initial tests for clojure.core/merge-with by perrygeo · Pull Request #896 · jank-lang/clojure-test-suite

perrygeo · 2026-05-24T18:18:09Z

Testing clojure.core/merge-with

Manually tested with

JVM
Clojurescript
Clojure CLR
Babashka
Basalisp (fails, see comment on nil handling)
PHEL (I don't have access to PHP tooling at the moment)

Dialect Differences

Clojurescript returns a nil in the zero-arg case. Clojure etc throw an exception.
Clojurescript: numeric keys have different equality semantics than the JVM.
Basalisp returns nil on (merge-with second-arg nil {}), rest return {}

Anything else I can do to improve this? Let me know. This is a really fun way to get to know the clojure standard library!

Resolves #361

CLAassistant · 2026-05-24T18:18:18Z

All committers have signed the CLA.

E-A-Griffin · 2026-05-24T19:55:46Z

basilisp and phel status is unknown, are the core 4 langs sufficient?

They are yeah, @jeaye disabled them for the time being, that being said noting how they fail is always nice for their respective maintainers to be aware of (and potentially fixed)

E-A-Griffin

Could you use the following reader conditional order: https://github.com/E-A-Griffin/clojure-test-suite/blob/e00fe2b8ff87fcf02020664cc57f11bbf4448904/doc/writing-tests.md#reader-conditional-order? This PR isn't merged yet but @jeaye and I agreed on it prior so going to try to maintain it going forward

E-A-Griffin · 2026-05-24T19:58:03Z

+     (testing "zero arg behavior"
+       ;; NOTE: clojurescript differs from the rest here, bug?
+       #?(:clj     (is (p/thrown? (merge-with)))
+          :cljs    (is (nil? (merge-with))) ; TODO how to suppress warning


What is this TODO for?

When I run bb test-cljs, everything passes but the output contains this verbose warning:

... Ran 223 tests containing 4957 assertions. 0 failures, 0 errors. =============================================== [:test] Build completed. (324 files, 323 compiled, 1 warnings, 4.22s) ------ WARNING #1 - :fn-arity -------------------------------------------------- File: /Users/matthew/projects/clojure-test-suite/test/clojure/core_test/merge_with.cljc:15:30 -------------------------------------------------------------------------------- 12 | (testing "zero arg behavior" 13 | ;; NOTE: clojurescript differs from the rest here, bug? 14 | #?(:clj (is (p/thrown? (merge-with))) 15 | :cljs (is (nil? (merge-with))) ; TODO how to suppress warning ------------------------------------^------------------------------------------- Wrong number of args (0) passed to cljs.core/merge-with -------------------------------------------------------------------------------- 16 | :default (is (p/thrown? (merge-with))))) 17 | 18 | (testing "`nil`ish behavior" 19 | (is (nil? (merge-with second-arg nil))) --------------------------------------------------------------------------------

Since we're explicitly testing the zero-args case, is there a way to silence this?

E-A-Griffin · 2026-05-24T20:06:10Z

I think sorted-map tests would also be nice

perrygeo · 2026-05-25T13:41:14Z

Thanks for the review @E-A-Griffin ! The reader conditional ordering actually matters, TIL. I added tests for sorted-map and cleaned up a bit.

The cljs zero-arg case is the only real question mark. I've noted the difference in the top PR description. The warning still bothers me though, hoping to find a way to suppress that.

Copilot

Pull request overview

Adds a new clojure.core compliance/characterization test namespace covering merge-with, with some dialect-conditional expectations (notably ClojureScript’s zero-arg behavior).

Changes:

Introduces a new merge-with test file with coverage for nil handling, collision semantics, fold direction, duplicate-key invocation, sorted-map preservation, and nested-map merging.
Adds negative tests around non-map inputs and exception behavior (with dialect conditionals).

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jeaye

Thanks for the PR!

jeaye · 2026-05-25T17:56:07Z

+  (:require [clojure.test :as t :refer [deftest is testing]]
+            [clojure.core-test.portability #?(:cljs :refer-macros :default :refer) [when-var-exists] :as p]))
+
+(defn second-arg [_a b] b)


We can delete this fn and just use second.

second takes one arg and returns the 2nd element. I guess we could write #(second [%1 %2]) but not sure if that's an improvement.

@perrygeo , right, this is not the same. second-arg is being used as the merge function and it is overwriting the first arg with the second during the merge.

dgr

Overall, looks good.

dgr · 2026-06-23T15:19:33Z

+  (:require [clojure.test :as t :refer [deftest is testing]]
+            [clojure.core-test.portability #?(:cljs :refer-macros :default :refer) [when-var-exists] :as p]))
+
+(defn second-arg [_a b] b)


@perrygeo , right, this is not the same. second-arg is being used as the merge function and it is overwriting the first arg with the second during the merge.

dgr · 2026-06-23T15:23:00Z

+     (testing "zero arg behavior"
+       #?(:cljs (is (nil? (merge-with)))
+          :clj  (is (p/thrown? (merge-with)))))
+


There's a missing case here, I think: (merge-with second-arg)

That is, no maps, just the required merge function present.

dgr · 2026-06-23T15:24:19Z

+ (deftest test-merge-with
+   (testing "`merge-with`"
+
+     (testing "zero arg behavior"


Let's be clear that there is no zero-arg case for the function legitimately. The doc string shows f as a required arg, so the CLJ case is throwing because it's short args.

dgr · 2026-06-23T15:27:34Z

+                                          {:a 1} {:a 2} {:b 3}))))
+
+     (testing "sorted"
+       (let [result (merge-with +


I would also try this where the first arg is a sorted map and the second is a regular hash map, to validate that the sorting behavior accrues with the first argument. I'd also test it with three maps, where the first is sorted to validate how the sorting behavior is propagated through the folding-in of the later maps.

dgr · 2026-06-23T16:28:39Z

+     (testing "falsey values still trigger f on collision"
+       (is (= {:a false} (merge-with second-arg {:a true} {:a false})))
+       (is (= {:a nil} (merge-with second-arg {:a true} {:a nil}))))


I'm not sure what this is supposed to be testing. The keys are both :a and the value doesn't participate in the decision to merge or not.

dgr · 2026-06-23T16:33:49Z

+     (testing "metadata progation, when keys are equal but not identical"
+       (is (= {:meta1 true}
+              (meta (ffirst (merge-with + {^:meta1 {} 1} {^:meta2 {} 2}))))))


This is good. I would add a test with metadata on the values as well and make sure that it's preserved through the merge and through the merge function.

initial tests for clojure.core/merge-with

cee0e09

E-A-Griffin suggested changes May 24, 2026

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perrygeo added 3 commits May 25, 2026 07:10

remove comments, addressed in PR

97e46ed

reader conditional order

1f8f37a

test sorted-map

1f87e10

reader conditional alignment

a0501c1

jeaye requested a review from Copilot May 25, 2026 17:42

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jeaye requested a review from dgr May 25, 2026 17:50

jeaye requested changes May 25, 2026

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address review comments

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dgr suggested changes Jun 23, 2026

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perrygeo commented May 24, 2026 •

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