%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
Utilities for desugaring
This module exports some utility functions of no great interest.
\begin{code}
module DsUtils (
EquationInfo(..),
firstPat, shiftEqns,
MatchResult(..), CanItFail(..), CaseAlt(..),
cantFailMatchResult, alwaysFailMatchResult,
extractMatchResult, combineMatchResults,
adjustMatchResult, adjustMatchResultDs,
mkCoLetMatchResult, mkViewMatchResult, mkGuardedMatchResult,
matchCanFail, mkEvalMatchResult,
mkCoPrimCaseMatchResult, mkCoAlgCaseMatchResult, mkCoSynCaseMatchResult,
wrapBind, wrapBinds,
mkErrorAppDs, mkCoreAppDs, mkCoreAppsDs,
seqVar,
mkLHsVarPatTup, mkLHsPatTup, mkVanillaTuplePat,
mkBigLHsVarTup, mkBigLHsTup, mkBigLHsVarPatTup, mkBigLHsPatTup,
mkSelectorBinds,
selectSimpleMatchVarL, selectMatchVars, selectMatchVar,
mkOptTickBox, mkBinaryTickBox
) where
#include "HsVersions.h"
import Match ( matchSimply )
import HsSyn
import TcHsSyn
import TcType( tcSplitTyConApp )
import CoreSyn
import DsMonad
import DsExpr ( dsLExpr )
import CoreUtils
import MkCore
import MkId
import Id
import Literal
import TyCon
import ConLike
import DataCon
import PatSyn
import Type
import Coercion
import TysPrim
import TysWiredIn
import BasicTypes
import UniqSet
import UniqSupply
import Module
import PrelNames
import Outputable
import SrcLoc
import Util
import DynFlags
import FastString
import TcEvidence
import Control.Monad ( zipWithM )
\end{code}
%************************************************************************
%* *
\subsection{ Selecting match variables}
%* *
%************************************************************************
We're about to match against some patterns. We want to make some
@Ids@ to use as match variables. If a pattern has an @Id@ readily at
hand, which should indeed be bound to the pattern as a whole, then use it;
otherwise, make one up.
\begin{code}
selectSimpleMatchVarL :: LPat Id -> DsM Id
selectSimpleMatchVarL pat = selectMatchVar (unLoc pat)
selectMatchVars :: [Pat Id] -> DsM [Id]
selectMatchVars ps = mapM selectMatchVar ps
selectMatchVar :: Pat Id -> DsM Id
selectMatchVar (BangPat pat) = selectMatchVar (unLoc pat)
selectMatchVar (LazyPat pat) = selectMatchVar (unLoc pat)
selectMatchVar (ParPat pat) = selectMatchVar (unLoc pat)
selectMatchVar (VarPat var) = return (localiseId var)
selectMatchVar (AsPat var _) = return (unLoc var)
selectMatchVar other_pat = newSysLocalDs (hsPatType other_pat)
\end{code}
Note [Localise pattern binders]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider module M where
[Just a] = e
After renaming it looks like
module M where
[Just M.a] = e
We don't generalise, since it's a pattern binding, monomorphic, etc,
so after desugaring we may get something like
M.a = case e of (v:_) ->
case v of Just M.a -> M.a
Notice the "M.a" in the pattern; after all, it was in the original
pattern. However, after optimisation those pattern binders can become
let-binders, and then end up floated to top level. They have a
different *unique* by then (the simplifier is good about maintaining
proper scoping), but it's BAD to have two top-level bindings with the
External Name M.a, because that turns into two linker symbols for M.a.
It's quite rare for this to actually *happen* -- the only case I know
of is tc003 compiled with the 'hpc' way -- but that only makes it
all the more annoying.
