%
\begin{code}
-module StixPrim ( primCode, amodeToStix, amodeToStix' ) where
+module StixPrim ( amodeToStix, amodeToStix', foreignCallCode )
+where
#include "HsVersions.h"
-import MachMisc
-import MachRegs
+-- import MachMisc
import Stix
-import StixInteger
+import PprAbsC ( pprAmode )
import AbsCSyn hiding ( spRel )
import AbsCUtils ( getAmodeRep, mixedTypeLocn )
-import Constants ( uF_UPDATEE )
import SMRep ( fixedHdrSize )
-import Const ( Literal(..) )
-import CallConv ( cCallConv )
-import PrimOp ( PrimOp(..) )
-import PrimRep ( PrimRep(..), isFloatingRep )
-import UniqSupply ( returnUs, thenUs, UniqSM )
-import Constants ( mIN_INTLIKE )
+import Literal ( Literal(..), word2IntLit )
+import MachOp ( MachOp(..) )
+import PrimRep ( PrimRep(..), getPrimRepSizeInBytes )
+import UniqSupply ( returnUs, thenUs, getUniqueUs, UniqSM )
+import Constants ( wORD_SIZE,
+ mIN_INTLIKE, mIN_CHARLIKE, uF_UPDATEE, bLOCK_SIZE,
+ rESERVED_STACK_WORDS )
+import CLabel ( mkIntlikeClosureLabel, mkCharlikeClosureLabel,
+ mkMAP_FROZEN_infoLabel,
+ mkForeignLabel )
+import ForeignCall ( ForeignCall(..), CCallSpec(..), CCallTarget(..),
+ CCallConv(..), playSafe, playThreadSafe )
import Outputable
+import Util ( notNull )
+import FastString
+import FastTypes
-import Char ( ord )
+#include "NCG.h"
\end{code}
-The main honcho here is primCode, which handles the guts of COpStmts.
+The main honchos here are primCode and foreignCallCode, which handle the guts of COpStmts.
\begin{code}
-primCode
+foreignCallCode
:: [CAddrMode] -- results
- -> PrimOp -- op
+ -> ForeignCall -- op
-> [CAddrMode] -- args
- -> UniqSM StixTreeList
+ -> UniqSM StixStmtList
\end{code}
+%************************************************************************
+%* *
+\subsubsection{Code for foreign calls}
+%* *
+%************************************************************************
+
First, the dreaded @ccall@. We can't handle @casm@s.
Usually, this compiles to an assignment, but when the left-hand side
btw Why not let programmer use casm to provide assembly code instead
of C code? ADR
-The (MP) integer operations are a true nightmare. Since we don't have
-a convenient abstract way of allocating temporary variables on the (C)
-stack, we use the space just below HpLim for the @MP_INT@ structures,
-and modify our heap check accordingly.
-
-\begin{code}
--- NB: ordering of clauses somewhere driven by
--- the desire to getting sane patt-matching behavior
-primCode res@[ar,sr,dr] IntegerNegOp arg@[aa,sa,da]
- = gmpNegate (ar,sr,dr) (aa,sa,da)
-\end{code}
-
-\begin{code}
-primCode [res] IntegerCmpOp args@[aa1,sa1,da1, aa2,sa2,da2]
- = gmpCompare res (aa1,sa1,da1, aa2,sa2,da2)
-
-primCode [res] Integer2IntOp arg@[aa,sa,da]
- = gmpInteger2Int res (aa,sa,da)
-
-primCode [res] Integer2WordOp arg@[aa,sa,da]
- = gmpInteger2Word res (aa,sa,da)
-
-primCode [res] Int2AddrOp [arg]
- = simpleCoercion AddrRep res arg
-
-primCode [res] Addr2IntOp [arg]
- = simpleCoercion IntRep res arg
-
-primCode [res] Int2WordOp [arg]
- = simpleCoercion IntRep{-WordRep?-} res arg
-
-primCode [res] Word2IntOp [arg]
- = simpleCoercion IntRep res arg
-\end{code}
-
-\begin{code}
-primCode [res] SameMutableArrayOp args
- = let
- compare = StPrim AddrEqOp (map amodeToStix args)
- assign = StAssign IntRep (amodeToStix res) compare
- in
- returnUs (\xs -> assign : xs)
-
-primCode res@[_] SameMutableByteArrayOp args
- = primCode res SameMutableArrayOp args
-\end{code}
-
-Freezing an array of pointers is a double assignment. We fix the
-header of the ``new'' closure because the lhs is probably a better
-addressing mode for the indirection (most likely, it's a VanillaReg).
-
-\begin{code}
-
-primCode [lhs] UnsafeFreezeArrayOp [rhs]
- = let
- lhs' = amodeToStix lhs
- rhs' = amodeToStix rhs
- header = StInd PtrRep lhs'
- assign = StAssign PtrRep lhs' rhs'
- freeze = StAssign PtrRep header mutArrPtrsFrozen_info
- in
- returnUs (\xs -> assign : freeze : xs)
-
-primCode [lhs] UnsafeFreezeByteArrayOp [rhs]
- = simpleCoercion PtrRep lhs rhs
-primCode [lhs] UnsafeThawByteArrayOp [rhs]
- = simpleCoercion PtrRep lhs rhs
-\end{code}
+ToDo: saving/restoring of volatile regs around ccalls.
-Returning the size of (mutable) byte arrays is just
-an indexing operation.
+JRS, 001113: always do the call of suspendThread and resumeThread as a ccall
+rather than inheriting the calling convention of the thing which we're really
+calling.
\begin{code}
-primCode [lhs] SizeofByteArrayOp [rhs]
- = let
- lhs' = amodeToStix lhs
- rhs' = amodeToStix rhs
- sz = StIndex IntRep rhs' fixedHS
- assign = StAssign IntRep lhs' (StInd IntRep sz)
- in
- returnUs (\xs -> assign : xs)
-
-primCode [lhs] SizeofMutableByteArrayOp [rhs]
- = let
- lhs' = amodeToStix lhs
- rhs' = amodeToStix rhs
- sz = StIndex IntRep rhs' fixedHS
- assign = StAssign IntRep lhs' (StInd IntRep sz)
- in
- returnUs (\xs -> assign : xs)
+foreignCallCode lhs call@(CCall (CCallSpec ctarget cconv safety)) rhs
-\end{code}
-
-Most other array primitives translate to simple indexing.
+ | not (playSafe safety)
+ = returnUs (\xs -> ccall : xs)
-\begin{code}
-primCode lhs@[_] IndexArrayOp args
- = primCode lhs ReadArrayOp args
-
-primCode [lhs] ReadArrayOp [obj, ix]
- = let
- lhs' = amodeToStix lhs
- obj' = amodeToStix obj
- ix' = amodeToStix ix
- base = StIndex IntRep obj' arrHS
- assign = StAssign PtrRep lhs' (StInd PtrRep (StIndex PtrRep base ix'))
- in
- returnUs (\xs -> assign : xs)
-
-primCode [] WriteArrayOp [obj, ix, v]
- = let
- obj' = amodeToStix obj
- ix' = amodeToStix ix
- v' = amodeToStix v
- base = StIndex IntRep obj' arrHS
- assign = StAssign PtrRep (StInd PtrRep (StIndex PtrRep base ix')) v'
- in
- returnUs (\xs -> assign : xs)
-
-primCode lhs@[_] (IndexByteArrayOp pk) args
- = primCode lhs (ReadByteArrayOp pk) args
-
--- NB: indexing in "pk" units, *not* in bytes (WDP 95/09)
-
-primCode [lhs] (ReadByteArrayOp pk) [obj, ix]
- = let
- lhs' = amodeToStix lhs
- obj' = amodeToStix obj
- ix' = amodeToStix ix
- base = StIndex IntRep obj' arrHS
- assign = StAssign pk lhs' (StInd pk (StIndex pk base ix'))
- in
- returnUs (\xs -> assign : xs)
-
-primCode [lhs] (IndexOffAddrOp pk) [obj, ix]
- = let
- lhs' = amodeToStix lhs
- obj' = amodeToStix obj
- ix' = amodeToStix ix
- assign = StAssign pk lhs' (StInd pk (StIndex pk obj' ix'))
- in
- returnUs (\xs -> assign : xs)
-
-primCode [lhs] (IndexOffForeignObjOp pk) [obj, ix]
- = let
- lhs' = amodeToStix lhs
- obj' = amodeToStix obj
- ix' = amodeToStix ix
- obj'' = StIndex PtrRep obj' fixedHS
- assign = StAssign pk lhs' (StInd pk (StIndex pk obj'' ix'))
- in
- returnUs (\xs -> assign : xs)
-
-primCode [] (WriteByteArrayOp pk) [obj, ix, v]
