Getting Started with Erlang pt.2

Erlang をはじめよう その2

 

前回 - mog project: Getting Started with Erlang pt.1 の続き

CHAPTER 3: Atoms, Tuples, and Pattern Matching

 

アトム

アトムを利用したパターンマッチ、ブール値を示す特別なアトム

> hello.
> F = (fun(apple, Num) -> 100 * Num; (banana, Num) -> 200 * Num end).
> F(apple, 2).
> F(banana, 3).
> F(candy, 4).    % error
> 3<2.
> 3>2.
> 10 == 10.
> true and true.
> true or false.
> false xor false.
> not false.

ガード

when句、アンダースコアの利用

> Abs = (fun (Num) when Num =< 0 -> -Num; (Num) when Num > 0 -> Num end).
> Abs(-10).
> Abs(0).
> Abs(3).
> Abs = (fun (Num) when Num  -Num; (Num) when Num >= 0 -> Num end).  % error
> Abs2 = (fun
> (Num) when Num < 0 -> -Num;
> (0) -> 0;
> (Num) -> Num
> end).
> Abs2(-5).
> _.
> _ = 20.
> _.
> F = (fun(apple, Num) -> 100 * Num; (_, Num) -> 500 * Num end).
> F(apple, 3).
> F(candy, 10).
> G = (fun(apple, Num) -> 100 * Num; (_, _) -> 1 end).
> G(hello, world).

タプル

タプルの操作、パターンマッチでの利用

> {atom, 123, "string"}.
> T = {atom, 123, "string"}.
> element(2, T).
> setelement(2, T, 456).
> tuple_size(T).
> {A, B, C} = T.
> A.
> B.
> C.
> F = (fun ({apple, Num}) -> 100 * Num; ({banana, Num}) -> 200 * Num end).
> F({apple, 3}).

タプルを使った実装の隠蔽 (カプセル化)

-module(shop).
-export([buy/1]).

buy({Item, Num}) -> buy(Item, Num).

buy(apple, Num)  when Num >= 0 -> 100 * Num;
buy(banana, Num) when Num >= 0 -> 200 * Num;
buy(_, Num)      when Num >= 0 -> 500 * Num;
buy(_, _)                      -> 0.

> c(shop).
> shop:buy({apple, 3}).
> shop:buy({banana, 2}).
> shop:buy({candy, 1}).
> shop:buy({apple, -1}).
> shop:buy(apple, 3).    % error

CHAPTER 4: Logic and Recursion

 

case 構成要素 (case construct)

case は値を返す。case 内部でガードを行うことも可能。

> F = fun (Item, Num) ->
>   case Item of
>     apple -> 100 * Num;
>     banana -> 200 * Num
>   end
> end.
> F(apple, 3).
> G = fun (Item, Num) ->
>   X = case Item of
>     apple -> 100;
>     banana when Num >= 5 -> 198;
>     banana -> 200
>   end,
>   X * Num
> end.
> G(banana, 3).
> G(banana, 10).

if 構成要素 (if construct)

> F = fun (Item, Num) ->
> X = 100.
> if X >= 99 -> 'good' end.
> if X =< 99 -> 'good' end.
> F = fun(X) ->
>   if
>     X =< 99 -> io:format("X is less than ~w.~n", [100]);
>     true -> true
>   end
> end.
> F(10).
> F(100).
> A = 10.
> B = if A == 5 -> 100; true -> 20 end.
> B.
> BadFun = fun (X) ->
>   if
>     X < 0 -> Y = 1;
>     X >= 0 -> Z = 2
>   end,
>   Y + Z
> end.

再帰

カウントダウン

-module(count).
-export([countdown/1]).

countdown(From) when From > 0 ->
  io:format("~w!~n", [From]),
  countdown(From - 1);

countdown(From) ->
  io:format("blastoff!~n").

> c(count).
> count:countdown(10).

階乗

-module(fact).
-export([factorial/1]).

factorial(N) when N =< 1 -> 1;
factorial(N) -> N * factorial(N - 1).

> c(fact).
> fact:factorial(10).

階乗 (アキュムレーター付き)

-module(fact).
-export([factorial/1]).

factorial(N) -> factorial(1, N, 1).

factorial(Current, N, Result) when Current =< N ->
  factorial(Current + 1, N, Result * Current);
factorial(Current, N, Result) -> Result.

> c(fact).
> fact:factorial(10).

 

 

 

References

• サンプルコード

 

Related Posts

• mog project: Getting Started with Erlang pt.1