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[TOC] ## chain 函数 ``` // chain :: Monad m => (a -> m b) -> m a -> m b var chain = curry(function(f, m){ return m.map(f).join(); // 或者 compose(join, map(f))(m) }); ``` 实例1 ``` // map/join var firstAddressStreet = compose( join, map(safeProp('street')), join, map(safeHead), safeProp('addresses') ); // chain var firstAddressStreet = compose( chain(safeProp('street')), chain(safeHead), safeProp('addresses') ); // map/join var applyPreferences = compose( join, map(setStyle('#main')), join, map(log), map(JSON.parse), getItem ); // chain var applyPreferences = compose( chain(setStyle('#main')), chain(log), map(JSON.parse), getItem ); ``` 实例2 就能以一种纯函数式的方式来表示*序列*(sequence) 和*变量赋值*(variable assignment) ``` // getJSON :: Url -> Params -> Task JSON // querySelector :: Selector -> IO DOM getJSON('/authenticate', {username: 'stale', password: 'crackers'}) .chain(function(user) { return getJSON('/friends', {user_id: user.id}); }); // Task([{name: 'Seimith', id: 14}, {name: 'Ric', id: 39}]); querySelector("input.username").chain(function(uname) { return querySelector("input.email").chain(function(email) { return IO.of( "Welcome " + uname.value + " " + "prepare for spam at " + email.value ); }); }); // IO("Welcome Olivia prepare for spam at olivia@tremorcontrol.net"); Maybe.of(3).chain(function(three) { return Maybe.of(2).map(add(three)); }); // Maybe(5); Maybe.of(null).chain(safeProp('address')).chain(safeProp('street')); // Maybe(null); ``` ## 炫耀 实例:读取并上传一个文件 声明式代码 ``` // readFile :: Filename -> Either String (Future Error String) // httpPost :: String -> Future Error JSON // upload :: String -> Either String (Future Error JSON) var upload = compose(map(chain(httpPost('/uploads'))), readFile); ``` 从类型签名可以看出,我们预防了三个错误 1. `readFile`使用`Either`来验证输入(或许还有确保文件名存在); 2. `readFile`在读取文件的时候可能会出错,错误通过`readFile`的`Future`表示; 3. 文件上传可能会因为各种各样的原因出错,错误通过`httpPost`的`Future`表示 所有这些操作都是在一个从左到右的线性流中完成的,是完完全全纯的、声明式的代码,是可以等式推导(equational reasoning)并拥有可靠特性(reliable properties)的代码 命令式代码 跟标准的命令式的实现对比一下 ``` // upload :: String -> (String -> a) -> Void var upload = function(filename, callback) { if(!filename) { throw "You need a filename!"; } else { readFile(filename, function(err, contents) { if(err) throw err; httpPost(contents, function(err, json) { if(err) throw err; callback(json); }); }); } } ```