Value-case
Case macro for the different kinds of value 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 value structure, or to split
into cases based on its type.
Short Form
In its short form, value-case allows you to safely check the type of
a value structure. For example:
(value-case x :object)
is essentially just a safer alternative to writing:
(equal (value-kind x) :object)
Why is using value-case safer? When we directly inspect the
kind with equal, there is no static checking being done to
ensure that, e.g., :object is a valid kind of value structure. That means there is nothing to save you
if, later, you change the kind keyword for this type from :object to something else. It also means you get no help
if you just make a typo when writing the :object
symbol. Over the course of developing VL, we found that such
issues were very frequent sources of errors!
Long Form
In its longer form, value-case allows you to split into cases based
on the kind of structure you are looking at. A typical example would be:
(value-case x
:object ...
:array ...
:number ...
:string ...
:true ...
:false ...
:null ...)
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 :object case,
you can use fty::defprod-style foo.bar style accessors for x
without having to explicitly add a object b*
binder.
