[Haskell-cafe] lazy A-star search

Anton Kholomiov anton.kholomiov at gmail.com
Sat Oct 22 14:28:44 CEST 2011

Recently I was looking for an A-star search algorithm. I've found a package
but I couldn't understand the code. Then I saw some blogposts but they
 were difficult to understand too. I thought about some easier solution that
relies on laziness. And I've come to this:

Heuristic search is like depth-first search but solutions in sub-trees
are concatenated with mergeBy function, that concatenates two
list by specific order:

module Search where

import Control.Applicative
import Data.Function(on)
import Control.Arrow(second)
import Data.Tree

-- | Heuristic search. Nodes are visited from smaller to greater.
searchBy :: (a -> a -> Ordering) -> Tree a -> [a]
searchBy  heur (Node v ts) =
    v : foldr (mergeBy heur) [] (searchBy heur <$> ts)

-- | Merge two lists. Elements concatenated in specified order.
mergeBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
mergeBy _ a         []      = a
mergeBy _ []        b       = b
mergeBy p (a:as)    (b:bs)
    | a `p` b == LT    = a : mergeBy p as (b:bs)
    | otherwise         = b : mergeBy p bs (a:as)

Now we can define specific heuristic search in terms of searchBy:

-- | Heuristic is distance to goal.
bestFirst :: Ord h => (a -> h) -> (a -> [a]) -> a -> [a]
bestFirst dist alts =
    searchBy (compare `on` dist) . unfoldTree (\a -> (a, alts a))

-- | A-star search.
-- Heuristic is estimated length of whole path.
astar :: (Ord h, Num h) => (a -> h) -> (a -> [(a, h)]) -> a -> [a]
astar dist alts s0 = fmap fst $
    searchBy (compare `on` astarDist) $ unfoldTree gen (s0, 0)
    where astarDist (a, d) = dist a + d
          gen (a, d)  = d `seq` ((a, d), second (+d) <$> alts a)

I'm wondering is it effective enough?

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