1) For case statements with integer labels, which often represent an
enumerated type, it's *desirable* to trap non-matching case expressions
as it may indicate a forgotten case label.
2) When using case statements with type labels, the programmer typically
relies on non-matching case expressions being ignored; for message
records the message "fall through", as described in "Programming in
Oberon", p. 61. Here it's *essential* that the implementation does not
halt the program with a trap, otherwise this technique will not work.
The language report says:
"First the case expression is evaluated, then the statement sequence is
executed whose case label list contains the obtained value."
As I interpret the sentence above, the expression is supposed to match
exactly one label, however that is not the case with "fall through"
messages. I think it needs to be clarified in the language report that
"fall through" is well-defined for case statements with type labels. Any
comments?
My interpretation is that the presence of the phrase case label list indicates that the remainder of the sentence only applies to numeric case statements.
The syntax for type case statements is a substantially reduced form of the syntax for numeric case statements:
TypeCaseStatement = CASE qualident OF case {"|" case} END.
case = [qualident ":" StatementSequence].
NumericCaseStatement = CASE expression OF case {"|" case} END.
case = [CaseLabelList ":" StatementSequence].
CaseLabelList = LabelRange {"," LabelRange}.
LabelRange = label [".." label].
label = integer | string | qualident.
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Regards,
Chris Burrows
CFB Software