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;; Copyright (c) Stephen C. Gilardi. All rights reserved. The use and
;; distribution terms for this software are covered by the Eclipse Public
;; License 1.0 ( which can
;; be found in the file epl-v10.html at the root of this distribution. By
;; using this software in any fashion, you are agreeing to be bound by the
;; terms of this license. You must not remove this notice, or any other,
;; from this software.
;; condition.clj
;; scgilardi (gmail)
;; Created 09 June 2009
(ns ^{:author "Stephen C. Gilardi"
:doc "Flexible raising and handling of conditions:
raise: raises a condition
handler-case: dispatches raised conditions to appropriate handlers
print-stack-trace: prints abbreviated or full condition stack traces
A condition is a map containing values for these keys:
- :type, a condition type specifier, typically a keyword
- :stack-trace, a stack trace to the site of the raise
- :message, a human-readable message (optional)
- :cause, a wrapped exception or condition (optional)
- other keys given as arguments to raise (optional)
Note: requires AOT compilation.
Based on an idea from Chouser:"}
(:require clojure.contrib.condition.Condition)
(:import clojure.contrib.condition.Condition
(:use (clojure.contrib
[def :only (defvar)]
[seq :only (separate)])))
(defvar *condition*
"While a handler is running, bound to the condition being handled")
(defvar *selector*
"While a handler is running, bound to the selector returned by the
handler-case dispatch-fn for *condition*")
(defvar *condition-object*
"While a handler is running, bound to the Condition object whose metadata
is the condition")
(defvar *full-stack-traces* false
"Bind to true to include clojure.{core,lang,main} frames in stack
(defmacro raise
"Raises a condition. With no arguments, re-raises the current condition.
With one argument (a map), raises the argument. With two or more
arguments, raises a map with keys and values from the arguments."
`(throw *condition-object*))
`(throw (Condition. ~m)))
([key val & keyvals]
`(raise (hash-map ~key ~val ~@keyvals))))
(defmacro handler-case
"Executes body in a context where raised conditions can be handled.
dispatch-fn accepts a raised condition (a map) and returns a selector
used to choose a handler. Commonly, dispatch-fn will be :type to dispatch
on the condition's :type value.
Handlers are forms within body:
(handle key
If a condition is raised, executes the body of the first handler whose
key satisfies (isa? selector key). If no handlers match, re-raises the
While a handler is running, *condition* is bound to the condition being
handled and *selector* is bound to to the value returned by dispatch-fn
that matched the handler's key."
[dispatch-fn & body]
(let [[handlers code]
(separate #(and (list? %) (= 'handle (first %))) body)]
(catch Condition c#
(binding [*condition-object* c#
*condition* (meta c#)
*selector* (~dispatch-fn (meta c#))]
(fn [[_ key & body]]
`[(isa? *selector* ~key) (do ~@body)])
:else (raise)))))))
(defmulti stack-trace-info
"Returns header, stack-trace, and cause info from conditions and
(defmethod stack-trace-info IPersistentMap
[(format "condition: %s, %s" (:type condition)
(dissoc condition :type :stack-trace :cause))
(:stack-trace condition)
(:cause condition)])
(defmethod stack-trace-info Condition
(stack-trace-info (meta condition)))
(defmethod stack-trace-info Throwable
[(str throwable)
(.getStackTrace throwable)
(.getCause throwable)])
(defn print-stack-trace
"Prints a stack trace for a condition or Throwable. Skips frames for
classes in clojure.{core,lang,main} unless the *full-stack-traces* is
bound to logical true"
(let [[header frames cause] (stack-trace-info x)]
(printf "%s\n" header)
(doseq [frame frames]
(let [classname (.getClassName frame)]
(if (or *full-stack-traces*
(not (re-matches
#"clojure.(?:core|lang|main).*" classname)))
(printf " at %s/%s(%s:%s)\n"
(.getMethodName frame)
(.getFileName frame)
(.getLineNumber frame)))))
(when cause
(printf "caused by: ")
(recur cause))))
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