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

Erdi, Gergo Gergo.Erdi at sc.com
Wed Apr 6 02:04:33 UTC 2022


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> On Behalf Of Gergo √Črdi
Sent: Saturday, April 2, 2022 11:31 AM
To: Simon Peyton Jones <simon.peytonjones at gmail.com>
Cc: GHC Devs <ghc-devs at haskell.org>; 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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