<\/p>\n\r\ndata MovieReview = MovieReview {\r\n revTitle :: String\r\n , revUser :: String\r\n , revReview :: String\r\n}\r\n\r\nsimpleReview :: [(String, String)] -> Maybe MovieReview\r\nsimpleReview alist = \r\n case lookup \"title\" alist of\r\n Just title@(_:_) ->\r\n case lookup \"user\" alist of\r\n Just user@(_:_) ->\r\n case lookup \"review\" alist of\r\n Just review@(_:_) ->\r\n Just (MovieReview title user review)\r\n _ -> Nothing -- no review\r\n _ -> Nothing -- no user\r\n _ -> Nothing -- no title\r\n<\/pre>\n<\/code><\/p>\n
\u8fd9\u4e00\u6bb5\u4ee3\u7801\u662f\u8bf4\uff0c\u6709\u4e00\u4e2a\u7535\u5f71\u5f71\u8bc4\u7684\u7c7b\u578bMovieReview\uff0c\u5305\u542brevTitle,revUser,revReview\u4e09\u4e2a\u5b57\u6bb5\u3002\u73b0\u5728\u7528\u4e00\u4e2aassociation list\u6765\u5bf9\u4ed6\u8fdb\u884c\u521d\u59cb\u5316\uff0csimpleReview\u51fd\u6570\u4e2d\u7684case\u8bed\u53e5\u7565\u4f3cif else\uff0c\u6211\u4eec\u53ef\u4ee5\u770b\u5230\u8fd9\u6bb5\u4ee3\u7801\u7684\u5927\u610f\u662f\u8bf4\u5982\u679calist\u4e2d\u542b\u6709title\u3001user\u3001review\u4e09\u4e2a\u952e\u503c\u5e76\u4e14\u5bf9\u5e94\u7684\u5b57\u6bb5\u4e0d\u4e3a\u7a7a\uff0c\u5219\u7528\u8fd9\u4e09\u4e2a\u5b57\u6bb5\u521d\u59cb\u5316MoviewReview\u7c7b\u578b\uff0c\u5426\u5219\u4f9d\u6b21\u8fd4\u56deNothing (Maybe\u662f\u4e00\u4e2a\u5305\u88c5\u7c7b\u578b\uff0c\u5b83\u7684\u503c\u53ef\u4ee5\u662f\u5bf9\u5e94\u7c7b\u578b\u7684\u503c\uff0c\u6216\u8005\u662fNothing)\u3002
\n\u8fd9\u91cc\u662f\u91c7\u53d6\u4e86\u6700\u4f20\u7edf\u7684\u65b9\u5f0f\uff0c\u5c55\u73b0\u4e86\u5199\u4ee3\u7801\u65f6\u7ecf\u5e38\u9047\u5230\u7684\u82e6\u607c\u3002\u4e09\u5c42if\u5224\u65ad\uff0c\u91cc\u9762\u7684\u64cd\u4f5c\u5f88\u96f7\u540c\uff0c\u5e94\u8be5\u6709\u529e\u6cd5\u62bd\u8c61\u51fa\u6765\u3002\u5728C\u8bed\u8a00\u4e2d\u6211\u4f1a\u91c7\u53d6\u7684\u529e\u6cd5\u662f\u628atitle, user, review\u4e09\u4e2a\u952e\u503c\u5b58\u5728\u6570\u7ec4\u91cc\uff0c\u5e76\u4e14\u901a\u8fc7\u4e00\u4e2a\u5faa\u73af\u6765\u4f9d\u6b21\u5224\u65ad\uff0c\u867d\u7136\u5b9e\u73b0\u4e86\u91cd\u7528\u4ee3\u7801\uff0c\u4f46\u662f\u4ee3\u7801\u53ef\u8bfb\u6027\u53d8\u5dee\u4e86\u3002\u4ee3\u7801\u4e3b\u4f53\u5185\u5145\u6ee1\u4e86\u7c7b\u4f3c
\n<\/p>\n\r\nauto moviereview = new MovieReview (alist[keylist[0]], \r\n alist[keylist[1]], \r\n alist[keylist[2]]);\r\n<\/pre>\n<\/code>
