Shadowing in toIface* output

Tue Apr 5 03:08:53 UTC 2022

PUBLIC

OK, I must be doing something wrong then. I am now looking at Tidy (not Prep) output, and I see Core like this:

  showsPrec :: forall a. Show a => Int -> a -> ShowS
  [GblId[ClassOp],
   Arity=1,
   Caf=NoCafRefs,
   Str=<S!P(SL,A,A)>,
   RULES: Built in rule for showsPrec: "Class op showsPrec"]
  showsPrec
    = \ (@a_a1G3) (v_B1 :: Show a_a1G3) ->
        case v_B1 of v_B1 { C:Show v_B2 v_B3 v_B4 -> v_B2 }

so if I'm reading this right, these vs are still shadowing.

I am using tidyProgram on the output of hscSimplify, and then taking the cg_binds of its result. What else should I do to get tidy (heh) Tidy output?

From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Simon Peyton Jones
Sent: Monday, April 4, 2022 4:28 PM
To: Gergő Érdi <gergo at erdi.hu>
Cc: GHC Devs <ghc-devs at haskell.org>
Subject: [External] Re: Shadowing in toIface* output

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So does that mean Tidy produces unique `occNameFS`s, and then `Prep`
breaks them?

Tidy does not produce unique OccNames.  Rather, it avoids shadowing, so that if you delete all the uniques and print out the program (which is precisely what happens in an .hi file) you'll still get something sensible.

I'm not sure whether or not Prep maintains this invariant.  There is no particular reason it should.  It might, but it is not (currently) a goal.

Simon

On Sat, 2 Apr 2022 at 04:35, Gergő Érdi <gergo at erdi.hu<mailto:gergo at erdi.hu>> wrote:
So does that mean Tidy produces unique `occNameFS`s, and then `Prep`
breaks them?

On Fri, Apr 1, 2022 at 10:35 PM Josh Meredith
<joshmeredith2008 at gmail.com<mailto:joshmeredith2008 at gmail.com>> wrote:
>
> Hi,
>
> I encountered this when we used that for Plutus. I'll have to dig up the details, but IIRC `toIfaceExpr` expects GHC to have already tidied the output, which deals with this issue of overlapping variable names.
>
> Cheers,
> Josh
>
> On Sat, 2 Apr 2022 at 01:26, ÉRDI Gergő <gergo at erdi.hu<mailto:gergo at erdi.hu>> wrote:
>>
>> Hi,
>>
>> I'm trying to save (Prep'd) Core bindings right next to the serialized
>> `ModIface` (so basically `put_`ing them into the same bytestream, after the
>> `ModIface`), and that's exactly what the functions in `GHC.CoreToIface`
>> seem to be for, so I expected it to Just Work. However, I noticed that I
>> very frequently get problems with shadowing. For example, Core that looks
>> like `\v{u1} v{u2} -> v{u1}` would get translated to `\v v -> v`, which is
>> disastrous since these locally bound `Var`s are represented as just their
>> `getOccFS` (i.e. the `FastString` `"v"`).
>>
>> But this can't be right: if `toIfaceExpr` &c. would fail this blatently,
>> then the unfoldings couldn't be saved & restored, which is something GHC
>> itself does as part of normal `.hi` file handling. So clearly I must be
>> doing something wrong.
>>
>> So I guess my question could be, what could be causing `toIfaceExpr` (a
>> pure function!) to behave this way for my Cores? But then, if I look at
>> the implementation of `toIface*`, I can see that it really doesn't do
>> anything smarter than just storing `getOccFS` in the interface (no
>> uniques in sight)-- so maybe my *real* question is, what is GHC itself
>> doing so that it doesn't have this same problem?
>>
>> Thanks,
>>         Gergo
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