DsListComp.lhs 24.2 KB
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% (c) The University of Glasgow 2006
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% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
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Desugaring list comprehensions and array comprehensions
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\begin{code}
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{-# OPTIONS -fno-warn-incomplete-patterns #-}
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-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and fix
-- any warnings in the module. See
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--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
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-- for details

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module DsListComp ( dsListComp, dsPArrComp ) where
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#include "HsVersions.h"

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import {-# SOURCE #-} DsExpr ( dsLExpr, dsLocalBinds )
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import HsSyn
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import TcHsSyn
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import CoreSyn
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import DsMonad		-- the monadery used in the desugarer
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import DsUtils
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import DynFlags
import CoreUtils
import Var
import Type
import TysWiredIn
import Match
import PrelNames
import PrelInfo
import SrcLoc
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import Outputable

import Control.Monad ( liftM2 )
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\end{code}

List comprehensions may be desugared in one of two ways: ``ordinary''
(as you would expect if you read SLPJ's book) and ``with foldr/build
turned on'' (if you read Gill {\em et al.}'s paper on the subject).

There will be at least one ``qualifier'' in the input.

\begin{code}
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dsListComp :: [LStmt Id] 
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	   -> LHsExpr Id
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	   -> Type		-- Type of list elements
	   -> DsM CoreExpr
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dsListComp lquals body elt_ty = do 
    dflags <- getDOptsDs
    let quals = map unLoc lquals
    
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    if not (dopt Opt_RewriteRules dflags) || dopt Opt_IgnoreInterfacePragmas dflags
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       -- Either rules are switched off, or we are ignoring what there are;
       -- Either way foldr/build won't happen, so use the more efficient
       -- Wadler-style desugaring
       || isParallelComp quals
       -- Foldr-style desugaring can't handle parallel list comprehensions
        then deListComp quals body (mkNilExpr elt_ty)
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        else mkBuildExpr elt_ty (\(c, _) (n, _) -> dfListComp c n quals body) 
             -- Foldr/build should be enabled, so desugar 
             -- into foldrs and builds
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  where 
    -- We must test for ParStmt anywhere, not just at the head, because an extension
    -- to list comprehensions would be to add brackets to specify the associativity
    -- of qualifier lists. This is really easy to do by adding extra ParStmts into the
    -- mix of possibly a single element in length, so we do this to leave the possibility open
    isParallelComp = any isParallelStmt
  
    isParallelStmt (ParStmt _) = True
    isParallelStmt _           = False
    
    
-- This function lets you desugar a inner list comprehension and a list of the binders
-- of that comprehension that we need in the outer comprehension into such an expression
-- and the type of the elements that it outputs (tuples of binders)
dsInnerListComp :: ([LStmt Id], [Id]) -> DsM (CoreExpr, Type)
dsInnerListComp (stmts, bndrs) = do
        expr <- dsListComp stmts (mkBigLHsVarTup bndrs) bndrs_tuple_type
        return (expr, bndrs_tuple_type)
    where
        bndrs_types = map idType bndrs
        bndrs_tuple_type = mkBigCoreTupTy bndrs_types
        
        
-- This function factors out commonality between the desugaring strategies for TransformStmt.
-- Given such a statement it gives you back an expression representing how to compute the transformed
-- list and the tuple that you need to bind from that list in order to proceed with your desugaring
dsTransformStmt :: Stmt Id -> DsM (CoreExpr, LPat Id)
dsTransformStmt (TransformStmt (stmts, binders) usingExpr maybeByExpr) = do
    (expr, binders_tuple_type) <- dsInnerListComp (stmts, binders)
    usingExpr' <- dsLExpr usingExpr
    
    using_args <- 
        case maybeByExpr of
            Nothing -> return [expr]
            Just byExpr -> do
                byExpr' <- dsLExpr byExpr
                
                us <- newUniqueSupply
                [tuple_binder] <- newSysLocalsDs [binders_tuple_type]
                let byExprWrapper = mkTupleCase us binders byExpr' tuple_binder (Var tuple_binder)
                
                return [Lam tuple_binder byExprWrapper, expr]

    let inner_list_expr = mkApps usingExpr' ((Type binders_tuple_type) : using_args)
    
    let pat = mkBigLHsVarPatTup binders
    return (inner_list_expr, pat)
    
