[Erlang 0110] Erlang Abstract Format , Part 1
阿新 • • 發佈：2018-12-29
Erlang Abstract Format並不難懂,只是枯燥一點罷了,如果把Abstract Format的文件翻譯出來,其實就是Erlang教科書中語法入門的部分. Erlang Abstract Format實際上是用Erlang程式碼的AST,下面通過一些真切的例項程式碼瞭解一下它的一些細節.
首先,Erlang Abstract Format裡面包含一些概念,我會在下面的描述中把涉及到的概念字型加粗.請注意概念之間的層次關係.Erlang程式碼本身使用非常扁平的module組織,每一個module是由一系列forms組成的.這些forms分成兩大類:attributes和函式宣告
Attribute
attributes相對比較簡單,我們先從一個只有attributes沒有任何函式宣告的module開始
-module(k). -compile(export_all). -compile({parse_transform,print_form}). -export([test/0]). -url("http://cnblogs.com/me-sa/"). -record(student,{class,id}). -record(player,{id=0,name=[],level}).上面程式碼對應的Abstract Format如下:
[{attribute,1,file,{"
我們可以一一對照上面思維導圖,可以看到上面每一行程式碼包括record定義在內在Abstract Format層面看都是attribute.裡面不斷出現的數字是程式碼所在行數,這個資訊是非常重要的,在編譯時提示程式碼出錯行,執行時報錯包含程式碼行數都要用到它.Erlang最新版報錯的時候已經包含了出錯的程式碼所在行,而之前這個功能是沒有的,工作在蠻荒紀的erlanger,搞了一個smart_exceptions的專案來實現這個功能: https://github.com/thomasl/smart_exceptions/tree/master/stable 言歸正傳,上面需要細說的form是record的定義,在有record欄位初始值的情況,結構會稍複雜一點:{record_field,LINE,Name,Value}.比如player的id=0,對應的Form是{record_field,7,{atom,7,id},{integer,7,0}},id的初始值0是一個字面常量,記作{integer,7,0},在Erlang Abstract Format文件裡面字面常量被稱為Atomic literals,包含四種:atom integer string float,見下圖
Function declarations
說過了attribute,下面我們看下函式宣告部分,這裡有一個概念Patterns,它特指function或fun的引數列表.我們做幾個簡單的方法出來:-module(a). -compile(export_all). -export([test/0]). -record(student,{class,id}). -record(player,{id=0,name=[],level}). test()-> "hello world!". test(a,[1,2]) -> "a:[1,2]"; test(12.5,100)-> "test". test([]) -> empty; test(abc) -> "atom test". foo(a)-> {b,100}. bar({1,2},12)-> [1,2,3,4,5,6]. k(Num) when Num >1000 -> bigger_than_100; k(Num) -> whatever. call(1000)-> k(1000); call(1002)-> erlang:now().它對應的Abstract Format是:
[{attribute,1,file,{"a.erl",1}},
{attribute,1,module,a},
{attribute,2,compile,export_all},
{attribute,4,export,[{test,0}]},
{attribute,6,record,
{student,
[{record_field,6,{atom,6,class}},{record_field,6,{atom,6,id}}]}},
{attribute,7,record,
{player,
[{record_field,7,{atom,7,id},{integer,7,0}},
{record_field,7,{atom,7,name},{nil,7}},
{record_field,7,{atom,7,level}}]}},
{function,10,test,0,[{clause,10,[],[],[{string,11,"hello world!"}]}]},
{function,13,test,2,
[{clause,13,
[{atom,13,a},
{cons,13,{integer,13,1},{cons,13,{integer,13,2},{nil,13}}}],
[],
[{string,14,"a:[1,2]"}]},
{clause,15,
[{float,15,12.5},{integer,15,100}],
[],
[{string,16,"test"}]}]},
{function,18,test,1,
[{clause,18,[{nil,18}],[],[{atom,19,empty}]},
{clause,20,[{atom,20,abc}],[],[{string,21,"atom test"}]}]},
{function,24,foo,1,
[{clause,24,
[{atom,24,a}],
[],
[{tuple,25,[{atom,25,b},{integer,25,100}]}]}]},
{function,27,bar,2,
[{clause,27,
[{tuple,27,[{integer,27,1},{integer,27,2}]},{integer,27,12}],
[],
[{cons,28,
{integer,28,1},
{cons,28,
{integer,28,2},
{cons,28,
{integer,28,3},
{cons,28,
{integer,28,4},
{cons,28,
{integer,28,5},
{cons,28,{integer,28,6},{nil,28}}}}}}}]}]},
{function,30,k,1,
[{clause,30,
[{var,30,'Num'}],
[[{op,30,'>',{var,30,'Num'},{integer,30,1000}}]],
[{atom,31,bigger_than_100}]},
{clause,32,[{var,32,'Num'}],[],[{atom,33,whatever}]}]},
{function,36,call,1,
[{clause,36,
[{integer,36,1000}],
[],
[{call,37,{atom,37,k},[{integer,37,1000}]}]},
{clause,38,
[{integer,38,1002}],
[],
[{call,39,{remote,39,{atom,39,erlang},{atom,39,now}},[]}]}]},
{eof,41}]
看上面的函式宣告,function裡面包含一個或者多個function clauses,比如:
{function,18,test,1,
[{clause,18,[{nil,18}],[],[{atom,19,empty}]},
{clause,20,[{atom,20,abc}],[],[{string,21,"atom test"}]}]},
這段程式碼就很好玩了,它在形式上已經非常貼近Clojure(好吧,Lisp行不行)裡面函式定義的語法了:
Clojure 1.4.0
user=> (defn make_a_set
([x] #{x})
([x,y] #{x,y}))
#'user/make_a_set
user=> (make_a_set 12)
#{12}
user=> (make_a_set 12 23)
#{12 23}
user=>
再看一下函式呼叫,erlang:now().這一句的呼叫對應的是: [{call,39,{remote,39,{atom,39,erlang},{atom,39,now}},[]}]}]} 看到這裡能想起來Erlang裡面對Local Call和Remote Call的定義了吧. 上面的程式碼裡面還可以看到Guard的表達形式:
[[{op,30,'>',{var,30,'Num'},{integer,30,1000}}]].除了function clasuse之外,還有 if clauses, case clauses and catch clauses 就不再細說.深入函式體內部就是各種表示式, 方法體裡面包含了一些表示式Expressions,Expressions和Patterns表示方式是一樣的.比如A = lists:seq(1,10).這樣一個表示式,它的Abstract Format其實很好的解釋了這裡等號其實是一個匹配運算.
