README.md

slingshot

Enhanced throw and catch for Clojure

Provides try+ and throw+. Each is 100% compatible with Clojure and Java's native try and throw both in source code and at runtime. Each also provides new capabilities intended to improve ease of use by leveraging Clojure's features like maps, records, and destructuring.

Clojure's native try and throw behave much like those of Java: throw can accept objects derived from java.lang.Throwable and try selects from among catch clauses based on the class of the thrown object.

In addition to fully supporting those uses (whether they originate from Clojure code or from Java code via interop), try+ and throw+ provide these enhanced capabilities:

• throw+ can throw any Java object, not just those whose class is derived from java.lang.Throwable.

  Clojure maps or records become an easy way to represent custom exceptions without requiring gen-class.

• catch clauses within try+ can catch any Java object thrown by throw+, Clojure's throw, or Java's throw. The first catch clause whose selector matches the thrown object will execute.

  a selector can be:

  • a class name: (e.g., RuntimeException, my.clojure.record), matches any instance of that class, or

  • a key-value pair: (two element vector), matches objects where (get object key) returns val, or

  • a predicate: (function of one argument like map?, set?), matches any Object for which the predicate returns a truthy value, or

  • a selector form: a form containing one or more instances of % to be replaced by the thrown object, matches any object for which the form evaluates to truthy.

  • the class name, key-value pair, and predicate selectors are shorthand for these selector forms:

    `<class name>  => (instance? <class name> %)`
    
    `[<key> <val>] => (= (get % <key>) <val>)`
    
    `<predicate>   => (<predicate> %)`
    

• the binding to the caught exception in a catch clause is not required to be a simple symbol. It is subject to destructuring so the body of the catch clause can use the contents of a thrown collection easily.

• in a catch clause, the context at the throw site is accessible via the hidden argument &throw-context.

• &throw-context is a map containing:

  for all caught objects:

  :object       the thrown object;
  :stack-trace  the stack trace;
  

  for Throwable caught objects:

  :message      the message, from .getMessage;
  :cause        the cause, from .getCause;
  

  for non-Throwable caught objects:

  :message      the message, from the optional argument to throw+;
  :cause        the cause, captured by throw+, see below;
  :wrapper      the outermost Throwable wrapper of the caught object,
                see below;
  :environment  a map of locals visible at the throw+ site: symbols
                mapped to their bound values.
  

To throw a non-Throwable object, throw+ wraps it with a Throwable object of class Stone. That Stone may in turn end up wrapped by other exceptions (e.g., instances of RuntimeException or java.util.concurrent.ExecutionException). try+ sees through all such wrappers to find the object wrapped by the first instance of Stone in the outermost wrapper's cause chain. If needed, the outermost wrapper is available within a catch clause a via the :wrapper key in &throw-context. Any nested wrappers are accessible via its cause chain.

When throw+ throws a non-Throwable object from within a try+ catch clause, the outermost wrapper of the caught object being processed is captured as the cause of the new throw+.

Usage

See the tests for examples

Credits

Based on clojure.contrib.condition, data-conveying-exception, discussions on the clojure mailing list and wiki and discussions and implementations by Steve Gilardi, Phil Hagelberg, and Kevin Downey.

License

Copyright © 2011 Stephen C. Gilardi, Kevin Downey, and Phil Hagelberg

Distributed under the Eclipse Public License, the same as Clojure.