CoreLint check for case with no alts

[Simon Peyton Jones]

|  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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