Clojure Q&A - Questions without answers in Compiler

Possibly avoidable identity call to force instance method on Class literals

Thu, 02 Jul 2026 23:47:10 +0000

As of Clojure 1.13.0-alpha2, (.method ClassName) expands to (.method ^Class (identity ClassName)).

This is to force (.method ClassName) to always be an instance method. Without this, it would expand to (. ClassName method), which may instead be interpreted as a static method.

For example, (.getMethods String) expands to (.getMethods ^Class (identity String)).

This was introduced in these two commits:
- force instance member interpretation of (.method ClassName), e.g. (.getMethods String) works
- added autohinting to Class in macroexpansion of (.instanceMethodOfClass Classname) calls

Since then, Clojure has added support for qualified methods which we can use to reliably propagate the tag without any runtime changes. For example, expanding to (Class/.getMethods String) is now equivalent to (.getMethods ^Class (identity String)) in terms of tag propagation.

I have a proof-of-concept here that includes disassembled and decompiled output before and after the change. The net effect of using qualified methods in this case under direct linking is a removal of a single method call to identity:

   3: invokestatic  #20                 // Method clojure/core$identity.invokeStatic:(Ljava/lang/Object;)Ljava/lang/Object;

The effect is more substantial without direct linking (not included in the PR), with clojure.core/identity also being added to the static initializer.

I've also experimented with expanding to (. (do String) getMethods) (see earlier commits in that PR), which seems to also work and might be another approach.

Feature Request:Make anonymous fn/reify class name IDs per namespace instead of one global counter

Wed, 18 Feb 2026 15:34:38 +0000

Compiled anonymous functions and reify classes get names like ns$fn__4532, where the numeric suffix comes from a global AtomicInteger counter
(RT.nextID()).

This counter is shared across the entire runtime and is consumed by fn classes, reify classes, constants tables, gensyms, etc.

The problem is that because the counter is global, any code change shifts the IDs for everything compiled after it. This makes profiling across builds
really painful, you can't meaningfully compare flame graphs because all the class names change even in code you didn't touch.

I'd like to propose scoping the ID counter per-namespace instead. Since the namespace is already part of the class name prefix, uniqueness is still
guaranteed. This would eliminate the cascading effect across unchanged namespaces and make profiling diffs actually useful.

I'm not sure what's the downsides of this, but from what I saw of AtomicInteger, it isn't used a lot to break things.

Thank you!

Compiler-generated `foo__init.class` files lack `SourceFile` attribute

Thu, 31 Jul 2025 22:21:18 +0000

Most .class files generated by Clojure contain the expected SourceFile attribute (you can verify with javap -v). But __init classfiles lack this metadata. I asked @alexmiller on Slack, and he suggested I bring it up here.

The same appears to be true of classes generated via proxy.

Can custom literal collections reach parity with clojure's built in collections?

Thu, 20 Mar 2025 21:46:42 +0000

Teaching Old `eval` New Tricks

Tagged Literals Promise Extensibility

EDN and Tagged Literals:
- In EDN, tagged literals deliver complete extensibility.
- When processing EDN data, custom types created with tagged readers achieve parity with built-in literals.
Clojure’s Syntax and Tagged Literals:
- Clojure's syntax, as a superset of EDN, includes tagged literals.
- Key Differences:
- EDN is for reading data, while Clojure's syntax is expected to be eval'ed.
- Tagged literals in Clojure code are treated as second-class citizens compared to built-in types because:
```
1. The compiler inherently knows how to compile built-in types,
   but it does not understand other types returned by tagged
   readers.
```
Compiler Behavior and Challenges:
- By default:
- The compiler embeds the instance as a string in the emitted bytecode, recreating the instance at runtime via the reader.
- Issues with Collections:
- Container types like third party collections are problematic because their interior data is not evaluated.
- Common advice suggests returning a form that, when evaluated, creates an instance of the type. However:
```
- This makes the result no longer a literal, as:
  - The reader does not produce the expected type.
  - Macros don't see the expected type
  - You only get the expected type after eval.
- For mixed environments (EDN without eval vs. Clojure with
  eval), separate tagged readers for the same tag may be
  required.
```

Towards a Solution

Compiler Extensions:
- A potential solution involves creating an extension point to teach the compiler how to compile new types.
- Special care is required for cross-compiling dialects, such as ClojureScript.
- JVM collections might need to self-compile for ClojureScript.
Backward Compatibility:
- Ideally, any solution would:
- Retain the existing "self-quoting" behavior.
- Allow types to opt into new functionality.

