Or-patterns is a way of grouping together patterns that match to the same value. A construct like

fun 0 _ = E fun _ 0 = E

Could more concisely be written as, for example

fun 0 _ || _ 0 = E

As a concrete example why this is beautiful and how it could look, see Red-black trees in a functional setting, C. Okasaki [1].

I don't know enough about GHC internals to know the obvious way to implement this, but I would gladly give it a try given pointers in the right direction.

[1] http://www.eecs.usma.edu/webs/people/okasaki/jfp99.ps