7> E= fun(Code)-> {_,Tokens,_}=erl_scan:string(Code),rp(erl_parse:parse_exprs(Tokens)) end.
#Fun<erl_eval.6.80484245>
8> E(" A = lists:seq(1,10).").
{ok,[{match,1,
{var,1,'A'},
{call,1,
{remote,1,{atom,1,lists},{atom,1,seq}},
[{integer,1,1},{integer,1,10}]}}]}
ok
10>
除了上面這些"普通"的東西,當然還有列表解析和二進位制資料處理相關的語法,下面通過兩個簡單的例子看下:
Eshell V5.10.2 (abort with ^G)
1> E= fun(Code)-> {_,Tokens,_}=erl_scan:string(Code),rp(erl_parse:parse_exprs(Tokens)) end.
#Fun<erl_eval.6.80484245>
2> E("[Item || Item<- [1,2,3,4],Item>2 ].").
{ok,[{lc,1,
{var,1,'Item'},
[{generate,1,
{var,1,'Item'},
{cons,1,
{integer,1,1},
{cons,1,
{integer,1,2},
{cons,1,{integer,1,3},{cons,1,{integer,1,4},{nil,1}}}}}},
{op,1,'>',{var,1,'Item'},{integer,1,2}}]}]}
ok
3>
7> E= fun(Code)-> {_,Tokens,_}=erl_scan:string(Code),rp(erl_parse:parse_exprs(Tokens)) end.
#Fun<erl_eval.6.80484245>
8> E("<<A:8,B/binary>> = <<1,2,3,4>>.").
{ok,[{match,1,
{bin,1,
[{bin_element,1,{var,1,'A'},{integer,1,8},default},
{bin_element,1,{var,1,'B'},default,[binary]}]},
{bin,1,
[{bin_element,1,{integer,1,1},default,default},
{bin_element,1,{integer,1,2},default,default},
{bin_element,1,{integer,1,3},default,default},
{bin_element,1,{integer,1,4},default,default}]}}]}
ok
對於Erlang資料結構中的王者List需要仔細觀察下,它的表達形式是:
4> E("[a,b,c,d].").
{ok,[{cons,1,
{atom,1,a},
{cons,1,
{atom,1,b},
{cons,1,{atom,1,c},{cons,1,{atom,1,d},{nil,1}}}}}]}
ok
說到這裡我們可以把概念之間的層次關係梳理出來了:
Run! Run!
上面囉嗦了那麼多細節,那從Abstract Format如何到可執行的程式碼呢?下面我們就完成這個過程:3> {ok, MTs, _} = erl_scan:string("-module(t)."). {ok,[{'-',1}, {atom,1,module}, {'(',1}, {atom,1,t}, {')',1}, {dot,1}], 1} 4> {ok, ETs, _} = erl_scan:string("-export([say/0])."). {ok,[{'-',1}, {atom,1,export}, {'(',1}, {'[',1}, {atom,1,say}, {'/',1}, {integer,1,0}, {']',1}, {')',1}, {dot,1}], 1} 5> {ok, FTs, _} = erl_scan:string("say() -> \"hello_world!!\"."). {ok,[{atom,1,say}, {'(',1}, {')',1}, {'->',1}, {string,1,"hello_world!!"}, {dot,1}], 1} 6> Forms= [begin {ok,R}=erl_parse:parse_form(Item),R end || Item<-[MTs,ETs,FTs]]. [{attribute,1,module,t}, {attribute,1,export,[{say,0}]}, {function,1,say,0, [{clause,1,[],[],[{string,1,"hello_world!!"}]}]}] 7> {ok, t, Bin} = compile:forms(Forms). {ok,t, <<70,79,82,49,0,0,1,224,66,69,65,77,65,116,111,109,0,0,0, 45,0,0,0,5,1,116,...>>} 8> code:load_binary(t,"nofile",Bin). {module,t} 9> t:say(). "hello_world!!" 10>好吧,好多好玩的東西還沒有說,暫時到這裡,下回繼續...... 最後小圖一張,經歷了25年,13季,大偵探波洛的故事結束了,"女士們,先生們,該收場了........."
