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2020/11/03に公開約3,700字

``````-- Lake Counting (POJ No.2386)
-- https://lvs7k.github.io/posts/2018/11/pccb-easy-1/
import Data.Array.ST
import Data.Array.Unboxed (UArray)
import Data.Array.IArray
import Data.Char
import Data.STRef
import qualified Data.Set as S
import Data.Typeable

solve :: Int -> Int -> [String] -> Int
solve n m xss = runST \$ do
as <- newListArray ((1, 1), (n, m)) (concat xss) :: ST s (STUArray s (Int, Int) Char)
refc <- newSTRef 0
forM_ [1 .. n] \$ \i -> do
forM_ [1 .. m] \$ \j -> do
c <- readArray as (i, j)
when (c == 'W') \$ do
dfs as (i, j)
modifySTRef' refc (+ 1)
where
dfs as (i, j) = do
c <- readArray as (i, j)
case c of
'.' -> return ()
_   -> do
writeArray as (i, j) '.'
mapM_ (dfs as) next
where
next = do
s <- [i - 1, i, i + 1]
t <- [j - 1, j, j + 1]
guard (inRange ((1, 1), (n, m)) (s, t))
return (s, t)

rInt :: String -> Int
rInt str = read str :: Int

main :: IO ()
main = do
n <- rInt <\$> getLine
m <- rInt <\$> getLine
inp <- getContents
let garden = lines inp
-- print (typeOf garden)
print garden
print \$ solve n m garden
``````

これをmain.hsで作成して、data.txtを作っておき

``````10
12
W........WW.
.WWW.....WWW
....WW...WW.
.........WW.
.........W..
..W......W..
.W.W.....WW.
W.W.W.....W.
.W.W......W.
..W.......W.
``````

``````stack runghc main.hs < data.txt
``````

だがしかし、そう簡単にはいかない。。エラーになっちゃう。

``````main.hs:27:9: error:
• Non type-variable argument in the constraint: MArray a Char m
(Use FlexibleContexts to permit this)
• When checking the inferred type
dfs :: forall (m :: * -> *) (a :: * -> * -> *).
MArray a Char m =>
a (Int, Int) Char -> (Int, Int) -> m ()
In an equation for ‘solve’:
solve n m xss
= runST
\$ do as <- newListArray ((1, 1), (n, m)) (concat xss) ::
ST s (STUArray s (Int, Int) Char)
refc <- newSTRef 0
forM_ [1 .. n] \$ \ i -> ...
....
where
dfs as (i, j)
= do c <- readArray as ...
....
where
next = ...
|
27 |         dfs as (i, j) = do
|         ^^^^^^^^^^^^^^^^^^...
``````

ちょっと状態系モナドを調べてきます。。

``````solve :: Int -> Int -> [String] -> Int
solve n m xss = runST \$ do
as <- newListArray ((1, 1), (n, m)) (concat xss) :: ST s (STUArray s (Int, Int) Char)
refc <- newSTRef 0
forM_ [1 .. n] \$ \i -> do
forM_ [1 .. m] \$ \j -> do
c <- readArray as (i, j)
when (c == 'W') \$ do
dfs as (i, j)
modifySTRef' refc (+ 1)
where
dfs as (i, j) = do
c <- readArray as (i, j)
case c of
-- '.' -> return ()
_   -> do
-- writeArray as (i, j) '.'
mapM_ (dfs as) next
where
next = do
s <- [i - 1, i, i + 1]
t <- [j - 1, j, j + 1]
guard (inRange ((1, 1), (n, m)) (s, t))
return (s, t)
``````

コメントアウトしている箇所を有効にするとビルドエラーになる。型が合わない。
'.'が駄目なようだがなぜだろう。。

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