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 <co...@conal.net>:
>
> 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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