\n\u8fd9\u6837\u7684\u4ee3\u7801\u3002\u6211\u4eec\u6765\u770b\u770bHaskell\u5982\u4f55\u901a\u8fc7monad\u5b9e\u73b0\u5bf9\u8fd9\u6837\u7684\u4ee3\u7801\u7684\u62bd\u8c61\u3002\u5b9e\u9645\u4e0aMaybe\u7c7b\u578b\u5c31\u662f\u4e00\u4e2aMonad\u3002\u4e00\u4e2aMonad\u53ef\u4ee5\u7b80\u5355\u7406\u89e3\u4e3a\u4e00\u4e2a\u5305\u88c5\u7c7b\u578b\uff0c\u4ed6\u5305\u88c5\u4e86\u4e00\u4e2a\u53d8\u91cf\uff08\u53ef\u80fd\u662f\u51fd\u6570\u3001\u64cd\u4f5c\u3001\u72b6\u6001\u7b49\u7b49\uff09\uff0c\u5e76\u4e14\u63d0\u4f9b\u4e00\u4e2a\u201c>>=\u201d\u8fd0\u7b97\uff0c\u8fd9\u4e2a\u8fd0\u7b97\u7684\u610f\u601d\u662f\uff0c\u628a\u5b83\u5305\u88c5\u7684\u53d8\u91cf\u63d0\u53d6\u51fa\u6765\uff0c\u5f53\u4f5c\u53c2\u6570\u4f20\u5165\u4e00\u4e2a\u53ef\u4ee5\u63a5\u53d7\u8fd9\u79cd\u7c7b\u578b\u53c2\u6570\u7684\u51fd\u6570\u4e2d\uff0c\u800c\u8fd9\u4e2a\u51fd\u6570\u7684\u8fd4\u56de\u503c\u5fc5\u987b\u4e5f\u662fmonad\u5305\u88c5\u7684\u3002
\n\u53e6\u5916Monad\u8fd8\u9700\u63d0\u4f9b\u4e00\u4e2areturn\u51fd\u6570\uff0c\u65b9\u4fbf\u5176\u4ed6\u51fd\u6570\u5c06\u539f\u59cb\u7c7b\u578b\u5305\u88c5\u4e3aMonad\u7c7b\u578b\u3002
\n<\/p>\n\r\nclass Monad m where\r\n -- chain\r\n (>>=) :: m a->(a -> m b) -> m b\r\n -- inject\r\n return :: a -> m a\r\n<\/pre>\n<\/code>
\n\u4e3a\u4f55\u8981\u8fd9\u6837\u5b9a\u4e49>>=\u8fd0\u7b97\uff0c\u4e00\u4e2amonad >>=\u8fd0\u7b97\u5c06\u4e00\u4e2aMonad (m a)\u4e2d\u7684\u53d8\u91cf(a)\u6254\u8fdb\u4e00\u4e2a(a -> m b)\u51fd\u6570\u4e2d\uff0c\u751f\u6210\u4e00\u4e2a\u65b0\u7684Monad\uff0c\u5b83\u5305\u542b\u7684\u53d8\u91cf\u7c7b\u578b\u8ddf\u4f20\u5165\u7684\u53ef\u80fd\u4e0d\u540c(m b)\u3002\u8fd9\u4e2am b\u8fd8\u662fMonad\uff0c\u5c31\u53c8\u53ef\u4ee5\u7ee7\u7eed\u901a\u8fc7>>=\u65b9\u6cd5\u6254\u8fdb\u65b0\u7684(a -> m b)\u4e2d\u3002\u5e76\u4e14\u8fd9\u91cc\u7279\u522b\u5c06\u8f93\u5165\u7c7b\u578bm a\u548c\u8f93\u51fa\u7c7b\u578bm b\u5206\u522b\u8868\u793a\uff0c\u4ece\u800c\u53ef\u4ee5\u5c06\u8bb8\u591a\u4e0d\u540c\u7684\u65b9\u6cd5\u901a\u8fc7>>=\u8fde\u63a5\u8d77\u6765\u3002\u5c31\u597d\u8c61\u4e00\u4e2aMonad\u88ab\u9001\u4e0a\u4e86\u751f\u4ea7\u7ebf\uff0c\u7ecf\u8fc7\u4e00\u9053\u9053\u5de5\u5e8f\u52a0\u5de5\u5b83\u672c\u8eab\u7684\u7c7b\u578b\u4e5f\u53ef\u4ee5\u7ecf\u8fc7\u8bb8\u591a\u53d8\u5316\uff0c\u6700\u7ec8\u8f93\u51fa\u4e00\u4e2am b\u3002<\/p>\n
\u8fd9\u4e2a\u5b9a\u4e49\u975e\u5e38\u4f18\u7f8e\uff0c\u6211\u76f8\u4fe1c++\u4e2d\u7684\u8f93\u5165\u8f93\u51fa\u6d41\u64cd\u4f5c\u7b26\u5c31\u662f\u4ece\u8fd9\u91cc\u501f\u9274\u7684\u3002\u4e0d\u8fc7\u8fd9\u4e2a\u6d41\u53ef\u4e0d\u4ec5\u4ec5\u80fd\u505a\u8f93\u5165\u8f93\u51fa\u7684\u64cd\u4f5c\uff0c\u5b83\u53ef\u4ee5\u63a7\u5236\u4efb\u4f55\u4e00\u79cd\u64cd\u4f5c\u6d41\uff0c\u975e\u5e38\u795e\u5947\u3002\u56e0\u6b64\u5728RWH\u4e2d\u79f0>>=\u7b97\u7b26\u662f\u4e00\u79cd\u53ef\u7f16\u7a0b\u7684