VDM-10 Language Manual

Chapter 6. Expressions
✞
state sigma of
numbers : seq of nat
index : nat
inv mk_sigma(numbers, index) ==
index not in set elems numbers
init s == s = mk_sigma([], 1)
end
✡✝
✆
For an example of the use of old names, consider the VDM++/VDM-RT instance variables
defined as:
✞
✡✝
instance variables
numbers: seq of nat := [];
index : nat := 1;
inv index not in set elems numbers;
✆
We can define an operation that increases the variable index in an implicit manner:
✞
✡✝
IncIndex()
ext wr index : nat
post index = index˜ + 1
✆
The operation IncIndex manipulates the variable index, indicated with the ext wr
clause. In the post condition, the new value of index is equal to the old value of index
plus 1. (See more about operations in section 12).
For a simple example of module/class names, suppose that a function called build rel is
defined (and exported) in a module/class called CGRel as follows:
✞
types
Cg = | | | | | |
| | | | | ;
CompatRel = map Cg to set of Cg
functions
✡✝
build_rel : set of (Cg * Cg) -> CompatRel
build_rel (s) == {|->}
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✆