DsMeta.hs 64 KB
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-----------------------------------------------------------------------------
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--
-- (c) The University of Glasgow 2006
--
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-- The purpose of this module is to transform an HsExpr into a CoreExpr which
-- when evaluated, returns a (Meta.Q Meta.Exp) computation analogous to the
-- input HsExpr. We do this in the DsM monad, which supplies access to
-- CoreExpr's of the "smart constructors" of the Meta.Exp datatype.
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--
-- It also defines a bunch of knownKeyNames, in the same way as is done
-- in prelude/PrelNames.  It's much more convenient to do it here, becuase
-- otherwise we have to recompile PrelNames whenever we add a Name, which is
-- a Royal Pain (triggers other recompilation).
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-----------------------------------------------------------------------------


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module DsMeta( dsBracket, 
	       templateHaskellNames, qTyConName, nameTyConName,
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	       liftName, expQTyConName, decQTyConName, typeQTyConName,
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	       decTyConName, typeTyConName, mkNameG_dName, mkNameG_vName, mkNameG_tcName
	        ) where
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#include "HsVersions.h"

import {-# SOURCE #-}	DsExpr ( dsExpr )

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import MatchLit
import DsUtils
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import DsMonad

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import qualified Language.Haskell.TH as TH
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import HsSyn
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import Class
import PrelNames
import OccName
-- To avoid clashes with DsMeta.varName we must make a local alias for
-- OccName.varName we do this by removing varName from the import of
-- OccName above, making a qualified instance of OccName and using
-- OccNameAlias.varName where varName ws previously used in this file.
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import qualified OccName
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import Module
import Id
import OccName
import Name
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import NameEnv
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import Type
import TcType
import TyCon
import TysWiredIn
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import CoreSyn
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import CoreUtils
import SrcLoc
import PackageConfig
import Unique
import BasicTypes
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import Outputable
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import Bag
import FastString
import ForeignCall
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import Data.Maybe
import Control.Monad
import Data.List
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-----------------------------------------------------------------------------
dsBracket :: HsBracket Name -> [PendingSplice] -> DsM CoreExpr
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-- Returns a CoreExpr of type TH.ExpQ
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-- The quoted thing is parameterised over Name, even though it has
-- been type checked.  We don't want all those type decorations!

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dsBracket brack splices
  = dsExtendMetaEnv new_bit (do_brack brack)
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  where
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    new_bit = mkNameEnv [(n, Splice (unLoc e)) | (n,e) <- splices]
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    do_brack (VarBr n)  = do { MkC e1  <- lookupOcc n ; return e1 }
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    do_brack (ExpBr e)  = do { MkC e1  <- repLE e     ; return e1 }
    do_brack (PatBr p)  = do { MkC p1  <- repLP p     ; return p1 }
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    do_brack (TypBr t)  = do { MkC t1  <- repLTy t    ; return t1 }
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    do_brack (DecBr ds) = do { MkC ds1 <- repTopDs ds ; return ds1 }
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{- -------------- Examples --------------------

  [| \x -> x |]
====>
  gensym (unpackString "x"#) `bindQ` \ x1::String ->
  lam (pvar x1) (var x1)


  [| \x -> $(f [| x |]) |]
====>
  gensym (unpackString "x"#) `bindQ` \ x1::String ->
  lam (pvar x1) (f (var x1))
-}


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-------------------------------------------------------
-- 			Declarations
-------------------------------------------------------

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repTopDs :: HsGroup Name -> DsM (Core (TH.Q [TH.Dec]))
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repTopDs group
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 = do { let { bndrs = map unLoc (groupBinders group) } ;
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	ss <- mkGenSyms bndrs ;
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	-- Bind all the names mainly to avoid repeated use of explicit strings.
	-- Thus	we get
	--	do { t :: String <- genSym "T" ;
	--	     return (Data t [] ...more t's... }
	-- The other important reason is that the output must mention
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	-- only "T", not "Foo:T" where Foo is the current module
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	decls <- addBinds ss (do {
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			val_ds  <- rep_val_binds (hs_valds group) ;
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			tycl_ds <- mapM repTyClD (hs_tyclds group) ;
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			inst_ds <- mapM repInstD' (hs_instds group) ;
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			for_ds <- mapM repForD (hs_fords group) ;
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			-- more needed
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			return (de_loc $ sort_by_loc $ val_ds ++ catMaybes tycl_ds ++ inst_ds ++ for_ds) }) ;
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	decl_ty <- lookupType decQTyConName ;
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	let { core_list = coreList' decl_ty decls } ;
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	dec_ty <- lookupType decTyConName ;
	q_decs  <- repSequenceQ dec_ty core_list ;
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	wrapNongenSyms ss q_decs
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	-- Do *not* gensym top-level binders
      }

groupBinders (HsGroup { hs_valds = val_decls, hs_tyclds = tycl_decls,
			hs_fords = foreign_decls })
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-- Collect the binders of a Group
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  = collectHsValBinders val_decls ++
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    [n | d <- tycl_decls, n <- tyClDeclNames (unLoc d)] ++
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    [n | L _ (ForeignImport n _ _) <- foreign_decls]
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{- 	Note [Binders and occurrences]
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we desugar [d| data T = MkT |]
we want to get
	Data "T" [] [Con "MkT" []] []
and *not*
	Data "Foo:T" [] [Con "Foo:MkT" []] []
That is, the new data decl should fit into whatever new module it is
asked to fit in.   We do *not* clone, though; no need for this:
	Data "T79" ....