\u5206\u53f7<\/strong> \u6bcf\u4e2a\u64cd\u4f5c\u540e\u9762\u7528>>=\u7ed3\u5c3e\u53ef\u4ee5\u5bf9\u5e94\u4e8e\u547d\u4ee4\u5f0f\u8bed\u8a00\u4e2d\u7684;\u5206\u53f7\uff0c\u5728haskell\u8fd9\u6837\u7684\u51fd\u6570\u5f0f\u8bed\u8a00\u4e2d\uff0c\u901a\u8fc7monad\u5b9e\u73b0\u4e86\u547d\u4ee4\u5f0f\u7684\u7f16\u7a0b\u8303\u5f0f\u3002\u66f4\u52a0\u5f3a\u5927\u7684\u662f\uff0c\u4ed6\u7684\u53ef\u6269\u5c55\u6027\u8fdc\u9ad8\u4e8e\u4f20\u7edf\u7684\u547d\u4ee4\u4f9d\u6b21\u6267\u884c\uff0c\u5177\u6709\u7075\u6d3b\u7684\u53ef\u64cd\u63a7\u6027\u3002<\/p>\nMaybe\u5c31\u662f\u4e00\u79cdmonad\u3002\u4ed6\u7684>>=\u65b9\u6cd5\u5c06\u5176\u5305\u88c5\u7684\u53d8\u91cf\u63d0\u53d6\u51fa\u6765\uff0c\u7ed9\u540e\u7eed\u7684\u51fd\u6570\u4f5c\u4e3a\u53c2\u6570\u3002\u5982\u679c\u4e3aNothing\u5219\u77ed\u8def\uff0c\u76f4\u63a5\u8fd4\u56deNothing
\n<\/p>\n\r\n(>>=) :: Maybe a ->(a -> Maybe b) -> Maybe b\r\nNothing >>= _ = Nothing\r\nJust v >>= f = f v\r\n<\/pre>\n<\/code>
\n\u901a\u8fc7\u8fd9\u4e00\u5c42\u5305\u88c5\u53ef\u4ee5\u5f88\u597d\u7684\u5c06\u524d\u9762\u7684\u4e00\u6bb5\u4ee3\u7801\u7b80\u5316\uff0c\u4f9d\u6b21\u5904\u7406title, user, review\u4e09\u4e2a\u5b57\u6bb5\uff0c\u901a\u8fc7>>=\u8fd9\u4e2a\u201c\u53ef\u7f16\u7a0b\u7684\u5206\u53f7\u201d\u6765\u63a7\u5236\u64cd\u4f5c\u6d41\u3002\u5982\u679c\u4efb\u4f55\u4e00\u4e2a\u5b57\u6bb5\u51fa\u73b0Nothing\uff0c\u5219\u540e\u7eed\u7684>>=\u51fd\u6570\u90fd\u4f1a\u8fd4\u56deNothing\uff0c\u4e0d\u4f1a\u7ee7\u7eed\u8c03\u7528lookup\u51fd\u6570\u3002
\n<\/p>\n\r\nsimpleReview alist = \r\n lookup \"title\" alist >>=\r\n \\title -> lookup \"user\" alist >>=\r\n \\user -> lookup \"review\" alist >>= \r\n \\review -> Just (MovieReview title user review) \r\n<\/pre>\n<\/code>
\n\u53ef\u4ee5\u770b\u5230\u4ee3\u7801\u5df2\u7ecf\u5145\u5206\u7b80\u5316\u4e86\u3002\u4f46Haskell\u7684\u8bbe\u8ba1\u8005\u8fd8\u662f\u4e0d\u6ee1\u610f\uff0c\u8fd9\u6837\u7684\u4ee3\u7801\u8fd8\u6709\u91cd\u590d\u4e4b\u7f6a\u3002\u5982\u679c\u6211\u4eec\u53ef\u4ee5\u76f4\u63a5\u5c06(lookup “string” alist)\u4f5c\u4e3a\u53c2\u6570\uff0c\u653e\u8fdbMovieReview\u7684\u6784\u9020\u51fd\u6570\u7684\u8bdd\uff0c\u5c31\u53ef\u4ee5\u4e00\u884c\u4ee3\u7801\u5b9e\u73b0\u8fd9\u4e2a\u4efb\u52a1\u4e86\u3002\u73b0\u5728\u9047\u5230\u7684\u56f0\u96be\u662f\uff0clookup\u8fd4\u56de\u5305\u88f9\u5728Maybe\u4e2d\u7684\u91cf\uff1a\u53ef\u80fd\u627e\u5f97\u5230\uff0c\u53ef\u80fd\u627e\u4e0d\u5230\u3002\u800cMovieReview\u7684\u6784\u9020\u51fd\u6570\uff0c\u4ec5\u5f53\u4e09\u4e2a\u67e5\u627e\u90fd\u4e0d\u4e3a\u7a7a\u7684\u65f6\u5019\u624d\u5e94\u5f53\u8c03\u7528\uff0c\u5e76\u4e14\u5176\u53c2\u6570\u5e94\u5f53\u662f\u89e3\u9664\u5305\u88c5\u7684\u5b57\u7b26\u4e32\u7c7b\u578b\u3002\u5982\u4f55\u5c06Monad\u5305\u88f9\u7684\u503c\u5f53\u4f5c\u53c2\u6570\u4f20\u5165\u666e\u901a\u7684Pure function\u4e2d\u5462\uff1f<\/p>\n
