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Outcome

Outcome-case

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

Short Form

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

(outcome-case x :function-success)

is essentially just a safer alternative to writing:

(equal (outcome-kind x) :function-success)

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

Long Form

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

(outcome-case x
  :function-success ...
  :type-success ...
  :specification-success ...
  :theorem-success ...
  :transformation-success ...
  :proof-obligation-failure ...
  :theorem-failure ...
  :transformation-failure ...
  :unexpected-failure ...)

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