Use type-level Set for exceptions

[chshersh]

Currently we just append lists. This can probably not be optimal for longer lists of exceptions.
We need to do something more efficient. Probably even use some GHC source plugin for type-level sets.

Using type-level sets has the following advantages:

• Order of exceptions doesn't matter
• All duplicates are removed
• If implemented via some good source-plugin, probably will be even more efficient than defining our own type families

[JonathanLorimer]

Does it make sense to use this package https://github.com/isovector/type-sets/ ?

[chshersh]

It's probably possible. But I'm afraid that the ergonomics will suffer. As far as I can tell from the library, the type-level set of exceptions needs to be constructed like this:

type Exceptions1 = Insert MyErr1 (Insert MyErr2 'Empty))

foo :: EIO Exceptions1 ()

While I was thinking more in the following design space:

foo :: EIO '[ MyErr1, MyErr2 ] ()

where the source plugin automatically creates sets for lists in the EIO type. So it will look more readable 🙂