\u8fd9\u91ccHaskell\u5f15\u5165\u4e86Functor\u7684\u6982\u5ff5\uff1a
\n<\/p>\n\r\nclass Functor f where\r\n fmap :: (a->b) -> f a -> f b\r\n<\/pre>\n<\/code>
\n\u4ed6\u5c06\u4e00\u4e2a\u666e\u901a\u7684\u51fd\u6570(a -> b)\u63d0\u5347\u4e3a\u5bf9\u5305\u88c5\u7c7b\u8fdb\u884c\u64cd\u4f5c\u7684Functor (f a -> f b)\u3002\u8fd9\u91ccf\u662f\u5305\u88c5\u51fd\u6570\u3002\u8fd9\u6837\u4f7f\u5f97\u4efb\u4f55\u5305\u88c5\u7c7b\u578b\u90fd\u53ef\u4ee5\u8f7b\u677e\u7ee7\u627f\u6240\u6709pure function\u7684\u4ee3\u7801\u3002\u5728Monad\u4e2d\u5b9a\u4e49\u4e86\u8fd9\u6837\u7684\u4e00\u4e2a\u63d0\u5347\u64cd\u4f5c\uff0c\u79f0\u4e4b\u4e3aliftM
\n<\/p>\n\r\nliftM :: (Monad m) => (a -> b) -> m a -> m b\r\nliftM f m = m >>= \\i -> return (f i)\r\n<\/pre>\n<\/code>
\n\u8fd9\u6837\u4efb\u4f55\u4e00\u4e2af\u90fd\u53ef\u4ee5\u5bf9Monad\u4e2d\u5305\u88c5\u7684\u503c\u901a\u8fc7\u201c\u63d0\u5347\u64cd\u4f5c\u201d\u8fdb\u884c\u8c03\u7528\u4e86\u3002\u8fd9\u91cc\u7684liftM\u4ec5\u9488\u5bf9\u5355\u53c2\u51fd\u6570(a -> b)\u3002\u6211\u4eec\u7684MovieReview\u9700\u89813\u4e2a\u53c2\u6570\uff0c\u8fd9\u91cc\u6211\u4eec\u53ef\u4ee5\u501f\u7528haskell\u51fd\u6570\u5e93\u4e2d\u7684liftM3\uff0c\u9488\u5bf93\u53c2\u6570\u51fd\u6570\u8fdb\u884c\u63d0\u5347\u3002
\n<\/p>\n\r\nliftM3 :: (Monad m) => (a -> b -> c -> d) -> m a -> m b -> m c -> m d\r\nliftM3 f m1 m2 m3 =\r\n m1 >>= \\a ->\r\n m2 >>= \\b ->\r\n m3 >>= \\c ->\r\n return (f a b c)\r\n<\/pre>\n<\/code><\/p>\n
\u90a3\u6211\u4eec\u7684\u4ee3\u7801\u53c8\u53ef\u4ee5\u8fdb\u4e00\u6b65\u7b80\u5316\u4e86
\n<\/p>\n\r\nliftReview alist =\r\n liftM3 MovieReview (lookup \"title\" alist)\r\n (lookup \"user\" alist)\r\n (lookup \"review\" alist)\r\n<\/pre>\n<\/code>
\n\u770b\uff0c\u7ecf\u8fc7\u63d0\u5347\u4e4b\u540eMovieReview\u5c31\u50cf\u662f\u53ef\u4ee5\u63a5\u53d7\u4e09\u4e2aMaybe\u4f5c\u4e3a\u53c2\u6570\u4e00\u6837\u5730\u8fdb\u884c\u8c03\u7528\u4e86\uff0c\u5e76\u4e14>>=\u7b26\u9690\u85cf\u5728\u4e86liftM3\u4e2d\uff0c\u5176\u4e2d\u63a7\u5236\u4e86\u4e00\u65e6\u4efb\u610f\u4e00\u4e2amaybe\u4e3anothing\u7684\u65f6\u5019\u77ed\u8def\u8fd9\u4e2a\u6d41\u7a0b\u3002\u867d\u7136\u8fd9\u6837client\u4ee3\u7801\u5df2\u7ecf\u975e\u5e38\u4f18\u7f8e\u4e86\uff0c\u4f46\u662flib\u4ee3\u7801\u4f1a\u6709\u4e9b\u95ee\u9898\u3002\u8fd9\u91cc\u7684liftM3\u5df2\u7ecf\u6709\u4e9b\u5446\u677f\u4e86\u3002\u5982\u679c\u662f\u8981\u63d0\u534710\u4e2a\u53c2\u6570\u7684\u51fd\u6570\u600e\u4e48\u529e\u3002\u7cbe\u76ca\u6c42\u7cbe\uff0cHaskell\u63d0\u4f9b\u4e86ap\u51fd\u6570\u8ba9\u63d0\u5347\u64cd\u4f5c\u53ef\u4ee5\u94fe\u72b6\u8c03\u7528\u3002
