DsUtils.lhs 29.4 KB
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%
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% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
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%
\section[DsUtils]{Utilities for desugaring}

This module exports some utility functions of no great interest.

\begin{code}
module DsUtils (
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	EquationInfo(..), 
	firstPat, shiftEqns,
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	mkDsLet, mkDsLets,
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	MatchResult(..), CanItFail(..), 
	cantFailMatchResult, alwaysFailMatchResult,
	extractMatchResult, combineMatchResults, 
	adjustMatchResult,  adjustMatchResultDs,
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	mkCoLetMatchResult, mkGuardedMatchResult, 
	matchCanFail,
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	mkCoPrimCaseMatchResult, mkCoAlgCaseMatchResult,
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	wrapBind, wrapBinds,
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	mkErrorAppDs, mkNilExpr, mkConsExpr, mkListExpr,
	mkIntExpr, mkCharExpr,
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	mkStringExpr, mkStringExprFS, mkIntegerExpr, 
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	mkSelectorBinds, mkTupleExpr, mkTupleSelector, 
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	mkTupleType, mkTupleCase, mkBigCoreTup,
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	mkCoreTup, mkCoreTupTy,
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	dsSyntaxTable, lookupEvidence,
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	selectSimpleMatchVarL, selectMatchVars
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    ) where

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#include "HsVersions.h"

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import {-# SOURCE #-}	Match ( matchSimply )
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import {-# SOURCE #-}	DsExpr( dsExpr )
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import HsSyn
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import TcHsSyn		( hsPatType )
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import CoreSyn
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import Constants	( mAX_TUPLE_SIZE )
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import DsMonad

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import CoreUtils	( exprType, mkIfThenElse, mkCoerce, bindNonRec )
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import MkId		( iRREFUT_PAT_ERROR_ID, mkReboxingAlt, mkNewTypeBody )
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import Id		( idType, Id, mkWildId, mkTemplateLocals, mkSysLocal )
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import Var		( Var )
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import Name		( Name )
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import Literal		( Literal(..), mkStringLit, inIntRange, tARGET_MAX_INT )
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import TyCon		( isNewTyCon, tyConDataCons )
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import DataCon		( DataCon, dataConSourceArity, dataConTyCon, dataConTag )
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import Type		( mkFunTy, isUnLiftedType, Type, splitTyConApp, mkTyVarTy )
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import TcType		( tcEqType )
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import TysPrim		( intPrimTy )
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import TysWiredIn	( nilDataCon, consDataCon, 
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                          tupleCon, mkTupleTy,
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			  unitDataConId, unitTy,
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                          charTy, charDataCon, 
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                          intTy, intDataCon, 
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			  isPArrFakeCon )
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import BasicTypes	( Boxity(..) )
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import UniqSet		( mkUniqSet, minusUniqSet, isEmptyUniqSet )
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import UniqSupply	( splitUniqSupply, uniqFromSupply, uniqsFromSupply )
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import PrelNames	( unpackCStringName, unpackCStringUtf8Name, 
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			  plusIntegerName, timesIntegerName, smallIntegerDataConName, 
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			  lengthPName, indexPName )
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import Outputable
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import SrcLoc		( Located(..), unLoc )
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import Util             ( isSingleton, zipEqual, sortWith )
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import ListSetOps	( assocDefault )
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import FastString
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import Data.Char	( ord )
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#ifdef DEBUG
import Util		( notNull )	-- Used in an assertion
#endif
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\end{code}

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%************************************************************************
%*									*
		Rebindable syntax
%*									*
%************************************************************************

\begin{code}
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dsSyntaxTable :: SyntaxTable Id 
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	       -> DsM ([CoreBind], 	-- Auxiliary bindings
		       [(Name,Id)])	-- Maps the standard name to its value

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dsSyntaxTable rebound_ids
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  = mapAndUnzipDs mk_bind rebound_ids	`thenDs` \ (binds_s, prs) ->
    return (concat binds_s, prs)
  where
	-- The cheapo special case can happen when we 
	-- make an intermediate HsDo when desugaring a RecStmt
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    mk_bind (std_name, HsVar id) = return ([], (std_name, id))
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    mk_bind (std_name, expr)
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 	 = dsExpr expr				`thenDs` \ rhs ->
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     	   newSysLocalDs (exprType rhs)		`thenDs` \ id ->
     	   return ([NonRec id rhs], (std_name, id))
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lookupEvidence :: [(Name, Id)] -> Name -> Id
lookupEvidence prs std_name
  = assocDefault (mk_panic std_name) prs std_name
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  where
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    mk_panic std_name = pprPanic "dsSyntaxTable" (ptext SLIT("Not found:") <+> ppr std_name)
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\end{code}