To avoid this, we craftily call 'localiseId' in the desugarer, which
simply turns the External Name for the Id into an Internal one, but
doesn't change the unique. So the desugarer produces this:
M.a{r8} = case e of (v:_) ->
case v of Just a{r8} -> M.a{r8}
The unique is still 'r8', but the binding site in the pattern
is now an Internal Name. Now the simplifier's usual mechanisms
will propagate that Name to all the occurrence sites, as well as
un-shadowing it, so we'll get
M.a{r8} = case e of (v:_) ->
case v of Just a{s77} -> a{s77}
In fact, even CoreSubst.simplOptExpr will do this, and simpleOptExpr
runs on the output of the desugarer, so all is well by the end of
the desugaring pass.
%************************************************************************
%* *
%* type synonym EquationInfo and access functions for its pieces *
%* *
%************************************************************************
\subsection[EquationInfo-synonym]{@EquationInfo@: a useful synonym}
The ``equation info'' used by @match@ is relatively complicated and
worthy of a type synonym and a few handy functions.
\begin{code}
firstPat :: EquationInfo -> Pat Id
firstPat eqn = ASSERT( notNull (eqn_pats eqn) ) head (eqn_pats eqn)
shiftEqns :: [EquationInfo] -> [EquationInfo]
shiftEqns eqns = [ eqn { eqn_pats = tail (eqn_pats eqn) } | eqn <- eqns ]
\end{code}
Functions on MatchResults
\begin{code}
matchCanFail :: MatchResult -> Bool
matchCanFail (MatchResult CanFail _) = True
matchCanFail (MatchResult CantFail _) = False
alwaysFailMatchResult :: MatchResult
alwaysFailMatchResult = MatchResult CanFail (\fail -> return fail)
cantFailMatchResult :: CoreExpr -> MatchResult
cantFailMatchResult expr = MatchResult CantFail (\_ -> return expr)
extractMatchResult :: MatchResult -> CoreExpr -> DsM CoreExpr
extractMatchResult (MatchResult CantFail match_fn) _
= match_fn (error "It can't fail!")
extractMatchResult (MatchResult CanFail match_fn) fail_expr = do
(fail_bind, if_it_fails) <- mkFailurePair fail_expr
body <- match_fn if_it_fails
return (mkCoreLet fail_bind body)
combineMatchResults :: MatchResult -> MatchResult -> MatchResult
combineMatchResults (MatchResult CanFail body_fn1)
(MatchResult can_it_fail2 body_fn2)
= MatchResult can_it_fail2 body_fn
where
body_fn fail = do body2 <- body_fn2 fail
(fail_bind, duplicatable_expr) <- mkFailurePair body2
body1 <- body_fn1 duplicatable_expr
return (Let fail_bind body1)
combineMatchResults match_result1@(MatchResult CantFail _) _
= match_result1
adjustMatchResult :: DsWrapper -> MatchResult -> MatchResult
adjustMatchResult encl_fn (MatchResult can_it_fail body_fn)
= MatchResult can_it_fail (\fail -> encl_fn <$> body_fn fail)
adjustMatchResultDs :: (CoreExpr -> DsM CoreExpr) -> MatchResult -> MatchResult
adjustMatchResultDs encl_fn (MatchResult can_it_fail body_fn)
= MatchResult can_it_fail (\fail -> encl_fn =<< body_fn fail)
wrapBinds :: [(Var,Var)] -> CoreExpr -> CoreExpr
wrapBinds [] e = e
wrapBinds ((new,old):prs) e = wrapBind new old (wrapBinds prs e)
wrapBind :: Var -> Var -> CoreExpr -> CoreExpr
wrapBind new old body
| new==old = body
| otherwise = Let (NonRec new (varToCoreExpr old)) body