-- This function factors out commonality between the desugaring strategies for GroupStmt.
-- Given such a statement it gives you back an expression representing how to compute the transformed
-- list and the tuple that you need to bind from that list in order to proceed with your desugaring
dsGroupStmt :: Stmt Id -> DsM (CoreExpr, LPat Id)
dsGroupStmt (GroupStmt (stmts, binderMap) groupByClause) = do
    let (fromBinders, toBinders) = unzip binderMap
        
        fromBindersTypes = map idType fromBinders
        toBindersTypes = map idType toBinders
        
        toBindersTupleType = mkBigCoreTupTy toBindersTypes
    
    -- Desugar an inner comprehension which outputs a list of tuples of the "from" binders
    (expr, fromBindersTupleType) <- dsInnerListComp (stmts, fromBinders)
    
    -- Work out what arguments should be supplied to that expression: i.e. is an extraction
    -- function required? If so, create that desugared function and add to arguments
    (usingExpr', usingArgs) <- 
        case groupByClause of
            GroupByNothing usingExpr -> liftM2 (,) (dsLExpr usingExpr) (return [expr])
            GroupBySomething usingExpr byExpr -> do
                usingExpr' <- dsLExpr (either id noLoc usingExpr)
                
                byExpr' <- dsLExpr byExpr
                
                us <- newUniqueSupply
                [fromBindersTuple] <- newSysLocalsDs [fromBindersTupleType]
                let byExprWrapper = mkTupleCase us fromBinders byExpr' fromBindersTuple (Var fromBindersTuple)
                
                return (usingExpr', [Lam fromBindersTuple byExprWrapper, expr])
    
    -- Create an unzip function for the appropriate arity and element types and find "map"
    (unzip_fn, unzip_rhs) <- mkUnzipBind fromBindersTypes
    map_id <- dsLookupGlobalId mapName

    -- Generate the expressions to build the grouped list
    let -- First we apply the grouping function to the inner list
        inner_list_expr = mkApps usingExpr' ((Type fromBindersTupleType) : usingArgs)
        -- Then we map our "unzip" across it to turn the lists of tuples into tuples of lists
        -- We make sure we instantiate the type variable "a" to be a list of "from" tuples and
        -- the "b" to be a tuple of "to" lists!
        unzipped_inner_list_expr = mkApps (Var map_id) 
            [Type (mkListTy fromBindersTupleType), Type toBindersTupleType, Var unzip_fn, inner_list_expr]
        -- Then finally we bind the unzip function around that expression
        bound_unzipped_inner_list_expr = Let (Rec [(unzip_fn, unzip_rhs)]) unzipped_inner_list_expr
    
    -- Build a pattern that ensures the consumer binds into the NEW binders, which hold lists rather than single values
    let pat = mkBigLHsVarPatTup toBinders
    return (bound_unzipped_inner_list_expr, pat)
    
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\end{code}

%************************************************************************
%*									*
\subsection[DsListComp-ordinary]{Ordinary desugaring of list comprehensions}
%*									*
%************************************************************************

Just as in Phil's chapter~7 in SLPJ, using the rules for
optimally-compiled list comprehensions.  This is what Kevin followed
as well, and I quite happily do the same.  The TQ translation scheme
transforms a list of qualifiers (either boolean expressions or
generators) into a single expression which implements the list
comprehension.  Because we are generating 2nd-order polymorphic
lambda-calculus, calls to NIL and CONS must be applied to a type
argument, as well as their usual value arguments.
\begin{verbatim}
TE << [ e | qs ] >>  =  TQ << [ e | qs ] ++ Nil (typeOf e) >>

(Rule C)
TQ << [ e | ] ++ L >> = Cons (typeOf e) TE <<e>> TE <<L>>

(Rule B)
TQ << [ e | b , qs ] ++ L >> =
    if TE << b >> then TQ << [ e | qs ] ++ L >> else TE << L >>