Incorrect result when evaluating `not=` on `##NaNs`

Sun, 08 Dec 2024 21:14:02 +0000

Alex Miller asked me to write this up here. I found a bug in clojure.core yesterday that involves not= when used to compare NaNs. This was reported on Slack and the discussion is here:
https://clojurians.slack.com/archives/C03S1KBA2/p1733612992809069

Here is a more highly summarized version of the discussion from Slack (at least from my perspective). If we fire up the Clojure CLI, we can evaluate the following.

 Clojure 1.12.0
 user=> (= ##NaN ##NaN)
 false
 user=> (not= ##NaN ##NaN)
 false
 user=> (not (= ##NaN ##NaN))
 true

The problem here is that =, not, and not= have a relationship between them. Specifically, for any x and y, if (= x y) returns a boolean, then (not (= x y)) should return the opposite boolean, and since not= is defined as (not (= x y)), it should return the same value as (not (= x y)) for all x and y. This doesn't happen if x and y are both ##NaN.

Note that there were a lot of calories burned on Slack with suggestions that doubles should never be compared for equality, that NaNs are shifty, not-quite-value objects and should be avoided, that anybody who wants to test for the presence of a NaN should use NaN? which is already in clojure.core, and that the documentation around equality should be updated to say some of those things. Many of those statements are true or good practice. But all of them miss the broader point.

This bug has nothing to do specifically to do with NaNs. It just seems that NaNs expose the bug. The real issue is with the contractual relationship between =, not, and not= which appears to be violated in the presence of NaNs. Specifically, not= is no longer referentially transparent with respect to (not (= ...)).

On Slack, @potetm decompiled the code generated for these cases and found the following.

For (not= ##NaN ##NaN) :

(clj-java-decompiler.core/decompile
  (not= ##NaN ##NaN))

// Decompiling class: cjd__init
import clojure.lang.*;

public class cjd__init
{
    public static final Var __not_EQ_;
    public static final Object const__1;
    
    public static void load() {
        ((IFn)cjd__init.__not_EQ_.getRawRoot()).invoke(cjd__init.const__1, cjd__init.const__1);
    }
    
    public static void __init0() {
        __not_EQ_ = RT.var("clojure.core", "not=");
        const__1 = Double.NaN;
    }
    
    static {
        __init0();
        Compiler.pushNSandLoader(RT.classForName("cjd__init").getClassLoader());
        try {
            load();
            Var.popThreadBindings();
        }
        finally {
            Var.popThreadBindings();
        }
    }
}

And then for (not (= ##NaN ##NaN)):

(clj-java-decompiler.core/decompile
  (not (= ##NaN ##NaN)))

// Decompiling class: cjd__init
import clojure.lang.*;

public class cjd__init
{
    public static final Var __not;
    
    public static void load() {
        ((IFn)cjd__init.__not.getRawRoot()).invoke(Util.equiv(Double.NaN, Double.NaN) ? Boolean.TRUE : Boolean.FALSE);
    }
    
    public static void __init0() {
        __not = RT.var("clojure.core", "not");
    }
    
    static {
        __init0();
        Compiler.pushNSandLoader(RT.classForName("cjd__init").getClassLoader());
        try {
            load();
            Var.popThreadBindings();
        }
        finally {
            Var.popThreadBindings();
        }
    }
}

It appears that the compiler optimizes the call to = and does not box the ##NaN values when compiling (not (= ##NaN ##NaN)), whereas the call to not= receives the ##NaNs boxed as Doubles. This then causes a subsequent call to clojure.lang.Util/equiv (after following the call chain through not= -> = -> clojure.lang.Util/equiv) to return true improperly (since NaNs are never equal to anything, even themselves).

IMO, this is a bug, albeit a low priority one. Most programmers successfully use floating point math without ever having to deal with NaNs. While NaNs seem to trigger the bug, there may be other cases that also trigger it. I can't speak to that.

Cannot invoke "clojure.lang.Var.isBound()" because "clojure.lang.Compiler.LOADER" is null, potential bug?

Thu, 18 Apr 2024 16:11:35 +0000

As per discussion on slack I'm posting here the findings regarding clojure.lang.Compiler.LOADER being null during the analysis phase of GraalVM native-image run.

The full reproduction can be found here and the link to issue on Oracle GraalVM side here.

What is this issue about?

Native Image cannot be produced and throws an error during analysis phase when Clojure 1.11.2 (although the previous version of Clojure might be also affected) is used and reflection entries for java.lang.UUID and clojure.lang.Keyword are present in reflectionconfig.json.

I can't however reproduce the error prior to Clojure 1.9.0. For more information please kindly take a look into a repro.