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%************************************************************************
%*									*
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\subsection{Building lets}
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%*									*
%************************************************************************

Use case, not let for unlifted types.  The simplifier will turn some
back again.

\begin{code}
mkDsLet :: CoreBind -> CoreExpr -> CoreExpr
mkDsLet (NonRec bndr rhs) body
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  | isUnLiftedType (idType bndr) 
  = Case rhs bndr (exprType body) [(DEFAULT,[],body)]
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mkDsLet bind body
  = Let bind body

mkDsLets :: [CoreBind] -> CoreExpr -> CoreExpr
mkDsLets binds body = foldr mkDsLet body binds
\end{code}


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%************************************************************************
%*									*
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\subsection{ Selecting match variables}
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%*									*
%************************************************************************

We're about to match against some patterns.  We want to make some
@Ids@ to use as match variables.  If a pattern has an @Id@ readily at
hand, which should indeed be bound to the pattern as a whole, then use it;
otherwise, make one up.

\begin{code}
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selectSimpleMatchVarL :: LPat Id -> DsM Id
selectSimpleMatchVarL pat = selectMatchVar (unLoc pat) (hsPatType pat)

-- (selectMatchVars ps tys) chooses variables of type tys
-- to use for matching ps against.  If the pattern is a variable,
-- we try to use that, to save inventing lots of fresh variables.
-- But even if it is a variable, its type might not match.  Consider
--	data T a where
--	  T1 :: Int -> T Int
--	  T2 :: a   -> T a
--
--	f :: T a -> a -> Int
--	f (T1 i) (x::Int) = x
--	f (T2 i) (y::a)   = 0
-- Then we must not choose (x::Int) as the matching variable!

selectMatchVars :: [Pat Id] -> [Type] -> DsM [Id]
selectMatchVars []     [] 	= return []
selectMatchVars (p:ps) (ty:tys) = do { v  <- selectMatchVar  p  ty
				     ; vs <- selectMatchVars ps tys
				     ; return (v:vs) }

selectMatchVar (LazyPat pat)   pat_ty  = selectMatchVar (unLoc pat) pat_ty
selectMatchVar (VarPat var)    pat_ty  = try_for var 	     pat_ty
selectMatchVar (AsPat var pat) pat_ty  = try_for (unLoc var) pat_ty
selectMatchVar other_pat       pat_ty  = newSysLocalDs pat_ty   -- OK, better make up one...

try_for var pat_ty 
  | idType var `tcEqType` pat_ty = returnDs var
  | otherwise			 = newSysLocalDs pat_ty
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\end{code}


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%************************************************************************
%*									*
%* type synonym EquationInfo and access functions for its pieces	*
%*									*
%************************************************************************
\subsection[EquationInfo-synonym]{@EquationInfo@: a useful synonym}

The ``equation info'' used by @match@ is relatively complicated and
worthy of a type synonym and a few handy functions.

\begin{code}
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firstPat :: EquationInfo -> Pat Id
firstPat eqn = head (eqn_pats eqn)
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shiftEqns :: [EquationInfo] -> [EquationInfo]
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-- Drop the first pattern in each equation
shiftEqns eqns = [ eqn { eqn_pats = tail (eqn_pats eqn) } | eqn <- eqns ]
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\end{code}

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Functions on MatchResults

\begin{code}
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matchCanFail :: MatchResult -> Bool
matchCanFail (MatchResult CanFail _)  = True
matchCanFail (MatchResult CantFail _) = False

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alwaysFailMatchResult :: MatchResult
alwaysFailMatchResult = MatchResult CanFail (\fail -> returnDs fail)

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cantFailMatchResult :: CoreExpr -> MatchResult
cantFailMatchResult expr = MatchResult CantFail (\ ignore -> returnDs expr)
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extractMatchResult :: MatchResult -> CoreExpr -> DsM CoreExpr
extractMatchResult (MatchResult CantFail match_fn) fail_expr
  = match_fn (error "It can't fail!")
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extractMatchResult (MatchResult CanFail match_fn) fail_expr
  = mkFailurePair fail_expr	 	`thenDs` \ (fail_bind, if_it_fails) ->
    match_fn if_it_fails		`thenDs` \ body ->
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    returnDs (mkDsLet fail_bind body)
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combineMatchResults :: MatchResult -> MatchResult -> MatchResult
combineMatchResults (MatchResult CanFail      body_fn1)
		    (MatchResult can_it_fail2 body_fn2)
  = MatchResult can_it_fail2 body_fn
  where
    body_fn fail = body_fn2 fail			`thenDs` \ body2 ->
		   mkFailurePair body2	 		`thenDs` \ (fail_bind, duplicatable_expr) ->
		   body_fn1 duplicatable_expr		`thenDs` \ body1 ->
		   returnDs (Let fail_bind body1)

