pphetra:
I would like to write a program that can do something like this.
;; lisp syntax * (my-flatten '(1 (2 (3 4) 5))) (1 2 3 4 5)
I end up like this.
data Store a = E a | S [Store a] deriving (Show)
flat :: [Store a] -> [a] flat [] = [] flat ((E x):xs) = [x] ++ flat xs flat ((S x):xs) = flat x ++ flat xs
so *Main> flat [E 1, S[E 2, S[E 3, E 4], E 5]] [1,2,3,4,5]
Compare to a Lisp solution, It 's not looking good. Any suggestion.
Since this data type:
data Store a = E a | S [Store a] deriving (Show)
Is isomorphic to the normal Data.Tree type anyway, so we'll use that:
data Tree a = N a [Tree a] deriving Show
to define a new tree:
tree = N 1 [N 2 [N 3 [], N 4 []], N 5 []]
Now we can flatten by folding:
flatten t = go t [] where go (N x ts) xs = x : foldr go xs ts
So we can flatten our test tree:
list = flatten tree
Even run it:
main = print (flatten tree)
Or in GHCi: *Main> flatten tree [1,2,3,4,5] Based on Data.Tree in the base library. -- Don