Executable 'java' not found on system path.

Wed, 03 Jan 2024 19:38:51 +0000

Hi guys I,m basically trying to run a clojure script app using a shadow file, to be honest i don't really know too much about clojure since this is a project that i downloaded, but i need it to make it run locally on windows 11 and i'm receiving the following error

shadow-cljs - config: C:\Appsmiths\i\src\winglue\webglue\shadow-cljs.edn
===== ERROR =================

Executable 'java' not found on system path.

I have java on my system of course and my environment variables are well configured, so i think it could be something inside of the shadow file, but i cannot identify what it is, if you can help me out with this it would be really helpful

this is basically all the shadow config:

;; shadow-cljs configuration
{:dependencies [
                [akiroz.re-frame/storage "0.1.3"]
                [bidi "2.1.5"]
                [com.cemerick/url "0.1.1"]
                [binaryage/devtools "1.0.2"]
                [binaryage/oops "0.7.1"]
                [com.cognitect/transit-cljs "0.8.256"]
                [com.rpl/specter "1.1.3"]
                [com.taoensso/timbre "4.10.0"]
                [cljs-http "0.1.46"]
                [day8.re-frame/async-flow-fx "0.1.0"]
                [day8.re-frame/http-fx "0.1.6"]
                [day8.re-frame/re-frame-10x "1.5.0"]
                [day8.re-frame/tracing "0.5.1"]
                [day8.re-frame/undo "0.3.2"]
                [district0x/graphql-query "1.0.6"]
                [district0x/re-frame-interval-fx "1.0.2"]
                [expound "0.8.5"]
                [funcool/cuerdas "2021.05.29-0"]
                [funcool/promesa "2.0.1"]
                [juji/editscript "0.4.6"]
                [maximgb/re-state "1.5.0"]
                [medley "1.2.0"]
                [metosin/spec-tools "0.10.4"]
                [org.clojure/data.json "0.2.6"]
                [org.clojure/spec.alpha "0.2.187"]
                [pez/clerk "1.0.0"]
                [prismatic/schema "1.1.12"]
                [re-frame "1.3.0"]
                [re-graph "0.1.15" :exclusions [cljs-http]]
                [com.andrewmcveigh/cljs-time "0.5.2"]
                [reagent "1.1.1"]
                [spyscope "0.1.6"]
                [venantius/accountant "0.2.5"]
                [lambdaisland/regal "0.0.143"]
                [cheshire "5.11.0"]
                [camel-snake-kebab "0.4.3"]
                [garden/garden-units "1.0.0-RC2"]
                [cljs-bean "1.8.0"]]

 :source-paths ["lib" "graphql" "src" "C:/Appsmiths/i/src/winglue-artifact/webglue" "test"]
 :nrepl {:port 41002}
 :open-file-command ["idea " :pwd " --line " :line :file]
 ;; webglue artifact are in /i/src/winglue-artifact/webglue/js
 :builds {:app {:output-dir "C:/Appsmiths/i/src/winglue-artifact/webglue/js"
                :asset-path "/js"
                :compiler-options
                {:optimizations :none
                 ;; use es2018 for recat-markdown-editor
                 :output-feature-set :es2018
                 :main webglue.core
                 :closure-warnings {:global-this :off}
                 :closure-defines {re-frame.trace/trace-enabled? true
                                   day8.re-frame-10x.debug?             true
                                   day8.re-frame.tracing.trace-enabled? true}
                 :external-config {:devtools/config {:features-to-install    [:formatters :hints]
                                                     :fn-symbol              "Fn"
                                                     :print-config-overrides true}}}
                :target :browser
                :js-options {:ignore-asset-requires true}
                :module-loader true
                :modules {:webglue
                          {:entries [webglue.core]}
                          :ag-grid
                          {:entries [webglue.components.ag-grid]
                           :depends-on #{:webglue}}
                          :dev
                          {:entries [webglue.pages.component-show-case webglue.pages.dev]
                           :depends-on #{:webglue}}
                          :plot
                          {:entries [webglue.component-generator.graph
                                     webglue.component-generator.ivsp-plot
                                     webglue.component-generator.pie-chart
                                     webglue.component-generator.bar-chart
                                     webglue.component-generator.bubble-chart]
                           :depends-on #{:webglue}}}
                :devtools {:repl-pprint true
                           :after-load webglue.core/reload!
                           :loader-mode :eval
                           :http-root "C:/Appsmiths/i/src/winglue-artifact/webglue"
                           :http-port 3333
                           :http-handler shadow.http.push-state/handle
                           :preloads [devtools.preload
                                      ; use only for debug re-frame, this option wil take 300 ms to reload and 330 file to watch
                                      ; run by uncomment next line, and save. shadow-cljs will do the rest
                                      day8.re-frame-10x.preload]}}
          :win-app {:output-dir "C:/Appsmiths/i/src/winglue-artifact/webglue"
                    :asset-path "/js"
                    :compiler-options
                    {:optimizations :none
                     :output-feature-set :es2018
                     :main webglue.core
                     :closure-warnings {:global-this :off}
                     :closure-defines {re-frame.trace/trace-enabled? true
                                       day8.re-frame-10x.debug?             true
                                       day8.re-frame.tracing.trace-enabled? true}
                     :external-config {:devtools/config {:features-to-install    [:formatters :hints]
                                                         :fn-symbol              "Fn"
                                                         :print-config-overrides true}}}
                    :target :browser
                    :js-options {:ignore-asset-requires true}
                    :modules {:webglue {:entries [webglue.core]}}
                    :devtools {:http-port 3434
                               :http-root "C:/Appsmiths/i/src/winglue-artifact/webglue"
                               :http-handler shadow.http.push-state/handle
                               :after-load webglue.core/reload!
                               :loader-mode :eval
                               :preloads [devtools.preload
                                          day8.re-frame-10x.preload]}}
          :test {:target :browser-test
                 :test-dir "test-assets"
                 :devtools {:http-port 9100
                            :http-root "test-assets"}
                 :runner-ns main-test-initial
                 :js-options {:ignore-asset-requires true}
                 :ns-regexp "-test$"}

