I've been trying to figure out how exceptions work in Racket, but I'm a bit puzzled on the exception predicates. To catch an exception of say exn:fail type, you test it with a predicate exn:fail? in a with-handlers expression. I've found the documentation on exn:fail, but I don't see where exn:fail? is documented. Why I'm so curious about this is according to this article, if you make a custom exception struct of say exn:no-vowels, you can still use exn:no-vowels? despite the fact that this predicate was never manually implemented. If I had to wager a guess, the predicate must have been generated automatically with a macro, but if so I'm curious to know where this "automatically generate predicates behavior" is documented and how the macro works exactly.

CodePudding user response：

exn and derived exception types like the exn:fail hierarchy are all instances of Racket structure types, and predicates, constructors and accessors (like exn-message) are automatically defined when they're created with the struct macro.

User defined exception types can be exported from a module with struct-out in a provide form, which exports all those generated functions without you having to list them manually.