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
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/ XMPP: nomeata@joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs