On Thu, Nov 27, 2008 at 8:29 PM, Simon Peyton-Jones
So this message is to ask you:
can you tell us about the most persuasive, fun application you've encountered, for type families or functional dependencies?
I only just discovered functional dependencies a week or two ago, and when it solved my problem it absolutely made my day. I was reading 'Bananas, Lenses, etc.' and wanted to implement paramorphic recursion with a type class. My initial attempt: class Paramorphic p where term :: p -> Bool this :: p -> a next ::: p -> p para :: (Paramorphic p) => t -> (a -> (p, t) -> t) ->p -> t para a f p | term p = a | otherwise = f (this p) (next p, para a f (next p)) instance Paramorphic Int where term = (== 0) this = id next = subtract 1 This is broken, since 'a' in 'this' is loose. Then I found multiple class parameters: class Paramorphic p a where ... But 'a' was still too loose--para's type became para :: (Paramorphic p a, Paramorphic p a1, Paramorphic p a2, Paramorphic p a3) => t -> (a1 -> (p, t) -> t) -> p -> t But a fundep solved it for me: class Paramorphic p a | p -> a where ... I could now pretty much do factorial and tails from the paper. fact = para 1 (\a (_, b) -> a * b) instance Paramorphic [a] a where term = null this = head next = tail tails = para [[]] (\a (as, b) -> (a : as) : b) Not exactly a 'fun' example, but I was smiling for hours after discovering this. :) Toby.