combineMatchResults match_result1@(MatchResult CantFail body_fn1) match_result2
  = match_result1

adjustMatchResult :: (CoreExpr -> CoreExpr) -> MatchResult -> MatchResult
adjustMatchResult encl_fn (MatchResult can_it_fail body_fn)
  = MatchResult can_it_fail (\fail -> body_fn fail	`thenDs` \ body ->
				      returnDs (encl_fn body))

adjustMatchResultDs :: (CoreExpr -> DsM CoreExpr) -> MatchResult -> MatchResult
adjustMatchResultDs encl_fn (MatchResult can_it_fail body_fn)
  = MatchResult can_it_fail (\fail -> body_fn fail	`thenDs` \ body ->
				      encl_fn body)

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wrapBinds :: [(Var,Var)] -> CoreExpr -> CoreExpr
wrapBinds [] e = e
wrapBinds ((new,old):prs) e = wrapBind new old (wrapBinds prs e)
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wrapBind :: Var -> Var -> CoreExpr -> CoreExpr
wrapBind new old body
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  | new==old    = body
  | isTyVar new = App (Lam new body) (Type (mkTyVarTy old))
  | otherwise   = Let (NonRec new (Var old)) body
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mkCoLetMatchResult :: CoreBind -> MatchResult -> MatchResult
mkCoLetMatchResult bind match_result
  = adjustMatchResult (mkDsLet bind) match_result
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mkGuardedMatchResult :: CoreExpr -> MatchResult -> MatchResult
mkGuardedMatchResult pred_expr (MatchResult can_it_fail body_fn)
  = MatchResult CanFail (\fail -> body_fn fail	`thenDs` \ body ->
				  returnDs (mkIfThenElse pred_expr body fail))
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mkCoPrimCaseMatchResult :: Id				-- Scrutinee
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                    -> Type                             -- Type of the case
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		    -> [(Literal, MatchResult)]		-- Alternatives
		    -> MatchResult
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mkCoPrimCaseMatchResult var ty match_alts
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  = MatchResult CanFail mk_case
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  where
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    mk_case fail
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      = mappM (mk_alt fail) sorted_alts		`thenDs` \ alts ->
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	returnDs (Case (Var var) var ty ((DEFAULT, [], fail) : alts))
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    sorted_alts = sortWith fst match_alts	-- Right order for a Case
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    mk_alt fail (lit, MatchResult _ body_fn) = body_fn fail	`thenDs` \ body ->
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					       returnDs (LitAlt lit, [], body)
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mkCoAlgCaseMatchResult :: Id					-- Scrutinee
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                    -> Type                                     -- Type of exp
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		    -> [(DataCon, [CoreBndr], MatchResult)]	-- Alternatives
		    -> MatchResult
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mkCoAlgCaseMatchResult var ty match_alts 
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  | isNewTyCon tycon		-- Newtype case; use a let
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  = ASSERT( null (tail match_alts) && null (tail arg_ids1) )
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    mkCoLetMatchResult (NonRec arg_id1 newtype_rhs) match_result1
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  | isPArrFakeAlts match_alts	-- Sugared parallel array; use a literal case 
  = MatchResult CanFail mk_parrCase

