[Haskell-cafe] Use cases of empty type classes

[David Feuer]

The constraints package currently uses

class Any => Bottom where
  no :: Dict a

It should probably actually use

no :: a

but that's a minor point.

There are two ideas:

1. The class constraint Any is a stuck type family, so it's utterly
impossible to create an instance of Bottom. Thus any entailment   c :-
Bottom  implies that c is unsatisfiable.
2. There's an entailment

bottom :: Bottom :- c
bottom = Sub no

On Thu, Mar 10, 2016 at 1:42 PM, Mitchell Rosen <mitchellwrosen at gmail.com>
wrote:

> One empty typeclass I use often not only has no methods, but also has no
> members.
>
> class Void
>
> This is useful in type families that return Constraint, when you want to
> say this constraint is "unsatisfiable". I suppose returning something like
> "True ~ False" would also suffice, but isn't as pretty.
>
> On Monday, March 7, 2016 at 10:10:48 PM UTC-8, Tomas Tauber wrote:
>>
>> Hi everyone,
>>
>> I have one question. What are current use cases of type classes with no
>> methods?
>>
>> I saw early uses in type-level programming (e.g. HList [1]).
>> In the OO world, interfaces with no methods are called marker interfaces
>> -- their use cases range from things that could be done with datatype
>> generic programming in Haskell (e.g. serialization) to metadata annotations
>> (e.g. RandomAccess [2]).
>>
>> Regards,
>>
>> Tomas Tauber
>>
>>
>> [1] http://okmij.org/ftp/Haskell/HList-ext.pdf
>> [2] https://docs.oracle.com/javase/8/docs/api/java/util/RandomAccess.html
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