Re: [Haskell-cafe] Embedded Functions in Algebraic Data Types?

On Feb 10, 2008 1:42 PM, Sebastian Sylvan wrote:

On Feb 10, 2008 1:34 PM, Michael Feathers 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).

Perhaps I should add an empty MyMap too: empty = M (\_-> Nothing ) -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862