VDM-10 Language Manual

VDM-10 Language Manual
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let p1 = e1, ..., pn = en in s
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where p1, ..., pn are patterns, e1, ..., en are expressions which match the corresponding
patterns pi, and s is a statement, of any type, involving the pattern identifiers
of p1, ..., pn. It denotes the evaluation of the statement s in the context in which the
patterns p1, ..., pn are matched against the corresponding expressions e1, ...,
en.
More advanced let statements can also be made by using local function definitions. The
semantics of doing that is simply that the scope of such locally defined functions is restricted
to the body of the let statement.
A let-be-such-that statement has the form
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let b be st e in s
where b is a binding of a pattern to a set value (or a type), e is a boolean expression, and
s is a statement, involving the pattern identifiers of the pattern in b. The be st e part is
optional. The expression denotes the evaluation of the statement s in the context where the
pattern from b has been matched against an element in the set (or type) from b 1 . If the be
st expression e is present, only such bindings where e evaluates to true in the matching
context are used.
Examples: An example of a let be st statement is provided in the operation GroupWinner
from the class GroupPhase which returns the winning team in a given group:
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GroupWinner : GroupName ==> Team
GroupWinner (gp) ==
let sc in set gps(gp) be st
forall sc’ in set} gps(gp) \ {sc} &
(sc.points > sc’.points) or
(sc.points = sc’.points and sc.won > sc’.won)
in
return sc.team
The companion operation GroupRunnerUp gives an example of a simple let statement as
well:
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GroupRunnerUp_expl : GroupName ==> Team
GroupRunnerUp_expl (gp) ==
1 Remember that only the set bindings can be executed by means of the interpreter.
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