Re: [Haskell-cafe] breadth first search one-liner?

On Mon, Mar 22, 2010 at 11:02:32AM +0100, Johannes Waldmann wrote:

Dear all, there is this neat "one-line" BFS implementation

bfs :: Eq a => ( a -> [a] ) -> a -> [a] bfs next start = let xs = nub $ start : ( xs >>= next ) in xs

but it has a problem: it only works for infinite graphs. This is fine:

take 20 $ bfs ( \ x -> [2*x, x+1] ) 1

but this is not:

take 20 $ bfs ( \ x -> filter (>0) [ div x 2, x - 1 ] ) 10

Is there a nice way to repair this?

bfs :: (a -> [a]) -> a -> [a] bfs f s = concat $ takeWhile (not . null) $ iterate (>>= f) [s]