try-make-sarna-happy🔗ℹ

This package was written for the 2021 Syntax Parse bee and to satisfy sarna’s complaints on Discord with with-handlers, which leads to a "backwards" looking program:

; say first what to do with the exception

(with-handlers ([exn:fail:syntax?

(λ (e) (displayln "got a syntax error"))])

; and only then what to actually do

(raise-syntax-error #f "a syntax error"))

(from the with-handlers examples).

1 The try macro🔗ℹ

The catch clauses use with-handlers, but in a different format: when pred-expr returns true for a thrown exception, exn-id is bound to the exception for the body handler-expr.

The catch/match clauses are match forms tested against the exception.

When both catch-style and catch/match-style clauses are present, all of the catch-style clauses are tried before any of the catch/match clauses.

If no pred-exprs succeed, testing proceeds to any catch/match clauses.

If no match-exprs succeed, the exception is re-raised.

1.1 Examples🔗ℹ

1.2 Before and After🔗ℹ

This is a "Code Cleaning" macro: it tidies up a common pattern and makes it read in a forward direction. In this it is similar to the threading library but for exceptions.

The following Before/After pairs do not show the call-with-continuation-barrier call because they assumes none of the shown procedures muck with continuations. It is needed in the general case, however, to prevent a captured continuation from re-entering the dynamic-wind and thus causing the finally clause to be run more than once.

The following are from Beautiful Racket: Errors and Exceptions.

1.3 Implementation🔗ℹ

The following is provided explicitly for the 2021 Syntax Parse bee.

2 License and Acknowledgements🔗ℹ

The code is licensed with the MIT license of the Racket project. The text of this documentation is licensed with the CCA 4.0 International License.