Re: Disappearing case alternative?

12 Mar 2016

      I think you put your finger on the problem here. That `evalP` is a
placeholder to get transformed away later. I thought that my `NOINLINE`
annotation would prevent GHC from noticing that `evalP` evaluates to
bottom. I'll do a better job.

Thank you!

-- Conal

On Fri, Mar 11, 2016 at 4:06 PM, Ömer Sinan Ağacan 
wrote:
...
Empty case expressions like `case evalP @ (Pair Int) ds of wild { }` are
only
valid if the scrutinee is bottom, see CoreLint.hs for how exprIsBottom is
used
and CoreUtils.hs for how it's defined. So it seems like the simplifier
somehow
figured that `evalP @ (Pair Int) ds` is bottom, and generated an empty
case.
Is the linter complaining about this case expression? (you can try it using
CoreLint.lintExpr)
2016-03-11 16:49 GMT-05:00 Conal Elliott :
...
I'm working on a GHC plugin, mainly in the form of a "builtin" rewrite
rule,
...
and I'm seeing a strange disappearance of a case alternative. With
-ddump-rule-rewrites, I can see before & after the disappearance: Here's
one
rule firing with the alternative present:
Rule fired
        Rule: reify
        Before: reifyP
                  TyArg Pair Int -> Int
                  ValArg \ (ds :: Pair Int) ->
                           case ds of _ { :# a b -> $fNumInt_$c+ a b }
        After:  lamP
                  @ (Pair Int)
                  @ Int
                  "ds"#
                  (\ (ds :: EP (Pair Int)) ->
                     reifyP
                       @ Int
                       (case evalP @ (Pair Int) ds of _ { :# a b ->
$fNumInt_$c+ a b }))
        Cont:   StrictArg toE
                Stop[BoringCtxt] E Prim (Pair Int -> Int)
The next thing I see is the rule firing again on the residual reifyP
application, but with the case alternative gone:
Rule fired
        Rule: reify
        Before: reifyP
                  TyArg Int ValArg case evalP @ (Pair Int) ds of wild { }
        After:  ...
Any ideas what's causing the disappearance?
Thanks.  - Conal
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