          :app-release {:target :browser
                        :modules {:webglue
                                  {:entries [webglue.core]}}
                        :js-options {:ignore-asset-requires true}
                        :compiler-options {:source-map false
                                           :output-feature-set :es2018
                                           :main  webglue.core
                                           :closure-defines {}}
                        :release  {:output-dir "C:/Appsmiths/i/src/winglue-artifact/webglue/js"}}

          :tao2py-release {:target :browser
                           :js-options {:ignore-asset-requires true}
                           :compiler-options
                           {:source-map false
                            :output-feature-set :es2018
                            :main  webglue.core
                            :closure-defines {webglue.config/tao2py-release true
                                              ;; landing page is page key in webglue.route_pages
                                              webglue.config/landing-page "field-overview"}}
                           :release  {:output-dir "C:/Appsmiths/i/src/winglue-artifact/webglue/js"
                                      :asset-path "/js"}
                           :module-loader true
                           :modules {:webglue
                                     {:entries [webglue.core]}
                                     :ag-grid
                                     {:entries [webglue.components.ag-grid]
                                      :depends-on #{:webglue}}
                                     :dev
                                     {:entries [webglue.pages.component-show-case webglue.pages.dev]
                                      :depends-on #{:webglue}}
                                     :plot
                                     {:entries [webglue.component-generator.graph
                                                webglue.component-generator.ivsp-plot
                                                webglue.component-generator.pie-chart
                                                webglue.component-generator.bar-chart
                                                webglue.component-generator.bubble-chart]
                                      :depends-on #{:webglue}}}}

          :tao2py-testing {:target :browser
                           :modules {:webglue
                                     {:entries [webglue.core]}}
                           :js-options {:ignore-asset-requires true}
                           :compiler-options {:source-map false
                                              :output-feature-set :es2018
                                              :main  webglue.core
                                              :closure-defines {}}
                           :release  {:output-dir "C:/Appsmiths/i/src/winglue-artifact/webglue/js"}}

          :budger-release {:target :browser
                           :modules {:webglue {:entries [webglue.core]}}
                           :release {:output-dir "release/js"}
                           :js-options {:ignore-asset-requires true}
                           :compiler-options {:source-map false
                                              :output-feature-set :es2018
                                              :main       webglue.core
                                              :closure-defines {}}}}}

Clojure Q&A - Questions without answers in Compiler

Possibly avoidable identity call to force instance method on Class literals

Feature Request:Make anonymous fn/reify class name IDs per namespace instead of one global counter

Compiler-generated `foo__init.class` files lack `SourceFile` attribute

Can custom literal collections reach parity with clojure's built in collections?

Teaching Old eval New Tricks

Tagged Literals Promise Extensibility

Towards a Solution

Incorrect result when evaluating `not=` on `##NaNs`

Cannot invoke "clojure.lang.Var.isBound()" because "clojure.lang.Compiler.LOADER" is null, potential bug?

Executable 'java' not found on system path.

Executable 'java' not found on system path.

Teaching Old `eval` New Tricks