlifted-type (cheap-eagerness) lets may well have a demanded flag on them
demanded_float (Rec _)	    = False
#endif
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\end{code}
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%************************************************************************
%*									*
\subsection{Completing a lazy binding}
%*									*
%************************************************************************

completeLazyBind
	* deals only with Ids, not TyVars
	* takes an already-simplified binder and RHS
	* is used for both recursive and non-recursive bindings
	* is used for both top-level and non-top-level bindings

It does the following:
  - tries discarding a dead binding
  - tries PostInlineUnconditionally
  - add unfolding [this is the only place we add an unfolding]
  - add arity

It does *not* attempt to do let-to-case.  Why?  Because it is used for
	- top-level bindings (when let-to-case is impossible) 
	- many situations where the "rhs" is known to be a WHNF
		(so let-to-case is inappropriate).

\begin{code}
completeLazyBind :: SimplEnv
		 -> TopLevelFlag	-- Flag stuck into unfolding
		 -> InId 		-- Old binder
		 -> OutId		-- New binder
	         -> OutExpr		-- Simplified RHS
	   	 -> SimplM (FloatsWith SimplEnv)
-- We return a new SimplEnv, because completeLazyBind may choose to do its work
-- by extending the substitution (e.g. let x = y in ...)
-- The new binding (if any) is returned as part of the floats.
-- NB: the returned SimplEnv has the right SubstEnv, but you should
--     (as usual) use the in-scope-env from the floats

completeLazyBind env top_lvl old_bndr new_bndr new_rhs
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  | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
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  = 		-- Drop the binding
    tick (PostInlineUnconditionally old_bndr)	`thenSmpl_`
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    -- pprTrace "Inline unconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
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    returnSmpl (emptyFloats env, extendIdSubst env old_bndr (DoneEx new_rhs))
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		-- Use the substitution to make quite, quite sure that the substitution
		-- will happen, since we are going to discard the binding

  |  otherwise
  = let
		-- Add arity info
  	new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs

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	-- Add the unfolding *only* for non-loop-breakers
	-- Making loop breakers not have an unfolding at all 
	-- means that we can avoid tests in exprIsConApp, for example.
	-- This is important: if exprIsConApp says 'yes' for a recursive
	-- thing, then we can get into an infinite loop

	-- If the unfolding is a value, the demand info may
	-- go pear-shaped, so we nuke it.  Example:
	--	let x = (a,b) in
	--	case x of (p,q) -> h p q x
	-- Here x is certainly demanded. But after we've nuked
	-- the case, we'll get just
	--	let x = (a,b) in h a b x
	-- and now x is not demanded (I'm assuming h is lazy)
	-- This really happens.  Similarly
	--	let f = \x -> e in ...f..f...
	-- After inling f at some of its call sites the original binding may
	-- (for example) be no longer strictly demanded.
	-- The solution here is a bit ad hoc...
 	info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
        final_info | loop_breaker		= new_bndr_info
		   | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
		   | otherwise			= info_w_unf

	final_id = new_bndr `setIdInfo` final_info
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    in
		-- These seqs forces the Id, and hence its IdInfo,
		-- and hence any inner substitutions
    final_id					`seq`
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    -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
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    returnSmpl (unitFloat env final_id new_rhs, env)

  where 
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    unfolding    = mkUnfolding (isTopLevel top_lvl) new_rhs
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    loop_breaker = isLoopBreaker occ_info
    old_info     = idInfo old_bndr
    occ_info     = occInfo old_info
\end{code}    



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%************************************************************************
%*									*
\subsection[Simplify-simplExpr]{The main function: simplExpr}
%*									*
%************************************************************************

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The reason for this OutExprStuff stuff is that we want to float *after*
simplifying a RHS, not before.  If we do so naively we get quadratic
behaviour as things float out.

To see why it's important to do it after, consider this (real) example:

	let t = f x
	in fst t
==>
	let t = let a = e1
		    b = e2
		in (a,b)
	in fst t
==>
	let a = e1
	    b = e2
	    t = (a,b)
	in
	a	-- Can't inline a this round, cos it appears twice
==>
	e1

Each of the ==> steps is a round of simplification.  We'd save a
whole round if we float first.  This can cascade.  Consider

	let f = g d
	in \x -> ...f...
==>
	let f = let d1 = ..d.. in \y -> e
	in \x -> ...f...
==>
	let d1 = ..d..
	in \x -> ...(\y ->e)...

Only in this second round can the \y be applied, and it 
might do the same again.


