Erlang (programming language)/Tutorials/otp design: Difference between revisions

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imported>Eric Evers
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Example program for the OTP behaviour, gen_server.
===Fail early and often===


Here is an example program using gen_server to create a simple text game.
Fail early and often is a strange and unusual programming method used in OTP.
 
Processes are programed to a specification about how to handle good input. If
<code>
bad input is seen by a worker process we let the process crash.  
 
The supervisor restarts the process and the bad input is ignored.  
  -module(rock).
Fail early and often works well with any type of data flow situation
  -author("Eric Evers").
when a program is processing a flow of messages.
  -behaviour(gen_server).
Logically, security and stability is increased by limiting
  % ---------------------------------------------------------------
the propagation of errors to other processes.
  % Example of a gen_server
  %  Purpose: a tutorial on gen_server
  %  Program: a program that plays "rock, paper, scissors"
  %    in spanish and english
  %  Topics covered:
  %    * starting a gen_server
  %    * stoping a gen_server
  %    * using a gen_server to play a simple game
  %    * make a call within a call
  %    * use calls and casts
  %
  % For more information see: 
  %
  % References:
  %  Author:  Ericsson AB
  %  Document: Erlang doumentation
  %  Section:  OTP Design Principles
  %  Page:    Gen_Server Behaviour 
  % ----------------------------------------------------------------
  %
  % - for internal use
  -export([init/1, handle_call/3, handle_cast/2]).  
  -export([handle_info/2, terminate/2, code_change/3]).
 
  % - quick halt (note: casts are Asynchronous)
  -export([stop/0]).
 
  % - for external use (note: calls are Synchronous)
  -export([start/0, translate/1, juego/1, list/0, play/1]).
 
  % @start the server
  start() ->            gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
  list()  ->            gen_server:call(?MODULE, {list}).
  translate(Word) ->    gen_server:call(?MODULE, {translate, Word}).
  %
  % use juego to play the game using spanish words
  juego(Word) ->        gen_server:call(?MODULE, {juego, Word}).
 
  % use play to play the game using english words
  play(Word) ->        gen_server:call(?MODULE, {play, Word}).
  %
  % init
  init([]) -> Dictionary = dict:from_list(
    [{rock,roca},{paper,papel},{scissors,tijeras}]),
    {ok, Dictionary}.
  %
  % handle_call
  handle_call({list}, _From, Dictionary) ->
    Response = dict:to_list(Dictionary),
    {reply, Response, Dictionary};
  %
  % Here we make a call within a call
  handle_call({play, Word}, _From, Dictionary) ->
    Status = dict:is_key(Word, Dictionary),
    if
        Status == true ->
            SWord = dict:fetch(Word, Dictionary),
            {reply, Response, _} =
                handle_call({juego, SWord}, _From, Dictionary);
        true ->
            Response = your_word_is_not_valid
    end,
    {reply, Response, Dictionary};
  % 
  handle_call({juego, Word}, _From, Dictionary) ->
    List = dict:to_list(Dictionary),
    {_English, Spanish} = lists:unzip(List),
    Indexes = lists:seq(1, length(Spanish)),
    Enumerated = lists:zip(Spanish, Indexes),
    Enumerated_dictionary = dict:from_list(Enumerated),
    {_H,_M,S} = time(),
    N = (S rem length(Spanish)) + 1,
    Pick = lists:nth(N, Spanish),
    io:format("you play: ~w \n",[Word]),
    P = dict:fetch(Word, Enumerated_dictionary),
    Delta = ((N+3)-P) rem 3,
    Response = case Delta of
        0 -> {'I',picked,Pick,its_a,draw};
        1 -> {'I',picked,Pick,you,loose};
        2 -> {'I',picked,Pick,you,win};
        true -> {unknown, result}
    end,
    {reply, Response, Dictionary};
  %   
  handle_call({translate, Word}, _From, Dictionary) ->
    Response = case dict:is_key(Word, Dictionary) of
        true ->
            dict:fetch(Word, Dictionary);
        false ->
            {word_not_known, Word}
    end,
    {reply, Response, Dictionary};
  %
  handle_call(_Message, _From, Dictionary) -> {reply, error, Dictionary}.
  % -
  stop() ->
    gen_server:cast(?MODULE, stop).
  %
  terminate(normal, _State) ->
    ok.
  %
  handle_cast(stop, State) ->
    {stop, normal, State};
  % 
  % - lets keep the compiler quiet with all the call-backs
  handle_cast(_Message, Dictionary) -> {noreply, Dictionary}.
  handle_info(_Message, Dictionary) -> {noreply, Dictionary}.
  code_change(_OldVersion, Dictionary, _Extra) -> {ok, Dictionary}.
 
  % - sample output -----------------------------
  % c(rock).
  %  <compile is done>
  % rock:start().
  %  <server started>
  %
  % 67> rock:juego(tijeras).
  % you play: tijeras
  % {'I',picked,roca,you,loose}
  %
  % 68> rock:juego(tijeras).
  % you play: tijeras
  % {'I',picked,papel,you,win}
  %
  % 69> rock:juego(tijeras).
  % you play: tijeras 
  % {'I',picked,tijeras,its_a,draw}
  % ----------------------------------------------------
</code>

Revision as of 11:15, 22 July 2009

Fail early and often

Fail early and often is a strange and unusual programming method used in OTP. Processes are programed to a specification about how to handle good input. If bad input is seen by a worker process we let the process crash. The supervisor restarts the process and the bad input is ignored. Fail early and often works well with any type of data flow situation when a program is processing a flow of messages. Logically, security and stability is increased by limiting the propagation of errors to other processes.

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