seqVar :: Var -> CoreExpr -> CoreExpr
seqVar var body = Case (Var var) var (exprType body)
[(DEFAULT, [], body)]
mkCoLetMatchResult :: CoreBind -> MatchResult -> MatchResult
mkCoLetMatchResult bind = adjustMatchResult (mkCoreLet bind)
mkViewMatchResult :: Id -> CoreExpr -> Id -> MatchResult -> MatchResult
mkViewMatchResult var' viewExpr var =
adjustMatchResult (mkCoreLet (NonRec var' (mkCoreAppDs viewExpr (Var var))))
mkEvalMatchResult :: Id -> Type -> MatchResult -> MatchResult
mkEvalMatchResult var ty
= adjustMatchResult (\e -> Case (Var var) var ty [(DEFAULT, [], e)])
mkGuardedMatchResult :: CoreExpr -> MatchResult -> MatchResult
mkGuardedMatchResult pred_expr (MatchResult _ body_fn)
= MatchResult CanFail (\fail -> do body <- body_fn fail
return (mkIfThenElse pred_expr body fail))
mkCoPrimCaseMatchResult :: Id
-> Type
-> [(Literal, MatchResult)]
-> MatchResult
mkCoPrimCaseMatchResult var ty match_alts
= MatchResult CanFail mk_case
where
mk_case fail = do
alts <- mapM (mk_alt fail) sorted_alts
return (Case (Var var) var ty ((DEFAULT, [], fail) : alts))
sorted_alts = sortWith fst match_alts
mk_alt fail (lit, MatchResult _ body_fn)
= ASSERT( not (litIsLifted lit) )
do body <- body_fn fail
return (LitAlt lit, [], body)
data CaseAlt a = MkCaseAlt{ alt_pat :: a,
alt_bndrs :: [CoreBndr],
alt_wrapper :: HsWrapper,
alt_result :: MatchResult }
mkCoAlgCaseMatchResult
:: DynFlags
-> Id
-> Type
-> [CaseAlt DataCon]
-> MatchResult
mkCoAlgCaseMatchResult dflags var ty match_alts
| isNewtype
= ASSERT( null (tail match_alts) && null (tail arg_ids1) )
mkCoLetMatchResult (NonRec arg_id1 newtype_rhs) match_result1
| isPArrFakeAlts match_alts
= MatchResult CanFail $ mkPArrCase dflags var ty (sort_alts match_alts)
| otherwise
= mkDataConCase var ty match_alts
where
isNewtype = isNewTyCon (dataConTyCon (alt_pat alt1))
alt1@MkCaseAlt{ alt_bndrs = arg_ids1, alt_result = match_result1 }
= ASSERT( notNull match_alts ) head match_alts
arg_id1 = ASSERT( notNull arg_ids1 ) head arg_ids1
var_ty = idType var
(tc, ty_args) = tcSplitTyConApp var_ty
newtype_rhs = unwrapNewTypeBody tc ty_args (Var var)
isPArrFakeAlts :: [CaseAlt DataCon] -> Bool
isPArrFakeAlts [alt] = isPArrFakeCon (alt_pat alt)
isPArrFakeAlts (alt:alts) =
case (isPArrFakeCon (alt_pat alt), isPArrFakeAlts alts) of
(True , True ) -> True
(False, False) -> False
_ -> panic "DsUtils: you may not mix `[:...:]' with `PArr' patterns"
isPArrFakeAlts [] = panic "DsUtils: unexpectedly found an empty list of PArr fake alternatives"
mkCoSynCaseMatchResult :: Id -> Type -> CaseAlt PatSyn -> MatchResult
mkCoSynCaseMatchResult var ty alt = MatchResult CanFail $ mkPatSynCase var ty alt
\end{code}
\begin{code}
sort_alts :: [CaseAlt DataCon] -> [CaseAlt DataCon]
sort_alts = sortWith (dataConTag . alt_pat)
mkPatSynCase :: Id -> Type -> CaseAlt PatSyn -> CoreExpr -> DsM CoreExpr
mkPatSynCase var ty alt fail = do
matcher <- dsLExpr $ mkLHsWrap wrapper $ nlHsTyApp matcher [ty]
let MatchResult _ mkCont = match_result
cont <- mkCoreLams bndrs <$> mkCont fail
return $ mkCoreAppsDs matcher [Var var, cont, fail]
where
MkCaseAlt{ alt_pat = psyn,
alt_bndrs = bndrs,
alt_wrapper = wrapper,
