OK, I attached an example for a specialization of Show for a special case of a custom type, like Ratio in GHC.Real. 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@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@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@joachim-breitner.de • https://www.joachim-breitner.de/
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