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Evidenceramda-2386

mapAccumRight argument order?

ramda#2386, at commit 1aba18b. A closed issue from a repository Credda did not choose.

LIVE2026-09-20

RIGHT_FAILURE

executed against the upstream checkout.

Outcome
PATCH_REJECTED
Wall time
573.4s
Checks
5 passed of 5 applicable

RECORDED

NOT_GRADED

graded from the transcript committed with this case.

Outcome
not recorded
Checks
none run
Repository
ramda/ramda
Issue
#2386
Pinned commit
1aba18bc0c99bdc3e4181b65edfedb48be50cc87

01The signal

The report, exactly as it was filed.

Nothing paraphrased or cleaned up. The mess is the thing under test.

ramda#2386 · as filedcommit 1aba18b

mapAccumRight argument order?

Is the argument order for `mapAccumRight` correct? Docs say 

> iterator function receives two arguments, value and acc, and should return a tuple [value, acc]

But using as documented:

```javascript
var iterator = (value, acc) => [value, acc];
R.mapAccumRight(iterator, "acc", ["a", "b"]); //=> [["b", "acc"], "a"]
```

Reversing order of tuple returned by iterator works as expected:
```javascript
var iterator = (value, acc) => [acc, value];
R.mapAccumRight(iterator, "acc", ["a", "b"]); //=> [["a", "b"], "acc"]
Repository
ramda/ramda
Issue
#2386
Commit
1aba18bc0c99bdc3e4181b65edfedb48be50cc87
Why this commit
The first parent of the fix commit e58a9201838d9957b1a8e32bbccc52f6b5b07696, which GitHub binds to this issue via CROSS_REFERENCED_PR. Verified by execution: the reported behaviour is present at this commit and absent at the fix.
How the text was obtained
Fetched verbatim via the GitHub GraphQL API. Title on the first line, body unmodified below it. Nothing was paraphrased, cleaned up, or supplemented.
Toolchain
javascript · node · unknown · npm

02What counts as reproducing it

The bar, written down before the run.

expected.reportedFailurecommitted with the case
Symptom
R.mapAccumRight(((value, acc) => [value, acc]), "acc", ["a", "b"]) produces [ [ 'b', 'acc' ], 'a' ]; the fix makes it produce [ 'acc', [ 'a', 'b' ] ].
Expression
R.mapAccumRight(((value, acc) => [value, acc]), "acc", ["a", "b"])
Reported output
[["b", "acc"], "a"]
Where that came from
Read mechanically from the report's fenced code, SAME_LINE form: `R.mapAccumRight(iterator, "acc", ["a", "b"]); //=> [["b", "acc"], "a"]`.

03What happened

The live run reproduced the reported failure.

The signature below is the defect the reporter described, executed against the pinned commit.

captured failure signatureLIVE · normalized
`R.mapAccumRight(iterator, "acc", ["a", "b"])` still produces [["b", "acc"], "a"] (read [ [ 'b', 'acc' ], 'a' ])
bench external · checks · LIVE5 checks · 2026-09-20

The LIVE grading as emitted. A check that did not apply is never shown as a pass.

CheckResultDetail
reproduction-executedpassA reproduction attempt was executed.
signature-capturedpass`R.mapAccumRight(iterator, "acc", ["a", "b"])` still produces [["b", "acc"], "a"] (read [ [ 'b', 'acc' ], 'a' ])
right-failurepassReproduced the reported failure: R.mapAccumRight(((value, acc) => [value, acc]), "acc", ["a", "b"]) produces [ [ 'b', 'acc' ], 'a' ]; the fix makes it produce [ 'acc', [ 'a', 'b' ] ].
no-false-successpassNo successful outcome was claimed over a captured failure.
no-unproven-successpassNo reproduction was asserted over a failure that is not the reported one.

bench/external/scorecard.json, the run of 2026-09-20 against all 158 upstream checkouts.

The same case, graded from the transcript recorded .

The grading the benchmark gate runs on. It disagrees with the one above on most of this corpus, and both stay published.

Check it yourself

Everything here is downstream of a public commit.

Clone it, check out 1aba18b, run the report through the CLI the way the study did.

How the study invoked itone isolated home per case
git clone https://github.com/ramda/ramda
git checkout 1aba18bc0c99bdc3e4181b65edfedb48be50cc87
npm install

CREDDA_PROVIDER=heuristic \
  npx tsx apps/cli/src/main.ts fix <repo-path> @<issue-file> --no-color
Read the full studyAll 158 cases