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    • Value

    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.