Phil Wadler wants to control the pretty-printer used to display output in GHCi. Something: adding a flag

  -pretty <function-name>

The supplied function will typically have type (C a => a -> String) for some class C. Using Razvan's libary, he could declare

  :set -pretty GenericPretty.pretty

for example.

Since this is GHCi-specific, one could use a new GHCi command instead:

:pretty GenericPretty.pretty

The latter design would presumably look up GenericPretty.pretty in the current context, and bleat if not found. The version with a flag might do that each time the pretty printer was used, which is arguably less efficient and (more important) more confusing. Personally I think I prefer the :pretty command best. (I'm not fussy about the name.)