| I wanted to ask: Is there a restriction on what kind empty case | expressions are supported by the code generator or can I just improve | the lint check and assume that the code will be handled by the code | generator correctly?
I believe the latter. See CoreToStg line 364. So feel free to make Lint cleverer; make sure you add a Note. But if it /needs/ to be cleverer, that suggests that the simplifier should be cleverer instead, and should simplify the code so that even a dumb Core Lint has no trouble. In your example why didn't the simplifier do case-of-case? I suppose that this might be after some pass and before a simplifier run. Simon | -----Original Message----- | From: ghc-devs [mailto:[email protected]] On Behalf Of Ömer | Sinan Agacan | Sent: 29 February 2016 03:16 | To: ghc-devs <[email protected]> | Subject: CoreLint check for case with no alts | | Hi all, | | CoreLint has a check that, when seeing a case expression with empty | list of alternatives, checks whether the scrutinee is bottom. This | "bottom-ness" check is, however, very simple and returning many false | negatives. For example, when it sees a case expression, all it does is: | | go _ (Case _ _ _ alts) = null alts | | Which is just too simple for some cases. (it could check if all the | RHSs are bottom, or if the scrutinee is bottom etc.) | | I guess this makes sense, since it's OK to generate unreachable code, | but it's not OK to not generate a code in a reachable path. | | But in my case this is becoming problem as it's rejecting my seemingly | valid program. One of the relevant parts in my code is this: | | case ww_s4C3 of ww_X4Fb { | (#_||#) ww_s4Ce -> | case case ww_s4Ce of ww_s4F1 { (# ww_s4F2, ww_s4F3 #) -> | lvl_s4F4 ww_s4F3 ww_s4F2 | } | of wild_00 { | -- empty | }; | | lvl_s4F4 :: Int# -> String -> Var | [LclId, Arity=2, Str=DmdType (args: <L,U><L,U>) (res: x)] | lvl_s4F4 = | \ (ww_s4F3 :: Int#) (ww_s4F2 :: String) -> | lvl_s3V4 (TyVar ww_s4F2 ww_s4F3) | | lvl_s3V4 :: Var -> Var | [LclId, Arity=1, CallArity=1, Str=DmdType (args: <B,1*H>) (res: x)] | lvl_s3V4 = \ (i_a1vO :: Var) -> lvl_s4EZ i_a1vO | | lvl_s4EZ :: Var -> Var | [LclId, Arity=1, Str=DmdType (args: <L,U>) (res: x)] | lvl_s4EZ = \ (i_a1vO :: Var) -> error ... | | The scrutinee part of the case expression in the alternative is clearly | bottom, but this expression is rejected by the linter. | | One easy solution is to implement a more precise test and use it in | linter. But I thought maybe the current implementation is deliberately | so. Maybe the code generator doesn't support that type of code etc. so | I wanted to ask: Is there a restriction on what kind empty case | expressions are supported by the code generator or can I just improve | the lint check and assume that the code will be handled by the code | generator correctly? | | Thanks. | _______________________________________________ | ghc-devs mailing list | [email protected] | https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.ha | skell.org%2fcgi-bin%2fmailman%2flistinfo%2fghc- | devs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7ce621b9bcb2774ff8 | 522208d340b6c5a7%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=cIiMVTgiH | ibh0SNksSZGLK75bZBgb3HA13USf78ohyQ%3d _______________________________________________ ghc-devs mailing list [email protected] http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