But if we see this:
	data T = MkT 
	foo = reifyDecl T

then we must desugar to
	foo = Data "Foo:T" [] [Con "Foo:MkT" []] []

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So in repTopDs we bring the binders into scope with mkGenSyms and addBinds.
And we use lookupOcc, rather than lookupBinder
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in repTyClD and repC.

-}

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repTyClD :: LTyClDecl Name -> DsM (Maybe (SrcSpan, Core TH.DecQ))
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repTyClD (L loc (TyData { tcdND = DataType, tcdCtxt = cxt, 
		    tcdLName = tc, tcdTyVars = tvs, 
		    tcdCons = cons, tcdDerivs = mb_derivs }))
 = do { tc1 <- lookupLOcc tc ;		-- See note [Binders and occurrences] 
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        dec <- addTyVarBinds tvs $ \bndrs -> do {
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      	       cxt1    <- repLContext cxt ;
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               cons1   <- mapM repC cons ;
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      	       cons2   <- coreList conQTyConName cons1 ;
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      	       derivs1 <- repDerivs mb_derivs ;
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	       bndrs1  <- coreList nameTyConName bndrs ;
      	       repData cxt1 tc1 bndrs1 cons2 derivs1 } ;
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        return $ Just (loc, dec) }
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repTyClD (L loc (TyData { tcdND = NewType, tcdCtxt = cxt, 
		    tcdLName = tc, tcdTyVars = tvs, 
		    tcdCons = [con], tcdDerivs = mb_derivs }))
 = do { tc1 <- lookupLOcc tc ;		-- See note [Binders and occurrences] 
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        dec <- addTyVarBinds tvs $ \bndrs -> do {
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      	       cxt1   <- repLContext cxt ;
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               con1   <- repC con ;
      	       derivs1 <- repDerivs mb_derivs ;
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	       bndrs1  <- coreList nameTyConName bndrs ;
      	       repNewtype cxt1 tc1 bndrs1 con1 derivs1 } ;
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        return $ Just (loc, dec) }

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repTyClD (L loc (TySynonym { tcdLName = tc, tcdTyVars = tvs, tcdSynRhs = ty }))
 = do { tc1 <- lookupLOcc tc ;		-- See note [Binders and occurrences] 
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        dec <- addTyVarBinds tvs $ \bndrs -> do {
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	       ty1     <- repLTy ty ;
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	       bndrs1  <- coreList nameTyConName bndrs ;
	       repTySyn tc1 bndrs1 ty1 } ;
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 	return (Just (loc, dec)) }
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repTyClD (L loc (ClassDecl { tcdCtxt = cxt, tcdLName = cls, 
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		      tcdTyVars = tvs, 
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		      tcdFDs = fds,
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		      tcdSigs = sigs, tcdMeths = meth_binds }))
 = do { cls1 <- lookupLOcc cls ;		-- See note [Binders and occurrences] 
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    	dec  <- addTyVarBinds tvs $ \bndrs -> do {
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 		  cxt1   <- repLContext cxt ;
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 		  sigs1  <- rep_sigs sigs ;
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 		  binds1 <- rep_binds meth_binds ;
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	          fds1 <- repLFunDeps fds;
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 		  decls1 <- coreList decQTyConName (sigs1 ++ binds1) ;
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	          bndrs1 <- coreList nameTyConName bndrs ;
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 		  repClass cxt1 cls1 bndrs1 fds1 decls1 } ;
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    	return $ Just (loc, dec) }
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-- Un-handled cases
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repTyClD (L loc d) = putSrcSpanDs loc $
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		     do { warnDs (hang ds_msg 4 (ppr d))
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			; return Nothing }
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-- represent fundeps
--
repLFunDeps :: [Located (FunDep Name)] -> DsM (Core [TH.FunDep])
repLFunDeps fds = do fds' <- mapM repLFunDep fds
                     fdList <- coreList funDepTyConName fds'
                     return fdList

repLFunDep :: Located (FunDep Name) -> DsM (Core TH.FunDep)
repLFunDep (L _ (xs, ys)) = do xs' <- mapM lookupBinder xs
                               ys' <- mapM lookupBinder ys
                               xs_list <- coreList nameTyConName xs'
                               ys_list <- coreList nameTyConName ys'
                               repFunDep xs_list ys_list
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repInstD' (L loc (InstDecl ty binds _ _))		-- Ignore user pragmas for now
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 = do	{ i <- addTyVarBinds tvs $ \tv_bndrs ->
		-- We must bring the type variables into scope, so their occurrences
		-- don't fail,  even though the binders don't appear in the resulting 
		-- data structure
		do {  cxt1 <- repContext cxt
		   ; inst_ty1 <- repPred (HsClassP cls tys)
		   ; ss <- mkGenSyms (collectHsBindBinders binds)
		   ; binds1 <- addBinds ss (rep_binds binds)
		   ; decls1 <- coreList decQTyConName binds1
		   ; decls2 <- wrapNongenSyms ss decls1
		   -- wrapNonGenSyms: do not clone the class op names!
		   -- They must be called 'op' etc, not 'op34'
		   ; repInst cxt1 inst_ty1 decls2 }

