VoidF

[langston-barrett]

What4 uses this one-off, uninhabited, kind-polymorphic Dummy type, and makes it an instance of several classes in this package.

https://github.com/GaloisInc/what4/blob/038e948817b092ac7750e073f1d430ee293f3c20/what4/src/What4/Expr/App.hs#L763

A few other projects make use of an isomorphic type, without the instances:

https://github.com/GaloisInc/grappa/blob/dd694520dfc5b6b90e01def72575235ab1c874ac/src/Language/Grappa/Interp/BayesNet.hs#L258

https://github.com/GaloisInc/ouroboros-network/blob/7286a2e6832fa3789fd9afc5968f1dce94555aea/ouroboros-consensus/src/Ouroboros/Consensus/Ledger/Basics.hs#L64

The barbies package, which has classes analogous to our TraversableF{,C} has a version of Void.

Perhaps we should consider adding such a type to this library, and making it (trivially) an instance of all the classes herein?