Transferring from
https://mail.haskell.org/pipermail/haskell-prime/2018-October/004396.html
On Sat, 13 Oct 2018 at 4:04 AM,
Consider for example:
class F a b where f:: a → b class X a where x:: a fx = f x
The type of fx, (F a b, X a) ⇒ b, should not be ambiguous: in distinct contexts, fx can have distinct types (if ambiguity, and 'improvement', are as defined below).
[See the -prime message for the "defined below".] I'm just not seeing why you think any Haskell should accept that. What value could inhabit `fx` except bottom? And indeed no Haskell does accept it, not even GHC with all sorts of nasty extensions switched on, including `AllowAmbiguousTypes`. GHC will accept class `F` with a Functional Dependency `b -> a`, but still I can't 'poke' any value into the `x` argument to `f` in the equation for `fx`. Note: agreeing with this view can lead to far-reaching consequences, e.g.
support of overloaded record fields [1,Section 7], ...
There are other ways to support overloaded/duplicate record fields, without doing this much violence to the type system. Look at the `HasField` class using Functional Dependencies, in Adam Gundry's work. polymonads [2] etc.
Note that's from a Trac ticket asking for 'dysfunctional' Functional Dependencies. There's a long discussion from the type-inference brains trust coming to no discernable conclusion as to whether it's broken type safety. (Catching incoherence in the Core lint typecheck is not a good look.) a) You've not shown any detailed working of how your proposal gives the type improvement required without also descending into incoherence. b) The combination of Functional Dependencies+Overlapping Instances+Undecidable Instances might be able to cover just enough, but not too much of the requirements (which were never made clear). See my worked examples on that ticket -- some of which are really quite scary. See some of Oleg's work on his ftp site with multi-directional FunDeps and overlaps to achieve injectivity. To try to tame the scariness while still supporting enough power, see the suggestion here https://ghc.haskell.org/trac/ghc/ticket/15632
Further examples can be discussed
I have yet to see a convincing use case (from the very lengthy discussions) which couldn't be handled already in GHC. I agree the combination of extensions in GHC (including its bogus consistency check/Trac #10675) can give alarming surprises; but they don't quite break type safety. but this example conveys the
main idea that ambiguity should be changed; unlike the example of (show . read), no type annotation can avoid ambiguity of polymorphic fx in current Haskell.
Since `fx` is not accepted in current Haskell, whether you can put a type annotation seems beside the point. AntC [1] Optional Type Classes for Haskell, Rodrigo Ribeiro, Carlos Camarão, Lucília Figueiredo, Cristiano Vasconcellos, SBLP'2016 (20th Brazilian Symposium on Programming Languages), Marília, SP, September 19-23, 2016. [2] https://github.com/carlos1camarao/ghc-proposals/blob/d81c1f26298961ac635ce07...