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	; return (loc, i)}
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 where
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   (tvs, cxt, cls, tys) = splitHsInstDeclTy (unLoc ty)
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repForD :: Located (ForeignDecl Name) -> DsM (SrcSpan, Core TH.DecQ)
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repForD (L loc (ForeignImport name typ (CImport cc s ch cn cis)))
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 = do MkC name' <- lookupLOcc name
      MkC typ' <- repLTy typ
      MkC cc' <- repCCallConv cc
      MkC s' <- repSafety s
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      cis' <- conv_cimportspec cis
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      MkC str <- coreStringLit $ static
                              ++ unpackFS ch ++ " "
                              ++ unpackFS cn ++ " "
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                              ++ cis'
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      dec <- rep2 forImpDName [cc', s', str, name', typ']
      return (loc, dec)
 where
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    conv_cimportspec (CLabel cls) = notHandled "Foreign label" (doubleQuotes (ppr cls))
    conv_cimportspec (CFunction DynamicTarget) = return "dynamic"
    conv_cimportspec (CFunction (StaticTarget fs)) = return (unpackFS fs)
    conv_cimportspec CWrapper = return "wrapper"
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    static = case cis of
                 CFunction (StaticTarget _) -> "static "
                 _ -> ""
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repForD decl = notHandled "Foreign declaration" (ppr decl)
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repCCallConv :: CCallConv -> DsM (Core TH.Callconv)
repCCallConv CCallConv = rep2 cCallName []
repCCallConv StdCallConv = rep2 stdCallName []

repSafety :: Safety -> DsM (Core TH.Safety)
repSafety PlayRisky = rep2 unsafeName []
repSafety (PlaySafe False) = rep2 safeName []
repSafety (PlaySafe True) = rep2 threadsafeName []

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ds_msg = ptext SLIT("Cannot desugar this Template Haskell declaration:")

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-------------------------------------------------------
-- 			Constructors
-------------------------------------------------------

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repC :: LConDecl Name -> DsM (Core TH.ConQ)
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repC (L loc (ConDecl con expl [] (L _ []) details ResTyH98 _))
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  = do { con1 <- lookupLOcc con ;		-- See note [Binders and occurrences] 
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	 repConstr con1 details }
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repC (L loc (ConDecl con expl tvs (L cloc ctxt) details ResTyH98 doc))
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  = do { addTyVarBinds tvs $ \bndrs -> do {
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             c' <- repC (L loc (ConDecl con expl [] (L cloc []) details ResTyH98 doc));
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             ctxt' <- repContext ctxt;
             bndrs' <- coreList nameTyConName bndrs;
             rep2 forallCName [unC bndrs', unC ctxt', unC c']
         }
       }
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repC (L loc con_decl)		-- GADTs
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  = putSrcSpanDs loc $
    notHandled "GADT declaration" (ppr con_decl) 
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repBangTy :: LBangType Name -> DsM (Core (TH.StrictTypeQ))
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repBangTy ty= do 
  MkC s <- rep2 str []
  MkC t <- repLTy ty'
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  rep2 strictTypeName [s, t]
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  where 
    (str, ty') = case ty of
		   L _ (HsBangTy _ ty) -> (isStrictName,  ty)
		   other	       -> (notStrictName, ty)
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-------------------------------------------------------
-- 			Deriving clause
-------------------------------------------------------

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repDerivs :: Maybe [LHsType Name] -> DsM (Core [TH.Name])
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repDerivs Nothing = coreList nameTyConName []
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repDerivs (Just ctxt)
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  = do { strs <- mapM rep_deriv ctxt ; 
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	 coreList nameTyConName strs }
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  where
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    rep_deriv :: LHsType Name -> DsM (Core TH.Name)
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	-- Deriving clauses must have the simple H98 form
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    rep_deriv (L _ (HsPredTy (HsClassP cls []))) = lookupOcc cls
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    rep_deriv other = notHandled "Non-H98 deriving clause" (ppr other)
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-------------------------------------------------------
--   Signatures in a class decl, or a group of bindings
-------------------------------------------------------

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rep_sigs :: [LSig Name] -> DsM [Core TH.DecQ]
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rep_sigs sigs = do locs_cores <- rep_sigs' sigs
                   return $ de_loc $ sort_by_loc locs_cores

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rep_sigs' :: [LSig Name] -> DsM [(SrcSpan, Core TH.DecQ)]
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	-- We silently ignore ones we don't recognise
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rep_sigs' sigs = do { sigs1 <- mapM rep_sig sigs ;
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		     return (concat sigs1) }

