VisibleTypeApplication and AllowAmbiguousTypes

[Ben Gamari]

Andrew Martin <andrew.thaddeus at gmail.com> writes:

> I'm posting this because Richard said it would be the best place to raise
> this issue. I know it's a little bit late in the GHC8 development process,
> but I would like to get the idea out there.
>
> To my knowledge, in GHC8, turning on VisibleTypeApplication will also turn
> on AllowAmbiguousTypes. I think that a better behavior for most end users
> would be to not have this happen.
>
> I need AllowAmbiguousTypes turned on in modules where I want to declare
> functions that are presently uncalled. At a call site though, I only need
> VisibleTypeApplication, not both extensions. The only reason I bring this
> up is because having AllowAmbiguousTypes on is usually bad. It makes it
> possible to write functions that don't work, but you don't learn that until
> you get to the call site. If I write libraries that require
> VisibleTypeAppliaction to call certain functions, I don't want users
> accidentally ending up turning on AllowAmbiguousTypes in their own code.
>
It sounds reasonable to me to require that those defining
functions to be used with type application enable AllowAmbiguousTypes.
In my opinion we should avoid adding extension implications unless
the extensions in question are essentially always used together and this
doesn't appear to be true in the case of TypeApplications and
AllowAmbiguousTypes.

Cheers,
- Ben
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