\n<\/p>\n\r\nap :: (Monad m) => m (a -> b) -> m a -> m b\r\nap = liftM2 id\r\n<\/pre>\n<\/code>
\n\u4ece\u51fd\u6570\u7c7b\u578b\u53ef\u4ee5\u770b\u51fa\uff0c\u4ed6\u8981\u6c42\u5c06pure function (a -> b)\u5305\u88c5\u5230monad\u4e2d\u3002\u8fd9\u6837\u505a\u53ef\u4ee5\u8fdb\u884c\u94fe\u5f0f\u8c03\u7528\u3002\u53ef\u662f\u4e3a\u4f55\u901a\u8fc7\u8fd9\u6837\u7684\u94fe\u5f0f\u8c03\u7528\u5c31\u80fd\u5c06\u591a\u53c2\u51fd\u6570\u94fe\u5f0f\u63d0\u5347\u4e86\u5462\uff1f\u8fd9\u662f\u56e0\u4e3a\u5b9e\u9645\u4e0ahaskell\u4ec5\u652f\u6301\u5355\u53c2\u51fd\u6570\u3002\u4f8b\u5982(a -> b -> c -> d)\u8fd9\u4e2a\u7c7b\u578b\uff0c\u4ed6\u8868\u793a\u63a5\u53d7(a, b, c)\u4e09\u4e2a\u53c2\u6570\uff0c\u5e76\u4e14\u8fd4\u56ded\u7c7b\u578b\u7684\u51fd\u6570\u3002\u8fd9\u4e2a\u51fd\u6570\u5b9e\u9645\u4e0a\u662f\u63a5\u53d7a\u7c7b\u578b\u7684\u53c2\u6570\uff0c\u5e76\u4e14\u8fd4\u56de\u4e00\u4e2a(b-> c -> d)\u7c7b\u578b\u7684\u51fd\u6570\u7684\u51fd\u6570\u3002\u8fd9\u6709\u70b9\u50cf\u662f\u901a\u8fc7\u63a5\u53d7\u4e86\u53c2\u6570a\u6765\u7ed1\u5b9a\u4e86\u4e09\u4e2a\u53c2\u6570\u4e2d\u7684\u4e00\u4e2a\uff0c\u751f\u6210\u4e86\u4e00\u4e2a\u504f\u7279\u5316\u7684\u51fd\u6570\uff08partial function\uff09\u3002
\nap\u51fd\u6570\u7c7b\u4f3c\u662f\u901a\u8fc7\u63d0\u5347\u5c06\u8fd9\u4e2a\u591a\u53c2\u51fd\u6570\u7684\u7b2c\u4e00\u4e2a\u53c2\u6570\u63d0\u5347\u4e86\uff0c\u8fd4\u56de\u7528monad\u5305\u88c5\u7684\u504f\u7279\u5316\u51fd\u6570(m b)\u3002\u8fd9\u4e2a\u65b0\u7684\u88abmonad\u5305\u88c5\u7684\u51fd\u6570\u53ef\u4ee5\u7ee7\u7eed\u4f20\u5165ap\uff0c\u7ee7\u7eed\u504f\u7279\u5316\u4e0b\u4e00\u4e2a\u53c2\u6570\u3002\u56e0\u6b64\u8fd9\u4e2a\u64cd\u4f5c\u53ef\u4ee5\u94fe\u5f0f\u63d0\u5347\u591a\u53c2\u51fd\u6570\u7684\u6bcf\u4e00\u4e2a\u53c2\u6570\uff0c\u6700\u7ec8\u5b8c\u6210\u63d0\u5347\u8fc7\u7a0b\u3002\u8fd9\u91ccap\u7684\u540d\u5b57\u610f\u601d\u662f\u201capply\u201d\uff0c\u5c31\u80fd\u6e05\u695a\u4ed6\u7684\u610f\u4e49\u4e86\u3002
\nap\u5b9e\u9645\u4e0a\u901a\u8fc7liftM2 id\u5b9e\u73b0\u3002id\u662f\u8fd4\u56de\u672c\u8eab
\n<\/p>\n\r\nid a = a\r\n<\/pre>\n<\/code>
\n\u800cliftM2\u63d0\u5347\u4e24\u4e2a\u53c2\u6570\u5e76\u8fd4\u56de\u8c03\u7528f\u7684\u5305\u88c5\u7ed3\u679c\uff0c\u73b0\u5728f\u662fid
