VDM-10 Language Manual

Chapter 12. Operation Definitions
specifiers can be found in section 14.4. Note that a static operation may not call non-static
operations or functions, and self expressions cannot be used in the definition of a static
operation.
The following example of an explicit operation updates the VDM-SL state GroupPhase
and the VDM++ instance variables of class GroupPhase when one team beats another.
✞
✡✝
Win : Team * Team ==> ()
Win (wt,lt) ==
let gp in set dom gps be st
{wt,lt} subset {sc.team | sc in set gps(gp)}
in gps := gps ++ { gp |->
{if sc.team = wt
then mu(sc, won |-> sc.won + 1,
points |-> sc.points + 3)
elseif sc.team = lt
then mu(sc, lost |-> sc.lost + 1)
else sc
| sc in set gps(gp)}}
pre exists gp in set dom gps &
{wt,lt} subset {sc.team | sc in set gps(gp)};
✆
An explicit operation consists of a statement (or several composed using a block statement),
as described in section 13. The statement may access any state/instance variables it wishes,
reading and writing to them as it sees fit.
An implicit operation is specified using an optional pre-condition, and a mandatory postcondition.
For example we could specify the Win operation implicitly:
✞
Win (wt,lt: Team)
ext wr gps : map GroupName to set of Score
pre exists gp in set dom gps &
{wt,lt} subset {sc.team | sc in set gps(gp)}
post exists gp in set dom} gps &
{wt,lt} subset {sc.team | sc in set gps(gp)}
and gps = gps˜ ++
{ gp |->
{if sc.team = wt
then mu(sc, won |-> sc.won + 1,
points |-> sc.points + 3)
elseif sc.team = lt
then mu(sc, lost |-> sc.lost + 1)
else sc
91