解釋器求值的順序可以分為應(yīng)用序和正則序，應(yīng)用序是先求值參數(shù)，再執(zhí)行表達式；正則序則是先將表達式按照實際參數(shù)展開，然后再執(zhí)行。具體可以看看過去寫的
這篇文章。
Clojure的求值可以肯定是應(yīng)用序的，如執(zhí)行
(defn mytest [a b]
(if (= a 0)
a
b))
(mytest 0 1/0)
盡管在(mytest 0 1/0)中a綁定為0，如果求值器是完全展開再求值，那應(yīng)該正常執(zhí)行并返回a，也就是1；但是因為clojure是應(yīng)用序，因此參數(shù)b的1/0會先計算，這顯然會報錯。
clojure的dosync用于將一些表達式包裝成事務(wù)，Ref的更新操作沒有包裝在事務(wù)里，會拋出異常
;;定義mutable的Ref
(def song (ref #{}))
;;添加一首歌
(alter song conj "dangerous")
alter用于向Ref查詢并添加元素，用conj將"dangerous"這首歌加入集合，但是alter要求執(zhí)行在一個事務(wù)里，因此上面的代碼會報錯
java.lang.IllegalStateException: No transaction running (NO_SOURCE_FILE:0)
如果你用dosync包裝就沒有問題
user=> (dosync (alter song conj "dangerous"))
#{"dangerous"}
返回更新后的結(jié)果集合。這個跟我們要談的正則序和應(yīng)用序有什么關(guān)系呢？可能你看出來了，如果說clojure是應(yīng)用序，那么在表達式
(dosync (alter song conj "dangerous"))中，alter也應(yīng)該先執(zhí)行，應(yīng)當照樣報"
No transaction running"的錯誤才對，為何卻沒有呢？難道dosync是按照正則序執(zhí)行？
查看dosync的文檔
user=> (doc dosync)
-------------------------
clojure.core/dosync
([& exprs])
Macro
Runs the exprs (in an implicit do) in a transaction that encompasses
exprs and any nested calls. Starts a transaction if none is already
running on this thread. Any uncaught exception will abort the
transaction and flow out of dosync. The exprs may be run more than
once, but any effects on Refs will be atomic.
這是一個宏，他的作用是將表達式包裝在一個事務(wù)里，如果當前線程沒有事務(wù)，那么就啟動一個。
查看源碼：
(defmacro dosync
"Runs the exprs (in an implicit do) in a transaction that encompasses
exprs and any nested calls. Starts a transaction if none is already
running on this thread. Any uncaught exception will abort the
transaction and flow out of dosync. The exprs may be run more than
once, but any effects on Refs will be atomic."
[& exprs]
`(sync nil ~@exprs))
本質(zhì)上dosync是調(diào)用了sync這個宏，sync干了些什么？
(defmacro sync
"transaction-flags => TBD, pass nil for now
Runs the exprs (in an implicit do) in a transaction that encompasses
exprs and any nested calls. Starts a transaction if none is already
running on this thread. Any uncaught exception will abort the
transaction and flow out of sync. The exprs may be run more than
once, but any effects on Refs will be atomic."
[flags-ignored-for-now & body]
`(. clojure.lang.LockingTransaction
(runInTransaction (fn [] ~@body))))
找到了，原來是調(diào)用了
clojure.lang.LockingTransaction.runInTransaction這個靜態(tài)方法，并且將exps包裝成一個匿名函數(shù)
fn [] ~@body
因此，dosync并非正則序,
dosync是個宏，(dosync (alter song conj "dangerous"))展開之后，其實是
(sync nil (fun [] (alter song conj "dangerous")))
這就解答了為什么
(dosync (alter song conj "dangerous"))可以正常運行的疑問。宏的使用，首先是展開，然后才是按照應(yīng)用序的順序求值。