alt_result = match_result} = alt
matcher = patSynMatcher psyn
mkDataConCase :: Id -> Type -> [CaseAlt DataCon] -> MatchResult
mkDataConCase _ _ [] = panic "mkDataConCase: no alternatives"
mkDataConCase var ty alts@(alt1:_) = MatchResult fail_flag mk_case
where
con1 = alt_pat alt1
tycon = dataConTyCon con1
data_cons = tyConDataCons tycon
match_results = map alt_result alts
sorted_alts :: [CaseAlt DataCon]
sorted_alts = sort_alts alts
var_ty = idType var
(_, ty_args) = tcSplitTyConApp var_ty
mk_case :: CoreExpr -> DsM CoreExpr
mk_case fail = do
alts <- mapM (mk_alt fail) sorted_alts
return $ mkWildCase (Var var) (idType var) ty (mk_default fail ++ alts)
mk_alt :: CoreExpr -> CaseAlt DataCon -> DsM CoreAlt
mk_alt fail MkCaseAlt{ alt_pat = con,
alt_bndrs = args,
alt_result = MatchResult _ body_fn }
= do { body <- body_fn fail
; case dataConBoxer con of {
Nothing -> return (DataAlt con, args, body) ;
Just (DCB boxer) ->
do { us <- newUniqueSupply
; let (rep_ids, binds) = initUs_ us (boxer ty_args args)
; return (DataAlt con, rep_ids, mkLets binds body) } } }
mk_default :: CoreExpr -> [CoreAlt]
mk_default fail | exhaustive_case = []
| otherwise = [(DEFAULT, [], fail)]
fail_flag :: CanItFail
fail_flag | exhaustive_case
= foldr orFail CantFail [can_it_fail | MatchResult can_it_fail _ <- match_results]
| otherwise
= CanFail
mentioned_constructors = mkUniqSet $ map alt_pat alts
un_mentioned_constructors
= mkUniqSet data_cons `minusUniqSet` mentioned_constructors
exhaustive_case = isEmptyUniqSet un_mentioned_constructors
mkPArrCase :: DynFlags -> Id -> Type -> [CaseAlt DataCon] -> CoreExpr -> DsM CoreExpr
mkPArrCase dflags var ty sorted_alts fail = do
lengthP <- dsDPHBuiltin lengthPVar
alt <- unboxAlt
return (mkWildCase (len lengthP) intTy ty [alt])
where
elemTy = case splitTyConApp (idType var) of
(_, [elemTy]) -> elemTy
_ -> panic panicMsg
panicMsg = "DsUtils.mkCoAlgCaseMatchResult: not a parallel array?"
len lengthP = mkApps (Var lengthP) [Type elemTy, Var var]
unboxAlt = do
l <- newSysLocalDs intPrimTy
indexP <- dsDPHBuiltin indexPVar
alts <- mapM (mkAlt indexP) sorted_alts
return (DataAlt intDataCon, [l], mkWildCase (Var l) intPrimTy ty (dft : alts))
where
dft = (DEFAULT, [], fail)
mkAlt indexP alt@MkCaseAlt{alt_result = MatchResult _ bodyFun} = do
body <- bodyFun fail
return (LitAlt lit, [], mkCoreLets binds body)
where
lit = MachInt $ toInteger (dataConSourceArity (alt_pat alt))
binds = [NonRec arg (indexExpr i) | (i, arg) <- zip [1..] (alt_bndrs alt)]
indexExpr i = mkApps (Var indexP) [Type elemTy, Var var, mkIntExpr dflags i]
\end{code}
%************************************************************************
%* *
\subsection{Desugarer's versions of some Core functions}
%* *
%************************************************************************
\begin{code}
mkErrorAppDs :: Id
-> Type
-> SDoc
-> DsM CoreExpr
mkErrorAppDs err_id ty msg = do
src_loc <- getSrcSpanDs
dflags <- getDynFlags
let
full_msg = showSDoc dflags (hcat [ppr src_loc, text "|", msg])
core_msg = Lit (mkMachString full_msg)
return (mkApps (Var err_id) [Type ty, core_msg])
\end{code}
'mkCoreAppDs' and 'mkCoreAppsDs' hand the special-case desugaring of 'seq'.
