OCamlテクニック/ghost のバックアップ差分(No.21)

追加された行はこの色です。
削除された行はこの色です。
[[OCamlテクニック/ghost]]

OCamlで幽霊型を応用してみました。
お金型に税込みかどうかの区別を付けて、二重に税金を計算することを
防ぐことができます。

まずシグネチャーを定義します。
ここでは、裸の金額と税込みの金額を区別した型を定義して、
tax関数は裸の金額にしか適用できないよう制限します。

 # module type MoneySig = sig
     type 'a money constraint 'a = [< `Naked | `Taxed ]
     val make : float -> [`Naked] money
     val add : 'a money -> 'a money -> 'a money
     val mul : 'a money -> 'a money -> 'a money
     val tax : [`Naked] money -> [`Taxed] money
     val show : 'a money -> string
   end
   ;;
 module type MoneySig =
   sig
     type 'a money constraint 'a = [< `Naked | `Taxed ]
     val make : float -> [ `Naked ] money
     val add : ([< `Naked | `Taxed ] as 'a) money -> 'a money -> 'a money
     val mul : ([< `Naked | `Taxed ] as 'a) money -> 'a money -> 'a money
     val tax : [ `Naked ] money -> [ `Taxed ] money
     val show : [< `Naked | `Taxed ] money -> string
   end

先ほどのシグネチャーを適用したモジュールを作ります。
実体をfloatとしています。

 # module Money : MoneySig = struct
     type 'a money = float constraint 'a = [< `Naked | `Taxed ]
     let make x = x
     let add = (+.)
     let mul = ( *.)
     let tax x = x *. 1.05
     let show = string_of_float
   end;;
 module Money : MoneySig

では、Moneyモジュールを使ってみましょう。

 # let juice = Money.make 120.0;; 
 val juice : [ `Naked ] Money.money = <abstr>

ジュースは120円、税抜きとします

 # Money.tax (Money.tax juice);;  
 This expression has type [ `Taxed ] Money.money but is here used with type
   [ `Naked ] Money.money
 These two variant types have no intersection

''二重に課税すると型エラーです。''

 # Money.add juice juice;;
 - : [ `Naked ] Money.money = <abstr>
 # let pay = Money.tax juice;;
 val pay : [ `Taxed ] Money.money = <abstr>
 # print_string (Money.show pay);;
 126.- : unit = ()

通常の使い方はこんな感じ。

 # Money.add juice pay;;
 Characters 16-19:
   Money.add juice pay;;
                   ^^^
 This expression has type [ `Taxed ] Money.money but is here used with type
   [ `Naked ] Money.money
 These two variant types have no intersection
 # 

''裸の金額と税込み金額をごちゃ混ぜにして足し算しようとしても、きちんとエラーになります''

- すごいなるほど！ところでconstraintというのは予約語ですか？こういう型の構文をはじめて見るのですが -- [[けいご]] &new{2006-12-22 (金) 21:09:12};
- ああ，'aが[`Naked]か[`Taxed]の場合しかないという意味ですね?． -- [[けいご]] &new{2006-12-22 (金) 21:10:50};
- constraint は予約語です。意味はそのまんまですね :) 今回は多相ヴァリアントを閉じた形でconstraintにしています。 -- [[ogasawara]] &new{2006-12-22 (金) 23:20:12};
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#comment