Generation of random shrinkable, showable functions. See the paper "Shrinking and showing functions" by Koen Claessen.

Example of use:

>>> :{
>>> let prop :: Fun String Integer -> Bool
>>> prop (Fun _ f) = f "monkey" == f "banana" || f "banana" == f "elephant"
>>> :}
>>> quickCheck prop
*** Failed! Falsifiable (after 3 tests and 134 shrinks):
{"elephant"->1, "monkey"->1, _->0}

To generate random values of type Fun a b, you must have an instance Function a. If your type has a Show instance, you can use functionShow to write the instance; otherwise, use functionMap to give a bijection between your type and a type that is already an instance of Function. See the Function [a] instance for an example of the latter.