diff options
Diffstat (limited to 'compiler/GHC/Core/DataCon.hs')
| -rw-r--r-- | compiler/GHC/Core/DataCon.hs | 24 |
1 files changed, 21 insertions, 3 deletions
diff --git a/compiler/GHC/Core/DataCon.hs b/compiler/GHC/Core/DataCon.hs index 13470c93af..c4563202a9 100644 --- a/compiler/GHC/Core/DataCon.hs +++ b/compiler/GHC/Core/DataCon.hs @@ -614,7 +614,7 @@ data DataConRep -- and *including* all evidence args , dcr_stricts :: [StrictnessMark] -- 1-1 with dcr_arg_tys - -- See also Note [Data-con worker strictness] in GHC.Types.Id.Make + -- See also Note [Data-con worker strictness] , dcr_bangs :: [HsImplBang] -- The actual decisions made (including failures) -- about the original arguments; 1-1 with orig_arg_tys @@ -715,8 +715,26 @@ filterEqSpec eq_spec instance Outputable EqSpec where ppr (EqSpec tv ty) = ppr (tv, ty) -{- Note [Bangs on data constructor arguments] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{- Note [Data-con worker strictness] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Notice that we do *not* say the worker Id is strict even if the data +constructor is declared strict + e.g. data T = MkT !(Int,Int) +Why? Because the *wrapper* $WMkT is strict (and its unfolding has case +expressions that do the evals) but the *worker* MkT itself is not. If we +pretend it is strict then when we see + case x of y -> MkT y +the simplifier thinks that y is "sure to be evaluated" (because the worker MkT +is strict) and drops the case. No, the workerId MkT is not strict. + +However, the worker does have StrictnessMarks. When the simplifier sees a +pattern + case e of MkT x -> ... +it uses the dataConRepStrictness of MkT to mark x as evaluated; but that's +fine... dataConRepStrictness comes from the data con not from the worker Id. + +Note [Bangs on data constructor arguments] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider data T = MkT !Int {-# UNPACK #-} !Int Bool |