Trying to get a Composite design pattern to work
I'm trying to implement hierarchical states in Haskell, following on from my work at doing them in C++. Here's what I've got so far: data StateNode a = CompositeState [ StateNode a ] | State a stateslist :: StateNode a -> [a] stateslist(State x) = [x] stateslist(CompositeState xs) = {- return list of objects of type a -} The following give errors (as they should) -- stateslist(CompositeState xs) = [ stateslist(x) | x <- xs ] -- stateslist(CompositeState xs) = map stateslist xs You see what I'm trying to do? This is how I want it to behave: sm1 = CompositeState [ State 1, State 2, State 3 ] stateslist(sm1) => [1, 2, 3] How can I get this to work? My mind has just gone all wobbly! Thanks
Asfand Yar Qazi wrote:
I'm trying to implement hierarchical states in Haskell, following on from my work at doing them in C++.
Here's what I've got so far:
data StateNode a = CompositeState [ StateNode a ] | State a stateslist :: StateNode a -> [a] stateslist(State x) = [x] stateslist(CompositeState xs) = {- return list of objects of type a -}
The following give errors (as they should) -- stateslist(CompositeState xs) = [ stateslist(x) | x <- xs ] -- stateslist(CompositeState xs) = map stateslist xs
You see what I'm trying to do? This is how I want it to behave:
sm1 = CompositeState [ State 1, State 2, State 3 ] stateslist(sm1) => [1, 2, 3]
Maybe... stateslist :: StateNode a -> [a] stateslist (State x) = [x] stateslist (CompositeState xs) = concatMap stateslist xs Greg Buchholz
On 3/13/06, Greg Buchholz <haskell@sleepingsquirrel.org> wrote:
Asfand Yar Qazi wrote:
I'm trying to implement hierarchical states in Haskell, following on from my work at doing them in C++.
Here's what I've got so far:
data StateNode a = CompositeState [ StateNode a ] | State a stateslist :: StateNode a -> [a] stateslist(State x) = [x] stateslist(CompositeState xs) = {- return list of objects of type a -}
The following give errors (as they should) -- stateslist(CompositeState xs) = [ stateslist(x) | x <- xs ] -- stateslist(CompositeState xs) = map stateslist xs
You see what I'm trying to do? This is how I want it to behave:
sm1 = CompositeState [ State 1, State 2, State 3 ] stateslist(sm1) => [1, 2, 3]
Maybe...
stateslist :: StateNode a -> [a] stateslist (State x) = [x] stateslist (CompositeState xs) = concatMap stateslist xs
Greg Buchholz
Actually, I'm trying to avoid library functions, so I can learn the language and the functional way of thinking. How would one implement the concatMap function? Thanks
G'day all. Quoting Asfand Yar Qazi <ayqazi@gmail.com>:
Actually, I'm trying to avoid library functions, so I can learn the language and the functional way of thinking. How would one implement the concatMap function?
See if you can work how how this one works. No library functions, apart from function composition. statesList :: StateNode a -> [a] statesList s = statesList' s [] where statesList' (State x) = (x:) statesList' (CompositeState xs) = statesLists xs statesLists [] = id statesLists (x:xs) = statesList' x . statesLists xs Cheers, Andrew Bromage
participants (3)
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ajb＠spamcop.net -
Asfand Yar Qazi -
Greg Buchholz