On Feb 10, 2008 1:34 PM, Michael Feathers <mfeathers@mindspring.com> wrote:

On a lark, I loaded this into Hugs this morning, and it didn't complain:


data Thing = Thing (Integer -> Integer)



But, I've never seen that sort of construct in an example.  Do people
ever embed functions in ADTs?

Yes, this is quite common. Here's a quick example (untested):

data MyMap key value = M (key -> Maybe value)

lookup (M m) key = m key
insert (M m) key value = M (\key' -> if key' == key then Just value else m key')

This is a naive data structure for storing a map as a function from the key to the value. Not very efficient, perhaps, but you use similar concepts in more useful scenarios (e.g. the State monad).