CoreLint check for case with no alts

[Simon Peyton Jones]

|     coreToStgExpr (Case scrut bndr ty [])
|       = coreToStgExpr (Case scrut bndr ty [(DEFAULT, [], mkImpossibleExpr
| ty)])

Yes that should be fine.

I'm afraid I have no idea why you are getting

|     Oops!  Entered absent arg w_sc1l FilePath

or seg-faults.

Simon

| 
| But that may be because of some libraries I need to recompile, although I'm
| not
| sure why this would be the case.
| 
| The problem I'm trying to solve is my lint-safe (after the smarter lint check
| about bottoming expressions) programs are failing with segfaults. Of course
| that may be because I'm doing something wrong in STG level, but I'm trying to
| make sure everything above STG is correct and bug-free. It'd be really great
| if
| I could somehow generate an erroring expression in some places that are
| supposed to be unreachable, just to make sure my segfaults are not related
| with
| those.
| 
| 2016-02-29 6:32 GMT-05:00 Simon Peyton Jones <simonpj at microsoft.com>:
| > |  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:ghc-devs-bounces at haskell.org] On Behalf Of Ömer
| > |  Sinan Agacan
| > |  Sent: 29 February 2016 03:16
| > |  To: ghc-devs <ghc-devs at haskell.org>
| > |  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.
| > |  _______________________________________________
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