(Rule A')
TQ << [ e | p <- L1, qs ]  ++  L2 >> =
  letrec
    h = \ u1 ->
    	  case u1 of
	    []        ->  TE << L2 >>
	    (u2 : u3) ->
		  (( \ TE << p >> -> ( TQ << [e | qs]  ++  (h u3) >> )) u2)
		    [] (h u3)
  in
    h ( TE << L1 >> )

"h", "u1", "u2", and "u3" are new variables.
\end{verbatim}

@deListComp@ is the TQ translation scheme.  Roughly speaking, @dsExpr@
is the TE translation scheme.  Note that we carry around the @L@ list
already desugared.  @dsListComp@ does the top TE rule mentioned above.

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To the above, we add an additional rule to deal with parallel list
comprehensions.  The translation goes roughly as follows:
     [ e | p1 <- e11, let v1 = e12, p2 <- e13
         | q1 <- e21, let v2 = e22, q2 <- e23]
     =>
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     [ e | ((x1, .., xn), (y1, ..., ym)) <-
               zip [(x1,..,xn) | p1 <- e11, let v1 = e12, p2 <- e13]
                   [(y1,..,ym) | q1 <- e21, let v2 = e22, q2 <- e23]]
where (x1, .., xn) are the variables bound in p1, v1, p2
      (y1, .., ym) are the variables bound in q1, v2, q2

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In the translation below, the ParStmt branch translates each parallel branch
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into a sub-comprehension, and desugars each independently.  The resulting lists
are fed to a zip function, we create a binding for all the variables bound in all
the comprehensions, and then we hand things off the the desugarer for bindings.
The zip function is generated here a) because it's small, and b) because then we
don't have to deal with arbitrary limits on the number of zip functions in the
prelude, nor which library the zip function came from.
The introduced tuples are Boxed, but only because I couldn't get it to work
with the Unboxed variety.
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\begin{code}
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deListComp :: [Stmt Id] -> LHsExpr Id -> CoreExpr -> DsM CoreExpr
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deListComp (ParStmt stmtss_w_bndrs : quals) body list
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  = do
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    exps_and_qual_tys <- mapM dsInnerListComp stmtss_w_bndrs
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    let (exps, qual_tys) = unzip exps_and_qual_tys
    
    (zip_fn, zip_rhs) <- mkZipBind qual_tys
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	-- Deal with [e | pat <- zip l1 .. ln] in example above
    deBindComp pat (Let (Rec [(zip_fn, zip_rhs)]) (mkApps (Var zip_fn) exps)) 
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		   quals body list
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  where 
	bndrs_s = map snd stmtss_w_bndrs

	-- pat is the pattern ((x1,..,xn), (y1,..,ym)) in the example above
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	pat  = mkBigLHsPatTup pats
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	pats = map mkBigLHsVarPatTup bndrs_s
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	-- Last: the one to return
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deListComp [] body list = do    -- Figure 7.4, SLPJ, p 135, rule C above
    core_body <- dsLExpr body
    return (mkConsExpr (exprType core_body) core_body list)
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	-- Non-last: must be a guard
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deListComp (ExprStmt guard _ _ : quals) body list = do  -- rule B above
    core_guard <- dsLExpr guard
    core_rest <- deListComp quals body list
    return (mkIfThenElse core_guard core_rest list)
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-- [e | let B, qs] = let B in [e | qs]
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deListComp (LetStmt binds : quals) body list = do
    core_rest <- deListComp quals body list
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    dsLocalBinds binds core_rest
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deListComp (stmt@(TransformStmt _ _ _) : quals) body list = do
    (inner_list_expr, pat) <- dsTransformStmt stmt
    deBindComp pat inner_list_expr quals body list

deListComp (stmt@(GroupStmt _ _) : quals) body list = do
    (inner_list_expr, pat) <- dsGroupStmt stmt
    deBindComp pat inner_list_expr quals body list

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deListComp (BindStmt pat list1 _ _ : quals) body core_list2 = do -- rule A' above
    core_list1 <- dsLExpr list1
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    deBindComp pat core_list1 quals body core_list2
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\end{code}