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rep_sig :: LSig Name -> DsM [(SrcSpan, Core TH.DecQ)]
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	-- Singleton => Ok
	-- Empty     => Too hard, signature ignored
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rep_sig (L loc (TypeSig nm ty)) = rep_proto nm ty loc
rep_sig other		        = return []
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rep_proto :: Located Name -> LHsType Name -> SrcSpan -> DsM [(SrcSpan, Core TH.DecQ)]
rep_proto nm ty loc = do { nm1 <- lookupLOcc nm ; 
		       ty1 <- repLTy ty ; 
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		       sig <- repProto nm1 ty1 ;
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		       return [(loc, sig)] }
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-------------------------------------------------------
-- 			Types
-------------------------------------------------------
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-- gensym a list of type variables and enter them into the meta environment;
-- the computations passed as the second argument is executed in that extended
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-- meta environment and gets the *new* names on Core-level as an argument
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--
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addTyVarBinds :: [LHsTyVarBndr Name]	         -- the binders to be added
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	      -> ([Core TH.Name] -> DsM (Core (TH.Q a))) -- action in the ext env
	      -> DsM (Core (TH.Q a))
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addTyVarBinds tvs m =
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  do
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    let names = map (hsTyVarName.unLoc) tvs
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    freshNames <- mkGenSyms names
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    term       <- addBinds freshNames $ do
		    bndrs <- mapM lookupBinder names 
		    m bndrs
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    wrapGenSyns freshNames term
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-- represent a type context
--
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repLContext :: LHsContext Name -> DsM (Core TH.CxtQ)
repLContext (L _ ctxt) = repContext ctxt

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repContext :: HsContext Name -> DsM (Core TH.CxtQ)
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repContext ctxt = do 
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	            preds    <- mapM repLPred ctxt
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		    predList <- coreList typeQTyConName preds
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		    repCtxt predList
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-- represent a type predicate
--
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repLPred :: LHsPred Name -> DsM (Core TH.TypeQ)
repLPred (L _ p) = repPred p

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repPred :: HsPred Name -> DsM (Core TH.TypeQ)
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repPred (HsClassP cls tys) = do
			       tcon <- repTy (HsTyVar cls)
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			       tys1 <- repLTys tys
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			       repTapps tcon tys1
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repPred p@(HsEqualP _ _) = notHandled "Equational constraint" (ppr p)
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repPred p@(HsIParam _ _) = notHandled "Implicit parameter constraint" (ppr p)
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-- yield the representation of a list of types
--
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repLTys :: [LHsType Name] -> DsM [Core TH.TypeQ]
repLTys tys = mapM repLTy tys
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-- represent a type
--
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repLTy :: LHsType Name -> DsM (Core TH.TypeQ)
repLTy (L _ ty) = repTy ty

