Finding out if a binding is externally visible

[Sebastian Graf]

Also I should add that this is probably not just happening for ids
mentioned by RULEs, but also for Unfoldings. E.g. although wrappers are
exported, associated workers are not, but still visible through unfoldings.
It's not as bad as with RULEs since they are still reachable, but it might
make for weird results... And also contradicts Note [ExportFlag on binders]
in Var.hs as I understand it.

On Mon, May 29, 2017 at 10:40 AM, Sebastian Graf <sgraf1337 at gmail.com>
wrote:

> OK, I attached an example for a specialization of Show for a special case
> of a custom type, like Ratio in GHC.Real
> <https://hackage.haskell.org/package/base-4.9.1.0/docs/src/GHC.Real.html>.
> My invokation with a custom GHC (8.0-based) was like this:
>
>     $ <ghc> Module.hs -ddump-simpl -O -fforce-recomp
>
> At the begin of the log, I listed all exported top-level identifiers, the
> only interesting of which is `$fShowRatio :: Show a => Show (Ratio a)`, the
> default implementation. Note that at the bottom, there are RULEs stating
> the specialization for `Show Integer`, but that the specialized dictionary
> `$fShowRatio_$s$fShowRatio :: Show (Ratio Integer)` isn't otherwise
> reachable from any exported top-level identifier. Same goes for any of the
> specialized dictionary methods.
>
> Now consider what happens if we request a `Show (Ratio Integer)`
> dictionary in another module: GHC finds the exported default dictionary
> `$fShowRatio :: Show a => Show (Ratio a)`, but then applies a
> specialization RULE that will effectively use the presumably absent
> `$fShowRatio_$s$fShowRatio :: Show (Ratio Integer)`.
>
> In my case, my custom GHC would substitute away the `showString " % "` for
> an `absentError "lvl [Char]"` and crash in subtle ways. The only reason
> this isn't happening for GHC master is that DmdAnal does consider all
> top-level arguments to be used, instead of only the exported ones (which is
> what CallArity does).
>
> On Sun, May 28, 2017 at 11:53 PM, Sebastian Graf <sgraf1337 at gmail.com>
> wrote:
>
>>
>> welcome on this list!
>>
>>
>> Thanks :)
>>
>>      isExportedId :: Var -> Bool
>>
>>
>> That's what I have been using so far, but I found that said RULE pragmas
>> mentioned non-exported identifiers. Or maybe it was just some temporary
>> build system mess-up, I'll work on a reproduction...
>>
>> On Sun, May 28, 2017 at 10:33 PM, Joachim Breitner <
>> mail at joachim-breitner.de> wrote:
>>
>>> Hi Sebastian,
>>>
>>> welcome on this list!
>>>
>>> Am Sonntag, den 28.05.2017, 17:52 +0200 schrieb Sebastian Graf:
>>> > Is there some function that tells me if an identifier is 'externally
>>> > visible' in that sense? I think https://downloads.haskell.org/~ghc/8.
>>> > 0.1/docs/html/libraries/ghc-8.0.1/GHC.html#v:isExternalName is what
>>> > I'm after, but I want to be sure. Does that function already work
>>> > when the Ids are still local?
>>>
>>> I would expect
>>>
>>>     isExportedId :: Var -> Bool
>>>
>>> to do precisely that.
>>>
>>> Greetings,
>>> Joachim
>>>
>>> --
>>> Joachim “nomeata” Breitner
>>>   mail at joachim-breitner.de • https://www.joachim-breitner.de/
>>>   XMPP: nomeata at joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F
>>>   Debian Developer: nomeata at debian.org
>>> _______________________________________________
>>> ghc-devs mailing list
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>>>
>>>
>>
>
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