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@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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