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\begin{code}
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simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
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simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
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		   where
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		     expr_ty' = substTy env (exprType expr)
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	-- The type in the Stop continuation, expr_ty', is usually not used
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	-- It's only needed when discarding continuations after finding
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	-- a function that returns bottom.
	-- Hence the lazy substitution
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simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
	-- Simplify an expression, given a continuation
simplExprC env expr cont 
  = simplExprF env expr cont	`thenSmpl` \ (floats, expr) ->
    returnSmpl (wrapFloats floats expr)
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simplExprF :: SimplEnv -> InExpr -> SimplCont -> SimplM FloatsWithExpr
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	-- Simplify an expression, returning floated binds
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simplExprF env (Var v)	        cont = simplVar env v cont
simplExprF env (Lit lit)	cont = rebuild env (Lit lit) cont
simplExprF env expr@(Lam _ _)   cont = simplLam env expr cont
simplExprF env (Note note expr) cont = simplNote env note expr cont
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simplExprF env (Cast body co)   cont = simplCast env body co cont
simplExprF env (App fun arg)    cont = simplExprF env fun 
				         (ApplyTo NoDup arg (Just env) cont)
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simplExprF env (Type ty) cont
  = ASSERT( contIsRhsOrArg cont )
    simplType env ty			`thenSmpl` \ ty' ->
    rebuild env (Type ty') cont
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simplExprF env (Case scrut bndr case_ty alts) cont
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  | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
  = 	-- Simplify the scrutinee with a Select continuation
    simplExprF env scrut (Select NoDup bndr alts env cont)
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  | otherwise
  = 	-- If case-of-case is off, simply simplify the case expression
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	-- in a vanilla Stop context, and rebuild the result around it
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    simplExprC env scrut case_cont	`thenSmpl` \ case_expr' ->
    rebuild env case_expr' cont
  where
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    case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
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    case_ty'  = substTy env case_ty	-- c.f. defn of simplExpr
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simplExprF env (Let (Rec pairs) body) cont
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  = simplRecBndrs env (map fst pairs) 		`thenSmpl` \ (env, bndrs') -> 
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	-- NB: bndrs' don't have unfoldings or rules
	-- We add them as we go down
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    simplRecBind env NotTopLevel pairs bndrs' 	`thenSmpl` \ (floats, env) ->
    addFloats env floats 			$ \ env ->
    simplExprF env body cont
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-- A non-recursive let is dealt with by simplNonRecBind
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simplExprF env (Let (NonRec bndr rhs) body) cont
  = simplNonRecBind env bndr rhs env (contResultType cont)	$ \ env ->
    simplExprF env body cont
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---------------------------------
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simplType :: SimplEnv -> InType -> SimplM OutType
	-- Kept monadic just so we can do the seqType
simplType env ty
  = seqType new_ty   `seq`   returnSmpl new_ty
  where
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    new_ty = substTy env ty
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\end{code}


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%************************************************************************
%*									*
\subsection{Lambdas}
%*									*
%************************************************************************

\begin{code}
simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont -> SimplM FloatsWithExpr
simplCast env body co cont
  = let
       addCoerce co cont 
         | (s1, k1) <- coercionKind co
         , s1 `tcEqType` k1 = cont
       addCoerce co1 (CoerceIt co2 cont)
         | (s1, k1) <- coercionKind co1
         , (l1, t1) <- coercionKind co2
                -- 	coerce T1 S1 (coerce S1 K1 e)
		-- ==>
		--	e, 			if T1=K1
		--	coerce T1 K1 e,		otherwise
		--
		-- For example, in the initial form of a worker
		-- we may find 	(coerce T (coerce S (\x.e))) y
		-- and we'd like it to simplify to e[y/x] in one round 
		-- of simplification
         , s1 `coreEqType` t1  = cont		 -- The coerces cancel out  
         | otherwise           = CoerceIt (mkTransCoercion co1 co2) cont
    
       addCoerce co (ApplyTo dup arg arg_se cont)
         | not (isTypeArg arg)    -- This whole case only works for value args
	                        -- Could upgrade to have equiv thing for type apps too	
         , Just (s1s2, t1t2) <- splitCoercionKind_maybe co
         , isFunTy s1s2
                -- co : s1s2 :=: t1t2
		--	(coerce (T1->T2) (S1->S2) F) E
		-- ===> 
		--	coerce T2 S2 (F (coerce S1 T1 E))
		--
		-- t1t2 must be a function type, T1->T2, because it's applied
		-- to something but s1s2 might conceivably not be
		--
		-- When we build the ApplyTo we can't mix the out-types
		-- with the InExpr in the argument, so we simply substitute
		-- to make it all consistent.  It's a bit messy.
		-- But it isn't a common case.
         = result
         where
           -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and 
           -- t2 :=: s2 with left and right on the curried form: 
           --    (->) t1 t2 :=: (->) s1 s2
           [co1, co2] = decomposeCo 2 co
           new_arg    = mkCoerce (mkSymCoercion co1) (substExpr arg_env arg)
           arg_env    = setInScope arg_se env
           result     = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
       addCoerce co cont = CoerceIt co cont
    in
    simplType env co		`thenSmpl` \ co' ->
    simplExprF env body (addCoerce co' cont)
\end{code}

