Clojure exceptions as data format

README.md

Clojure exceptions are really just Java exceptions under the hood. However, at times clojure shows them formatted as a map data structure. For instance, that's what you get if you inspect them in the REBL. This is because clojure implements the Datafy protocol for Throwable objects, delegating to clojure.core/Throwable->map.

This gist is a reminder of how to interpret that data assuming familiary with Java printed stack traces.

commented_exception.edn

{:cause          ; <- Exception message form the root (innermost) exception in the exception chain
 "rootcause"     

 :data           ; <- ex-data from the root (innermost) exception in the exception chain
 {:data-from :a} 

 :trace          ; <- stack trace of the root (innermost) exception in the exceptions chain
 [[playground$a invokeStatic "playground.clj" 99] ; <- stack-frame that threw the exception is the first (topmost)
  [playground$a invoke "playground.clj" 22]
  [playground$b invokeStatic "NO_SOURCE_FILE" 102]
  [playground$b invoke "NO_SOURCE_FILE" 101]
  [playground$bar invokeStatic "NO_SOURCE_FILE" 105]
  [playground$bar invoke "NO_SOURCE_FILE" 104]
  [playground$foo invokeStatic "NO_SOURCE_FILE" 109]
  [playground$foo invoke "NO_SOURCE_FILE" 108]
  [playground$fn__24983 invokeStatic "playground.clj" 112]
  [playground$fn__24983 invoke "playground.clj" 112]
  [clojure.lang.AFn applyToHelper "AFn.java" 152]
  [clojure.lang.AFn applyTo "AFn.java" 144]
  [clojure.lang.Compiler$InvokeExpr eval "Compiler.java" 3702]
  [clojure.lang.Compiler$DefExpr eval "Compiler.java" 457]
  [clojure.lang.Compiler eval "Compiler.java" 7182]
  [clojure.lang.Compiler eval "Compiler.java" 7132]
  [clojure.core$eval invokeStatic "core.clj" 3214]
  [clojure.core.server$prepl$fn__8941 invoke "server.clj" 232]
  [clojure.core.server$prepl invokeStatic "server.clj" 228]
  [clojure.core.server$prepl doInvoke "server.clj" 191]
  [clojure.lang.RestFn invoke "RestFn.java" 425]
  [cognitect.rebl$repl invokeStatic "rebl.clj" 122]
  [cognitect.rebl$repl invoke "rebl.clj" 96]
  [cognitect.rebl$_main invokeStatic "rebl.clj" 125]
  [cognitect.rebl$_main invoke "rebl.clj" 124]
  [clojure.lang.AFn applyToHelper "AFn.java" 152]
  [clojure.lang.AFn applyTo "AFn.java" 144]
  [clojure.lang.Var applyTo "Var.java" 705]
  [clojure.core$apply invokeStatic "core.clj" 665]
  [clojure.main$main_opt invokeStatic "main.clj" 514]
  [clojure.main$main_opt invoke "main.clj" 510]
  [clojure.main$main invokeStatic "main.clj" 664]
  [clojure.main$main doInvoke "main.clj" 616]
  [clojure.lang.RestFn applyTo "RestFn.java" 137]
  [clojure.lang.Var applyTo "Var.java" 705]
  [clojure.main main "main.java" 40]]             ; <- stack-frame of the beggining of the thread call stack is the last (bottom)

 :via            ; <- for each exception in the chain: 
 [{:type clojure.lang.ExceptionInfo,                         ; <- It's class
   :message "wrapping",                                      ; <- It's message
   :data {:data-from :bar},                                  ; <- It's ex-data
   :at [playground$bar invokeStatic "NO_SOURCE_FILE" 106]}   ; <- Stack frame that threw the exception (i.e. the topmost stack frame)
  {:type clojure.lang.ExceptionInfo,
   :message "rootcause",
   :data {:data-from :a},
   :at [playground$a invokeStatic "playground.clj" 99]}]
}

java_printed_stacktrace.txt

clojure.lang.ExceptionInfo: wrapping {:data-from :bar}
	at playground$bar.invokeStatic(NO_SOURCE_FILE:106)
	at playground$bar.invoke(NO_SOURCE_FILE:104)
	at playground$foo.invokeStatic(NO_SOURCE_FILE:109)
	at playground$foo.invoke(NO_SOURCE_FILE:108)
	at playground$fn__24983.invokeStatic(playground.clj:112)
	at playground$fn__24983.invoke(playground.clj:112)
	at clojure.lang.AFn.applyToHelper(AFn.java:152)
	at clojure.lang.AFn.applyTo(AFn.java:144)
	at clojure.lang.Compiler$InvokeExpr.eval(Compiler.java:3702)
	at clojure.lang.Compiler$DefExpr.eval(Compiler.java:457)
	at clojure.lang.Compiler.eval(Compiler.java:7182)
	at clojure.lang.Compiler.eval(Compiler.java:7132)
	at clojure.core$eval.invokeStatic(core.clj:3214)
	at clojure.core.server$prepl$fn__8941.invoke(server.clj:232)
	at clojure.core.server$prepl.invokeStatic(server.clj:228)
	at clojure.core.server$prepl.doInvoke(server.clj:191)
	at clojure.lang.RestFn.invoke(RestFn.java:425)
	at cognitect.rebl$repl.invokeStatic(rebl.clj:122)
	at cognitect.rebl$repl.invoke(rebl.clj:96)
	at cognitect.rebl$_main.invokeStatic(rebl.clj:125)
	at cognitect.rebl$_main.invoke(rebl.clj:124)
	at clojure.lang.AFn.applyToHelper(AFn.java:152)
	at clojure.lang.AFn.applyTo(AFn.java:144)
	at clojure.lang.Var.applyTo(Var.java:705)
	at clojure.core$apply.invokeStatic(core.clj:665)
	at clojure.main$main_opt.invokeStatic(main.clj:514)
	at clojure.main$main_opt.invoke(main.clj:510)
	at clojure.main$main.invokeStatic(main.clj:664)
	at clojure.main$main.doInvoke(main.clj:616)
	at clojure.lang.RestFn.applyTo(RestFn.java:137)
	at clojure.lang.Var.applyTo(Var.java:705)
	at clojure.main.main(main.java:40)
Caused by: clojure.lang.ExceptionInfo: rootcause {:data-from :a}
	at playground$a.invokeStatic(playground.clj:99)
	at playground$a.invoke(playground.clj:22)
	at playground$b.invokeStatic(NO_SOURCE_FILE:102)
	at playground$b.invoke(NO_SOURCE_FILE:101)
	at playground$bar.invokeStatic(NO_SOURCE_FILE:105)
	... 31 more

sample.clj

(defn a []
  (throw (ex-info "rootcause" {:data-from :a})))

(defn b []
  (a))

(defn bar []
  (try (b)
       (catch Exception b (throw (ex-info "wrapping" {:data-from :bar} b)))))

(defn foo []
  (bar))

(def e (try (foo) (catch Exception e e)))