VDM-10 Language Manual

Chapter 17. Top-level Specification (VDM-SL)
Semantics: The exports section must be used to make constructs visible to other modules. Some
or all of the defined constructs from a module can be exported. In the latter case, the keyword
all is used. However, imported constructs are not exported from the module. If only part of
the constructs are exported, the visible constructs with the appropriate signatures are stated.
Normally, if a construct is visible to another module, that construct can be considered to be
defined inside the module. However, with types and operations there are some exceptions:
Types: If a type T is defined in module A and this type is also going to be used in module
B, the type from module A has to be exported. This can be done in two ways:
1. The name of the type is exported.
2. The structure of the type is exported.
If only the name of the type is exported, the other module cannot create values of type
T. This means that the exporting module (A) must provide functions and/or operations
to directly create and manipulate values of type T by means of the constructors related
to the representation of T.
If we export the structure of the type by using the keyword struct, the other module
can create and manipulate values of type T (it can also use mk keyword and the is
keyword for this type if it is a record type).
If the type also defines an invariant, the invariant predicate function is only exported if
the structure of the type is exported.
Operations: In a module, a state that is global for the module can be defined. All operations
within the module can manipulate that state. If operations are exported from a module,
they manipulate the state in the exporting module, i.e. the state in the module where
they are defined.
If an exported function or an operation defines a pre- and/or post-condition, the corresponding
predicate functions (see section 5) are also exported.
Examples: Consider a model of a bank account. An account is characterised by the name of the
holder, the account number, the bank branch at which the account is maintained, the balance,
and an encrypted PIN code for the ATM card. We might model this as follows:
✞
module BankAccount
exports types digit; account
functions digval: digit -> nat;
withdrawal: account * real -> account;
isPin: account * nat -> bool;
requestWithdrawal: account * nat -> bool
definitions
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