On Feb 7, 2016, at 4:22 AM, Evan Laforge
1, Just to clarify, there are no actual known uses for a TypeRep-polymorphic ($), right? I thought I saw someone say that.
Here is where this treatment of ($) was introduced: https://ghc.haskell.org/trac/ghc/ticket/8739
2, ($) has had a fib in its type for a very long time, but did it ever hurt anyone? The closest I saw was a generalized concern about it being bad when people report a bug and then hear that things are more general than they thought and I guess this makes their bug not a bug or something? Results in some confusing back-and-forth? It would be nice to get more specific about how much trouble the ($) lie has caused.
I don't have data, but there is a real cost to lying. It shows up in the slow-ish but steady stream of posts / questions / bug reports that are produced saying something is weird. I've seen a good number of these come up in my years in the Haskell community. I'll note that there is also a real cost to telling the truth: witness this thread. This all adds up to a need to do both, which is what we would get by having a richer REPL environment.
3, Is anything other than ($) TypeRep-polymorphic?
Yes: undefined and error. These really are used at unlifted types.
If 2 is "not very much", an obvious solution is to keep lying and not worry about it. But clearly someone had a reason strong enough to make this change... what was it?
The change to the user-visible type of ($) is due to updates around TypeInType. But ($)'s ability to handle this case has been around since 7.8. Note that if you say :i $ in GHCi 7.10 you get a mention of OpenKind, which is the ancestor of current representation-polymorphism. It's just that :t ($) did a nice job of hiding it.
I hesitate to get distracted from the specifics, but I agree with Ben Lippemeier in that I like how haskell doesn't make me worry about boxed vs. unboxed. I'm not saying one little type annotation on one operator is suddenly making me worry since I've never even thought to run :t ($). But I'm happy that # is mostly off in its own little corner and you don't even have to know it exists until you need to get into low level optimization.
And this is what the new -fshow-runtime-rep flag will (re-)enable. Richard