- = let
- obj' = amodeToStix obj
- ix' = amodeToStix ix
- v' = amodeToStix v
- base = StIndex IntRep obj' arrHS
- assign = StAssign pk (StInd pk (StIndex pk base ix')) v'
- in
- returnUs (\xs -> assign : xs)
-\end{code}
-
-\begin{code}
---primCode lhs (CCallOp fn is_asm may_gc) rhs
-primCode lhs (CCallOp (Left fn) is_asm may_gc cconv) rhs
- | is_asm = error "ERROR: Native code generator can't handle casm"
- | may_gc = error "ERROR: Native code generator can't handle _ccall_GC_\n"
| otherwise
- = case lhs of
- [] -> returnUs (\xs -> (StCall fn cconv VoidRep args) : xs)
- [lhs] ->
- let lhs' = amodeToStix lhs
- pk = if isFloatingRep (getAmodeRep lhs) then DoubleRep else IntRep
- call = StAssign pk lhs' (StCall fn cconv pk args)
- in
- returnUs (\xs -> call : xs)
- where
- args = map amodeCodeForCCall rhs
- amodeCodeForCCall x =
- let base = amodeToStix' x
- in
- case getAmodeRep x of
- ArrayRep -> StIndex PtrRep base arrHS
- ByteArrayRep -> StIndex IntRep base arrHS
- ForeignObjRep -> StIndex PtrRep base fixedHS
- _ -> base
-\end{code}
-
-DataToTagOp won't work for 64-bit archs, as it is.
-
-\begin{code}
-primCode [lhs] DataToTagOp [arg]
- = let lhs' = amodeToStix lhs
- arg' = amodeToStix arg
- infoptr = StInd PtrRep arg'
- word_32 = StInd WordRep (StIndex PtrRep infoptr (StInt (-1)))
- masked_le32 = StPrim SrlOp [word_32, StInt 16]
- masked_be32 = StPrim AndOp [word_32, StInt 65535]
-#ifdef WORDS_BIGENDIAN
- masked = masked_be32
-#else
- masked = masked_le32
-#endif
- assign = StAssign IntRep lhs' masked
+ = save_thread_state `thenUs` \ save ->
+ load_thread_state `thenUs` \ load ->
+ getUniqueUs `thenUs` \ uniq ->
+ let
+ id = StixTemp (StixVReg uniq IntRep)
+
+ is_threadSafe
+ | playThreadSafe safety = 1
+ | otherwise = 0
+
+ suspend = StAssignReg IntRep id
+ (StCall (Left FSLIT("suspendThread")) {-no:cconv-} CCallConv
+ IntRep [StReg stgBaseReg, StInt is_threadSafe ])
+ resume = StVoidable
+ (StCall (Left FSLIT("resumeThread")) {-no:cconv-} CCallConv
+ VoidRep [StReg id, StInt is_threadSafe ])
in
- returnUs (\xs -> assign : xs)
-\end{code}
-
-Now the more mundane operations.
-
-\begin{code}
-primCode lhs op rhs
- = let
- lhs' = map amodeToStix lhs
- rhs' = map amodeToStix' rhs
- pk = getAmodeRep (head lhs)
- in
- returnUs (\ xs -> simplePrim pk lhs' op rhs' : xs)
-\end{code}
-
-\begin{code}
-simpleCoercion
- :: PrimRep
- -> CAddrMode
- -> CAddrMode
- -> UniqSM StixTreeList
-
-simpleCoercion pk lhs rhs
- = returnUs (\xs -> StAssign pk (amodeToStix lhs) (amodeToStix rhs) : xs)
-\end{code}
-
-Here we try to rewrite primitives into a form the code generator can
-understand. Any primitives not handled here must be handled at the
-level of the specific code generator.
-
-\begin{code}
-simplePrim
- :: PrimRep -- Rep of first destination
- -> [StixTree] -- Destinations
- -> PrimOp
- -> [StixTree]
- -> StixTree
-\end{code}
+ returnUs (\xs -> save (suspend : ccall : resume : load xs))
-Now look for something more conventional.
