VDM-10 Language Manual

Chapter 17. Top-level Specification (VDM-SL)
exports all clause.
The module Keypad given below models the keypad interface of an ATM machine. The
state variable maintains a buffer of data typed at the keypad by the user.
✞
module Keypad
imports
from BankAccount types digit
exports all
definitions
state buffer of
data : seq of BankAccount‘digit
end
operations
DataAvailable : () ==> bool
DataAvailable () ==
return(data []);
ReadData : () ==> seq of BankAccount‘digit
ReadData () ==
return (data);
WriteData : seq of BankAccount‘digit ==> ()
WriteData (d) ==
data := dataˆd
end Keypad
✡✝
✆
In this module all constructs are exported. Since the only entities defined are the state and
operations on it, this means that all of the operations may be accessed by an importing
module. The state is not accessible to importing modules, but remains private to this module.
However the state constructor mk Keypad‘buffer is accessible.
17.2.3 The Imports Section
Syntax: interface = [ import definition list ],
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