VDM-10 Language Manual

VDM-10 Language Manual
✡✝
| sc in set gps(gp)}};
✆
The externals field lists the state/instance variables that the operation will manipulate. The
state/instance variables listed after the reserved word rd can only be read whereas the operation
can both read and write the variables listed after wr.
In VDM-SL these pre- and post-conditions the interpreter also creates new functions as with
the pre- and post-conditions of operation definitions. However, if a specification contains a
global state, the state is also part of the newly created functions. Thus, functions with the
following signatures are created for operations with pre- and/or post-conditions 1 :
✞
pre_Op : InType * State +> bool
✡✝
post_Op : InType * OutType * State * State +> bool
✆
with the following exceptions:
• If the operation does not take any arguments, the InType part of the signature is left
out in both the pre Op and post Op signatures.
• If the operation does not return a value, the OutType part is left out in the post Op
signature.
• If the specification does not define a state, the State part(s) of both signatures are left
out.
In the post Op signature, the first State part is for the old state, whereas the second
State part is for the state after the operation call.
For instance, consider the following specifications:
✞
module A
definitions
state St of
n : nat
end
operations
Op1 (a : nat) b :nat
1 However, you should remember that these pre and post condition predicates for an operation are simply boolean
functions and the state components are thus not changed by calling such a predicate.
92