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+;;; trace.clj -- simple call-tracing macros for Clojure
+
+;; by Stuart Sierra, http://stuartsierra.com/
+;; June 9, 2008
+
+;; Copyright (c) 2008 Stuart Sierra. All rights reserved.  The use and
+;; distribution terms for this software are covered by the Common
+;; Public License 1.0 (http://www.opensource.org/licenses/cpl1.0.php)
+;; which can be found in the file CPL.TXT 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.
+
+
+;; This file defines simple "tracing" macros to help you see what your
+;; code is doing.
+
+
+(clojure/in-ns 'trace)
+(clojure/refer 'clojure)
+
+(def
+ #^{:doc "PrintStream for trace output.  Defaults to System.err."}
+ *trace-out* (. System err))
+
+(defmacro trace
+  "Prints value of expr to standard error and returns it.  Can be
+  inserted anywhere without affecting surrounding code.  Optional
+  'name' argument can be used to identify what is being traced."
+  ([expr]
+     `(let [value# ~expr]
+        (. *trace-out* (println
+                        (str "TRACE: " (pr-str value#))))
+        value#))
+  ([name expr]
+     `(let [value# ~expr]
+        (. *trace-out* (println
+                        (str "TRACE " ~name ": " (pr-str value#))))
+        value#)))
+
+(defmacro deftrace
+  "Use in place of defn; traces each call/return of this fn, including
+  arguments."
+  [name & definition]
+  `(let [f# (fn ~@definition)]
+     (defn ~name [& args#]
+       (let [id# (gensym "t")]  ; identifier for this invocation
+         (. *trace-out*
+            (println (str "TRACE " id# ": " ~(str name)
+                          " called with " (pr-str args#))))
+         (let [value# (apply f# args#)]  ; call original fn
+           (. *trace-out*
+              (println (str "TRACE " id# ": " ~(str name)
+                            " returned " (pr-str value#))))
+           value#)))))