VDM-10 Language Manual

VDM-10 Language Manual
Semantics: A pattern is always used in a context where it is matched to a value of a particular
type. Matching consists of checking that the pattern can be matched to the value, and binding
any pattern identifiers in the pattern to the corresponding values, i.e. making the identifiers
denote those values throughout their scope. In some cases where a pattern can be used, a
bind can be used as well (see next section). If a bind is used it simply means that additional
information (a type or a set expression) is used to constrain the possible values which can
match the given pattern.
Matching is defined as follows
1. A pattern identifier fits any type and can be matched to any value. If it is an identifier,
that identifier is bound to the value; if it is the don’t-care symbol ‘-’, no binding occurs.
2. A match value can only be matched against the value of itself; no binding occurs. If a
match value is not a literal like e.g. 7 or it must be an expression enclosed in
parentheses in order to discriminate it to a pattern identifier.
3. A set enumeration pattern fits only set values. The patterns are matched to distinct
elements of a set; all elements must be matched.
4. A set union pattern fits only set values. The two patterns are matched to a partition of
two subsets of a set. In the Toolbox the two subsets will always be chosen such that
they are non-empty and disjoint.
5. A sequence enumeration pattern fits only sequence values. Each pattern is matched
against its corresponding element in the sequence value; the length of the sequence
value and the number of patterns must be equal.
6. A sequence concatenation pattern fits only sequence values. The two patterns are
matched against two subsequences which together can be concatenated to form the
original sequence value. In the Toolbox the two subsequences will always be chosen
so that they are non-empty.
7. A tuple pattern fits only tuples with the same number of elements. Each of the patterns
are matched against the corresponding element in the tuple value.
8. A record pattern fits only record values with the same tag. Each of the patterns are
matched against the field of the record value. All the fields of the record must be
matched.
Examples: The simplest kind of pattern is the pattern identifier. An example of this is given in the
following let expression:
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let top = GroupA(1)
in top.sc
Here the identifier top is bound to the head of the sequence GroupA and the identifier may
then be used in the body of the let expression.
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