[Haskell] Trying to get a Composite design pattern to work

Asfand Yar Qazi ayqazi at gmail.com
Mon Mar 13 16:26:08 EST 2006

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 ]
	=> [1, 2, 3]

How can I get this to work?  My mind has just gone all wobbly!


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