To round off Simon's email with a concrete example and explanation. ``` {-# LANGUAGE BangPatterns #-} module Test where mid x = x {-# NOINLINE mid #-} data Foo = Foo1 () test :: Foo -> () test x = case x of !_ -> mid (case x of Foo1 x1 -> x1) ``` refineDefaultAlt fills in the DEFAULT here with `Foo ip1` and then x becomes bound to `Foo ip1` so is inlined into the other case which causes the KnownBranch optimisation to kick in. Simon J's point also seems plausible. Especially as it's called just before combineIdenticalAlts. Thanks everyone! On Fri, May 11, 2018 at 4:17 PM, Simon Jakobi wrote:

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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