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  | otherwise			-- Datatype case; use a case
  = MatchResult fail_flag mk_case
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  where
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    tycon = dataConTyCon con1
	-- [Interesting: becuase of GADTs, we can't rely on the type of 
	--  the scrutinised Id to be sufficiently refined to have a TyCon in it]
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	-- Stuff for newtype
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    (con1, arg_ids1, match_result1) = head match_alts
    arg_id1 	= head arg_ids1
    newtype_rhs = mkNewTypeBody tycon (idType arg_id1) (Var var)
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	-- Stuff for data types
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    data_cons      = tyConDataCons tycon
    match_results  = [match_result | (_,_,match_result) <- match_alts]
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    fail_flag | exhaustive_case
	      = foldr1 orFail [can_it_fail | MatchResult can_it_fail _ <- match_results]
	      | otherwise
	      = CanFail
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    wild_var = mkWildId (idType var)
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    sorted_alts  = sortWith get_tag match_alts
    get_tag (con, _, _) = dataConTag con
    mk_case fail = mappM (mk_alt fail) sorted_alts	`thenDs` \ alts ->
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		   returnDs (Case (Var var) wild_var ty (mk_default fail ++ alts))
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    mk_alt fail (con, args, MatchResult _ body_fn)
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	= body_fn fail				`thenDs` \ body ->
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	  newUniqueSupply			`thenDs` \ us ->
	  returnDs (mkReboxingAlt (uniqsFromSupply us) con args body)
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    mk_default fail | exhaustive_case = []
		    | otherwise       = [(DEFAULT, [], fail)]
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    un_mentioned_constructors
        = mkUniqSet data_cons `minusUniqSet` mkUniqSet [ con | (con, _, _) <- match_alts]
    exhaustive_case = isEmptyUniqSet un_mentioned_constructors
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	-- Stuff for parallel arrays
	-- 
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	--  * the following is to desugar cases over fake constructors for
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	--   parallel arrays, which are introduced by `tidy1' in the `PArrPat'
	--   case
	--
	-- Concerning `isPArrFakeAlts':
	--
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	--  * it is *not* sufficient to just check the type of the type
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	--   constructor, as we have to be careful not to confuse the real
	--   representation of parallel arrays with the fake constructors;
	--   moreover, a list of alternatives must not mix fake and real
	--   constructors (this is checked earlier on)
	--
	-- FIXME: We actually go through the whole list and make sure that
	--	  either all or none of the constructors are fake parallel
	--	  array constructors.  This is to spot equations that mix fake
	--	  constructors with the real representation defined in
	--	  `PrelPArr'.  It would be nicer to spot this situation
	--	  earlier and raise a proper error message, but it can really
	--	  only happen in `PrelPArr' anyway.
	--
    isPArrFakeAlts [(dcon, _, _)]      = isPArrFakeCon dcon
    isPArrFakeAlts ((dcon, _, _):alts) = 
      case (isPArrFakeCon dcon, isPArrFakeAlts alts) of
        (True , True ) -> True
        (False, False) -> False
	_	       -> 
	  panic "DsUtils: You may not mix `[:...:]' with `PArr' patterns"
    --
    mk_parrCase fail = 		   
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      dsLookupGlobalId lengthPName			`thenDs` \lengthP  ->
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      unboxAlt						`thenDs` \alt      ->
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      returnDs (Case (len lengthP) (mkWildId intTy) ty [alt])
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      where
	elemTy      = case splitTyConApp (idType var) of
		        (_, [elemTy]) -> elemTy
		        _	        -> panic panicMsg
        panicMsg    = "DsUtils.mkCoAlgCaseMatchResult: not a parallel array?"
	len lengthP = mkApps (Var lengthP) [Type elemTy, Var var]
	--
	unboxAlt = 
	  newSysLocalDs intPrimTy			`thenDs` \l	   ->
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	  dsLookupGlobalId indexPName			`thenDs` \indexP   ->
	  mappM (mkAlt indexP) sorted_alts              `thenDs` \alts     ->
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	  returnDs (DataAlt intDataCon, [l], (Case (Var l) wild ty (dft : alts)))
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          where
	    wild = mkWildId intPrimTy
	    dft  = (DEFAULT, [], fail)
	--
	-- each alternative matches one array length (corresponding to one
	-- fake array constructor), so the match is on a literal; each
	-- alternative's body is extended by a local binding for each
	-- constructor argument, which are bound to array elements starting
	-- with the first
	--
	mkAlt indexP (con, args, MatchResult _ bodyFun) = 
	  bodyFun fail					`thenDs` \body     ->
	  returnDs (LitAlt lit, [], mkDsLets binds body)
	  where
	    lit   = MachInt $ toInteger (dataConSourceArity con)
	    binds = [NonRec arg (indexExpr i) | (i, arg) <- zip [1..] args]
	    --
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	    indexExpr i = mkApps (Var indexP) [Type elemTy, Var var, mkIntExpr i]
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\end{code}
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%************************************************************************
%*									*
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\subsection{Desugarer's versions of some Core functions}
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%*									*
%************************************************************************

\begin{code}
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mkErrorAppDs :: Id 		-- The error function
	     -> Type		-- Type to which it should be applied
	     -> String		-- The error message string to pass
	     -> DsM CoreExpr

mkErrorAppDs err_id ty msg
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  = getSrcSpanDs		`thenDs` \ src_loc ->
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    let
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	full_msg = showSDoc (hcat [ppr src_loc, text "|", text msg])
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	core_msg = Lit (mkStringLit full_msg)
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	-- mkStringLit returns a result of type String#
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    in
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    returnDs (mkApps (Var err_id) [Type ty, core_msg])
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\end{code}


