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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