VDM-10 Language Manual

VDM-10 Language Manual
In another module/class we can access this function by in VDM-SL first importing the module
CGRel then by using the following call
✞
✡✝
CGRel‘build_rel({mk_(, )})
Note that in VDM++ and VDM-RT build rel function must have an access modifier
allowing access to it outside the defining class.
✆
6.25 The Undefined Expression
Syntax: expression = . . .
| undefined expression ;
undefined expression = ‘undefined’ ;
Semantics: The undefined expression is used to state explicitly that the result of an expression
is undefined. This could for instance be used if it has not been decided what the result of
evaluating the else-branch of an if-then-else expression should be. When an undefined expression
is evaluated the interpreter will terminate the execution and report that an undefined
expression was evaluated.
Pragmatically use of undefined expressions differs from pre-conditions: use of a pre-condition
means it is the caller’s responsibility to ensure that the pre-condition is satisfied when the
function is called; if an undefined expression is used it is the called function’s responsibility
to deal with error handling.
Examples: We can check that the type invariant holds before building Score values:
✞
✡✝
build_score : Team * nat * nat * nat * nat -> Score
build_score (t,w,d,l,p) ==
if 3 * w + d = p
then mk_Score(t,w,d,l,p)
else undefined
✆
6.26 The Precondition Expression
Syntax: expression = . . .
| precondition expression ;
precondition expression = ‘pre ’, ‘(’, expression,
[ { ‘,’, expression } ], ‘)’ ;
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