*************************************************************
%*									*
\subsection{Making literals}
%*									*
%************************************************************************

\begin{code}
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mkCharExpr     :: Char	     -> CoreExpr      -- Returns	C# c :: Int
mkIntExpr      :: Integer    -> CoreExpr      -- Returns	I# i :: Int
mkIntegerExpr  :: Integer    -> DsM CoreExpr  -- Result :: Integer
mkStringExpr   :: String     -> DsM CoreExpr  -- Result :: String
mkStringExprFS :: FastString -> DsM CoreExpr  -- Result :: String
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mkIntExpr  i = mkConApp intDataCon  [mkIntLit i]
mkCharExpr c = mkConApp charDataCon [mkLit (MachChar c)]

mkIntegerExpr i
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  | inIntRange i  	-- Small enough, so start from an Int
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  = dsLookupDataCon  smallIntegerDataConName	`thenDs` \ integer_dc ->
    returnDs (mkSmallIntegerLit integer_dc i)
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-- Special case for integral literals with a large magnitude:
-- They are transformed into an expression involving only smaller
-- integral literals. This improves constant folding.

  | otherwise 		-- Big, so start from a string
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  = dsLookupGlobalId plusIntegerName		`thenDs` \ plus_id ->
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    dsLookupGlobalId timesIntegerName		`thenDs` \ times_id ->
    dsLookupDataCon  smallIntegerDataConName	`thenDs` \ integer_dc ->
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    let 
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	lit i = mkSmallIntegerLit integer_dc i
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        plus a b  = Var plus_id  `App` a `App` b
        times a b = Var times_id `App` a `App` b

	-- Transform i into (x1 + (x2 + (x3 + (...) * b) * b) * b) with abs xi <= b
	horner :: Integer -> Integer -> CoreExpr
	horner b i | abs q <= 1 = if r == 0 || r == i 
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				  then lit i 
				  else lit r `plus` lit (i-r)
	           | r == 0     =               horner b q `times` lit b
	           | otherwise  = lit r `plus` (horner b q `times` lit b)
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  		   where
		     (q,r) = i `quotRem` b

    in
    returnDs (horner tARGET_MAX_INT i)

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mkSmallIntegerLit small_integer_data_con i = mkConApp small_integer_data_con [mkIntLit i]
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mkStringExpr str = mkStringExprFS (mkFastString str)
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mkStringExprFS str
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  | nullFS str
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  = returnDs (mkNilExpr charTy)

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  | lengthFS str == 1
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  = let
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	the_char = mkCharExpr (headFS str)
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    in
    returnDs (mkConsExpr charTy the_char (mkNilExpr charTy))

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  | all safeChar chars
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  = dsLookupGlobalId unpackCStringName	`thenDs` \ unpack_id ->
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    returnDs (App (Var unpack_id) (Lit (MachStr str)))

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  | otherwise
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  = dsLookupGlobalId unpackCStringUtf8Name	`thenDs` \ unpack_id ->
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    returnDs (App (Var unpack_id) (Lit (MachStr str)))
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  where
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    chars = unpackFS str
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    safeChar c = ord c >= 1 && ord c <= 0x7F
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\end{code}

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%************************************************************************
%*									*
\subsection[mkSelectorBind]{Make a selector bind}
%*									*
%************************************************************************

This is used in various places to do with lazy patterns.
For each binder $b$ in the pattern, we create a binding:
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\begin{verbatim}
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    b = case v of pat' -> b'
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\end{verbatim}
where @pat'@ is @pat@ with each binder @b@ cloned into @b'@.
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ToDo: making these bindings should really depend on whether there's
much work to be done per binding.  If the pattern is complex, it
should be de-mangled once, into a tuple (and then selected from).
Otherwise the demangling can be in-line in the bindings (as here).