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repTy :: HsType Name -> DsM (Core TH.TypeQ)
repTy (HsForAllTy _ tvs ctxt ty)  = 
  addTyVarBinds tvs $ \bndrs -> do
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    ctxt1  <- repLContext ctxt
    ty1    <- repLTy ty
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    bndrs1 <- coreList nameTyConName bndrs
    repTForall bndrs1 ctxt1 ty1
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repTy (HsTyVar n)
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  | isTvOcc (nameOccName n)       = do 
				      tv1 <- lookupBinder n
				      repTvar tv1
  | otherwise		          = do 
				      tc1 <- lookupOcc n
				      repNamedTyCon tc1
repTy (HsAppTy f a)               = do 
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				      f1 <- repLTy f
				      a1 <- repLTy a
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				      repTapp f1 a1
repTy (HsFunTy f a)               = do 
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				      f1   <- repLTy f
				      a1   <- repLTy a
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				      tcon <- repArrowTyCon
				      repTapps tcon [f1, a1]
repTy (HsListTy t)		  = do
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				      t1   <- repLTy t
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				      tcon <- repListTyCon
				      repTapp tcon t1
repTy (HsPArrTy t)                = do
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				      t1   <- repLTy t
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				      tcon <- repTy (HsTyVar (tyConName parrTyCon))
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				      repTapp tcon t1
repTy (HsTupleTy tc tys)	  = do
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				      tys1 <- repLTys tys 
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				      tcon <- repTupleTyCon (length tys)
				      repTapps tcon tys1
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repTy (HsOpTy ty1 n ty2) 	  = repLTy ((nlHsTyVar (unLoc n) `nlHsAppTy` ty1) 
					   `nlHsAppTy` ty2)
repTy (HsParTy t)  	       	  = repLTy t
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repTy (HsPredTy pred)             = repPred pred
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repTy ty@(HsNumTy _)              = notHandled "Number types (for generics)" (ppr ty)
repTy ty			  = notHandled "Exotic form of type" (ppr ty)
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-----------------------------------------------------------------------------
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-- 		Expressions
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-----------------------------------------------------------------------------
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repLEs :: [LHsExpr Name] -> DsM (Core [TH.ExpQ])
repLEs es = do { es'  <- mapM repLE es ;
		 coreList expQTyConName es' }
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-- FIXME: some of these panics should be converted into proper error messages
--	  unless we can make sure that constructs, which are plainly not
--	  supported in TH already lead to error messages at an earlier stage
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repLE :: LHsExpr Name -> DsM (Core TH.ExpQ)
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repLE (L loc e) = putSrcSpanDs loc (repE e)
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repE :: HsExpr Name -> DsM (Core TH.ExpQ)
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repE (HsVar x)            =
  do { mb_val <- dsLookupMetaEnv x 
     ; case mb_val of
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	Nothing	         -> do { str <- globalVar x
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			       ; repVarOrCon x str }
	Just (Bound y)   -> repVarOrCon x (coreVar y)
	Just (Splice e)  -> do { e' <- dsExpr e
			       ; return (MkC e') } }
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repE e@(HsIPVar x) = notHandled "Implicit parameters" (ppr e)
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	-- Remember, we're desugaring renamer output here, so
	-- HsOverlit can definitely occur
repE (HsOverLit l) = do { a <- repOverloadedLiteral l; repLit a }
repE (HsLit l)     = do { a <- repLiteral l;           repLit a }
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repE (HsLam (MatchGroup [m] _)) = repLambda m
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repE (HsApp x y)   = do {a <- repLE x; b <- repLE y; repApp a b}
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repE (OpApp e1 op fix e2) =
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  do { arg1 <- repLE e1; 
       arg2 <- repLE e2; 
       the_op <- repLE op ;
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       repInfixApp arg1 the_op arg2 } 
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repE (NegApp x nm)        = do
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			      a         <- repLE x
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			      negateVar <- lookupOcc negateName >>= repVar
			      negateVar `repApp` a
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repE (HsPar x)            = repLE x
repE (SectionL x y)       = do { a <- repLE x; b <- repLE y; repSectionL a b } 
repE (SectionR x y)       = do { a <- repLE x; b <- repLE y; repSectionR a b } 
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repE (HsCase e (MatchGroup ms _)) = do { arg <- repLE e
				       ; ms2 <- mapM repMatchTup ms
				       ; repCaseE arg (nonEmptyCoreList ms2) }
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repE (HsIf x y z)         = do
			      a <- repLE x
			      b <- repLE y
			      c <- repLE z
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			      repCond a b c
repE (HsLet bs e)         = do { (ss,ds) <- repBinds bs
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			       ; e2 <- addBinds ss (repLE e)
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			       ; z <- repLetE ds e2
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			       ; wrapGenSyns ss z }
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-- FIXME: I haven't got the types here right yet
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repE (HsDo DoExpr sts body ty) 
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 = do { (ss,zs) <- repLSts sts; 
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	body'	<- addBinds ss $ repLE body;
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	ret	<- repNoBindSt body';	
        e       <- repDoE (nonEmptyCoreList (zs ++ [ret]));
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        wrapGenSyns ss e }
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repE (HsDo ListComp sts body ty) 
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 = do { (ss,zs) <- repLSts sts; 
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	body'	<- addBinds ss $ repLE body;
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	ret	<- repNoBindSt body';	
        e       <- repComp (nonEmptyCoreList (zs ++ [ret]));
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        wrapGenSyns ss e }
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repE e@(HsDo _ _ _ _) = notHandled "mdo and [: :]" (ppr e)
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repE (ExplicitList ty es) = do { xs <- repLEs es; repListExp xs } 
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repE e@(ExplicitPArr ty es) = notHandled "Parallel arrays" (ppr e)
repE e@(ExplicitTuple es boxed) 
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  | isBoxed boxed         = do { xs <- repLEs es; repTup xs }
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  | otherwise		  = notHandled "Unboxed tuples" (ppr e)
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repE (RecordCon c _ flds)
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 = do { x <- lookupLOcc c;
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        fs <- repFields flds;
        repRecCon x fs }
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repE (RecordUpd e flds _ _)
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 = do { x <- repLE e;
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        fs <- repFields flds;
        repRecUpd x fs }
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repE (ExprWithTySig e ty) = do { e1 <- repLE e; t1 <- repLTy ty; repSigExp e1 t1 }
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repE (ArithSeq _ aseq) =
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  case aseq of
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    From e              -> do { ds1 <- repLE e; repFrom ds1 }
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    FromThen e1 e2      -> do 
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		             ds1 <- repLE e1
			     ds2 <- repLE e2
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			     repFromThen ds1 ds2
    FromTo   e1 e2      -> do 
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			     ds1 <- repLE e1
			     ds2 <- repLE e2
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			     repFromTo ds1 ds2
    FromThenTo e1 e2 e3 -> do 
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			     ds1 <- repLE e1
			     ds2 <- repLE e2
			     ds3 <- repLE e3
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			     repFromThenTo ds1 ds2 ds3
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repE (HsSpliceE (HsSplice n _)) 
  = do { mb_val <- dsLookupMetaEnv n
       ; case mb_val of
		 Just (Splice e) -> do { e' <- dsExpr e
				       ; return (MkC e') }
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		 other -> pprPanic "HsSplice" (ppr n) }
			-- Should not happen; statically checked
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repE e@(PArrSeq {})      = notHandled "Parallel arrays" (ppr e)
repE e@(HsCoreAnn {})    = notHandled "Core annotations" (ppr e)
repE e@(HsSCC {})        = notHandled "Cost centres" (ppr e)
repE e@(HsBracketOut {}) = notHandled "TH brackets" (ppr e)
repE e 			 = notHandled "Expression form" (ppr e)
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-----------------------------------------------------------------------------
-- Building representations of auxillary structures like Match, Clause, Stmt, 

