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Enhanced try and throw for Clojure leveraging Clojure's capabilities

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# 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 other capabilities like record types and
destructuring.

Clojure's 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 records become an easy, natural way to represent custom
  exceptions without requiring gen-class. Clojure hash-maps with :type
  metadata also work well.

* catch clauses within try+ can catch any java object thrown by throw+
  Clojure's throw, or Java's throw.

  the selector can be a class name (e.g., Integer, my.clojure.record,
  IllegalArgumentException), a keyword representing a clojure type (as
  used by isa?, derive, etc.), or an arbitrary predicate applied to
  the thrown object. The first clause whose type or predicate matches
  is executed.

* the binding to the caught exception in a catch clause is no longer
  required to be a simple symbol. It is now subject to destructuring so
  the body of the catch clause can use the contents of a thrown
  collection (record, set, map, vector) easily.

* in a catch clause, the context at the throw site is accessible via
  the hidden argument &throw-context. The context includes the stack
  trace and the environment: bound locals and their values.

## 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 (C) 2011 Stephen Gilardi, Kevin Downey, and Phil Hagelberg

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