\n<\/p>\n\r\nliftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r\r\nliftM2 f m1 m2 =\r\n m1 >>= \\a1 ->\r\n m2 >>= \\a2 ->\r\n return (f a1 a2)\r\n\r\nliftM2 id = \r\n m1 >>= \\a1 ->\r\n m2 >>= \\a2 ->\r\n return (a1 a2) -- id means self\r\n<\/pre>\n<\/code>
\n\u8fd9\u91cc\u6709\u4e2a\u4e0d\u592a\u7b26\u5408\u76f4\u89c9\u7684\u5730\u65b9\uff0creturn (a1 a2)\u5c31\u662fm (a1 a2)\uff0c \u4ed6\u600e\u4e48\u5c31\u53d8\u6210ap\u7684m (a -> b) -> m a -> m b\u4e86\uff1f
\n\u8fd9\u8981\u770b\u4f20\u5165\u7684\u53c2\u6570\u3002ap\u63a5\u53d7\u4e24\u4e2a\u53c2\u6570\u5206\u522b\u662fm (a -> b)\u548c m a\u3002\u7528\u8fd9\u4e24\u4e2a\u53c2\u6570\u66ff\u6362liftM2\u7684a1\u548ca2\u540e
\n<\/p>\n\r\nliftM2 id (m (a -> b)) (m a) = \r\n m (a -> b) >>= \\(a -> b) ->\r\n m a >>= \\a ->\r\n return ( (a -> b) a)\r\n<\/pre>\n<\/code>
\n\u7ecf\u8fc7\u8fd9\u4e2a\u63a8\u5bfc\uff0c\u53ef\u4ee5\u6e05\u6670\u5730\u770b\u51fa\uff0cliftM2 id\u5c06ap\u7684\u7b2c\u4e00\u4e2a\u53c2\u6570m (a -> b)\u63d0\u5347\u540e\u7ed1\u5b9a\u4e86\u7b2c\u4e00\u4e2a\u53c2\u6570a\uff0c\u4e5f\u5c31\u662fap\u7684\u7b2c\u4e8c\u4e2a\u53c2\u6570m a\u3002\u5e76\u4e14\u8fd4\u56de\u4e86\u7ed1\u5b9a\u53c2\u6570\u540e\u7684\u504f\u7279\u5316\u51fd\u6570b\u7684\u5305\u88c5monad, m b\u3002\u8fd9\u4e2a\u7ed1\u5b9a\u7684\u8fc7\u7a0b\u53ef\u4ee5\u5728\u6700\u540e\u4e00\u53e5\u4f53\u73b0\u51fa\u6765\uff1areturn ( ( a -> b ) a)\u3002\u5bf9\uff08a->b\uff09\u5e94\u7528\u53c2\u6570a\u3002
\n\u4ece\u8fd9\u91cc\u53ef\u4ee5\u770b\u51famonad\u662f\u4e00\u4e2a\u6781\u7aef\u62bd\u8c61\u7684\u6982\u5ff5\uff0c\u8ba8\u8bba\u4ed6\u7ecf\u5e38\u4f1a\u9047\u5230\u4e0d\u6613\u7406\u89e3\u7684\u5730\u65b9\uff0c\u53ea\u6709\u5e26\u4e0a\u5b9e\u9645\u4f7f\u7528\u7684\u53c2\u6570\u540e\u624d\u80fd\u5bb9\u6613\u7406\u89e3\u3002<\/p>\n
\u5229\u7528ap\u51fd\u6570\u7684\u4ee3\u7801\u5982\u4e0b\uff0c\u8fd9\u91cc\u4f7f\u7528`\u7b26\u53f7\u5305\u88f9\u4e00\u4e2a\u53cc\u53c2\u6570\u7684\u51fd\u6570\uff0c\u5c06\u51fd\u6570\u5f53\u4f5c\u64cd\u4f5c\u7b26\u6765\u4f7f\u7528\uff0c\u4ece\u800c\u5f97\u5230\u94fe\u5f0f\u8c03\u7528\u7684\u4ee3\u7801
\n<\/p>\n\r\nliftReview alist =\r\n MovieReview `liftM` lookup \"title\" alist\r\n `ap` lookup \"user\" alist\r\n `ap` lookup \"review\" alist\r\n<\/pre>\n<\/code><\/p>\n