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repMatchTup ::  LMatch Name -> DsM (Core TH.MatchQ) 
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repMatchTup (L _ (Match [p] ty (GRHSs guards wheres))) =
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  do { ss1 <- mkGenSyms (collectPatBinders p) 
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     ; addBinds ss1 $ do {
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     ; p1 <- repLP p
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     ; (ss2,ds) <- repBinds wheres
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     ; addBinds ss2 $ do {
     ; gs    <- repGuards guards
     ; match <- repMatch p1 gs ds
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     ; wrapGenSyns (ss1++ss2) match }}}
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repMatchTup other = panic "repMatchTup: case alt with more than one arg"
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repClauseTup ::  LMatch Name -> DsM (Core TH.ClauseQ)
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repClauseTup (L _ (Match ps ty (GRHSs guards wheres))) =
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  do { ss1 <- mkGenSyms (collectPatsBinders ps) 
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     ; addBinds ss1 $ do {
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       ps1 <- repLPs ps
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     ; (ss2,ds) <- repBinds wheres
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     ; addBinds ss2 $ do {
       gs <- repGuards guards
     ; clause <- repClause ps1 gs ds
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     ; wrapGenSyns (ss1++ss2) clause }}}
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repGuards ::  [LGRHS Name] ->  DsM (Core TH.BodyQ)
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repGuards [L _ (GRHS [] e)]
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  = do {a <- repLE e; repNormal a }
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repGuards other 
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  = do { zs <- mapM process other;
     let {(xs, ys) = unzip zs};
	 gd <- repGuarded (nonEmptyCoreList ys);
     wrapGenSyns (concat xs) gd }
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  where 
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    process :: LGRHS Name -> DsM ([GenSymBind], (Core (TH.Q (TH.Guard, TH.Exp))))
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    process (L _ (GRHS [L _ (ExprStmt e1 _ _)] e2))
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           = do { x <- repLNormalGE e1 e2;
                  return ([], x) }
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    process (L _ (GRHS ss rhs))
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           = do (gs, ss') <- repLSts ss
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		rhs' <- addBinds gs $ repLE rhs
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                g <- repPatGE (nonEmptyCoreList ss') rhs'
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                return (gs, g)
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repFields :: [(Located Name, LHsExpr Name)] -> DsM (Core [TH.Q TH.FieldExp])
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repFields flds = do
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        fnames <- mapM lookupLOcc (map fst flds)
        es <- mapM repLE (map snd flds)
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        fs <- zipWithM repFieldExp fnames es
        coreList fieldExpQTyConName fs
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-----------------------------------------------------------------------------
-- Representing Stmt's is tricky, especially if bound variables
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-- shadow each other. Consider:  [| do { x <- f 1; x <- f x; g x } |]
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-- First gensym new names for every variable in any of the patterns.
-- both static (x'1 and x'2), and dynamic ((gensym "x") and (gensym "y"))
-- if variables didn't shaddow, the static gensym wouldn't be necessary
-- and we could reuse the original names (x and x).
--
-- do { x'1 <- gensym "x"
--    ; x'2 <- gensym "x"   
--    ; doE [ BindSt (pvar x'1) [| f 1 |]
--          , BindSt (pvar x'2) [| f x |] 
--          , NoBindSt [| g x |] 
--          ]
--    }

-- The strategy is to translate a whole list of do-bindings by building a
-- bigger environment, and a bigger set of meta bindings 
-- (like:  x'1 <- gensym "x" ) and then combining these with the translations
-- of the expressions within the Do
      
-----------------------------------------------------------------------------
-- The helper function repSts computes the translation of each sub expression
-- and a bunch of prefix bindings denoting the dynamic renaming.

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repLSts :: [LStmt Name] -> DsM ([GenSymBind], [Core TH.StmtQ])
repLSts stmts = repSts (map unLoc stmts)

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repSts :: [Stmt Name] -> DsM ([GenSymBind], [Core TH.StmtQ])
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repSts (BindStmt p e _ _ : ss) =
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   do { e2 <- repLE e 
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      ; ss1 <- mkGenSyms (collectPatBinders p) 
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      ; addBinds ss1 $ do {
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      ; p1 <- repLP p; 
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      ; (ss2,zs) <- repSts ss
      ; z <- repBindSt p1 e2
      ; return (ss1++ss2, z : zs) }}
repSts (LetStmt bs : ss) =
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   do { (ss1,ds) <- repBinds bs
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      ; z <- repLetSt ds
      ; (ss2,zs) <- addBinds ss1 (repSts ss)
      ; return (ss1++ss2, z : zs) } 
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repSts (ExprStmt e _ _ : ss) =       
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   do { e2 <- repLE e
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      ; z <- repNoBindSt e2 
      ; (ss2,zs) <- repSts ss
      ; return (ss2, z : zs) }
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repSts []    = return ([],[])
repSts other = notHandled "Exotic statement" (ppr other)
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-----------------------------------------------------------
--			Bindings
-----------------------------------------------------------
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repBinds :: HsLocalBinds Name -> DsM ([GenSymBind], Core [TH.DecQ]) 
repBinds EmptyLocalBinds
  = do	{ core_list <- coreList decQTyConName []
	; return ([], core_list) }

