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ch02-Erlang速览

Hello Wrold

$ erl
Eshell V12.0.3  (abort with ^G)
1> c(hello). # 编译
{ok,hello}
2> hello:start().
Hello world
ok

$ erlc hello.erl
$ erl -noshell -s hello start -s init stop
Hello world

文件服务进程

Eshell V12.0.3  (abort with ^G)
1> c(afile_server).
{ok,afile_server}
2> FileServer = afile_server:start(".").
<0.86.0>
3> FileServer ! {self(), list_dir}.
{<0.79.0>,list_dir}
4> receive X -> X end.
{<0.86.0>,
 {ok,["hello.erl","README.md","afile_server.beam",
      "hello.beam","afile_server.erl"]}}

Eshell V12.0.3  (abort with ^G)
1> c(afile_server).
{ok,afile_server}
2> c(afile_client).
{ok,afile_client}
3> FileServer = afile_server:start(".").
<0.91.0>
4> afile_client:get_file(FileServer, "missing").
{error,enoent}
5> afile_client:get_file(FileServer, "afile_server.erl").
{ok,<<"-module(afile_server).\n-export([start/1, loop/1]).\n\nstart(Dir) -> spawn(afile_server, loop, [Dir]).\n\nloop(Di"...>>}

ch03-基本概念

• 变量只能赋值一次
• = 是一个模式匹配操作符
• 变量不可变
• 原子是全局性的
• 没有类型声明
• 元组会在声明时自动创建，不再使用时则被销毁
• 列表里的元素可以是任意类型

ch04-模块与函数

• 当函数包含多个不同的子句，会用第一个与调用参数相匹配的子句开始执行

• 逗号分隔函数调用、数据构造和模式中的参数

• 分号分隔子句，如函数定义，以及case、if、try…catch和receive表达式

• 句号分隔函数整体，以及shell里的表达式

练习

(1)

area({triangle, Length, Width}) -> Length * Width / 2;
area({circle, Radius}) -> 3.14 * Radius * Radius;
area({rectangle, Width, Height}) -> Width * Height;
area({square, Side}) -> Side * Side.

(2)

• 思路是获取tuple的长度，然后从最后面开始构造list

my_tuple_to_list(T) ->
  my_tuple_to_list(T, tuple_size(T), []).

my_tuple_to_list(T, Size, L) when Size =:= 0 ->
  L;
my_tuple_to_list(T, Size, L)  ->
  my_tuple_to_list(T, Size-1, [element(Size, T)|L]).

(3)

my_time_func(F) ->
{Hour1, Minute1, Second1} = erlang:time(),
F,
{Hour2, Minute2, Second2} = erlang:time(),
io:format(“runtime:_fn”, [((Hour2-Hour1)/3600+(Minute2-Minute1)/60+(Second2-Second1))/1000]).
my_date_string() ->
{Year, Month, Day} = erlang:date(),
{Hour, Minute, Second} = erlang:time(),
io:format(“_p年p月_p日p时_p分p秒~n”, [Year, Month, Day, Hour, Minute, Second]).

(5)

-module(math_functions).
-export([even/1, odd/1]).

even(X) ->
  (X rem 2) =:= 0.

odd(X) ->
  (X rem 2) =:= 1.

(6)

filter(F, L) ->
  [X || X <- L, F(X) =:= true].

测试：

 math_functions:filter(fun(X) -> math_functions:even(X) end, [1, 2, 3, 4, 5]).

(7)

split(L) ->
  {filter(fun(X) -> even(X) end, L), filter(fun(X) -> odd(X) end, L)}.

测试：

40> math_functions:split([1,2,3,4,5,4]).
{[2,4,4],[1,3,5]}