VDM-10 Language Manual

Chapter 7. Patterns
In the following examples we use match values:
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let a =
in cases GroupA(1).team:
-> "Brazil are winners",
(a) -> "France are winners",
others -> "Neither France nor Brazil are winners"
end;
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Match values can only match against their own values, so here if the team at the head of
GroupA is then the first clause is matched; if the team at the head of GroupA
is then the second clause is matched. Otherwise the others clause is matched.
Note here that the use of brackets around a forces a to be considered as a match value.
Set enumerations match patterns to elements of a set. For instance in
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let {sc1, sc2, sc3, sc4} = elems GroupA
in
sc1.points + sc2.points + sc3.points + sc4.points;
the identifiers sc1, sc2, sc3 and sc4 are bound to the four elements of GroupA. Note that
the choice of binding is loose - for instance sc1 may be bound to [any] element of elems
GroupA. In this case if elems GroupA does not contain precisely four elements, then the
expression is not well-formed.
A set union pattern can be used to decompose a set for recursive function calls. An example
of this is the function set2seq which converts a set into a sequence (with arbitrary order):
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set2seq[@elem] : set of @elem -> seq of @elem
set2seq(s) ==
cases s:
{} -> [],
{x}
-> [x],
s1 union s2 -> (set2seq[@elem](s1))ˆ(set2seq[@elem](s2))
end
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In the third cases alternative we see the use of a set union pattern. This binds s1 and s2 to
arbitrary subsets of s such that they partition s. The Toolbox interpreter always ensures a
disjoint partition.
Sequence enumeration patterns can be used to extract specific elements from a sequence. An
example of this is the function promoted which extracts the first two elements of a seqnce
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