\u597d\u4e86\uff0c\u505a\u4e2a\u5c0f\u7ed3\u3002\u6211\u521a\u5b66haskell\u4e0d\u4e45\uff0c\u5bf9monad\u6709\u4e86\u4e00\u4e2a\u521d\u6b65\u7684\u4e86\u89e3\u3002\u53ef\u4ee5\u770b\u5230monad\u662f\u4e00\u4e2a\u5b9e\u8df5\u6027\u975e\u5e38\u5f3a\u7684\u62bd\u8c61\uff0c\u6d89\u53ca\u5230\u7684lift, functor\u7b49\u6982\u5ff5\uff0c\u5982\u679c\u53ea\u662f\u5f62\u5f0f\u4e0a\u5730\u770b\u4ed6\u7684\u8bbe\u8ba1\uff0c\u5f88\u96be\u7406\u89e3\u8bbe\u8ba1\u7684\u610f\u56fe\uff0c\u5c31\u50cf\u662f\u4e00\u5768\u62bd\u8c61\u5e9f\u8bdd\u3002\u4f46\u662f\u4e00\u65e6\u4ece\u5b9e\u8df5\u4e2d\u591a\u5229\u7528\u8fd9\u4e9b\u5de5\u5177\uff0c\u5c31\u80fd\u8f7b\u677e\u7684\u907f\u514d\u5404\u79cd\u5404\u6837\u7684\u4ee3\u7801\u574f\u5473\uff0c\u5f97\u5230\u7f8e\u89c2\u81ea\u7136\u548c\u53ef\u8bfb\u6027\u975e\u5e38\u5f3a\u7684\u4ee3\u7801\u3002Haskell\u8fd9\u6837\u7684\u8bed\u8a00\uff0c\u53ea\u8981\u7528\u5fc3\u8bbe\u8ba1\u597d\u51fd\u6570\u548c\u53d8\u91cf\u540d\uff0c\u5373\u4f7f\u4e0d\u5199\u6ce8\u91ca\u4e5f\u80fd\u660e\u767d\u4ee3\u7801\u7684\u610f\u56fe\uff0c\u5e76\u4e14\u5f88\u96be\u51fa\u9519\u3002\u56e0\u4e3a\u4ed6\u628a\u5b9e\u73b0\u9690\u85cf\u8d77\u6765\uff0c\u4ee3\u7801\u5c31\u662f\u76f4\u63a5\u8868\u8fbe\u4e86\u610f\u56fe\u3002\u6211\u8ba4\u4e3a\u8fd9\u662f\u975e\u5e38\u6709\u6f5c\u529b\u7684\u8bed\u8a00\uff0c\u4eca\u540e\u4f1a\u505a\u8fdb\u4e00\u6b65\u7684\u5b66\u4e60\u548c\u7814\u7a76\uff0c\u5e76\u5c3d\u5feb\u5c06\u5176\u7528\u4e8e\u5b9e\u6218\u3002<\/p>\n","protected":false},"excerpt":{"rendered":"
\u4ece\u4e09\u6708\u4efd\u5f00\u59cb\u770bReal World Haskell\u8fd9\u672c\u4e66\uff0c\u65ad\u65ad\u7eed\u7eed\u770b\u52307\u6708\u4efd\uff0c\u603b\u7b97\u521d\u6b65\u5bf9Haskell\u6709\u4e86\u4e00\u4e9b\u8ba4\u8bc6\u3002 \u6211\u60f3\uff0c\u5b66\u4e60Haskell\u8fd9\u95e8\u8bed\u8a00\uff0c\u7b2c\u4e00\u4e2a\u95e8\u69db\u5c31\u662fMonad\u8fd9\u4e2a\u6982\u5ff5\u3002\u4eca\u5929\u521d\u6b65\u6765\u505a\u4e00\u70b9\u603b\u7ed3\u3002 \u5728\u8bfb\u4e66\u548c\u8bd5\u7740\u7528haskell\u505a\u4e00\u4e9b\u4e60\u9898\u7684\u65f6\u5019\uff0c\u5c31\u4f1a\u611f\u89c9\u5230\uff0chaskell\u662f\u4e00\u95e8\u5b9e\u8df5\u6027\u6781\u5f3a\u7684\u8bed\u8a00\u3002\u5f53\u7136\uff0c\u4ed6\u540c\u65f6\u4e5f\u662f\u4e00\u95e8\u7406\u8bba\u6027\u6781\u5f3a\u7684\u8bed\u8a00\uff0c\u4ed6\u5141\u8bb8\u751a\u81f3\u9f13\u52b1\u4f60\u7528\u6570\u5b66\u65b9\u6cd5\u53bb\u63a8\u5bfc\u51fd\u6570\u7b7e\u540d\uff0c\u901a\u8fc7\u5b9a\u4e49\u4e00\u7cfb\u5217\u516c\u7406\u53ef\u4ee5\u5bf9\u7279\u5b9a\u7c7b\u578b\u7684\u51fd\u6570\u5f0f\u8fdb\u884c\u6570\u5b66\u53d8\u5f62\uff0c\u4ee5\u8fbe\u5230\u6700\u77ed\u6700\u4f18\u7f8e\u7684\u4ee3\u7801\u5f62\u5f0f\u3002 \u4f46\u5bf9\u4e8e\u6211\u800c\u8a00\uff0c\u90a3\u4e9b\u8fc7\u5206\u62bd\u8c61\u7684\u51fd\u6570\u548c\u6982\u5ff5\uff0c\u975e\u5e38\u96be\u4ee5\u7406\u89e3\uff0c\u5e76\u4e14\u5373\u4f7f\u662f\u5f62\u5f0f\u4e0a\u7406\u89e3\u4e86\u4ed6\u7684\u5b9a\u4e49\uff0c\u8fd8\u662f\u65e0\u6cd5\u7406\u89e3\u4ed6\u4e3a\u4f55\u8981\u5982\u6b64\u5b9a\u4e49\uff0c\u6709\u4ec0\u4e48\u610f\u4e49\u3002Haskell\u4ee3\u7801\u7684\u7279\u70b9\u5728\u4e8e\u6781\u7aef\u7684\u7cbe\u70bc\uff0c\u8bb8\u591a\u51fd\u6570\u5b9a\u4e49\u770b\u4e0a\u53bb\u4ec0\u4e48\u90fd\u6ca1\u505a\uff0c\u50cf\u662f\u5728\u8bf4\u5e9f\u8bdd\u3002\u5e78\u597dRWH\u662f\u4e00\u672c\u975e\u5e38\u597d\u7684\u6559\u6750\uff0c\u4ed6\u901a\u8fc7\u8bb8\u591a\u5b9e\u4f8b\u6765\u8bf4\u660e\u8fd9\u4e9b\u62bd\u8c61\u5b9a\u4e49\u5728\u5b9e\u8df5\u4e2d\u7684\u7528\u9014\uff0c\u8ba9\u4f60\u770b\u5230\u8bb8\u8bb8\u591a\u591a\u5728\u5b9e\u9645\u4ee3\u7801\u4e2d\u7ecf\u5e38\u9047\u5230\u7684\u75db\u70b9\uff0c\u5728Haskell\u4e2d\u90fd\u6709\u89e3\u836f\u3002\u5982\u540c\u918d\u9190\u704c\u9876\u822c\u723d\u5feb\u3002\uff08\u8bf7\u53c2\u89c1\u4ee3\u7801\u4ea4\u53c9\u62f7\u8d1d\u6096\u8bba\uff09 Monad\u5c31\u662fHaskell\u7684\u4e00\u4e2a\u7ecf\u5178\u8bbe\u8ba1\u3002\u5b83\u6765\u6e90\u4e8e\u201c\u8303\u7574\u8bba\u201d\uff0c\u53ef\u4ee5\u8bf4\u662f\u6570\u5b66\u4e2d\u7684\u6570\u5b66\uff0c\u4e00\u5768\u201c\u62bd\u8c61\u5e9f\u8bdd\u201d\u3002\u6211\u5148\u4e0d\u53bb\u8003\u8651\u4ed6\u7684\u6570\u5b66\u542b\u4e49\uff0c\u5355\u770b\u4ed6\u5728\u4ee3\u7801\u4e2d\u5982\u4f55\u5316\u7e41\u4e3a\u7b80\u3002\u8fd9\u91cc\u6211\u6284\u4e00\u6bb5RWH\u4e2d\u7684\u4ee3\u7801(\u7565\u7b80\u5316)\u3002 data MovieReview = MovieReview { revTitle :: String , revUser :: String , revReview :: String } simpleReview :: [(String, String)] -> Maybe MovieReview simpleReview alist = case lookup “title” alist of Just title@(_:_) -> case lookup “user” alist of Just user@(_:_) -> case lookup “review” alist of …<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3],"tags":[68,35],"class_list":["post-880","post","type-post","status-publish","format-standard","hentry","category-post","tag-haskell","tag-35"],"_links":{"self":[{"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/posts\/880","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/comments?post=880"}],"version-history":[{"count":8,"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/posts\/880\/revisions"}],"predecessor-version":[{"id":885,"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/posts\/880\/revisions\/885"}],"wp:attachment":[{"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/media?parent=880"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/categories?post=880"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.hawkwithwind.net\/blog\/wp-json\/wp\/v2\/tags?post=880"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}