Boring!  Boring!  One error message per binder.  The above ToDo is
even more helpful.  Something very similar happens for pattern-bound
expressions.

\begin{code}
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mkSelectorBinds :: LPat Id	-- The pattern
		-> CoreExpr	-- Expression to which the pattern is bound
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		-> DsM [(Id,CoreExpr)]
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mkSelectorBinds (L _ (VarPat v)) val_expr
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  = returnDs [(v, val_expr)]

mkSelectorBinds pat val_expr
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  | isSingleton binders || is_simple_lpat pat
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  = 	-- Given   p = e, where p binds x,y
	-- we are going to make
	--	v = p	(where v is fresh)
	--	x = case v of p -> x
	--	y = case v of p -> x

	-- Make up 'v'
	-- NB: give it the type of *pattern* p, not the type of the *rhs* e.
	-- This does not matter after desugaring, but there's a subtle 
	-- issue with implicit parameters. Consider
	--	(x,y) = ?i
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	-- Then, ?i is given type {?i :: Int}, a PredType, which is opaque
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	-- to the desugarer.  (Why opaque?  Because newtypes have to be.  Why
	-- does it get that type?  So that when we abstract over it we get the
	-- right top-level type  (?i::Int) => ...)
	--
	-- So to get the type of 'v', use the pattern not the rhs.  Often more
	-- efficient too.
    newSysLocalDs (hsPatType pat)	`thenDs` \ val_var ->
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	-- For the error message we make one error-app, to avoid duplication.
	-- But we need it at different types... so we use coerce for that
    mkErrorAppDs iRREFUT_PAT_ERROR_ID 
		 unitTy (showSDoc (ppr pat))	`thenDs` \ err_expr ->
    newSysLocalDs unitTy			`thenDs` \ err_var ->
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    mappM (mk_bind val_var err_var) binders	`thenDs` \ binds ->
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    returnDs ( (val_var, val_expr) : 
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	       (err_var, err_expr) :
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	       binds )
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  | otherwise
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  = mkErrorAppDs iRREFUT_PAT_ERROR_ID 
		 tuple_ty (showSDoc (ppr pat))			`thenDs` \ error_expr ->
    matchSimply val_expr PatBindRhs pat local_tuple error_expr	`thenDs` \ tuple_expr ->
    newSysLocalDs tuple_ty					`thenDs` \ tuple_var ->
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    let
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	mk_tup_bind binder
	  = (binder, mkTupleSelector binders binder tuple_var (Var tuple_var))
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    in
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    returnDs ( (tuple_var, tuple_expr) : map mk_tup_bind binders )
  where
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    binders	= collectPatBinders pat
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    local_tuple = mkTupleExpr binders
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    tuple_ty    = exprType local_tuple
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    mk_bind scrut_var err_var bndr_var
    -- (mk_bind sv err_var) generates
    --		bv = case sv of { pat -> bv; other -> coerce (type-of-bv) err_var }
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    -- Remember, pat binds bv
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      = matchSimply (Var scrut_var) PatBindRhs pat
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		    (Var bndr_var) error_expr			`thenDs` \ rhs_expr ->
        returnDs (bndr_var, rhs_expr)
      where
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        error_expr = mkCoerce (idType bndr_var) (Var err_var)
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    is_simple_lpat p = is_simple_pat (unLoc p)

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    is_simple_pat (TuplePat ps Boxed)      = all is_triv_lpat ps
    is_simple_pat (ConPatOut _ _ _ _ ps _) = all is_triv_lpat (hsConArgs ps)
    is_simple_pat (VarPat _)	       	   = True
    is_simple_pat (ParPat p)		   = is_simple_lpat p
    is_simple_pat other		       	   = False
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    is_triv_lpat p = is_triv_pat (unLoc p)

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    is_triv_pat (VarPat v)  = True
    is_triv_pat (WildPat _) = True
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    is_triv_pat (ParPat p)  = is_triv_lpat p
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    is_triv_pat other       = False
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\end{code}


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%************************************************************************
%*									*
		Tuples
%*									*
%************************************************************************

@mkTupleExpr@ builds a tuple; the inverse to @mkTupleSelector@.  

* If it has only one element, it is the identity function.

* If there are more elements than a big tuple can have, it nests 
  the tuples.  

Nesting policy.  Better a 2-tuple of 10-tuples (3 objects) than
a 10-tuple of 2-tuples (11 objects).  So we want the leaves to be big.
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\begin{code}
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mkTupleExpr :: [Id] -> CoreExpr
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mkTupleExpr ids = mkBigCoreTup (map Var ids)