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%************************************************************************
%*									*
\subsection{Lambdas}
%*									*
%************************************************************************
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\begin{code}
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simplLam env fun cont
  = go env fun cont
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  where
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    zap_it  = mkLamBndrZapper fun (countArgs cont)
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    cont_ty = contResultType cont

      	-- Type-beta reduction
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    go env (Lam bndr body) (ApplyTo _ (Type ty_arg) mb_arg_se body_cont)
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      =	ASSERT( isTyVar bndr )
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	do { tick (BetaReduction bndr)
	   ; ty_arg' <- case mb_arg_se of
			  Just arg_se -> simplType (setInScope arg_se env) ty_arg
			  Nothing     -> return ty_arg
	   ; go (extendTvSubst env bndr ty_arg') body body_cont }
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	-- Ordinary beta reduction
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    go env (Lam bndr body) cont@(ApplyTo _ arg (Just arg_se) body_cont)
      = do { tick (BetaReduction bndr)	
	   ; simplNonRecBind env (zap_it bndr) arg arg_se cont_ty	$ \ env -> 
	     go env body body_cont }

    go env (Lam bndr body) cont@(ApplyTo _ arg Nothing body_cont)
      = do { tick (BetaReduction bndr)	
	   ; simplNonRecX env (zap_it bndr) arg 	$ \ env -> 
	     go env body body_cont }
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	-- Not enough args, so there are real lambdas left to put in the result
    go env lam@(Lam _ _) cont
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      = do { (env, bndrs') <- simplLamBndrs env bndrs
	   ; body' <- simplExpr env body
	   ; (floats, new_lam) <- mkLam env bndrs' body' cont
	   ; addFloats env floats		$ \ env -> 
	     rebuild env new_lam cont }
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      where
	(bndrs,body) = collectBinders lam
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	-- Exactly enough args
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    go env expr cont = simplExprF env expr cont
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mkLamBndrZapper :: CoreExpr 	-- Function
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		-> Int		-- Number of args supplied, *including* type args
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		-> Id -> Id	-- Use this to zap the binders
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mkLamBndrZapper fun n_args
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  | n_args >= n_params fun = \b -> b		-- Enough args
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  | otherwise		   = \b -> zapLamIdInfo b
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  where
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	-- NB: we count all the args incl type args
	-- so we must count all the binders (incl type lambdas)
    n_params (Note _ e) = n_params e
    n_params (Lam b e)  = 1 + n_params e
    n_params other	= 0::Int
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\end{code}

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%************************************************************************
%*									*
\subsection{Notes}
%*									*
%************************************************************************

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\begin{code}
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-- Hack: we only distinguish subsumed cost centre stacks for the purposes of
-- inlining.  All other CCCSs are mapped to currentCCS.
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simplNote env (SCC cc) e cont
  = simplExpr (setEnclosingCC env currentCCS) e 	`thenSmpl` \ e' ->
    rebuild env (mkSCC cc e') cont

-- See notes with SimplMonad.inlineMode
simplNote env InlineMe e cont
  | contIsRhsOrArg cont		-- Totally boring continuation; see notes above
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  =				-- Don't inline inside an INLINE expression
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    simplExpr (setMode inlineMode env )  e	`thenSmpl` \ e' ->
    rebuild env (mkInlineMe e') cont
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  | otherwise  	-- Dissolve the InlineMe note if there's
		-- an interesting context of any kind to combine with
		-- (even a type application -- anything except Stop)
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  = simplExprF env e cont
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simplNote env (CoreNote s) e cont
  = simplExpr env e    `thenSmpl` \ e' ->
    rebuild env (Note (CoreNote s) e') cont
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\end{code}


