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).
-- Sebastian Sylvan
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