Commit 8612b811 authored by's avatar
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Fix Trac #4396, by localising pattern binders in the desugarer

See Note [Localise pattern binders]
parent 4e04af2f
......@@ -144,12 +144,49 @@ 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 (VarPat var) = return (localiseId var) -- Note [Localise pattern binders]
selectMatchVar (AsPat var _) = return (unLoc var)
selectMatchVar other_pat = newSysLocalDs (hsPatType other_pat)
-- OK, better make up one...
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.
%* *
......@@ -551,10 +588,11 @@ mkSelectorBinds pat val_expr
tuple_expr <- matchSimply val_expr PatBindRhs pat local_tuple error_expr
tuple_var <- newSysLocalDs tuple_ty
let mk_tup_bind binder
= (binder, mkTupleSelector binders binder tuple_var (Var tuple_var))
= (binder, mkTupleSelector local_binders binder tuple_var (Var tuple_var))
return ( (tuple_var, tuple_expr) : map mk_tup_bind binders )
binders = collectPatBinders pat
local_binders = map localiseId binders -- See Note [Localise pattern binders]
local_tuple = mkBigCoreVarTup binders
tuple_ty = exprType local_tuple
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