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\begin{code}
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deBindComp :: OutPat Id
           -> CoreExpr
           -> [Stmt Id]
           -> LHsExpr Id
           -> CoreExpr
           -> DsM (Expr Id)
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deBindComp pat core_list1 quals body core_list2 = do
    let
        u3_ty@u1_ty = exprType core_list1	-- two names, same thing
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        -- u1_ty is a [alpha] type, and u2_ty = alpha
        u2_ty = hsLPatType pat
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        res_ty = exprType core_list2
        h_ty   = u1_ty `mkFunTy` res_ty
        
    [h, u1, u2, u3] <- newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty]
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    -- the "fail" value ...
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    let
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        core_fail   = App (Var h) (Var u3)
        letrec_body = App (Var h) core_list1
        
    rest_expr <- deListComp quals body core_fail
    core_match <- matchSimply (Var u2) (StmtCtxt ListComp) pat rest_expr core_fail	
    
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    let
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        rhs = Lam u1 $
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	      Case (Var u1) u1 res_ty
		   [(DataAlt nilDataCon,  [], 	    core_list2),
		    (DataAlt consDataCon, [u2, u3], core_match)]
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			-- Increasing order of tag
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    return (Let (Rec [(h, rhs)]) letrec_body)
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\end{code}

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%************************************************************************
%*									*
\subsection[DsListComp-foldr-build]{Foldr/Build desugaring of list comprehensions}
%*									*
%************************************************************************

@dfListComp@ are the rules used with foldr/build turned on:
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\begin{verbatim}
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TE[ e | ]            c n = c e n
TE[ e | b , q ]      c n = if b then TE[ e | q ] c n else n
TE[ e | p <- l , q ] c n = let 
				f = \ x b -> case x of
						  p -> TE[ e | q ] c b
						  _ -> b
			   in
			   foldr f n l
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\end{verbatim}
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\begin{code}
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dfListComp :: Id -> Id -- 'c' and 'n'
        -> [Stmt Id]   -- the rest of the qual's
        -> LHsExpr Id
        -> DsM CoreExpr
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	-- Last: the one to return
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dfListComp c_id n_id [] body = do
    core_body <- dsLExpr body
    return (mkApps (Var c_id) [core_body, Var n_id])
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	-- Non-last: must be a guard
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dfListComp c_id n_id (ExprStmt guard _ _  : quals) body = do
    core_guard <- dsLExpr guard
    core_rest <- dfListComp c_id n_id quals body
    return (mkIfThenElse core_guard core_rest (Var n_id))

dfListComp c_id n_id (LetStmt binds : quals) body = do
    -- new in 1.3, local bindings
    core_rest <- dfListComp c_id n_id quals body
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    dsLocalBinds binds core_rest
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dfListComp c_id n_id (stmt@(TransformStmt _ _ _) : quals) body = do
    (inner_list_expr, pat) <- dsTransformStmt stmt
    -- Anyway, we bind the newly transformed list via the generic binding function
    dfBindComp c_id n_id (pat, inner_list_expr) quals body

dfListComp c_id n_id (stmt@(GroupStmt _ _) : quals) body = do
    (inner_list_expr, pat) <- dsGroupStmt stmt
    -- Anyway, we bind the newly grouped list via the generic binding function
    dfBindComp c_id n_id (pat, inner_list_expr) quals body
    
dfListComp c_id n_id (BindStmt pat list1 _ _ : quals) body = do
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    -- evaluate the two lists
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    core_list1 <- dsLExpr list1
    
    -- Do the rest of the work in the generic binding builder
    dfBindComp c_id n_id (pat, core_list1) quals body
               
dfBindComp :: Id -> Id	        -- 'c' and 'n'
       -> (LPat Id, CoreExpr)
	   -> [Stmt Id] 	        -- the rest of the qual's
	   -> LHsExpr Id
	   -> DsM CoreExpr
dfBindComp c_id n_id (pat, core_list1) quals body = do
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    -- find the required type
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    let x_ty   = hsLPatType pat
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        b_ty   = idType n_id
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    -- create some new local id's
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    [b, x] <- newSysLocalsDs [b_ty, x_ty]
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    -- build rest of the comprehesion
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    core_rest <- dfListComp c_id b quals body
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    -- build the pattern match
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    core_expr <- matchSimply (Var x) (StmtCtxt ListComp)
		pat core_rest (Var b)
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    -- now build the outermost foldr, and return
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    mkFoldrExpr x_ty b_ty (mkLams [x, b] core_expr) (Var n_id) core_list1
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\end{code}

