[Haskell-cafe] Re: [Haskell] Trying to get a Composite design pattern to work

[Asfand Yar Qazi]

On 3/13/06, Jared Updike <jupdike at gmail.com> wrote:
>
> If you want a place to start, I would challenge you to expand the
> concatMap definition above with the definitions of concat and map, and
> then plug in the definition of foldr and see if you can make your own
> concatMap function. Maybe that will help you understand things better.

MUHAHAHAHA I AM TEH HAXXELL GENEUS.  Ahem...

How's this?

myhead :: [a] -> a
myhead (x:xs) = x

mytail :: [a] -> [a]
mytail (x:xs) = xs

mymap :: (a -> b) -> [a] -> [b]
mymap fn [] = []
mymap fn (x:xs) = fn x : mymap fn xs

myconcat :: [[a]] -> [a]
myconcat (x:xs) = x ++ myconcat xs
myconcat [] = []

-- For each a in [a], produce a [b] in another list, then concat the
-- list.
my_concat_map :: (a -> [b]) -> [a] -> [b]
my_concat_map fn [] = []
my_concat_map fn xs = myconcat (mymap fn xs)

stateslist :: StateNode a -> [a]
stateslist(State x) = [x]
stateslist (CompositeState xs) = my_concat_map stateslist xs