-
-\begin{code}
-simplePrim pk [lhs] op rest = StAssign pk lhs (StPrim op rest)
-simplePrim pk as op bs = simplePrim_error op
-
-simplePrim_error op
- = error ("ERROR: primitive operation `"++show op++"'cannot be handled\nby the native-code generator. Workaround: use -fvia-C.\n(Perhaps you should report it as a GHC bug, also.)\n")
+ where
+ (cargs, stix_target)
+ = case ctarget of
+ StaticTarget nm -> (rhs, Left nm)
+ DynamicTarget | notNull rhs -- an assertion
+ -> (tail rhs, Right (amodeToStix (head rhs)))
+ CasmTarget _
+ -> ncgPrimopMoan "Native code generator can't handle foreign call"
+ (ppr call)
+
+ stix_args = map amodeToStix' cargs
+
+ ccall = case lhs of
+ [] -> StVoidable (StCall stix_target cconv VoidRep stix_args)
+ [lhs] -> mkStAssign pk lhs' (StCall stix_target cconv pk stix_args)
+ where
+ lhs' = amodeToStix lhs
+ pk = case getAmodeRep lhs of
+ FloatRep -> FloatRep
+ DoubleRep -> DoubleRep
+ Int64Rep -> Int64Rep
+ Word64Rep -> Word64Rep
+ other -> IntRep
+
+-- a bit late to catch this here..
+foreignCallCode _ DNCall{} _
+ = panic "foreignCallCode: .NET interop not supported via NCG; compile with -fvia-C"
\end{code}
-%---------------------------------------------------------------------
-
-Here we generate the Stix code for CAddrModes.
+%************************************************************************
+%* *
+\subsubsection{Code for @CAddrMode@s}
+%* *
+%************************************************************************
When a character is fetched from a mixed type location, we have to do
an extra cast. This is reflected in amodeCode', which is for rhs
amodes that might possibly need the extra cast.
\begin{code}
-amodeToStix, amodeToStix' :: CAddrMode -> StixTree
+amodeToStix, amodeToStix' :: CAddrMode -> StixExpr
amodeToStix'{-'-} am@(CVal rr CharRep)
- | mixedTypeLocn am = StPrim ChrOp [amodeToStix am]
- | otherwise = amodeToStix am
-
-amodeToStix' am = amodeToStix am
+ | mixedTypeLocn am = StMachOp MO_NatS_to_32U [amodeToStix am]
+ | otherwise = amodeToStix am
+amodeToStix' am
+ = amodeToStix am
-----------
amodeToStix am@(CVal rr CharRep)
amodeToStix (CVal rr pk) = StInd pk (amodeToStix (CAddr rr))
amodeToStix (CAddr (SpRel off))
- = StIndex PtrRep stgSp (StInt (toInteger IBOX(off)))
+ = StIndex PtrRep (StReg stgSp) (StInt (toInteger (iBox off)))
amodeToStix (CAddr (HpRel off))
- = StIndex IntRep stgHp (StInt (toInteger (- IBOX(off))))
+ = StIndex IntRep (StReg stgHp) (StInt (toInteger (- (iBox off))))
amodeToStix (CAddr (NodeRel off))
- = StIndex IntRep stgNode (StInt (toInteger IBOX(off)))
+ = StIndex IntRep (StReg stgNode) (StInt (toInteger (iBox off)))
amodeToStix (CAddr (CIndex base off pk))
= StIndex pk (amodeToStix base) (amodeToStix off)
amodeToStix (CReg magic) = StReg (StixMagicId magic)
-amodeToStix (CTemp uniq pk) = StReg (StixTemp uniq pk)
+amodeToStix (CTemp uniq pk) = StReg (StixTemp (StixVReg uniq pk))
amodeToStix (CLbl lbl _) = StCLbl lbl
-- For CharLike and IntLike, we attempt some trivial constant-folding here.
