This module exports some utility functions of no great interest.
\begin{code}
-
-- | Utility functions for constructing Core syntax, principally for desugaring
module DsUtils (
EquationInfo(..),
mkCoPrimCaseMatchResult, mkCoAlgCaseMatchResult,
wrapBind, wrapBinds,
- mkErrorAppDs,
+ mkErrorAppDs, mkCoreAppDs, mkCoreAppsDs,
seqVar,
import ListSetOps
import FastString
import StaticFlags
-
-import Data.Char
\end{code}
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 var
+selectMatchVar (BangPat pat) = selectMatchVar (unLoc pat)
+selectMatchVar (LazyPat pat) = selectMatchVar (unLoc pat)
+selectMatchVar (ParPat pat) = selectMatchVar (unLoc pat)
+selectMatchVar (VarPat var) = return var
selectMatchVar (AsPat var _) = return (unLoc var)
-selectMatchVar other_pat = newSysLocalDs (hsPatType other_pat)
+selectMatchVar other_pat = newSysLocalDs (hsPatType other_pat)
-- OK, better make up one...
\end{code}
-- let var' = viewExpr var in mr
mkViewMatchResult :: Id -> CoreExpr -> Id -> MatchResult -> MatchResult
mkViewMatchResult var' viewExpr var =
- adjustMatchResult (mkCoreLet (NonRec var' (mkCoreApp viewExpr (Var var))))
+ adjustMatchResult (mkCoreLet (NonRec var' (mkCoreAppDs viewExpr (Var var))))
mkEvalMatchResult :: Id -> Type -> MatchResult -> MatchResult
mkEvalMatchResult var ty
| otherwise
= CanFail
- wild_var = mkWildId (idType var)
sorted_alts = sortWith get_tag match_alts
get_tag (con, _, _) = dataConTag con
mk_case fail = do alts <- mapM (mk_alt fail) sorted_alts
- return (Case (Var var) wild_var ty (mk_default fail ++ alts))
+ return (mkWildCase (Var var) (idType var) ty (mk_default fail ++ alts))
mk_alt fail (con, args, MatchResult _ body_fn) = do
body <- body_fn fail
mk_parrCase fail = do
lengthP <- dsLookupGlobalId lengthPName
alt <- unboxAlt
- return (Case (len lengthP) (mkWildId intTy) ty [alt])
+ return (mkWildCase (len lengthP) intTy ty [alt])
where
elemTy = case splitTyConApp (idType var) of
(_, [elemTy]) -> elemTy
l <- newSysLocalDs intPrimTy
indexP <- dsLookupGlobalId indexPName
alts <- mapM (mkAlt indexP) sorted_alts
- return (DataAlt intDataCon, [l], (Case (Var l) wild ty (dft : alts)))
+ return (DataAlt intDataCon, [l], mkWildCase (Var l) intPrimTy ty (dft : alts))
where
- wild = mkWildId intPrimTy
dft = (DEFAULT, [], fail)
--
-- each alternative matches one array length (corresponding to one
\begin{code}
mkErrorAppDs :: Id -- The error function
-> Type -- Type to which it should be applied
- -> String -- The error message string to pass
+ -> SDoc -- The error message string to pass
-> DsM CoreExpr
mkErrorAppDs err_id ty msg = do
src_loc <- getSrcSpanDs
let
- full_msg = showSDoc (hcat [ppr src_loc, text "|", text msg])
+ full_msg = showSDoc (hcat [ppr src_loc, text "|", msg])
core_msg = Lit (mkMachString full_msg)
-- mkMachString returns a result of type String#
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 #2231
+~~~~~~~~~~~~~~~~~~~~~~~~~
+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. So we desugar to:
+ let chp = case b of { True -> fst x; False -> 0 }
+ case chp of chp { I# -> ...chp... }
+Notice the shadowing of the case binder! 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 -- Note [Desugaring seq (1), (2)]
+ = Case arg1 case_bndr ty2 [(DEFAULT,[],arg2)]
+ where
+ case_bndr = case arg1 of
+ Var v1 | isLocalId v1 -> v1 -- Note [Desugaring seq (2) and (3)]
+ _ -> mkWildBinder ty1
+
+mkCoreAppDs fun arg = mkCoreApp fun arg -- The rest is done in MkCore
+
+mkCoreAppsDs :: CoreExpr -> [CoreExpr] -> CoreExpr
+mkCoreAppsDs fun args = foldl mkCoreAppDs fun args
+\end{code}
+
+
%************************************************************************
%* *
\subsection[mkSelectorBind]{Make a selector bind}
-- For the error message we make one error-app, to avoid duplication.
-- But we need it at different types... so we use coerce for that
- err_expr <- mkErrorAppDs iRREFUT_PAT_ERROR_ID unitTy (showSDoc (ppr pat))
+ err_expr <- mkErrorAppDs iRREFUT_PAT_ERROR_ID unitTy (ppr pat)
err_var <- newSysLocalDs unitTy
binds <- mapM (mk_bind val_var err_var) binders
return ( (val_var, val_expr) :
| otherwise = do
- error_expr <- mkErrorAppDs iRREFUT_PAT_ERROR_ID tuple_ty (showSDoc (ppr pat))
+ 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
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