Note [Desugaring seq (1)] cf Trac #1031
~~~~~~~~~~~~~~~~~~~~~~~~~
f x y = x `seq` (y `seq` (# x,y #))
The [CoreSyn let/app invariant] means that, other things being equal, because
the argument to the outer 'seq' has an unlifted type, we'll use call-by-value thus:
f x y = case (y `seq` (# x,y #)) of v -> x `seq` v
But that is bad for two reasons:
(a) we now evaluate y before x, and
(b) we can't bind v to an unboxed pair
Seq is very, very special! So we recognise it right here, and desugar to
case x of _ -> case y of _ -> (# x,y #)
Note [Desugaring seq (2)] cf Trac #2273
~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
let chp = case b of { True -> fst x; False -> 0 }
in chp `seq` ...chp...
Here the seq is designed to plug the space leak of retaining (snd x)
for too long.
If we rely on the ordinary inlining of seq, we'll get
let chp = case b of { True -> fst x; False -> 0 }
case chp of _ { I# -> ...chp... }
But since chp is cheap, and the case is an alluring contet, we'll
inline chp into the case scrutinee. Now there is only one use of chp,
so we'll inline a second copy. Alas, we've now ruined the purpose of
the seq, by re-introducing the space leak:
case (case b of {True -> fst x; False -> 0}) of
I# _ -> ...case b of {True -> fst x; False -> 0}...
We can try to avoid doing this by ensuring that the binder-swap in the
case happens, so we get his at an early stage:
case chp of chp2 { I# -> ...chp2... }
But this is fragile. The real culprit is the source program. Perhaps we
should have said explicitly
let !chp2 = chp in ...chp2...
But that's painful. So the code here does a little hack to make seq
more robust: a saturated application of 'seq' is turned *directly* into
the case expression, thus:
x `seq` e2 ==> case x of x -> e2 -- Note shadowing!
e1 `seq` e2 ==> case x of _ -> e2
So we desugar our example to:
let chp = case b of { True -> fst x; False -> 0 }
case chp of chp { I# -> ...chp... }
And now all is well.
The reason it's a hack is because if you define mySeq=seq, the hack
won't work on mySeq.
Note [Desugaring seq (3)] cf Trac #2409
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The isLocalId ensures that we don't turn
True `seq` e
into
case True of True { ... }
which stupidly tries to bind the datacon 'True'.
\begin{code}
mkCoreAppDs :: CoreExpr -> CoreExpr -> CoreExpr
mkCoreAppDs (Var f `App` Type ty1 `App` Type ty2 `App` arg1) arg2
| f `hasKey` seqIdKey
= Case arg1 case_bndr ty2 [(DEFAULT,[],arg2)]
where
case_bndr = case arg1 of
Var v1 | isLocalId v1 -> v1
_ -> mkWildValBinder ty1
mkCoreAppDs fun arg = mkCoreApp fun arg
mkCoreAppsDs :: CoreExpr -> [CoreExpr] -> CoreExpr
mkCoreAppsDs fun args = foldl mkCoreAppDs fun args
\end{code}
%************************************************************************
%* *
\subsection[mkSelectorBind]{Make a selector bind}
%* *
%************************************************************************
This is used in various places to do with lazy patterns.
For each binder $b$ in the pattern, we create a binding:
\begin{verbatim}
b = case v of pat' -> b'
\end{verbatim}
where @pat'@ is @pat@ with each binder @b@ cloned into @b'@.
ToDo: making these bindings should really depend on whether there's
much work to be done per binding. If the pattern is complex, it
should be de-mangled once, into a tuple (and then selected from).
Otherwise the demangling can be in-line in the bindings (as here).
Boring! Boring! One error message per binder. The above ToDo is
even more helpful. Something very similar happens for pattern-bound
expressions.
Note [mkSelectorBinds]
~~~~~~~~~~~~~~~~~~~~~~
Given p = e, where p binds x,y
we are going to make EITHER
EITHER (A) v = e (where v is fresh)
x = case v of p -> x
y = case v of p -> y
OR (B) t = case e of p -> (x,y)
x = case t of (x,_) -> x
y = case t of (_,y) -> y
We do (A) when
* Matching the pattern is cheap so we don't mind
doing it twice.
