VDM-10 Language Manual

VDM-10 Language Manual
of the entire state on the left-hand side of the equality and the right-hand side must evaluate
to a record value of the correct type. This enables the interpreter to evaluate the initpred
condition. A simple example of an initialisation predicate is shown below:
✞
✡✝
state St of
x: nat
y: nat
l: seq1 of nat
init s == s = mk_St(0,0,[1])
end
In the specification of both the invariant and the initial value the state must be manipulated
as a whole, and this is done by referring to it as a record tagged with the state name (see the
example). When a field in the state is manipulated in some operation, the field must however
be referenced to directly by the field name without pre-fixing it with the state name.
Examples: In the following example we create one state variable:
✞
types
GroupName = | | | | | | |
state GroupPhase of
gps : map GroupName to set of Score
inv mk_GroupPhase(gps) ==
forall gp in set rng gps &
(card gp = 4 and
{forall sc in set gp & sc.won + sc.lost + sc.drawn init_sc({, ,
, }),
...})
end
functions
✆
init_sc : set of Team -> set of Score
init_sc (ts) ==
{ mk_Score (t,0,0,0,0) | t in set ts }
✡✝
✆
In the invariant we state that each group has four teams, and no team plays more than three
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