-- corresponding type
mkTupleType :: [Id] -> Type
mkTupleType ids = mkBigTuple mkCoreTupTy (map idType ids)

mkBigCoreTup :: [CoreExpr] -> CoreExpr
mkBigCoreTup = mkBigTuple mkCoreTup

mkBigTuple :: ([a] -> a) -> [a] -> a
mkBigTuple small_tuple as = mk_big_tuple (chunkify as)
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  where
	-- Each sub-list is short enough to fit in a tuple
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    mk_big_tuple [as] = small_tuple as
    mk_big_tuple as_s = mk_big_tuple (chunkify (map small_tuple as_s))
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chunkify :: [a] -> [[a]]
-- The sub-lists of the result all have length <= mAX_TUPLE_SIZE
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-- But there may be more than mAX_TUPLE_SIZE sub-lists
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chunkify xs
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  | n_xs <= mAX_TUPLE_SIZE = {- pprTrace "Small" (ppr n_xs) -} [xs] 
  | otherwise		   = {- pprTrace "Big"   (ppr n_xs) -} (split xs)
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  where
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    n_xs     = length xs
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    split [] = []
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    split xs = take mAX_TUPLE_SIZE xs : split (drop mAX_TUPLE_SIZE xs)
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\end{code}


@mkTupleSelector@ builds a selector which scrutises the given
expression and extracts the one name from the list given.
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If you want the no-shadowing rule to apply, the caller
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is responsible for making sure that none of these names
are in scope.

If there is just one id in the ``tuple'', then the selector is
just the identity.

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If it's big, it does nesting
	mkTupleSelector [a,b,c,d] b v e
	  = case e of v { 
		(p,q) -> case p of p {
			   (a,b) -> b }}
We use 'tpl' vars for the p,q, since shadowing does not matter.

In fact, it's more convenient to generate it innermost first, getting

	case (case e of v 
		(p,q) -> p) of p
	  (a,b) -> b

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\begin{code}
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mkTupleSelector :: [Id]		-- The tuple args
		-> Id		-- The selected one
		-> Id		-- A variable of the same type as the scrutinee
		-> CoreExpr	-- Scrutinee
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		-> CoreExpr
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mkTupleSelector vars the_var scrut_var scrut
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  = mk_tup_sel (chunkify vars) the_var
  where
    mk_tup_sel [vars] the_var = mkCoreSel vars the_var scrut_var scrut
    mk_tup_sel vars_s the_var = mkCoreSel group the_var tpl_v $
				mk_tup_sel (chunkify tpl_vs) tpl_v
	where
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	  tpl_tys = [mkCoreTupTy (map idType gp) | gp <- vars_s]
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	  tpl_vs  = mkTemplateLocals tpl_tys
	  [(tpl_v, group)] = [(tpl,gp) | (tpl,gp) <- zipEqual "mkTupleSelector" tpl_vs vars_s,
					 the_var `elem` gp ]
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\end{code}

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A generalization of @mkTupleSelector@, allowing the body
of the case to be an arbitrary expression.

If the tuple is big, it is nested:

	mkTupleCase uniqs [a,b,c,d] body v e
	  = case e of v { (p,q) ->
	    case p of p { (a,b) ->
	    case q of q { (c,d) ->
	    body }}}

To avoid shadowing, we use uniqs to invent new variables p,q.

ToDo: eliminate cases where none of the variables are needed.

\begin{code}
mkTupleCase
	:: UniqSupply	-- for inventing names of intermediate variables
	-> [Id]		-- the tuple args
	-> CoreExpr	-- body of the case
	-> Id		-- a variable of the same type as the scrutinee
	-> CoreExpr	-- scrutinee
	-> CoreExpr

mkTupleCase uniqs vars body scrut_var scrut
  = mk_tuple_case uniqs (chunkify vars) body
  where
    mk_tuple_case us [vars] body
      = mkSmallTupleCase vars body scrut_var scrut
    mk_tuple_case us vars_s body
      = let
	    (us', vars', body') = foldr one_tuple_case (us, [], body) vars_s
	in
	mk_tuple_case us' (chunkify vars') body'
    one_tuple_case chunk_vars (us, vs, body)
      = let
	    (us1, us2) = splitUniqSupply us
	    scrut_var = mkSysLocal FSLIT("ds") (uniqFromSupply us1)
			(mkCoreTupTy (map idType chunk_vars))
	    body' = mkSmallTupleCase chunk_vars body scrut_var (Var scrut_var)
	in (us2, scrut_var:vs, body')
\end{code}

The same, but with a tuple small enough not to need nesting.

\begin{code}
mkSmallTupleCase
	:: [Id]		-- the tuple args
	-> CoreExpr	-- body of the case
	-> Id		-- a variable of the same type as the scrutinee
	-> CoreExpr	-- scrutinee
	-> CoreExpr

mkSmallTupleCase [var] body _scrut_var scrut
  = bindNonRec var scrut body
mkSmallTupleCase vars body scrut_var scrut
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-- One branch no refinement?
  = Case scrut scrut_var (exprType body) [(DataAlt (tupleCon Boxed (length vars)), vars, body)]
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\end{code}
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%************************************************************************
%*									*
\subsection[mkFailurePair]{Code for pattern-matching and other failures}
%*									*
%************************************************************************