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%************************************************************************
%*									*
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\subsection{Dealing with calls}
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%*									*
%************************************************************************
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\begin{code}
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simplVar env var cont
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  = case substId env var of
	DoneEx e	 -> simplExprF (zapSubstEnv env) e cont
	ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
	DoneId var1 occ  -> completeCall (zapSubstEnv env) var1 occ cont
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		-- Note [zapSubstEnv]
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		-- The template is already simplified, so don't re-substitute.
		-- This is VITAL.  Consider
		--	let x = e in
		--	let y = \z -> ...x... in
		--	\ x -> ...y...
		-- We'll clone the inner \x, adding x->x' in the id_subst
		-- Then when we inline y, we must *not* replace x by x' in
		-- the inlined copy!!
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---------------------------------------------------------
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--	Dealing with a call site
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completeCall env var occ_info cont
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  =     -- Simplify the arguments
    getDOptsSmpl					`thenSmpl` \ dflags ->
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    let
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	chkr		  = getSwitchChecker env
	(args, call_cont) = getContArgs chkr var cont
	fn_ty		  = idType var
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    in
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    simplifyArgs env fn_ty (interestingArgContext var call_cont) args 
		 (contResultType call_cont)	$ \ env args ->
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	-- Next, look for rules or specialisations that match
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	--
	-- It's important to simplify the args first, because the rule-matcher
	-- doesn't do substitution as it goes.  We don't want to use subst_args
	-- (defined in the 'where') because that throws away useful occurrence info,
	-- and perhaps-very-important specialisations.
	--
	-- Some functions have specialisations *and* are strict; in this case,
	-- we don't want to inline the wrapper of the non-specialised thing; better
	-- to call the specialised thing instead.
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	-- We used to use the black-listing mechanism to ensure that inlining of 
	-- the wrapper didn't occur for things that have specialisations till a 
	-- later phase, so but now we just try RULES first
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	--
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	-- You might think that we shouldn't apply rules for a loop breaker: 
	-- doing so might give rise to an infinite loop, because a RULE is
	-- rather like an extra equation for the function:
	--	RULE:		f (g x) y = x+y
	--	Eqn:		f a     y = a-y
	--
	-- But it's too drastic to disable rules for loop breakers.  
	-- Even the foldr/build rule would be disabled, because foldr 
	-- is recursive, and hence a loop breaker:
	--	foldr k z (build g) = g k z
	-- So it's up to the programmer: rules can cause divergence
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    let
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	in_scope   = getInScope env
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	rules	   = getRules env
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	maybe_rule = case activeRule env of
			Nothing     -> Nothing	-- No rules apply
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			Just act_fn -> lookupRule act_fn in_scope rules var args 
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    in
    case maybe_rule of {
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	Just (rule_name, rule_rhs) -> 
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		tick (RuleFired rule_name)			`thenSmpl_`
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		(if dopt Opt_D_dump_inlinings dflags then
		   pprTrace "Rule fired" (vcat [
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			text "Rule:" <+> ftext rule_name,
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			text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
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			text "After: " <+> pprCoreExpr rule_rhs,
			text "Cont:  " <+> ppr call_cont])
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		 else
			id)		$
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		simplExprF env rule_rhs call_cont ;
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	Nothing -> 		-- No rules
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	-- Next, look for an inlining
    let
	arg_infos = [ interestingArg arg | arg <- args, isValArg arg]
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	interesting_cont = interestingCallContext (notNull args)
						  (notNull arg_infos)
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						  call_cont
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    	active_inline = activeInline env var occ_info
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	maybe_inline  = callSiteInline dflags active_inline occ_info
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				       var arg_infos interesting_cont
    in
    case maybe_inline of {
	Just unfolding  	-- There is an inlining!
	  ->  tick (UnfoldingDone var)		`thenSmpl_`
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		(if dopt Opt_D_dump_inlinings dflags then
		   pprTrace "Inlining done" (vcat [
			text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
			text "Inlined fn: " <+> ppr unfolding,
			text "Cont:  " <+> ppr call_cont])
		 else
			id)		$
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	      simplExprF env unfolding (pushContArgs args call_cont)
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	;
	Nothing -> 		-- No inlining!

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	-- Done
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    rebuild env (mkApps (Var var) args) call_cont
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    }}
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\end{code}
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%************************************************************************
%*									*
\subsection{Arguments}
%*									*
%************************************************************************

\begin{code}
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---------------------------------------------------------
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--	Simplifying the arguments of a call

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simplifyArgs :: SimplEnv 
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	     -> OutType				-- Type of the function
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	     -> Bool				-- True if the fn has RULES
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	     -> [(InExpr, Maybe SimplEnv, Bool)] -- Details of the arguments
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	     -> OutType				-- Type of the continuation
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	     -> (SimplEnv -> [OutExpr] -> SimplM FloatsWithExpr)
	     -> SimplM FloatsWithExpr