amodeToStix (CCharLike (CLit (MachChar c)))
- = StLitLbl ((<>) (ptext SLIT("CHARLIKE_closure+")) (int off))
+ = StIndex Word8Rep cHARLIKE_closure (StInt (toInteger off))
where
- off = charLikeSize * ord c
+ off = charLikeSize * (c - mIN_CHARLIKE)
amodeToStix (CCharLike x)
- = StIndex PtrRep charLike off
- where
- off = StPrim IntMulOp [amodeToStix x, StInt (toInteger charLikeSize)]
+ = panic "amodeToStix.CCharLike"
-amodeToStix (CIntLike (CLit (MachInt i _)))
- = StLitLbl ((<>) (ptext SLIT("INTLIKE_closure+")) (int off))
+amodeToStix (CIntLike (CLit (MachInt i)))
+ = StIndex Word8Rep iNTLIKE_closure (StInt (toInteger off))
where
off = intLikeSize * (fromInteger (i - mIN_INTLIKE))
amodeToStix (CIntLike x)
- = panic "CIntLike"
+ = panic "amodeToStix.CIntLike"
amodeToStix (CLit core)
= case core of
- MachChar c -> StInt (toInteger (ord c))
+ MachChar c -> StInt (toInteger c)
MachStr s -> StString s
- MachAddr a -> StInt a
- MachInt i _ -> StInt (toInteger i)
- MachLitLit s _ -> {-trace (_UNPK_ s ++ "\n")-} (litLitToStix (_UNPK_ s))
- MachFloat d -> StDouble d
+ MachNullAddr -> StInt 0
+ MachInt i -> StInt i
+ MachWord w -> case word2IntLit core of MachInt iw -> StInt iw
+ MachLitLit s _ -> litLitErr
+ -- dreadful, but rare.
+ MachLabel l (Just x) -> StCLbl (mkForeignLabel (mkFastString (unpackFS l ++ '@':show x)) False)
+ MachLabel l _ -> StCLbl (mkForeignLabel l False{-ToDo: dynamic-})
+ MachFloat d -> StFloat d
MachDouble d -> StDouble d
_ -> panic "amodeToStix:core literal"
-amodeToStix (CLitLit s _)
- = litLitToStix (_UNPK_ s)
-
amodeToStix (CMacroExpr _ macro [arg])
- = case macro of
- ENTRY_CODE -> amodeToStix arg
- ARG_TAG -> amodeToStix arg -- just an integer no. of words
- GET_TAG -> StPrim SrlOp
- [StInd WordRep (StPrim IntSubOp [amodeToStix arg,
- StInt 1]),
+ = let
+ arg_amode = amodeToStix arg
+ in
+ case macro of
+ ENTRY_CODE -> arg_amode
+ ARG_TAG -> arg_amode -- just an integer no. of words
+ GET_TAG ->
+#ifdef WORDS_BIGENDIAN
+ StMachOp MO_Nat_And
+ [StInd WordRep (StIndex PtrRep arg_amode
+ (StInt (toInteger (-1)))),
+ StInt 65535]
+#else
+ StMachOp MO_Nat_Shr
+ [StInd WordRep (StIndex PtrRep arg_amode
+ (StInt (toInteger (-1)))),
StInt 16]
+#endif
UPD_FRAME_UPDATEE
- -> StInd PtrRep (StIndex PtrRep (amodeToStix arg)
+ -> StInd PtrRep (StIndex PtrRep arg_amode
(StInt (toInteger uF_UPDATEE)))
--- XXX!!!
--- GET_TAG(info_ptr) is supposed to be get_itbl(info_ptr)->srt_len,
--- which we've had to hand-code here.
-
-litLitToStix :: String -> StixTree
-litLitToStix nm
- = case nm of
- "stdout" -> stixFor_stdout
- "stderr" -> stixFor_stderr
- "stdin" -> stixFor_stdin
- other -> error ("\nlitLitToStix: can't handle `" ++ nm ++ "'\n"
- ++ "suggested workaround: use flag -fvia-C\n")
+
+ BYTE_ARR_CTS -> StIndex IntRep arg_amode arrWordsHS
+ PTRS_ARR_CTS -> StIndex PtrRep arg_amode arrPtrsHS
+ ForeignObj_CLOSURE_DATA -> StInd PtrRep (StIndex PtrRep arg_amode fixedHS)
+
+
+amodeToStix other
+ = pprPanic "StixPrim.amodeToStix" (pprAmode other)
+
+litLitErr
+ = ncgPrimopMoan "native code generator can't handle lit-lits" empty
\end{code}
Sizes of the CharLike and IntLike closures that are arranged as arrays
\begin{code}
-- The INTLIKE base pointer
-intLikePtr :: StixTree
-
-intLikePtr = StInd PtrRep (sStLitLbl SLIT("INTLIKE_closure"))
+iNTLIKE_closure :: StixExpr
+iNTLIKE_closure = StCLbl mkIntlikeClosureLabel
-- The CHARLIKE base
-charLike :: StixTree
-
-charLike = sStLitLbl SLIT("CHARLIKE_closure")
-
--- Trees for the ErrorIOPrimOp
+cHARLIKE_closure :: StixExpr
+cHARLIKE_closure = StCLbl mkCharlikeClosureLabel
-topClosure, errorIO :: StixTree
+mutArrPtrsFrozen_info = StCLbl mkMAP_FROZEN_infoLabel
-topClosure = StInd PtrRep (sStLitLbl SLIT("TopClosure"))
-errorIO = StJump (StInd PtrRep (sStLitLbl SLIT("ErrorIO_innards")))
+-- these are the sizes of charLike and intLike closures, in _bytes_.
