On Thu, Jan 22, 2015 at 4:31 AM, Adam Gundry <adam@well-typed.com> wrote:

Actually, the simplifications I recently came up with could allow us to
make uses of the field work as van Laarhoven lenses, other lenses *and*
selector functions. In practice, however, I suspect this might lead to
somewhat confusing error messages, so it might not be desirable.

Interesting. Have you actually tried this with a composition of your simplified form, because I don't see how that can work.

When we tried this before we showed that there was a fundamental limitation in the way the functional dependencies had to flow information down the chain, also, "foo.bar.baz" has very different interpretations, between the lens and normal accessors, and both are producing functions, so its hard to see how this doesn't yield overlapping instance hell.

-Edward