-- [CPS-like because of strict arguments]
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-- Simplify the arguments to a call.
-- This part of the simplifier may break the no-shadowing invariant
-- Consider
--	f (...(\a -> e)...) (case y of (a,b) -> e')
-- where f is strict in its second arg
-- If we simplify the innermost one first we get (...(\a -> e)...)
-- Simplifying the second arg makes us float the case out, so we end up with
--	case y of (a,b) -> f (...(\a -> e)...) e'
-- So the output does not have the no-shadowing invariant.  However, there is
-- no danger of getting name-capture, because when the first arg was simplified
-- we used an in-scope set that at least mentioned all the variables free in its
-- static environment, and that is enough.
--
-- We can't just do innermost first, or we'd end up with a dual problem:
--	case x of (a,b) -> f e (...(\a -> e')...)
--
-- I spent hours trying to recover the no-shadowing invariant, but I just could
-- not think of an elegant way to do it.  The simplifier is already knee-deep in
-- continuations.  We have to keep the right in-scope set around; AND we have
-- to get the effect that finding (error "foo") in a strict arg position will
-- discard the entire application and replace it with (error "foo").  Getting
-- all this at once is TOO HARD!

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simplifyArgs env fn_ty has_rules args cont_ty thing_inside
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  = go env fn_ty args thing_inside
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  where
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    go env fn_ty []	    thing_inside = thing_inside env []
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    go env fn_ty (arg:args) thing_inside = simplifyArg env fn_ty has_rules arg cont_ty	$ \ env arg' ->
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					   go env (applyTypeToArg fn_ty arg') args 	$ \ env args' ->
					   thing_inside env (arg':args')
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simplifyArg env fn_ty has_rules (arg, Nothing, _) cont_ty thing_inside
  = thing_inside env arg	-- Already simplified

simplifyArg env fn_ty has_rules (Type ty_arg, Just se, _) cont_ty thing_inside
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  = simplType (setInScope se env) ty_arg 	`thenSmpl` \ new_ty_arg ->
    thing_inside env (Type new_ty_arg)
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simplifyArg env fn_ty has_rules (val_arg, Just arg_se, is_strict) cont_ty thing_inside 
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  | is_strict 
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  = simplStrictArg AnArg env val_arg arg_se arg_ty cont_ty thing_inside
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  | otherwise	-- Lazy argument
		-- DO NOT float anything outside, hence simplExprC
		-- There is no benefit (unlike in a let-binding), and we'd
		-- have to be very careful about bogus strictness through 
		-- floating a demanded let.
  = simplExprC (setInScope arg_se env) val_arg
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	       (mkLazyArgStop arg_ty has_rules)		`thenSmpl` \ arg1 ->
    thing_inside env arg1
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  where
    arg_ty = funArgTy fn_ty
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simplStrictArg ::  LetRhsFlag
	        -> SimplEnv		-- The env of the call
		-> InExpr -> SimplEnv	-- The arg plus its env
		-> OutType		-- arg_ty: type of the argument
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	        -> OutType		-- cont_ty: Type of thing computed by the context
	        -> (SimplEnv -> OutExpr -> SimplM FloatsWithExpr)	
	 			 	-- Takes an expression of type rhs_ty, 
		 			-- returns an expression of type cont_ty
					-- The env passed to this continuation is the
					-- env of the call, plus any new in-scope variables
	        -> SimplM FloatsWithExpr	-- An expression of type cont_ty

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simplStrictArg is_rhs call_env arg arg_env arg_ty cont_ty thing_inside
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  = simplExprF (setInScope arg_env call_env) arg
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	       (ArgOf is_rhs arg_ty cont_ty (\ new_env -> thing_inside (setInScope call_env new_env)))
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  -- Notice the way we use arg_env (augmented with in-scope vars from call_env) 
  --	to simplify the argument
  -- and call-env (augmented with in-scope vars from the arg) to pass to the continuation
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\end{code}
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%************************************************************************
%*									*
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\subsection{mkAtomicArgs}
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%*									*
%************************************************************************
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mkAtomicArgs takes a putative RHS, checks whether it's a PAP or
constructor application and, if so, converts it to ANF, so that the 
resulting thing can be inlined more easily.  Thus
	x = (f a, g b)
becomes
	t1 = f a
	t2 = g b
	x = (t1,t2)
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There are three sorts of binding context, specified by the two
boolean arguments
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Strict
   OK-unlifted
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N  N	Top-level or recursive			Only bind args of lifted type
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N  Y	Non-top-level and non-recursive,	Bind args of lifted type, or
		but lazy			unlifted-and-ok-for-speculation
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Y  Y	Non-top-level, non-recursive,		Bind all args
		 and strict (demanded)
	
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For example, given
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	x = MkC (y div# z)
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there is no point in transforming to
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	x = case (y div# z) of r -> MkC r
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