seqVar,
-- LHs tuples
- mkLHsVarTup, mkLHsTup, mkLHsVarPatTup, mkLHsPatTup,
+ mkLHsVarPatTup, mkLHsPatTup, mkVanillaTuplePat,
mkBigLHsVarTup, mkBigLHsTup, mkBigLHsVarPatTup, mkBigLHsPatTup,
mkSelectorBinds,
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
+Note [Desugaring seq (2)] cf Trac #2273
~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
let chp = case b of { True -> fst x; False -> 0 }
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:
+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... }
-Notice the shadowing of the case binder! And now all is well.
+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.
\end{code}
-Creating tuples and their types for full Haskell expressions
+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}
-
--- Smart constructors for source tuple expressions
-mkLHsVarTup :: [Id] -> LHsExpr Id
-mkLHsVarTup ids = mkLHsTup (map nlHsVar ids)
-
-mkLHsTup :: [LHsExpr Id] -> LHsExpr Id
-mkLHsTup [] = nlHsVar unitDataConId
-mkLHsTup [lexp] = lexp
-mkLHsTup lexps = L (getLoc (head lexps)) $
- ExplicitTuple lexps Boxed
-
--- Smart constructors for source tuple patterns
-mkLHsVarPatTup :: [Id] -> LPat Id
-mkLHsVarPatTup bs = mkLHsPatTup (map nlVarPat bs)
-
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
+-- A vanilla tuple pattern simply gets its type from its sub-patterns
+mkVanillaTuplePat pats box
+ = TuplePat pats box (mkTupleTy box (map hsLPatType pats))
+
-- The Big equivalents for the source tuple expressions
mkBigLHsVarTup :: [Id] -> LHsExpr Id
mkBigLHsVarTup ids = mkBigLHsTup (map nlHsVar ids)
mkBigLHsTup :: [LHsExpr Id] -> LHsExpr Id
-mkBigLHsTup = mkChunkified mkLHsTup
-
+mkBigLHsTup = mkChunkified mkLHsTupleExpr
-- The Big equivalents for the source tuple patterns
mkBigLHsVarPatTup :: [Id] -> LPat Id
\begin{code}
mkFailurePair :: CoreExpr -- Result type of the whole case expression
-> DsM (CoreBind, -- Binds the newly-created fail variable
- -- to either the expression or \ _ -> expression
- CoreExpr) -- Either the fail variable, or fail variable
- -- applied to unit tuple
+ -- to \ _ -> expression
+ CoreExpr) -- Fail variable applied to realWorld#
+-- See Note [Failure thunks and CPR]
mkFailurePair expr
- | isUnLiftedType ty = do
- fail_fun_var <- newFailLocalDs (unitTy `mkFunTy` ty)
- fail_fun_arg <- newSysLocalDs unitTy
- return (NonRec fail_fun_var (Lam fail_fun_arg expr),
- App (Var fail_fun_var) (Var unitDataConId))
-
- | otherwise = do
- fail_var <- newFailLocalDs ty
- return (NonRec fail_var expr, Var fail_var)
+ = do { fail_fun_var <- newFailLocalDs (realWorldStatePrimTy `mkFunTy` ty)
+ ; fail_fun_arg <- newSysLocalDs realWorldStatePrimTy
+ ; return (NonRec fail_fun_var (Lam fail_fun_arg expr),
+ App (Var fail_fun_var) (Var realWorldPrimId)) }
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 (Int,[Id]) -> CoreExpr -> DsM CoreExpr
mkOptTickBox Nothing e = return e