Why are Clojure stacktraces so long?

Question

I've written some toy interpreters/compilers in the past, so I associate stacktraces referencing lines in compiler source files with compiler bugs.

If I add the following bad function definition to my compojure project:

(defn dodgy-map []
  {:1 :2 :3})

Lein refuses to start:

$ lein ring server-headless
Exception in thread "main" java.lang.RuntimeException: Map literal must contain an even number of forms, compiling:(one_man_wiki/views.clj:19)
        at clojure.lang.Compiler.load(Compiler.java:6958)
        at clojure.lang.RT.loadResourceScript(RT.java:359)
        at clojure.lang.RT.loadResourceScript(RT.java:350)
        at clojure.lang.RT.load(RT.java:429)
        at clojure.lang.RT.load(RT.java:400)
        at clojure.core$load$fn__4890.invoke(core.clj:5415)
        at clojure.core$load.doInvoke(core.clj:5414)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at clojure.core$load_one.invoke(core.clj:5227)
        at clojure.core$load_lib.doInvoke(core.clj:5264)
        at clojure.lang.RestFn.applyTo(RestFn.java:142)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$load_libs.doInvoke(core.clj:5298)
        at clojure.lang.RestFn.applyTo(RestFn.java:137)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$require.doInvoke(core.clj:5381)
        at clojure.lang.RestFn.invoke(RestFn.java:457)
        at one_man_wiki.handler$eval1564$loading__4784__auto____1565.invoke(handler.clj:1)
        at one_man_wiki.handler$eval1564.invoke(handler.clj:1)
        at clojure.lang.Compiler.eval(Compiler.java:6511)
        at clojure.lang.Compiler.eval(Compiler.java:6501)
        at clojure.lang.Compiler.load(Compiler.java:6952)
        at clojure.lang.RT.loadResourceScript(RT.java:359)
        at clojure.lang.RT.loadResourceScript(RT.java:350)
        at clojure.lang.RT.load(RT.java:429)
        at clojure.lang.RT.load(RT.java:400)
        at clojure.core$load$fn__4890.invoke(core.clj:5415)
        at clojure.core$load.doInvoke(core.clj:5414)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at clojure.core$load_one.invoke(core.clj:5227)
        at clojure.core$load_lib.doInvoke(core.clj:5264)
        at clojure.lang.RestFn.applyTo(RestFn.java:142)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$load_libs.doInvoke(core.clj:5298)
        at clojure.lang.RestFn.applyTo(RestFn.java:137)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$require.doInvoke(core.clj:5381)
        at clojure.lang.RestFn.invoke(RestFn.java:421)
        at user$eval1.invoke(NO_SOURCE_FILE:1)
        at clojure.lang.Compiler.eval(Compiler.java:6511)
        at clojure.lang.Compiler.eval(Compiler.java:6500)
        at clojure.lang.Compiler.eval(Compiler.java:6477)
        at clojure.core$eval.invoke(core.clj:2797)
        at clojure.main$eval_opt.invoke(main.clj:297)
        at clojure.main$initialize.invoke(main.clj:316)
        at clojure.main$null_opt.invoke(main.clj:349)
        at clojure.main$main.doInvoke(main.clj:427)
        at clojure.lang.RestFn.invoke(RestFn.java:421)
        at clojure.lang.Var.invoke(Var.java:419)
        at clojure.lang.AFn.applyToHelper(AFn.java:163)
        at clojure.lang.Var.applyTo(Var.java:532)
        at clojure.main.main(main.java:37)
Caused by: java.lang.RuntimeException: Map literal must contain an even number of forms
        at clojure.lang.Util.runtimeException(Util.java:170)
        at clojure.lang.LispReader$MapReader.invoke(LispReader.java:1071)
        at clojure.lang.LispReader.readDelimitedList(LispReader.java:1126)
        at clojure.lang.LispReader$ListReader.invoke(LispReader.java:962)
        at clojure.lang.LispReader.read(LispReader.java:180)
        at clojure.lang.Compiler.load(Compiler.java:6949)
        ... 51 more

If I reference a variable that doesn't exist:

(defn no-such-variable []
  i-dont-exist)

I get an equally ginormous traceback:

