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Plexeme

Plexeme-case

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

Short Form

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

(plexeme-case x :header)

is essentially just a safer alternative to writing:

(equal (plexeme-kind x) :header)

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

Long Form

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

(plexeme-case x
  :header ...
  :ident ...
  :number ...
  :char ...
  :string ...
  :punctuator ...
  :other ...
  :block-comment ...
  :line-comment ...
  :newline ...
  :spaces ...
  :horizontal-tab ...
  :vertical-tab ...
  :form-feed ...)

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