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Expression

Expression-case

Case macro for the different kinds of expression 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 expression structure, or to split into cases based on its type.

Short Form

In its short form, expression-case allows you to safely check the type of a expression structure. For example:

(expression-case x :literal)

is essentially just a safer alternative to writing:

(equal (expression-kind x) :literal)

Why is using expression-case safer? When we directly inspect the kind with equal, there is no static checking being done to ensure that, e.g., :literal is a valid kind of expression structure. That means there is nothing to save you if, later, you change the kind keyword for this type from :literal to something else. It also means you get no help if you just make a typo when writing the :literal symbol. Over the course of developing VL, we found that such issues were very frequent sources of errors!

Long Form

In its longer form, expression-case allows you to split into cases based on the kind of structure you are looking at. A typical example would be:

(expression-case x
  :literal ...
  :variable ...
  :unary ...
  :binary ...
  :if ...
  :when ...
  :unless ...
  :cond ...
  :call ...
  :multi ...
  :component ...
  :bind ...
  :product-construct ...
  :product-field ...
  :product-update ...
  :sum-construct ...
  :sum-field ...
  :sum-update ...
  :sum-test ...
  :bad-expression ...)

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 :literal case, you can use fty::defprod-style foo.bar style accessors for x without having to explicitly add a literal b* binder.