[](https://travis-ci.org/scgilardi/slingshot) [](https://www.versioneye.com/user/projects/54d57a7b3ca0840b1900063a)
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 additional 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+`,
- any map passed to `ex-info` and thrown by `throw` or `throw+`, or
- any `Throwable` thrown by 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-values** vector: (e.g., `[key val & kvs]`), matches
objects where `(and (= (get object key) 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-values, and predicate selectors are
shorthand for these selector forms:
`<class name> => (instance? <class name> %)`
`[<key> <val> & <kvs>] => (and (= (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 Throwable caught objects:
:object the caught object;
:message the message, from .getMessage;
:cause the cause, from .getCause;
:stack-trace the stack trace, from .getStackTrace;
:throwable the caught object;
for non-Throwable caught objects (including maps passed to ex-info)
:object the caught object;
:message the message, from throw+ or ex-info;
:cause the cause, from throw+ or ex-info, see below;
:stack-trace the stack trace, captured by throw+ or ex-info;
:wrapper the Throwable wrapper that carried the object;
:throwable the outermost Throwable whose cause chain contains
the wrapper, see below.
To throw a non-`Throwable` object, `throw+` wraps it in a
`Throwable` wrapper by calling `ex-info`. Every instance of
`IExceptionInfo` (whether generated by throw+ or by a direct call to
`ex-info`) is treated as a wrapper.
The wrapper is available via the `:wrapper` key in `&throw-context`.
Between being thrown and caught, a wrapper may be further wrapped by
other Exceptions (e.g., instances of `RuntimeException` or
`java.util.concurrent.ExecutionException`). `try+` sees through all
such wrappers to find the thrown object. The outermost wrapper is
available within a catch clause a via the `:throwable` key in
`&throw-context`.
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+`. This can be
overridden by providing an explicit `cause` argument to `throw+`.
- an optional `else` clause may appear after all `catch` clauses and
before any `finally` clause. Its contents will be executed (for
side effects) immediately after the code in the `try+` body
completes only if nothing was thrown.
Usage
-----
project.clj
[](http://clojars.org/slingshot)
tensor/parse.clj
```clojure
(ns tensor.parse
(:use [slingshot.slingshot :only [throw+]]))
(defn parse-tree [tree hint]
(if (bad-tree? tree)
(throw+ {:type ::bad-tree :tree tree :hint hint})
(parse-good-tree tree hint)))
```
math/expression.clj
```clojure
(ns math.expression
(:require [tensor.parse]
[clojure.tools.logging :as log])
(:use [slingshot.slingshot :only [throw+ try+]]))
(defn read-file [file]
(try+
[...]
(tensor.parse/parse-tree tree)
[...]
(catch [:type :tensor.parse/bad-tree] {:keys [tree hint]}
(log/error "failed to parse tensor" tree "with hint" hint)
(throw+))
(catch Object _
(log/error (:throwable &throw-context) "unexpected error")
(throw+))))
```
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 © 2011-2014 Stephen C. Gilardi, Kevin Downey, and
Phil Hagelberg
Distributed under the Eclipse Public License, the same as Clojure.