* Or if the pattern binds only one variable (so we'll only
match once)
* AND the pattern can't fail (else we tiresomely get two inexhaustive
pattern warning messages)
Otherwise we do (B). Really (A) is just an optimisation for very common
cases like
Just x = e
(p,q) = e
\begin{code}
mkSelectorBinds :: [Maybe (Tickish Id)]
-> LPat Id
-> CoreExpr
-> DsM [(Id,CoreExpr)]
mkSelectorBinds ticks (L _ (VarPat v)) val_expr
= return [(v, case ticks of
[t] -> mkOptTickBox t val_expr
_ -> val_expr)]
mkSelectorBinds ticks pat val_expr
| null binders
= return []
| isSingleton binders || is_simple_lpat pat
= do { val_var <- newSysLocalDs (hsLPatType pat)
; err_expr <- mkErrorAppDs iRREFUT_PAT_ERROR_ID unitTy (ppr pat)
; err_var <- newSysLocalDs unitTy
; binds <- zipWithM (mk_bind val_var err_var) ticks' binders
; return ( (val_var, val_expr) :
(err_var, err_expr) :
binds ) }
| otherwise
= do { error_expr <- mkErrorAppDs iRREFUT_PAT_ERROR_ID tuple_ty (ppr pat)
; tuple_expr <- matchSimply val_expr PatBindRhs pat local_tuple error_expr
; tuple_var <- newSysLocalDs tuple_ty
; let mk_tup_bind tick binder
= (binder, mkOptTickBox tick $
mkTupleSelector local_binders binder
tuple_var (Var tuple_var))
; return ( (tuple_var, tuple_expr) : zipWith mk_tup_bind ticks' binders ) }
where
binders = collectPatBinders pat
ticks' = ticks ++ repeat Nothing
local_binders = map localiseId binders
local_tuple = mkBigCoreVarTup binders
tuple_ty = exprType local_tuple
mk_bind scrut_var err_var tick bndr_var = do
rhs_expr <- matchSimply (Var scrut_var) PatBindRhs pat
(Var bndr_var) error_expr
return (bndr_var, mkOptTickBox tick rhs_expr)
where
error_expr = mkCast (Var err_var) co
co = mkUnsafeCo (exprType (Var err_var)) (idType bndr_var)
is_simple_lpat p = is_simple_pat (unLoc p)
is_simple_pat (TuplePat ps Boxed _) = all is_triv_lpat ps
is_simple_pat pat@(ConPatOut{}) = case unLoc (pat_con pat) of
RealDataCon con -> isProductTyCon (dataConTyCon con)
&& all is_triv_lpat (hsConPatArgs (pat_args pat))
PatSynCon _ -> False
is_simple_pat (VarPat _) = True
is_simple_pat (ParPat p) = is_simple_lpat p
is_simple_pat _ = False
is_triv_lpat p = is_triv_pat (unLoc p)
is_triv_pat (VarPat _) = True
is_triv_pat (WildPat _) = True
is_triv_pat (ParPat p) = is_triv_lpat p
is_triv_pat _ = False
\end{code}
Creating big tuples and their types for full Haskell expressions.
They work over *Ids*, and create tuples replete with their types,
which is whey they are not in HsUtils.
