VDM-10 Language Manual

VDM-10 Language Manual
Quote expression
pre Op1(1,mk St(2))
post Op1(1,2,mk St(1), mk St(2))
post Op2(2,mk St(1), mk St(2))
post Op3(mk St(1), mk St(2))
Explanation
a bound to 1 in state St with n bound
to 2
a bound to 1, b bound to 2, state before
with n bound to 1, state after with n
bound to 2
b bound to 2, state before with n bound
to 1, state after with n bound to 2
state before with n bound to 1, state after
with n bound to 2
For module B we can quote the pre and post conditions defined in this specification as
illustrated below
Quote expression
Explanation
pre Op1(1) a bound to 1
post Op1(1,2) a bound to 1, b bound to 2
post Op2(2) b bound to 2
post Op3()
No binding at all
The exceptions clause can be used to describe how an operation should deal with error situations.
The rationale for having the exception clause is to give the user the ability to separate
the exceptional cases from the normal cases. The specification using exceptions does not
give any commitment as to how exceptions are to be signalled, but it gives the means to
show under which circumstances an error situation can occur and what the consequences are
for the result of calling the operation.
The exception clause has the form:
✞
errs COND1: c1 -> r1
...
CONDn: cn -> rn
✡✝
The condition names COND1, . . . , CONDn are identifiers which describe the kind of error
which can be raised 2 . The condition expressions c1, . . . , cn can be considered as preconditions
for the different kinds of errors. Thus, in these expressions the identifiers from
the arguments list and the variables from the externals list can be used (they have the same
scope as the pre-condition). The result expressions r1, . . . , rn can correspondingly be
considered as post-conditions for the different kinds of errors. In these expressions the result
identifier and old values of global variables (which can be written to) can also be used. Thus,
the scope corresponds to the scope of the post-condition.
Superficially there appears to be some redundancy between exceptions and pre-conditions
2 Notice that these names are purely of mnemonic value, i.e. semantically they are not important.
✆
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