Feat-bits-case
Case macro for the different kinds of feat-bits structures.
This is an ACL2::fty sum-type case macro,
typically introduced by fty::defflexsum or fty::deftagsum. It
allows you to safely check the type of a feat-bits structure, or to split
into cases based on its type.
Short Form
In its short form, feat-bits-case allows you to safely check the type of
a feat-bits structure. For example:
(feat-bits-case x :32)
is essentially just a safer alternative to writing:
(equal (feat-bits-kind x) :32)
Why is using feat-bits-case safer? When we directly inspect the
kind with equal, there is no static checking being done to
ensure that, e.g., :32 is a valid kind of feat-bits structure. That means there is nothing to save you
if, later, you change the kind keyword for this type from :32 to something else. It also means you get no help
if you just make a typo when writing the :32
symbol. Over the course of developing VL, we found that such
issues were very frequent sources of errors!
Long Form
In its longer form, feat-bits-case allows you to split into cases based
on the kind of structure you are looking at. A typical example would be:
(feat-bits-case x
:32 ...
:64 ...)
It is also possible to consolidate ``uninteresting'' cases using
:otherwise.
For convenience, the case macro automatically binds the fields of x for
you, as appropriate for each case. That is, in the :32 case,
you can use fty::defprod-style foo.bar style accessors for x
without having to explicitly add a 32 b*
binder.
