Tony Whyman via fpc-pascal wrote:
Back in the early eighties, I worked at ICL and we made extensive use of
the Prospero Pascal compiler building embedded systems based on the Z80
microprocessor. I still have a 1988 edition of the language
specification, and this uses EBNF to define the case statement as:
case-statement = "CASE" case-index "OF"
case-list-element {";" case-list-element }
[ ";" OTHERWISE statement][";"] "END"
case-index = expression
case-list-element = case-range-list ":" statement
case-range-list = case-range {"," case-range }
case-range = case-constant [".." case-constant ]
case-constant = constant
What is interesting about the above is not just that it uses "otherwise"
but that it insists on a semi-colon before the "otherwise". This may not
have been strictly necessary but it does enforce the separation between
the case -list-elements and the "otherwise", making the "otherwise"
clause into another case-list-element. It also aids readability and it
may be why I have always added a semi-colon after the final
case-list-element. Note that the final optional ";" is probably needed
to allow those that always like to end a statement in a semi-colon.
Prospero Pascal was close to ISO Pascal (although I have lost my
original copy of ISO Pascal) and I would guess that the above is copied
from ISO Pascal.
ISO-7185 Pascal doesn't have an else/otherwise in the case-statement, ISO-10206 Extended Pascal
does, but it is of later date.
The change from "otherwise" to "else" is probably a Borland Pascal invention
preferring a shorter word and then overlooking the importance of the semi-colon case-list-element
separator and the resulting ambiguity. The same error then flowed through to exception handling.
As the otherwise-clause is not in ISO-7185 Pascal, it seems more plausible that Borland invented
the else-clause (without semicolon) independently. All other Pascals I have looked at, use an
otherwise-clause (with an obligatory semicolon). The motivation for this, given in IBM Pascal is
interesting. The manual says that the statement-part of the otherwise-clause can be intentionally
"left blank" and be used "to prevent possible errors during execution". I recall that in ISO-7185
Pascal it is an error if no case discriminator matches at runtime. So, the otherwise-clause was
seen as a way to get around that !
Regards,
Adriaan van Os
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