Shadowing in toIface* output

Josh Meredith joshmeredith2008 at
Fri Apr 1 14:33:39 UTC 2022


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.


On Sat, 2 Apr 2022 at 01:26, ÉRDI Gergő <gergo at> 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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