strobogrammatic numbers

Main.hs

import Control.Applicative
import Control.Monad
import Control.DeepSeq
import Data.List
import System.Environment

--

{-
strobo' :: Int -> [String]
strobo' n
    | n <= 0    = [""]
    | n == 1    = ["0", "1", "8"]
    | otherwise =
        let surrounders = fmap surround [('0', '0'), ('1', '1'), ('6', '9'), ('8', '8'), ('9', '6')]
        in surrounders <*> strobo' (n - 2)
    where
        surround (x, y) xs = x : xs ++ [y]

strobo :: Int -> [String]
strobo = filter predicate . strobo'
    where predicate = (||) <$> (/=) '0' . head <*> (==) 1 . length
-}

strobo'' :: Int -> [String]
strobo'' n
    | n <= 0    = error "Must be positive"
    | n <= 2    = ["1", "9", "8", "6"]
    | otherwise = (flip (:)) <$> strobo'' (n - 2) <*> ['0', '1', '9', '8', '6']

strobo' :: Int -> [String]
strobo' n
    | n <= 0    = error "Must be positive"
    | n == 1    = ["0", "1", "8"]
    | n == 2    = ["1", "9", "8", "6"]
    | otherwise = (flip (:)) <$> strobo'' (n - 2) <*> (if odd n then ['0', '1', '8'] else ['0', '1', '9', '8', '6'])

strobo :: Int -> [String]
strobo n
    | odd n     = map (foldl' f <$> tail <*> id) $ strobo' n
    | otherwise = map (foldl' f <$> id <*> id) $ strobo' n
    where
        f acc '6' = '9' : acc
        f acc '9' = '6' : acc
        f acc x   = x : acc

sn :: [String]
sn = join $ map strobo [1..]

--

main :: IO ()
main = do
    args <- getArgs
    case args of
        "interactive":_ -> forever $ do
            n <- read <$> getLine
            putStrLn $ sn !! (n - 1)
        "digits":n:_ ->
            (join $ map strobo [1..read n]) `deepseq` return ()
        _ ->
            mapM_ putStrLn $ sn

Makefile

HC := ghc
HFLAGS := -O2 -Wall
LDFLAGS := -package deepseq -rtsopts

SRCS := Main.hs
OBJS := $(SRCS:.hs=.o)
INTS := $(SRCS:.hs=.hi)
PROG := run

.PHONY: all clean

all: $(PROG)

$(PROG): $(OBJS)
        $(HC) $(LDFLAGS) $^ -o $@

%.o: %.hs
        $(HC) $(HFLAGS) -c $< -o $@

clean:
        -rm $(PROG) $(INTS) $(OBJS)