%************************************************************************
%*									*
\subsection[DsFunGeneration]{Generation of zip/unzip functions for use in desugaring}
%*									*
%************************************************************************

\begin{code}

mkZipBind :: [Type] -> DsM (Id, CoreExpr)
-- mkZipBind [t1, t2] 
-- = (zip, \as1:[t1] as2:[t2] 
--	   -> case as1 of 
--		[] -> []
--		(a1:as'1) -> case as2 of
--				[] -> []
--				(a2:as'2) -> (a1, a2) : zip as'1 as'2)]

mkZipBind elt_tys = do
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    ass  <- mapM newSysLocalDs  elt_list_tys
    as'  <- mapM newSysLocalDs  elt_tys
    as's <- mapM newSysLocalDs  elt_list_tys
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    zip_fn <- newSysLocalDs zip_fn_ty
    
    let inner_rhs = mkConsExpr elt_tuple_ty 
			(mkBigCoreVarTup as')
			(mkVarApps (Var zip_fn) as's)
        zip_body  = foldr mk_case inner_rhs (zip3 ass as' as's)
    
    return (zip_fn, mkLams ass zip_body)
  where
    elt_list_tys      = map mkListTy elt_tys
    elt_tuple_ty      = mkBigCoreTupTy elt_tys
    elt_tuple_list_ty = mkListTy elt_tuple_ty
    
    zip_fn_ty         = mkFunTys elt_list_tys elt_tuple_list_ty

    mk_case (as, a', as') rest
	  = Case (Var as) as elt_tuple_list_ty
		  [(DataAlt nilDataCon,  [],        mkNilExpr elt_tuple_ty),
		   (DataAlt consDataCon, [a', as'], rest)]
			-- Increasing order of tag
            
            
mkUnzipBind :: [Type] -> DsM (Id, CoreExpr)
-- mkUnzipBind [t1, t2] 
-- = (unzip, \ys :: [(t1, t2)] -> foldr (\ax :: (t1, t2) axs :: ([t1], [t2])
--     -> case ax of
--      (x1, x2) -> case axs of
--                (xs1, xs2) -> (x1 : xs1, x2 : xs2))
--      ([], [])
--      ys)
-- 
-- We use foldr here in all cases, even if rules are turned off, because we may as well!
mkUnzipBind elt_tys = do
    ax  <- newSysLocalDs elt_tuple_ty
    axs <- newSysLocalDs elt_list_tuple_ty
    ys  <- newSysLocalDs elt_tuple_list_ty
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    xs  <- mapM newSysLocalDs elt_tys
    xss <- mapM newSysLocalDs elt_list_tys
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    unzip_fn <- newSysLocalDs unzip_fn_ty

    [us1, us2] <- sequence [newUniqueSupply, newUniqueSupply]

    let nil_tuple = mkBigCoreTup (map mkNilExpr elt_tys)
        
        concat_expressions = map mkConcatExpression (zip3 elt_tys (map Var xs) (map Var xss))
        tupled_concat_expression = mkBigCoreTup concat_expressions
        
        folder_body_inner_case = mkTupleCase us1 xss tupled_concat_expression axs (Var axs)
        folder_body_outer_case = mkTupleCase us2 xs folder_body_inner_case ax (Var ax)
        folder_body = mkLams [ax, axs] folder_body_outer_case
        