+charLikeSize = (fixedHdrSize + 1) * (getPrimRepSizeInBytes PtrRep)
+intLikeSize = (fixedHdrSize + 1) * (getPrimRepSizeInBytes PtrRep)
+\end{code}
-mutArrPtrsFrozen_info = sStLitLbl SLIT("MUT_ARR_PTRS_FROZEN_info")
-charLikeSize = (fixedHdrSize + 1) * (fromInteger (sizeOf PtrRep))
-intLikeSize = (fixedHdrSize + 1) * (fromInteger (sizeOf PtrRep))
+\begin{code}
+save_thread_state
+ = getUniqueUs `thenUs` \ tso_uq ->
+ let tso = StixTemp (StixVReg tso_uq PtrRep) in
+ returnUs (\xs ->
+ StAssignReg PtrRep tso (StReg stgCurrentTSO)
+ : StAssignMem PtrRep
+ (StMachOp MO_Nat_Add
+ [StReg tso, StInt (toInteger (TSO_SP*BYTES_PER_WORD))])
+ (StReg stgSp)
+ : StAssignMem PtrRep
+ (StMachOp MO_Nat_Add
+ [StReg stgCurrentNursery,
+ StInt (toInteger (BDESCR_FREE * BYTES_PER_WORD))])
+ (StMachOp MO_Nat_Add
+ [StReg stgHp, StInt (toInteger (1 * BYTES_PER_WORD))])
+ : xs
+ )
+
+load_thread_state
+ = getUniqueUs `thenUs` \ tso_uq ->
+ let tso = StixTemp (StixVReg tso_uq PtrRep) in
+ returnUs (\xs ->
+ StAssignReg PtrRep tso (StReg stgCurrentTSO)
+ : StAssignReg PtrRep
+ stgSp
+ (StInd PtrRep
+ (StMachOp MO_Nat_Add
+ [StReg tso, StInt (toInteger (TSO_SP*BYTES_PER_WORD))]))
+ : StAssignReg PtrRep
+ stgSpLim
+ (StMachOp MO_Nat_Add
+ [StReg tso,
+ StInt (toInteger ((TSO_STACK + rESERVED_STACK_WORDS)
+ *BYTES_PER_WORD))])
+ : StAssignReg PtrRep
+ stgHp
+ (StMachOp MO_Nat_Sub
+ [StInd PtrRep
+ (StMachOp MO_Nat_Add
+ [StReg stgCurrentNursery,
+ StInt (toInteger (BDESCR_FREE * BYTES_PER_WORD))]),
+ StInt (toInteger (1 * BYTES_PER_WORD))
+ ])
+ : StAssignReg PtrRep
+ stgHpLim
+ (StIndex Word8Rep
+ (StInd PtrRep
+ (StIndex PtrRep (StReg stgCurrentNursery)
+ (StInt (toInteger BDESCR_START))
+ )
+ )
+ (StMachOp MO_Nat_Sub
+ [StMachOp MO_NatU_Mul
+ [StInd WordRep
+ (StIndex PtrRep (StReg stgCurrentNursery)
+ (StInt (toInteger BDESCR_BLOCKS))),
+ StInt (toInteger bLOCK_SIZE{-in bytes-})
+ ],
+ StInt (1 * BYTES_PER_WORD)
+ ]
+ )
+
+ )
+
+ : xs
+ )
\end{code}