Editing Vienna Development Method (section)

===Bank system example===
As a very simple example of a VDM-SL model, consider a system for maintaining details of customer bank account. Customers are modelled by customer numbers (''CustNum''), accounts are modelled by account numbers (''AccNum''). The representations of customer numbers are held to be immaterial and so are modelled by a token type. Balances and overdrafts are modelled by numeric types.
<syntaxhighlight lang="rsl">
 AccNum = token;
 CustNum = token;
 Balance = int;
 Overdraft = nat;

 AccData :: owner : CustNum
            balance : Balance

 state Bank of
   accountMap : map AccNum to AccData
   overdraftMap : map CustNum to Overdraft
 inv mk_Bank(accountMap,overdraftMap) == for all a in set rng accountMap & a.owner in set dom overdraftMap and
                                         a.balance >= -overdraftMap(a.owner)
</syntaxhighlight>
With operations:
''NEWC'' allocates a new customer number:
<syntaxhighlight lang="rsl">
 operations
 NEWC(od : Overdraft)r : CustNum
 ext wr overdraftMap : map CustNum to Overdraft
 post r not in set dom ~overdraftMap and overdraftMap = ~overdraftMap ++ { r |-> od};
</syntaxhighlight>
''NEWAC'' allocates a new account number and sets the balance to zero:
<syntaxhighlight lang="rsl">
 NEWAC(cu : CustNum)r : AccNum
 ext wr accountMap : map AccNum to AccData
     rd overdraftMap map CustNum to Overdraft
 pre cu in set dom overdraftMap
 post r not in set dom accountMap~ and accountMap = accountMap~ ++ {r|-> mk_AccData(cu,0)}
</syntaxhighlight>
''ACINF'' returns all the balances of all the accounts of a customer, as a map of account number to balance:
<syntaxhighlight lang="rsl">
 ACINF(cu : CustNum)r : map AccNum to Balance
 ext rd accountMap : map AccNum to AccData
 post r = {an |-> accountMap(an).balance | an in set dom accountMap & accountMap(an).owner = cu}
</syntaxhighlight>