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repBinds b@(HsIPBinds _) = notHandled "Implicit parameters" (ppr b)
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repBinds (HsValBinds decs)
 = do	{ let { bndrs = map unLoc (collectHsValBinders decs) }
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		-- No need to worrry about detailed scopes within
		-- the binding group, because we are talking Names
		-- here, so we can safely treat it as a mutually 
		-- recursive group
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	; ss        <- mkGenSyms bndrs
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	; prs       <- addBinds ss (rep_val_binds decs)
	; core_list <- coreList decQTyConName 
				(de_loc (sort_by_loc prs))
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	; return (ss, core_list) }
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rep_val_binds :: HsValBinds Name -> DsM [(SrcSpan, Core TH.DecQ)]
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-- Assumes: all the binders of the binding are alrady in the meta-env
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rep_val_binds (ValBindsOut binds sigs)
 = do { core1 <- rep_binds' (unionManyBags (map snd binds))
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      ;	core2 <- rep_sigs' sigs
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      ;	return (core1 ++ core2) }
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rep_val_binds (ValBindsOut binds sigs)
 = panic "rep_val_binds: ValBindsOut"
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rep_binds :: LHsBinds Name -> DsM [Core TH.DecQ]
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rep_binds binds = do { binds_w_locs <- rep_binds' binds
		     ; return (de_loc (sort_by_loc binds_w_locs)) }

rep_binds' :: LHsBinds Name -> DsM [(SrcSpan, Core TH.DecQ)]
rep_binds' binds = mapM rep_bind (bagToList binds)
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rep_bind :: LHsBind Name -> DsM (SrcSpan, Core TH.DecQ)
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-- Assumes: all the binders of the binding are alrady in the meta-env
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-- Note GHC treats declarations of a variable (not a pattern) 
-- e.g.  x = g 5 as a Fun MonoBinds. This is indicated by a single match 
-- with an empty list of patterns
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rep_bind (L loc (FunBind { fun_id = fn, 
			   fun_matches = MatchGroup [L _ (Match [] ty (GRHSs guards wheres))] _ }))
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 = do { (ss,wherecore) <- repBinds wheres
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	; guardcore <- addBinds ss (repGuards guards)
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	; fn'  <- lookupLBinder fn
	; p    <- repPvar fn'
	; ans  <- repVal p guardcore wherecore
	; ans' <- wrapGenSyns ss ans
	; return (loc, ans') }
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rep_bind (L loc (FunBind { fun_id = fn, fun_matches = MatchGroup ms _ }))
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 =   do { ms1 <- mapM repClauseTup ms
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	; fn' <- lookupLBinder fn
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        ; ans <- repFun fn' (nonEmptyCoreList ms1)
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rep_bind (L loc (PatBind { pat_lhs = pat, pat_rhs = GRHSs guards wheres }))
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        ; (ss,wherecore) <- repBinds wheres
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	; guardcore <- addBinds ss (repGuards guards)
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        ; ans  <- repVal patcore guardcore wherecore
	; ans' <- wrapGenSyns ss ans
        ; return (loc, ans') }
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rep_bind (L loc (VarBind { var_id = v, var_rhs = e}))
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	; e2 <- repLE e
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        ; x <- repNormal e2
        ; patcore <- repPvar v'
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	; empty_decls <- coreList decQTyConName [] 
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        ; ans <- repVal patcore x empty_decls
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        ; return (srcLocSpan (getSrcLoc v), ans) }
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rep_bind other = panic "rep_bind: AbsBinds"

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-----------------------------------------------------------------------------
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-- Since everything in a Bind is mutually recursive we need rename all
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-- all the variables simultaneously. For example: 
-- [| AndMonoBinds (f x = x + g 2) (g x = f 1 + 2) |] would translate to
-- do { f'1 <- gensym "f"
--    ; g'2 <- gensym "g"
--    ; [ do { x'3 <- gensym "x"; fun f'1 [pvar x'3] [| x + g2 |]},
--        do { x'4 <- gensym "x"; fun g'2 [pvar x'4] [| f 1 + 2 |]}
--      ]}
-- This requires collecting the bindings (f'1 <- gensym "f"), and the 
-- environment ( f |-> f'1 ) from each binding, and then unioning them 
-- together. As we do this we collect GenSymBinds's which represent the renamed 
-- variables bound by the Bindings. In order not to lose track of these 
-- representations we build a shadow datatype MB with the same structure as 
-- MonoBinds, but which has slots for the representations


-----------------------------------------------------------------------------
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-- GHC allows a more general form of lambda abstraction than specified
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-- by Haskell 98. In particular it allows guarded lambda's like : 
-- (\  x | even x -> 0 | odd x -> 1) at the moment we can't represent this in
-- Haskell Template's Meta.Exp type so we punt if it isn't a simple thing like
-- (\ p1 .. pn -> exp) by causing an error.  

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repLambda :: LMatch Name -> DsM (Core TH.ExpQ)
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repLambda (L _ (Match ps _ (GRHSs [L _ (GRHS [] e)] EmptyLocalBinds)))
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 = do { let bndrs = collectPatsBinders ps ;
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      ; ss  <- mkGenSyms bndrs
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      ; lam <- addBinds ss (
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		do { xs <- repLPs ps; body <- repLE e; repLam xs body })
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      ; wrapGenSyns ss lam }
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repLambda (L _ m) = notHandled "Guarded labmdas" (pprMatch LambdaExpr m)
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-----------------------------------------------------------------------------
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--			Patterns
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-- repP deals with patterns.  It assumes that we have already
-- walked over the pattern(s) once to collect the binders, and 
-- have extended the environment.  So every pattern-bound 
-- variable should already appear in the environment.