Call the constructor Ids when building explicit lists, so that they
interact well with rules.

\begin{code}
mkNilExpr :: Type -> CoreExpr
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mkNilExpr ty = mkConApp nilDataCon [Type ty]
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mkConsExpr :: Type -> CoreExpr -> CoreExpr -> CoreExpr
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mkConsExpr ty hd tl = mkConApp consDataCon [Type ty, hd, tl]
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mkListExpr :: Type -> [CoreExpr] -> CoreExpr
mkListExpr ty xs = foldr (mkConsExpr ty) (mkNilExpr ty) xs
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-- The next three functions make tuple types, constructors and selectors,
-- with the rule that a 1-tuple is represented by the thing itselg
mkCoreTupTy :: [Type] -> Type
mkCoreTupTy [ty] = ty
mkCoreTupTy tys  = mkTupleTy Boxed (length tys) tys

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mkCoreTup :: [CoreExpr] -> CoreExpr			    
-- Builds exactly the specified tuple.
-- No fancy business for big tuples
mkCoreTup []  = Var unitDataConId
mkCoreTup [c] = c
mkCoreTup cs  = mkConApp (tupleCon Boxed (length cs))
			 (map (Type . exprType) cs ++ cs)
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mkCoreSel :: [Id]	-- The tuple args
	  -> Id		-- The selected one
	  -> Id		-- A variable of the same type as the scrutinee
	  -> CoreExpr	-- Scrutinee
	  -> CoreExpr
-- mkCoreSel [x,y,z] x v e
-- ===>  case e of v { (x,y,z) -> x
mkCoreSel [var] should_be_the_same_var scrut_var scrut
  = ASSERT(var == should_be_the_same_var)
    scrut

mkCoreSel vars the_var scrut_var scrut
  = ASSERT( notNull vars )
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    Case scrut scrut_var (idType the_var)
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	 [(DataAlt (tupleCon Boxed (length vars)), vars, Var the_var)]
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\end{code}


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%************************************************************************
%*									*
\subsection[mkFailurePair]{Code for pattern-matching and other failures}
%*									*
%************************************************************************

Generally, we handle pattern matching failure like this: let-bind a
fail-variable, and use that variable if the thing fails:
\begin{verbatim}
	let fail.33 = error "Help"
	in
	case x of
		p1 -> ...
		p2 -> fail.33
		p3 -> fail.33
		p4 -> ...
\end{verbatim}
Then
\begin{itemize}
\item
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If the case can't fail, then there'll be no mention of @fail.33@, and the
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simplifier will later discard it.

\item
If it can fail in only one way, then the simplifier will inline it.

\item
Only if it is used more than once will the let-binding remain.
\end{itemize}

There's a problem when the result of the case expression is of
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unboxed type.  Then the type of @fail.33@ is unboxed too, and
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there is every chance that someone will change the let into a case:
\begin{verbatim}
	case error "Help" of
	  fail.33 -> case ....
\end{verbatim}

which is of course utterly wrong.  Rather than drop the condition that
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only boxed types can be let-bound, we just turn the fail into a function
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for the primitive case:
\begin{verbatim}
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	let fail.33 :: Void -> Int#
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	    fail.33 = \_ -> error "Help"
	in
	case x of
		p1 -> ...
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		p2 -> fail.33 void
		p3 -> fail.33 void
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		p4 -> ...
\end{verbatim}

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Now @fail.33@ is a function, so it can be let-bound.
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\begin{code}
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mkFailurePair :: CoreExpr	-- Result type of the whole case expression
	      -> DsM (CoreBind,	-- Binds the newly-created fail variable
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				-- to either the expression or \ _ -> expression
		      CoreExpr)	-- Either the fail variable, or fail variable
				-- applied to unit tuple
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mkFailurePair expr
  | isUnLiftedType ty
  = newFailLocalDs (unitTy `mkFunTy` ty)	`thenDs` \ fail_fun_var ->
    newSysLocalDs unitTy			`thenDs` \ fail_fun_arg ->
    returnDs (NonRec fail_fun_var (Lam fail_fun_arg expr),
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	      App (Var fail_fun_var) (Var unitDataConId))
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  | otherwise
  = newFailLocalDs ty 		`thenDs` \ fail_var ->
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    returnDs (NonRec fail_var expr, Var fail_var)
  where
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    ty = exprType expr
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\end{code}

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