Re: VisibleTypeApplication and AllowAmbiguousTypes

On 15/03/16 14:53, Richard Eisenberg wrote:

I'm happy to change the setting, but my logic was this: With -XTypeApplications, there are no ambiguous types. Of course, there may be types that are unambiguous locally but might be ambiguous in downstream modules that don't use -XTypeApplications, and this is what Andrew is (quite validly) getting at.

Yes. I think the key point here is that TypeApplications is typically required at use sites, whereas AllowAmbiguousTypes is needed at definition sites. AllowAmbiguousTypes is a heuristic: ambiguity can arise without it, and even before TypeApplications there were cases in which AllowAmbiguousTypes was necessary to permit polymorphic definitions that could be used unambiguously in some contexts. With TypeApplications, any definition can be used given a suitable context, but ambiguity is still a useful rule for deciding whether a definition is likely to lead to type inference trouble. TL;DR I agree that we should drop the implication. All the best, Adam

Richard

On Mar 15, 2016, at 9:16 AM, Ben Gamari wrote:

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Andrew Martin writes:

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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

-- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/