Quelle LocalResource.vdmpp Sprache: VDM

\section{The Class LocalResource}

A LocalResource acts as a virtually seamless interface for a Till to a Channel.
Thus it provides the same calling interface as a CentralResource, except that
its operations are able to return the value <Fail> to represent a communication
failure.

\begin{vdm_al}
class LocalResource
\end{vdm_al}

A LocalResource has only one instance variable: a reference to a Channel.

\begin{vdm_al}
instance variables
  c : Channel := new Channel();

operations
\end{vdm_al}

The Create operation is used to initialize the Channel.

\begin{vdm_al}
  public LocalResource : Channel ==> LocalResource
  LocalResource(nc) ==
    c := nc;
\end{vdm_al}

The operation GetBalance shadows the corresponding operation in CentralResource.
The argument received by the method is converted into a value of type Channel`ReqData,
and then a request is constructed. This is sent to the channel and then the Wait
operation is called.

\begin{vdm_al}
  public GetBalance : Account`AccountId ==> [nat]| <Fail>
  GetBalance(accountId) ==
    let req = mk_Channel`Request(<GetBalance>,
                                 {mk_Channel`AccountId(accountId)}) in
    (c.SendRequest(req);
     Wait(<GetBalance>));
\end{vdm_al}

The Wait operation waits for a particular response from the channel. If the
response is nil or does not match the expected result, a failure is signalled
(corresponding to a timeout in the Channel). Otherwise the data value in the
response is returned.

\begin{vdm_al}
  Wait : Channel`Command ==> Channel`RespData | <Fail>
  Wait(comm) ==
    (c.Wait();
     let resp = c.ReceiveResponse() in
     if resp = nil
     then return <Fail>
     elseif resp.command <> comm
     then return <Fail>
     else return resp.data);
\end{vdm_al}

The remaining operations follow the same basic approach as that of GetBalance,
and need no further explanation.

\begin{vdm_al}
  public Withdrawal : Account`AccountId * Card`CardId * nat ==> bool | <Fail>
  Withdrawal(accountId,cardId,amount) ==
    let req = mk_Channel`Request(<Withdrawal>,
                                 {mk_Channel`AccountId(accountId),
                                  mk_Channel`CardId(cardId),
                                  mk_Channel`Amount(amount)}) in
    (c.SendRequest(req);
     Wait(<Withdrawal>));

  public PostStatement : Account`AccountId * Card`CardId ==> bool | <Fail>
  PostStatement(accountId,cardId) ==
    let req = mk_Channel`Request(<PostStmt>,
                                 {mk_Channel`AccountId(accountId),
                                  mk_Channel`CardId(cardId)}) in
    (c.SendRequest(req);
     Wait(<PostStmt>));

  public IsLegalCard : Account`AccountId * Card`CardId ==> bool | <Fail>
  IsLegalCard(accountId,cardId) ==
    let req = mk_Channel`Request(<IsLegalCard>,
                                 {mk_Channel`AccountId(accountId),
                                  mk_Channel`CardId(cardId)}) in
    (c.SendRequest(req);
     Wait(<IsLegalCard>));

  public NumberOfTriesExceeded : Card`CardId ==> bool | <Fail>
  NumberOfTriesExceeded(cardId) ==
    let req = mk_Channel`Request(<TriesExceeded>,
                                 {mk_Channel`CardId(cardId)}) in
    (c.SendRequest(req);
     Wait(<TriesExceeded>));

  public ResetNumberOfTries : Card`CardId ==> [<Fail>]
  ResetNumberOfTries(cardId) ==
    let req = mk_Channel`Request(<ResetTries>,
                                 {mk_Channel`CardId(cardId)}) in
    (c.SendRequest(req);
     Wait(<ResetTries>));

  public IncrNumberOfTries : Card`CardId ==> [<Fail>]
  IncrNumberOfTries(cardId) ==
    let req = mk_Channel`Request(<IncTries>,
                                 {mk_Channel`CardId(cardId)}) in
    (c.SendRequest(req);
     Wait(<IncTries>));

end LocalResource
\end{vdm_al}

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