Re: Motivation for refineDefaultAlt

Hi! I thought refineDefaultAlt was about scenarios like this: data D = C0 | C1 | C2 case e of DEFAULT -> e0 C0 -> e1 C1 -> e1 When we apply combineIdenticalAlts to this expression, it can't combine the alts for C0 and C1, as we already have a default case. If we apply refineDefaultAlt first, we get case e of C0 -> e1 C1 -> e1 C2 -> e0 and combineIdenticalAlts can turn that into case e of DEFAULT -> e1 C2 -> e0 But that's just my own interpretation and possibly not the original motivation. Cheers, Simon 2018-05-11 17:03 GMT+02:00 Simon Peyton Jones via ghc-devs :

Because if e contains

…(case x of Foo p q -> e2)…

as a sub-expression, we’d like to simplify it.

Sorry that is not documented; please do add that to the comments in the source code.

Simon

From: ghc-devs On Behalf Of Matthew Pickering Sent: 11 May 2018 15:54 To: GHC developers Subject: Motivation for refineDefaultAlt

Hi all,

Does anyone know the motivation for refineDefaultAlt?

The comment states

- -- | Refine the default alternative to a 'DataAlt', if there is a unique way to do so.

OK - so the code transforms something like

case x of { DEFAULT -> e } ===>

case x of { Foo a1 a2 a3 -> e }

but why is this necessary or desirable?

Perhaps you know Simon (Jakobi)?

Cheers,

Matt

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