Quite frequently. Here are a few examples from my own code: For "functional references" (representing a bidirectional function from a data type to a part of itself (for example the first element of a pair)).
data Accessor s a = Accessor { get :: s -> a , set :: a -> s -> s }
My quantum computation arrow (really in the realm of "concrete, useful things", huh? :-)
data Operator b c = Op (forall d. QStateVec (b,d) -> IO (QStateVec (c,d))) | ...
The ubiquitous FRP Behavior, comprising a current value and a function which takes a timestep and returns the next value.
data Behavior a = Behavior a (Double -> Behavior a)
The "suspend" monad, representing a computation which can either finish now with a value of type a, or suspends to request a value of type v before continuing.
data Suspend v a = Return a | Suspend (v -> Suspend v a)
It seems that most frequently, functions in ADTs are used when
implementing monads or arrows, but that doens't need to be the case.
A lot of times a function is the best way to represent a particular
part of a data structure :-)
Luke
On Feb 10, 2008 1:34 PM, Michael Feathers
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?
Michael
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe