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

[Neil Mitchell]

> How's this?

What about ++, in Haskell thats just an ordinary function, yet you are
using the library one in this case.

The other thing is that the first definition of my_concat_map is
entirely redundant, the second one handles both cases anyway.

Also, for completeness, you might be interested to find that in some
compilers the list is actually defined as:

data [] a = [] | (:) a ([] a)

This isn't legal Haskell 98, but nhc and Yhc both define a list
similarly to this.

Thanks

Neil

>
> 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
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