Avoiding `OtherCon []` unfoldings, restoring definitions from unfoldings

[Erdi, Gergo]

PUBLIC

Please see this question in my previous email:


  *   Unfortunately, I am unable to reproduce this from the command line using the GHC executable, so before I put in the effort of making a minimal example of using the GHC API to get this result, I would first like to know if this is even an interesting case, or if there is some explanation for why one of these two would have an unfolding when the other doesn't, in certain cases.

So I can NOT reproduce it with just a GHC command line, only with my program (that uses GHC as a library). So it will require extra effort to get something you or others can just run, hence my question: is there anything here that is worth pursuing at all? Or who cares, since I'm not using unfoldings anymore (I've switched over to storing IfaceExprs directly), and it works in GHC-the-program?

From: Simon Peyton Jones <simon.peytonjones at gmail.com>
Sent: Wednesday, April 6, 2022 3:37 PM
To: Erdi, Gergo <Gergo.Erdi at sc.com>
Cc: Gergo Érdi <gergo at erdi.hu>; GHC Devs <ghc-devs at haskell.org>; clash-language at googlegroups.com
Subject: [External] Re: Avoiding `OtherCon []` unfoldings, restoring definitions from unfoldings

Hi Gergo

If you think GHC has a but, can you open a ticket about this?   It's all getting lost in a maze of emails.

Can you also give precise repro instructions?  In an attempt to reproduce, I have just compiled this module (the code you gave)

{-# OPTIONS_GHC -fexpose-all-unfoldings #-}

module Foo where
nonEmptySubsequences :: [a] -> [[a]]
nonEmptySubsequences [] = []
nonEmptySubsequences (x:xs) = [x] : foldr f [] (nonEmptySubsequences xs)
   where
     f ys r = ys : (x:ys) : r

subsequences            :: [a] -> [[a]]
subsequences xs         =  [] : nonEmptySubsequences xs

with HEAD, and -O.  Unlike you, I see both unfoldings in the interface file.  So clearly you and I are doing different things -- but I can't work out what is different without more precision about what you are doing.   (It would be harder if it only happens when you use GHC as a library, but I think you are seeing this behaviour in regular GHC.)

Thanks

Simon

On Wed, 6 Apr 2022 at 03:05, Erdi, Gergo <Gergo.Erdi at sc.com<mailto:Gergo.Erdi at sc.com>> wrote:
PUBLIC

Just so this isn't prematurely all lost, I went back and looked for this example. With the following two definitions:

subsequences            :: [a] -> [[a]]
subsequences xs         =  [] : nonEmptySubsequences xs

nonEmptySubsequences         :: [a] -> [[a]]
nonEmptySubsequences []      =  []
nonEmptySubsequences (x:xs)  =  [x] : foldr f [] (nonEmptySubsequences xs)
  where f ys r = ys : (x : ys) : r

If I save the interface, load it back and look at the unfoldings, I see that `subsequences` has a useful unfolding:

  subsequences Unf{Src=<vanilla>, TopLvl=True, Value=True,
                   ConLike=True, WorkFree=True, Expandable=True,
                   Guidance=IF_ARGS [0] 30 10}

Whereas `nonEmptySubsequenes` has a trivial one:

nonEmptySubsequences OtherCon []

The interface I save is created from the ModDetails returned by tidyProgram. ExposeAllUnfoldings is set.

Unfortunately, I am unable to reproduce this from the command line using the GHC executable, so before I put in the effort of making a minimal example of using the GHC API to get this result, I would first like to know if this is even an interesting case, or if there is some explanation for why one of these two would have an unfolding when the other doesn't, in certain cases.

-----Original Message-----
From: ghc-devs <ghc-devs-bounces at haskell.org<mailto:ghc-devs-bounces at haskell.org>> On Behalf Of Gergo Érdi
Sent: Saturday, April 2, 2022 11:31 AM
To: Simon Peyton Jones <simon.peytonjones at gmail.com<mailto:simon.peytonjones at gmail.com>>
Cc: GHC Devs <ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>>; clash-language at googlegroups.com<mailto:clash-language at googlegroups.com>
Subject: [External] Re: Avoiding `OtherCon []` unfoldings, restoring definitions from unfoldings


I'm using Prep's output (mostly so that it's in ANF) in my full compilation pipeline, so ideally I would save Prep'd Core in my .hi-equivalents so that I don't have to rerun Prep on them every time I use them.

I'll get back to you with some concrete examples of `OtherCon []` vs.
meaningful unfoldings next week.

Merging with my other question about shadowing problems with `toIface*`, in summary it seems that what I really should be doing, is compiling up to Tidy, taking the `CoreBinding`s from there and using `toIfaceBinding` on them to save the definitions.

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