-- Process a list of patterns
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repLPs :: [LPat Name] -> DsM (Core [TH.PatQ])
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repLPs ps = do { ps' <- mapM repLP ps ;
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		 coreList patQTyConName ps' }
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repLP :: LPat Name -> DsM (Core TH.PatQ)
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repLP (L _ p) = repP p
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repP :: Pat Name -> DsM (Core TH.PatQ)
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repP (WildPat _)       = repPwild 
repP (LitPat l)        = do { l2 <- repLiteral l; repPlit l2 }
repP (VarPat x)        = do { x' <- lookupBinder x; repPvar x' }
repP (LazyPat p)       = do { p1 <- repLP p; repPtilde p1 }
repP (AsPat x p)       = do { x' <- lookupLBinder x; p1 <- repLP p; repPaspat x' p1 }
repP (ParPat p)        = repLP p 
repP (ListPat ps _)    = do { qs <- repLPs ps; repPlist qs }
repP (TuplePat ps _ _) = do { qs <- repLPs ps; repPtup qs }
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repP (ConPatIn dc details)
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 = do { con_str <- lookupLOcc dc
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      ; case details of
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         PrefixCon ps   -> do { qs <- repLPs ps; repPcon con_str qs }
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         RecCon pairs -> do { vs <- sequence $ map lookupLOcc (map hsRecFieldId pairs)
                            ; ps <- sequence $ map repLP (map hsRecFieldArg pairs)
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                            ; fps <- zipWithM (\x y -> rep2 fieldPatName [unC x,unC y]) vs ps
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                            ; fps' <- coreList fieldPatQTyConName fps
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                            ; repPrec con_str fps' }
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         InfixCon p1 p2 -> do { p1' <- repLP p1;
                                p2' <- repLP p2;
                                repPinfix p1' con_str p2' }
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   }
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repP (NPat l Nothing _ _)  = do { a <- repOverloadedLiteral l; repPlit a }
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repP p@(NPat l (Just _) _ _) = notHandled "Negative overloaded patterns" (ppr p)
repP p@(SigPatIn {})  = notHandled "Type signatures in patterns" (ppr p)
	-- The problem is to do with scoped type variables.
	-- To implement them, we have to implement the scoping rules
	-- here in DsMeta, and I don't want to do that today!
	--	 do { p' <- repLP p; t' <- repLTy t; repPsig p' t' }
	--	repPsig :: Core TH.PatQ -> Core TH.TypeQ -> DsM (Core TH.PatQ)
	--	repPsig (MkC p) (MkC t) = rep2 sigPName [p, t]

repP other = notHandled "Exotic pattern" (ppr other)
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----------------------------------------------------------
-- Declaration ordering helpers

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sort_by_loc :: [(SrcSpan, a)] -> [(SrcSpan, a)]
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sort_by_loc xs = sortBy comp xs
    where comp x y = compare (fst x) (fst y)

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de_loc :: [(a, b)] -> [b]
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de_loc = map snd

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----------------------------------------------------------
--	The meta-environment

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-- A name/identifier association for fresh names of locally bound entities
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type GenSymBind = (Name, Id)	-- Gensym the string and bind it to the Id
				-- I.e.		(x, x_id) means
				--	let x_id = gensym "x" in ...

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-- Generate a fresh name for a locally bound entity
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mkGenSyms :: [Name] -> DsM [GenSymBind]
-- We can use the existing name.  For example:
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--	[| \x_77 -> x_77 + x_77 |]
-- desugars to
--	do { x_77 <- genSym "x"; .... }
-- We use the same x_77 in the desugared program, but with the type Bndr
-- instead of Int
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--
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-- We do make it an Internal name, though (hence localiseName)
--
-- Nevertheless, it's monadic because we have to generate nameTy
mkGenSyms ns = do { var_ty <- lookupType nameTyConName
		  ; return [(nm, mkLocalId (localiseName nm) var_ty) | nm <- ns] }

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addBinds :: [GenSymBind] -> DsM a -> DsM a
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-- Add a list of fresh names for locally bound entities to the 
-- meta environment (which is part of the state carried around 
-- by the desugarer monad) 
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addBinds bs m = dsExtendMetaEnv (mkNameEnv [(n,Bound id) | (n,id) <- bs]) m

-- Look up a locally bound name
--
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lookupLBinder :: Located Name -> DsM (Core TH.Name)
lookupLBinder (L _ n) = lookupBinder n

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lookupBinder :: Name -> DsM (Core TH.Name)
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lookupBinder n 
  = do { mb_val <- dsLookupMetaEnv n;
	 case mb_val of
	    Just (Bound x) -> return (coreVar x)
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	    other	   -> failWithDs msg }
  where
    msg = ptext SLIT("DsMeta: failed binder lookup when desugaring a TH bracket:") <+> ppr n
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-- Look up a name that is either locally bound or a global name
--
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</