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    unzip_body <- mkFoldrExpr elt_tuple_ty elt_list_tuple_ty folder_body nil_tuple (Var ys)
    return (unzip_fn, mkLams [ys] unzip_body)
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  where
    elt_tuple_ty       = mkBigCoreTupTy elt_tys
    elt_tuple_list_ty  = mkListTy elt_tuple_ty
    elt_list_tys       = map mkListTy elt_tys
    elt_list_tuple_ty  = mkBigCoreTupTy elt_list_tys
    
    unzip_fn_ty        = elt_tuple_list_ty `mkFunTy` elt_list_tuple_ty
            
    mkConcatExpression (list_element_ty, head, tail) = mkConsExpr list_element_ty head tail
            
            

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\end{code}

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%************************************************************************
%*									*
\subsection[DsPArrComp]{Desugaring of array comprehensions}
%*									*
%************************************************************************

\begin{code}

-- entry point for desugaring a parallel array comprehension
--
--   [:e | qss:] = <<[:e | qss:]>> () [:():]
--
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dsPArrComp :: [Stmt Id] 
            -> LHsExpr Id
            -> Type		    -- Don't use; called with `undefined' below
            -> DsM CoreExpr
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dsPArrComp [ParStmt qss] body _  =  -- parallel comprehension
  dePArrParComp qss body
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dsPArrComp qs            body _  = do -- no ParStmt in `qs'
    sglP <- dsLookupGlobalId singletonPName
    let unitArray = mkApps (Var sglP) [Type unitTy, mkCoreTup []]
    dePArrComp qs body (mkLHsPatTup []) unitArray
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-- the work horse
--
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dePArrComp :: [Stmt Id] 
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	   -> LHsExpr Id
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	   -> LPat Id		-- the current generator pattern
	   -> CoreExpr		-- the current generator expression
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	   -> DsM CoreExpr
--
--  <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
--
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dePArrComp [] e' pa cea = do
    mapP <- dsLookupGlobalId mapPName
    let ty = parrElemType cea
    (clam, ty'e') <- deLambda ty pa e'
    return $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea]
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--
--  <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
--
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dePArrComp (ExprStmt b _ _ : qs) body pa cea = do
    filterP <- dsLookupGlobalId filterPName
    let ty = parrElemType cea
    (clam,_) <- deLambda ty pa b
    dePArrComp qs body pa (mkApps (Var filterP) [Type ty, clam, cea])
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--
--  <<[:e' | p <- e, qs:]>> pa ea =
--    let ef = \pa -> e
--    in
--    <<[:e' | qs:]>> (pa, p) (crossMap ea ef)
--
-- if matching again p cannot fail, or else
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--
--  <<[:e' | p <- e, qs:]>> pa ea = 
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--    let ef = \pa -> filterP (\x -> case x of {p -> True; _ -> False}) e
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--    in
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--    <<[:e' | qs:]>> (pa, p) (crossMapP ea ef)
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--
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dePArrComp (BindStmt p e _ _ : qs) body pa cea = do
    filterP <- dsLookupGlobalId filterPName
    crossMapP <- dsLookupGlobalId crossMapPName
    ce <- dsLExpr e
    let ety'cea = parrElemType cea
        ety'ce  = parrElemType ce
        false   = Var falseDataConId
        true    = Var trueDataConId
    v <- newSysLocalDs ety'ce
    pred <- matchSimply (Var v) (StmtCtxt PArrComp) p true false
    let cef | isIrrefutableHsPat p = ce
            | otherwise            = mkApps (Var filterP) [Type ety'ce, mkLams [v] pred, ce]
    (clam, _) <- mkLambda ety'cea pa cef
    let ety'cef = ety'ce		    -- filter doesn't change the element type
        pa'     = mkLHsPatTup [pa, p]

