Real World Haskell
実戦で学ぶ関数型言語プログラミング
(オライリージャパン)
Bryan O'Sullivan (著) John Goerzen (著)
Don Stewart (著)
山下 伸夫 (翻訳) 伊東 勝利 (翻訳)
株式会社タイムインターメディア (翻訳)
開発環境
- OS X Mavericks - Apple(OS)
- BBEdit - Bare Bones Software, Inc., Emacs (Text Editor)
- Haskell (純粋関数型プログラミング言語)
- GHC (The Glasgow Haskell Compiler) (処理系)
- The Haskell Platform (インストール方法、モジュール等)
Real World Haskell―実戦で学ぶ関数型言語プログラミング(Bryan O'Sullivan (著)、 John Goerzen (著)、 Don Stewart (著)、山下 伸夫 (翻訳)、伊東 勝利 (翻訳)、株式会社タイムインターメディア (翻訳)、オライリージャパン)の10章(コード事例研究: バイナリデータフォーマットの構文解析)、10.3(「ボイラープレート」コードの除去)、10.9(今後の方向性)の練習問題 1.を解いてみる。
その他参考書籍
- すごいHaskellたのしく学ぼう!(オーム社) Miran Lipovača(著)、田中 英行、村主 崇行(翻訳)
- プログラミングHaskell (オーム社) Graham Hutton(著) 山本 和彦(翻訳)
練習問題 1.
コード(BBEdit, Emacs)
ParseP2.hs
{-# OPTIONS -Wall -Werror #-}
module ParseP2 where
import Control.Applicative ((<$>))
import Data.Char (isSpace, isDigit)
import System.Environment (getArgs)
main :: IO ()
main = do
(a:_) <- getArgs
contents <- readFile a
let g = parseP2 (ParseState contents)
print $ fmap fst g
putStrLn "データ"
print $ greyData <$> fst <$> g
-- プレーンフォーマット
data GreymapP2 = GreymapP2 {greyWith :: Int,
greyHeight :: Int,
greyMax :: Int,
greyData :: [Int]}
deriving (Eq)
instance Show GreymapP2 where
show (GreymapP2 w h m _) = "GreymapP2 " ++ show w ++ "x" ++ show h ++
" " ++ show m
-- 解析状態の型、ParseState型を定義
-- stringは未処理の文字列
data ParseState = ParseState {string :: String} deriving (Show)
{--
タプル(ペア)のパターンマッチを除去
各解析関数(matchHeader, skipSpaces, parseNat, parseInts)関数を
解析状態を第一引数としてとり、解析結果に使われる中間的なもの
(幅、高さ、最大グレー値等)と解析状態を返す関数に修正
ParseState -> Maybe (a, ParseState)
(aが中間的なもの)
--}
parseP2 :: ParseState -> Maybe (GreymapP2, ParseState)
parseP2 =
parseHeader ==> \header -> skipSpaces ==>&
assert (header == "P2") ==>&
parseNat ==> \width -> skipSpaces ==>&
parseNat ==> \height -> skipSpaces ==>&
parseNat ==> \maxGrey -> skipSpaces ==>&
assert (maxGrey <= 255) ==>&
parseInts (width * height) ==> \ns ->
\s -> Just (GreymapP2 width height maxGrey ns, s)
(==>) :: (ParseState -> Maybe (a, ParseState)) ->
(a -> ParseState -> Maybe (b, ParseState)) ->
ParseState -> Maybe (b, ParseState)
firstParser ==> secondParser = chainedParser
where chainedParser initState =
case firstParser initState of
Nothing -> Nothing
Just (firstResult, newState) ->
secondParser firstResult newState
(==>&) :: (ParseState -> Maybe (a, ParseState)) ->
(ParseState -> Maybe (b, ParseState)) ->
ParseState -> Maybe (b, ParseState)
p ==>& f = p ==> \_ -> f
skipSpaces :: ParseState -> Maybe ((), ParseState)
skipSpaces initState =
Just ((), ParseState (dropWhile isSpace (string initState)))
assert :: Bool -> (ParseState -> Maybe ((), ParseState))
assert True = \s -> Just ((), s)
assert False = \_ -> Nothing
parseHeader :: ParseState -> Maybe (String, ParseState)
parseHeader initState =
let str = string initState
header = takeWhile (not . isSpace) str
in Just (header, ParseState (drop (length header) str))
-- nat(natural number)
parseNat :: ParseState -> Maybe (Int, ParseState)
parseNat initState = let (num, str) = span isDigit (string initState)
in if null num
then Nothing
else Just (read num :: Int, ParseState str)
parseInts :: Int -> ParseState -> Maybe ([Int], ParseState)
parseInts n initState = iter n (' ':string initState) []
iter :: Int -> String -> [Int] -> Maybe ([Int], ParseState)
iter n s ns
| n == 0 && (null s || null (dropWhile isSpace s))
= Just (ns, ParseState s)
| n /= 0 && null s = Nothing
| n /= 0 && (isSpace (head s)) =
let (num, str) = span isDigit (tail s)
in if null num
then Nothing
else iter (n - 1) str (ns ++ [read num :: Int])
| otherwise = Nothing
入出力結果(Terminal, runghc)
$ runghc ParseP2.hs test.pgm Just GreymapP2 24x7 15 データ Just [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,3,3,3,3,0,0,7,7,7,7,0,0,11,11,11,11,0,0,15,15,15,15,0,0,3,0,0,0,0,0,7,0,0,0,0,0,11,0,0,0,0,0,15,0,0,15,0,0,3,3,3,0,0,0,7,7,7,0,0,0,11,11,11,0,0,0,15,15,15,15,0,0,3,0,0,0,0,0,7,0,0,0,0,0,11,0,0,0,0,0,15,0,0,0,0,0,3,0,0,0,0,0,7,7,7,7,0,0,11,11,11,11,0,0,15,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0] $ runghc ParseP2.hs ParseP2.hs Nothing データ Nothing $ cat test.pgm P2 24 7 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 3 3 3 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 15 15 15 0 0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 15 0 0 3 3 3 0 0 0 7 7 7 0 0 0 11 11 11 0 0 0 15 15 15 15 0 0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 0 0 0 3 0 0 0 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 $
次回は解析器の型の詳細を隠す。
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