Exception in thread "main" java.lang.RuntimeException: Unable to resolve symbol: i-dont-exist in this context, compiling:(one_man_wiki/views.clj:18)
        at clojure.lang.Compiler.analyze(Compiler.java:6281)
        at clojure.lang.Compiler.analyze(Compiler.java:6223)
        at clojure.lang.Compiler$BodyExpr$Parser.parse(Compiler.java:5618)
        at clojure.lang.Compiler$FnMethod.parse(Compiler.java:5054)
        at clojure.lang.Compiler$FnExpr.parse(Compiler.java:3674)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6453)
        at clojure.lang.Compiler.analyze(Compiler.java:6262)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6443)
        at clojure.lang.Compiler.analyze(Compiler.java:6262)
        at clojure.lang.Compiler.access$100(Compiler.java:37)
        at clojure.lang.Compiler$DefExpr$Parser.parse(Compiler.java:518)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6455)
        at clojure.lang.Compiler.analyze(Compiler.java:6262)
        at clojure.lang.Compiler.analyze(Compiler.java:6223)
        at clojure.lang.Compiler.eval(Compiler.java:6515)
        at clojure.lang.Compiler.load(Compiler.java:6952)
        at clojure.lang.RT.loadResourceScript(RT.java:359)
        at clojure.lang.RT.loadResourceScript(RT.java:350)
        at clojure.lang.RT.load(RT.java:429)
        at clojure.lang.RT.load(RT.java:400)
        at clojure.core$load$fn__4890.invoke(core.clj:5415)
        at clojure.core$load.doInvoke(core.clj:5414)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at clojure.core$load_one.invoke(core.clj:5227)
        at clojure.core$load_lib.doInvoke(core.clj:5264)
        at clojure.lang.RestFn.applyTo(RestFn.java:142)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$load_libs.doInvoke(core.clj:5298)
        at clojure.lang.RestFn.applyTo(RestFn.java:137)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$require.doInvoke(core.clj:5381)
        at clojure.lang.RestFn.invoke(RestFn.java:457)
        at one_man_wiki.handler$eval1564$loading__4784__auto____1565.invoke(handler.clj:1)
        at one_man_wiki.handler$eval1564.invoke(handler.clj:1)
        at clojure.lang.Compiler.eval(Compiler.java:6511)
        at clojure.lang.Compiler.eval(Compiler.java:6501)
        at clojure.lang.Compiler.load(Compiler.java:6952)
        at clojure.lang.RT.loadResourceScript(RT.java:359)
        at clojure.lang.RT.loadResourceScript(RT.java:350)
        at clojure.lang.RT.load(RT.java:429)
        at clojure.lang.RT.load(RT.java:400)
        at clojure.core$load$fn__4890.invoke(core.clj:5415)
        at clojure.core$load.doInvoke(core.clj:5414)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at clojure.core$load_one.invoke(core.clj:5227)
        at clojure.core$load_lib.doInvoke(core.clj:5264)
        at clojure.lang.RestFn.applyTo(RestFn.java:142)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$load_libs.doInvoke(core.clj:5298)
        at clojure.lang.RestFn.applyTo(RestFn.java:137)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$require.doInvoke(core.clj:5381)
        at clojure.lang.RestFn.invoke(RestFn.java:421)
        at user$eval1.invoke(NO_SOURCE_FILE:1)
        at clojure.lang.Compiler.eval(Compiler.java:6511)
        at clojure.lang.Compiler.eval(Compiler.java:6500)
        at clojure.lang.Compiler.eval(Compiler.java:6477)
        at clojure.core$eval.invoke(core.clj:2797)
        at clojure.main$eval_opt.invoke(main.clj:297)
        at clojure.main$initialize.invoke(main.clj:316)
        at clojure.main$null_opt.invoke(main.clj:349)
        at clojure.main$main.doInvoke(main.clj:427)
        at clojure.lang.RestFn.invoke(RestFn.java:421)
        at clojure.lang.Var.invoke(Var.java:419)
        at clojure.lang.AFn.applyToHelper(AFn.java:163)
        at clojure.lang.Var.applyTo(Var.java:532)
        at clojure.main.main(main.java:37)
Caused by: java.lang.RuntimeException: Unable to resolve symbol: i-dont-exist in this context
        at clojure.lang.Util.runtimeException(Util.java:170)
        at clojure.lang.Compiler.resolveIn(Compiler.java:6766)
        at clojure.lang.Compiler.resolve(Compiler.java:6710)
        at clojure.lang.Compiler.analyzeSymbol(Compiler.java:6671)
        at clojure.lang.Compiler.analyze(Compiler.java:6244)
        ... 66 more

Why doesn't the Clojure compiler raise a ClojureSyntaxError and a ClojureNameError that could be caught at the top level and a simple error shown? These are common developer errors during development.

If the long tracebacks are useful in some circumstances, why are they truncated?

Edit: What I'm looking for in an answer:

Are there situations (some metaprogramming with Java interop perhaps?) when getting a traceback referencing clojure.lang is useful?
(Related) Are there any technical constraints preventing adding a ClojureSyntaxError as I described above? Is it worth me opening an issue on the Clojure bug tracker? (update: I've opened CLJ-1155)
How do experience Clojure programmers read these tracebacks? Are there tools to help? Does everyone use clj-stacktrace?

Could you edit the question to more clearly explain what an acceptable answer would look like? — Arthur Ulfeldt
mods - while it could certainly be improved, IMHO question is reasonable and can be answered constructively (as JohnJ demonstrates below). Voted to re-open. — mikera
+1 for reopening, taking note of the high number of question votes so far. — JohnJ

JohnJ JohnJ · Accepted Answer · 2013-01-12T22:24:55

Replying to your numbered points,

it is idiomatic in Clojure to interoperate directly with Java libraries, so getting the full Java stacktrace can be helpful if you are calling into Java objects in some unexpected or unsupported way.
Sounds like a good idea; I've often at least wished for a settable option which would allow me to see only the parts of stacktraces originating in my own code, suppressing all the underlying language layers.
I usually just do it the hard way and pore over the stacktrace to get the line in my program that barfed, tuning out the clojure.* parts (and I usually test each minute change so I have a pretty good idea what change caused the problem). Some of the Emacs and Eclipse tools I've used show you only the actual error and not the whole stacktrace; I generally find this to be more helpful.

At Clojure/west in 2012, @chouser gave a nice talk [PDF] on the anatomy of stacktraces, explained how to read them, and introduced a promising-looking tool, which apparently still has not seen the light of day yet.

Compared with, say, Python, whose stacktraces I find pretty user-friendly, I think stacktraces are a rough edge in Clojure, particularly for beginners. This is partly due to the "hosted" nature of the language, though I expect there are improvements which could be made without adding incidental complexity.

Why are Clojure stacktraces so long?

2 Answers