[Oberon] Non-matching expressions in case statements

August Karlstrom fusionfile at gmail.com
Tue Aug 29 16:52:11 CEST 2023

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 



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