\begin{code}
mkLHsPatTup :: [LPat Id] -> LPat Id
mkLHsPatTup [] = noLoc $ mkVanillaTuplePat [] Boxed
mkLHsPatTup [lpat] = lpat
mkLHsPatTup lpats = L (getLoc (head lpats)) $
mkVanillaTuplePat lpats Boxed
mkLHsVarPatTup :: [Id] -> LPat Id
mkLHsVarPatTup bs = mkLHsPatTup (map nlVarPat bs)
mkVanillaTuplePat :: [OutPat Id] -> Boxity -> Pat Id
mkVanillaTuplePat pats box
= TuplePat pats box (mkTupleTy (boxityNormalTupleSort box) (map hsLPatType pats))
mkBigLHsVarTup :: [Id] -> LHsExpr Id
mkBigLHsVarTup ids = mkBigLHsTup (map nlHsVar ids)
mkBigLHsTup :: [LHsExpr Id] -> LHsExpr Id
mkBigLHsTup = mkChunkified mkLHsTupleExpr
mkBigLHsVarPatTup :: [Id] -> LPat Id
mkBigLHsVarPatTup bs = mkBigLHsPatTup (map nlVarPat bs)
mkBigLHsPatTup :: [LPat Id] -> LPat Id
mkBigLHsPatTup = mkChunkified mkLHsPatTup
\end{code}
%************************************************************************
%* *
\subsection[mkFailurePair]{Code for pattern-matching and other failures}
%* *
%************************************************************************
Generally, we handle pattern matching failure like this: let-bind a
fail-variable, and use that variable if the thing fails:
\begin{verbatim}
let fail.33 = error "Help"
in
case x of
p1 -> ...
p2 -> fail.33
p3 -> fail.33
p4 -> ...
\end{verbatim}
Then
\begin{itemize}
\item
If the case can't fail, then there'll be no mention of @fail.33@, and the
simplifier will later discard it.
\item
If it can fail in only one way, then the simplifier will inline it.
\item
Only if it is used more than once will the let-binding remain.
\end{itemize}
There's a problem when the result of the case expression is of
unboxed type. Then the type of @fail.33@ is unboxed too, and
there is every chance that someone will change the let into a case:
\begin{verbatim}
case error "Help" of
fail.33 -> case ....
\end{verbatim}
which is of course utterly wrong. Rather than drop the condition that
only boxed types can be let-bound, we just turn the fail into a function
for the primitive case:
\begin{verbatim}
let fail.33 :: Void -> Int#
fail.33 = \_ -> error "Help"
in
case x of
p1 -> ...
p2 -> fail.33 void
p3 -> fail.33 void
p4 -> ...
\end{verbatim}
Now @fail.33@ is a function, so it can be let-bound.
\begin{code}
mkFailurePair :: CoreExpr
-> DsM (CoreBind,
CoreExpr)
mkFailurePair expr
= do { fail_fun_var <- newFailLocalDs (voidPrimTy `mkFunTy` ty)
; fail_fun_arg <- newSysLocalDs voidPrimTy
; let real_arg = setOneShotLambda fail_fun_arg
; return (NonRec fail_fun_var (Lam real_arg expr),
App (Var fail_fun_var) (Var voidPrimId)) }
where
ty = exprType expr
\end{code}
Note [Failure thunks and CPR]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we make a failure point we ensure that it
does not look like a thunk. Example:
let fail = \rw -> error "urk"
in case x of
[] -> fail realWorld#
(y:ys) -> case ys of
[] -> fail realWorld#
(z:zs) -> (y,z)
Reason: we know that a failure point is always a "join point" and is
entered at most once. Adding a dummy 'realWorld' token argument makes
it clear that sharing is not an issue. And that in turn makes it more
CPR-friendly. This matters a lot: if you don't get it right, you lose
the tail call property. For example, see Trac #3403.
\begin{code}
mkOptTickBox :: Maybe (Tickish Id) -> CoreExpr -> CoreExpr
mkOptTickBox Nothing e = e
mkOptTickBox (Just tickish) e = Tick tickish e
mkBinaryTickBox :: Int -> Int -> CoreExpr -> DsM CoreExpr
mkBinaryTickBox ixT ixF e = do
uq <- newUnique
this_mod <- getModule
let bndr1 = mkSysLocal (fsLit "t1") uq boolTy
let
falseBox = Tick (HpcTick this_mod ixF) (Var falseDataConId)
trueBox = Tick (HpcTick this_mod ixT) (Var trueDataConId)
return $ Case e bndr1 boolTy
[ (DataAlt falseDataCon, [], falseBox)
, (DataAlt trueDataCon, [], trueBox)
]
\end{code}