    dePArrComp qs body pa' (mkApps (Var crossMapP) 
                                 [Type ety'cea, Type ety'cef, cea, clam])
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--
--  <<[:e' | let ds, qs:]>> pa ea = 
--    <<[:e' | qs:]>> (pa, (x_1, ..., x_n)) 
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--		      (mapP (\v@pa -> let ds in (v, (x_1, ..., x_n))) ea)
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--  where
--    {x_1, ..., x_n} = DV (ds)		-- Defined Variables
--
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dePArrComp (LetStmt ds : qs) body pa cea = do
    mapP <- dsLookupGlobalId mapPName
    let xs     = map unLoc (collectLocalBinders ds)
        ty'cea = parrElemType cea
    v <- newSysLocalDs ty'cea
    clet <- dsLocalBinds ds (mkCoreTup (map Var xs))
    let'v <- newSysLocalDs (exprType clet)
    let projBody = mkDsLet (NonRec let'v clet) $ 
                   mkCoreTup [Var v, Var let'v]
        errTy    = exprType projBody
        errMsg   = "DsListComp.dePArrComp: internal error!"
    cerr <- mkErrorAppDs pAT_ERROR_ID errTy errMsg
    ccase <- matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr
    let pa'    = mkLHsPatTup [pa, mkLHsPatTup (map nlVarPat xs)]
        proj   = mkLams [v] ccase
    dePArrComp qs body pa' (mkApps (Var mapP) 
                                   [Type ty'cea, Type errTy, proj, cea])
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--
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-- The parser guarantees that parallel comprehensions can only appear as
-- singeltons qualifier lists, which we already special case in the caller.
-- So, encountering one here is a bug.
--
dePArrComp (ParStmt _ : _) _ _ _ = 
  panic "DsListComp.dePArrComp: malformed comprehension AST"

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--  <<[:e' | qs | qss:]>> pa ea = 
--    <<[:e' | qss:]>> (pa, (x_1, ..., x_n)) 
--		       (zipP ea <<[:(x_1, ..., x_n) | qs:]>>)
--    where
--      {x_1, ..., x_n} = DV (qs)
--
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dePArrParComp :: [([LStmt Id], [Id])] -> LHsExpr Id -> DsM CoreExpr
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dePArrParComp qss body = do
    (pQss, ceQss) <- deParStmt qss
    dePArrComp [] body pQss ceQss
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  where
    deParStmt []             =
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      -- empty parallel statement lists have no source representation
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      panic "DsListComp.dePArrComp: Empty parallel list comprehension"
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    deParStmt ((qs, xs):qss) = do        -- first statement
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      let res_expr = mkLHsVarTup xs
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      cqs <- dsPArrComp (map unLoc qs) res_expr undefined
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      parStmts qss (mkLHsVarPatTup xs) cqs
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    ---
    parStmts []             pa cea = return (pa, cea)
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    parStmts ((qs, xs):qss) pa cea = do  -- subsequent statements (zip'ed)
      zipP <- dsLookupGlobalId zipPName
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      let pa'      = mkLHsPatTup [pa, mkLHsVarPatTup xs]
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          ty'cea   = parrElemType cea
          res_expr = mkLHsVarTup xs
      cqs <- dsPArrComp (map unLoc qs) res_expr undefined
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      let ty'cqs = parrElemType cqs
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          cea'   = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
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      parStmts qss pa' cea'
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-- generate Core corresponding to `\p -> e'
--
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deLambda :: Type			-- type of the argument
	  -> LPat Id			-- argument pattern
	  -> LHsExpr Id			-- body
	  -> DsM (CoreExpr, Type)
deLambda ty p e =
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    mkLambda ty p =<< dsLExpr e
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-- generate Core for a lambda pattern match, where the body is already in Core
--
mkLambda :: Type			-- type of the argument
	 -> LPat Id			-- argument pattern
	 -> CoreExpr			-- desugared body
	 -> DsM (CoreExpr, Type)
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mkLambda ty p ce = do
    v <- newSysLocalDs ty
    let errMsg = do "DsListComp.deLambda: internal error!"
        ce'ty  = exprType ce
    cerr <- mkErrorAppDs pAT_ERROR_ID ce'ty errMsg
    res <- matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr
    return (mkLams [v] res, ce'ty)
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-- obtain the element type of the parallel array produced by the given Core
-- expression
--
parrElemType   :: CoreExpr -> Type
parrElemType e  = 
  case splitTyConApp_maybe (exprType e) of
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    Just (tycon, [ty]) | tycon == parrTyCon -> ty
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    _							  -> panic
      "DsListComp.parrElemType: not a parallel array type"
\end{code}