On 05/07/2019 13:29, Sven Barth via fpc-devel wrote:
J. Gareth Moreton <gar...@moreton-family.com
<mailto:gar...@moreton-family.com>> schrieb am Fr., 5. Juli 2019, 14:02:
I'm glad we agree on the "is" operator. Personally I would like to
extend it so it accepts "if (I is TMyEnum) then" when I is also of
type
TMyEnum, since it may be the case that you're reading I directly
from a
stream, either individually or as part of a record, in which case
converting I from an Integer to TMyEnum is a little inconvenient.
When
you get down to the compiled assembly language it shouldn't really
matter anyway.
It might not matter from the assembly point of view, but it matters
from the language point of view. When we say that enum values only
contain valid values then "myenum is TMyEnum" *must* always evaluate
to true, because otherwise there's an exception of the rule that enums
only contain valid values.
Regards,
Sven
In this situation, the exception is kind of needed, and was suggested
for this purpose in the first place. If you read an enum from a stream
or any kind of external source, then there is a chance it will take on
an invalid value and needs to be trapped. I don't think anyone wants to
be forced to ditch enumerations for integers and C-style constants so as
to not fall foul of this quirk. "is" is to catch an enumeration taking
on an invalid value, thus allowing customised error handling to be
executed, so other expressions can be optimised on the assumption that
the enum is definitely valid by that point.
Gareth aka. Kit
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