[[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}; - GtNuuJzZF -- [[luwuwv]] &new{2008-09-26 (金) 22:49:44}; - , http://www.syracuse.com/forums/profile.ssf?nickname=n5CpCPqK ambien, xzf, http://www.syracuse.com/forums/profile.ssf?nickname=352Ov5jH diflucan, :-[[[, http://www.pennlive.com/forums/profile.ssf?nickname=dZ3l5jrx tramadol, 588, http://www.oregonlive.com/forums/profile.ssf?nickname=xzIEQnqc zoloft, ndre, http://www.silive.com/forums/profile.ssf?nickname=wJtcq63c actos, 54679, http://www.silive.com/forums/profile.ssf?nickname=soXPpVeZ ventolin, 59287, http://www.pennlive.com/forums/profile.ssf?nickname=ajOOVrbb allegra, 841, http://www.masslive.com/forums/profile.ssf?nickname=tBL6Kn60 celebrex, zsoijr, http://www.oregonlive.com/forums/profile.ssf?nickname=7likbuhv seroquel, 621, http://www.pennlive.com/forums/profile.ssf?nickname=9HV7TuHa plavix, 5205, http://www.masslive.com/forums/profile.ssf?nickname=fY8Neqtw vytorin, 142321, http://www.oregonlive.com/forums/profile.ssf?nickname=pvDctzlh codeine, 7893, http://www.nj.com/forums/profile.ssf?nickname=dNb4YjcI percocet, zxzvyr, http://www.syracuse.com/forums/profile.ssf?nickname=Ql09AvBD clomid, %-[[, http://www.oregonlive.com/forums/profile.ssf?nickname=SlMYjyZK prozac, :-), http://www.mlive.com/forums/profile.ssf?nickname=xXUJAGcZ nexium, =-[[[, http://www.cleveland.com/forums/profile.ssf?nickname=QCgYczZ1 actos, 8-OOO, http://www.syracuse.com/forums/profile.ssf?nickname=VBw2PwXg tenuate, 239112, http://www.oregonlive.com/forums/profile.ssf?nickname=paYyhkkb proscar, esr, http://www.mlive.com/forums/profile.ssf?nickname=O8t5r7jC phentermine, aid, http://www.mlive.com/forums/profile.ssf?nickname=V1n8IRdn aciphex, xenlu, http://www.silive.com/forums/profile.ssf?nickname=9v8TzeNT femara, 134, http://www.masslive.com/forums/profile.ssf?nickname=pDQqupAD lexapro, %[, http://www.syracuse.com/forums/profile.ssf?nickname=nNicgS92 kamagra, hzp, -- [[synthroid]] &new{2008-10-16 (木) 13:52:22}; - , http://www.syracuse.com/forums/profile.ssf?nickname=n5CpCPqK ambien, 22832, http://www.nj.com/forums/profile.ssf?nickname=dugzjd4N norvasc, fnrms, http://www.oregonlive.com/forums/profile.ssf?nickname=xzIEQnqc zoloft, ivwy, http://www.silive.com/forums/profile.ssf?nickname=iCSVS9UG lamictal, 67556, http://www.silive.com/forums/profile.ssf?nickname=soXPpVeZ ventolin, drts, http://www.mlive.com/forums/profile.ssf?nickname=mhAzjcZ9 hyzaar, qmtr, http://www.oregonlive.com/forums/profile.ssf?nickname=nHt2kT1X lexapro, dhmy, http://www.pennlive.com/forums/profile.ssf?nickname=uBTAPP3n viagra, ypmv, http://www.pennlive.com/forums/profile.ssf?nickname=CpxXfmBR antabuse, =DDD, http://www.oregonlive.com/forums/profile.ssf?nickname=GHH690bm femara, zdwo, http://www.syracuse.com/forums/profile.ssf?nickname=tvJkPgGB zyban, %-)), http://www.nola.com/forums/profile.ssf?nickname=WSLwtH8P dostinex, 971596, http://www.silive.com/forums/profile.ssf?nickname=VLtmccTu celebrex, nzsg, http://www.mlive.com/forums/profile.ssf?nickname=zXIW5on9 codeine, =], http://www.silive.com/forums/profile.ssf?nickname=DJLQGtiM adipex, 5399, http://www.nj.com/forums/profile.ssf?nickname=w6bItVe4 tenuate, :-), http://www.al.com/forums/profile.ssf?nickname=tb0iR7W7 geodon, 755496, http://www.cleveland.com/forums/profile.ssf?nickname=gkK7Itcq inderal, dmdjur, http://www.silive.com/forums/profile.ssf?nickname=U6TUNPzv klonopin, 181, http://www.silive.com/forums/profile.ssf?nickname=CPy1wiVd prozac, bpj, http://www.masslive.com/forums/profile.ssf?nickname=b8KFskUf proscar, mgj, http://www.nola.com/forums/profile.ssf?nickname=DdToM9pY antabuse, ivg, http://www.silive.com/forums/profile.ssf?nickname=jZbG7Z4p flonase, >:(, -- [[meridia]] &new{2008-10-16 (木) 20:39:16}; #comment