The key words must, required, shall should, recommend, may, and
optional in this document are to be interpreted as described in RFC 2119:
http://www.ietf.org/rfc/rfc2119.txt
-
must
-
When used alone, this word, or the term required, means that the
definition is an absolute requirement of the specification.
When followed by not (“must not” ), the phrase means that the
definition is an absolute prohibition of the specification.
-
should
-
When used alone, this word, or the adjective recommended, means that there
may exist valid reasons in particular circumstances to ignore a particular
item, but the full implications must be understood and carefully weighed
before choosing a different course.
When followed by not (“should not”), the phrase means that there may
exist valid reasons in particular circumstances when the particular behavior
is acceptable or even useful, but the full implications should be
understood and the case carefully weighed before implementing any behavior
described with this label.
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may
-
This word, or the adjective optional, means that an item is truly
optional.
One vendor may choose to include the item because a particular marketplace
requires it or because the vendor feels that it enhances the product while
another vendor may omit the same item.
An implementation which does not include a particular option must be
prepared to interoperate with another implementation which does include the
option, though perhaps with reduced functionality.
In the same vein an implementation which does include a particular option
must be prepared to interoperate with another implementation which does not
include the option (except, of course, for the feature the option provides).
The additional terms can and cannot are to be interpreted as follows:
-
can
-
This word means that the particular behavior described is a valid choice for
an application, and is never used to refer to implementation behavior.
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cannot
-
This word means that the particular behavior described is not achievable by
an application.
For example, an entry point does not exist, or shader code is not capable of
expressing an operation.
![[Note]](images/icons/note.png) | Note |
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There is an important distinction between cannot and must not, as used
in this Specification.
Cannot means something the application literally is unable to express or
accomplish through the API, while must not means something that the
application is capable of expressing through the API, but that the
consequences of doing so are undefined and potentially unrecoverable for the
implementation. |
![[Note]](images/icons/note.png) | editing-note |
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TODO (Jon) - We might need to augment the RFC 2119 definition of must not
to include some of the previous note, since at present it is defined solely
in terms of implementation behavior.
See Gitlab issue #9. |