implement freeze

[radarroark]

• Adds a freeze! function which makes data created in a transaction immutable, along with unit tests.
• Adds a bunch of fixes to LinkedArrayList which should've been in a separate PR but oh well :^D
• Fixes the cons implementations; I found that they were still not correct because they were not returning the correct type. Now they all call conj (which the HashMap is already doing).
• Adds print-method implementations for the writeable data structures.
• Updates the README to show how to use freeze!.

[codeboost]

@claude Can you review the PR and let us know if there's anything worth addressing ?

[claude]

Claude finished @codeboost's task in 1m 35s —— View job

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Code Review Complete ✓

I've reviewed the PR and the implementation looks solid overall! Here are my findings:

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Summary

This PR successfully implements the freeze! function to make data structures immutable within a transaction, along with several other improvements to the data structure implementations. The code is well-tested and the README documentation is clear.

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Positive Findings

1. Well-tested implementation: The test suite in freeze_test.clj covers all four data structure types (ArrayList, LinkedArrayList, HashMap, HashSet) with both "without freeze" and "with freeze" scenarios. This demonstrates the issue and validates the fix.

2. Consistent implementation: The IReadOnly protocol and -read-only method are consistently implemented across all writable data structures.

3. Good documentation: The README provides a clear example of the surprising behavior and how freeze! solves it.

4. Correct cons fix: The change from (cons o ...) to (. clojure.lang.RT (conj ... o)) is correct - cons always returns a clojure.lang.Cons type, while conj preserves the collection type.

5. Useful print-method implementations: These will make debugging much easier in the REPL.

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Issues Found

1. Potential Issue: freeze! implementation doesn't validate input (Minor)

Location: src/xitdb/db.clj:234-241

The freeze! function calls common/-unwrap and then accesses .cursor without checking if the value implements the required protocols. If someone calls freeze! on a regular Clojure collection or primitive value, this will throw an unclear error.

Suggestion: Add validation to provide a clearer error message:

(defn freeze!
  "Prevents all data written in the current transaction from
  being mutated by any remaining changes. Throws if called
  outside of a transaction. Returns a read-only version of the
  given writeable data structure."
  [x]
  (when-not (satisfies? common/IUnwrap x)
    (throw (IllegalArgumentException. 
            (str "freeze! requires a writeable XITDB data structure, got: " (type x)))))
  (-> x common/-unwrap .cursor .db .freeze)
  (common/-read-only x))

2. Duplicate protocol implementation for XITDBLinkedArrayList (Bug)

Location: src/xitdb/linked_list.clj:238-243

There are two IMaterialize protocol implementations for XITDBLinkedArrayList - one at line 112-116 and another at line 238-243. The second one returns a list while the first returns a vector. This is likely a copy-paste error.

The second implementation (lines 238-243) should probably be deleted, as it:

• Is redundant (protocol can only be extended once)
• Returns a different type (list vs vector)
• May not even be taking effect depending on load order

Fix: Remove the duplicate at lines 238-243, or if the intent was to return a list, update the first implementation instead.

3. Missing containsKey and entryAt for write types (Minor Inconsistency)

Location: src/xitdb/linked_list.clj:126+

The read-only XITDBLinkedArrayList doesn't implement clojure.lang.Associative, but the writable XITDBWriteLinkedArrayList does (lines 165-170). However, the writable version is missing containsKey and entryAt methods that are part of the Associative interface.

Looking at XITDBWriteArrayList in array_list.clj:187-199, it correctly implements all three methods: assoc, containsKey, and entryAt.

Fix: Add the missing methods to XITDBWriteLinkedArrayList:

clojure.lang.Associative
(assoc [this k v]
  (when-not (integer? k)
    (throw (IllegalArgumentException. "Key must be integer")))
  (operations/linked-array-list-assoc-value! wlal k (common/unwrap v))
  this)

(containsKey [this k]
  (and (integer? k) (>= k 0) (< k (.count wlal))))

(entryAt [this k]
  (when (.containsKey this k)
    (clojure.lang.MapEntry. k (.valAt this k))))

4. Inconsistent Associative implementation for read-only LinkedArrayList (Minor)

Location: src/xitdb/linked_list.clj:14-107

The read-only XITDBLinkedArrayList implements IPersistentVector and Indexed (lines 46-62) but doesn't implement Associative, unlike XITDBArrayList which implements all three (see array_list.clj:34-50).

According to the PR description: "Implements clojure.lang.IPersistentVector and clojure.lang.Associative for LinkedArrayList", but only IPersistentVector was added to the read-only version.

Fix: Add Associative implementation to XITDBLinkedArrayList similar to XITDBArrayList.

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Recommendations

1. Fix the duplicate IMaterialize protocol - This is the most important issue as it's a bug.

2. Complete the Associative implementation - For consistency with ArrayList and with the PR description.

3. Add validation to freeze! - For better developer experience with clear error messages.

4. Consider adding a test for calling freeze! with invalid input to ensure good error messages.

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Questions

1. Line 48 in freeze_test.clj: The expected result shows ["fish" "rice" "eggs" "apple" "pear" "grape"] - is this the intended order for lists? It appears elements are being prepended (LIFO) which makes sense for a linked list, but the order is interesting since conj is being used.

2. Structural sharing comment in README (line 170): The comment says "this is not doing an expensive copy" - is this guaranteed by the Java implementation? It would be good to verify that freeze indeed creates a read-only view with structural sharing rather than a deep copy.

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Overall Assessment

The implementation is well-designed and solves a real usability issue. The test coverage is excellent. The issues identified are minor and mostly about consistency and error handling. The core functionality appears correct.

Recommendation: Merge after addressing the duplicate IMaterialize protocol bug. The other issues are nice-to-haves that could be addressed in follow-up PRs.

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[radarroark]

@codeboost I got all of claude's suggestions in. I also added a bunch of linked array list fixes. I removed some interface impls from the read-only LinkedArrayList (specificaly IPersistentList and Associative) because they were forcing me to return a vector, whereas we want the LinkedArrayList to materialize to a clojure linked list. That's a small detail tho...I am still implementing those interfaces on the WriteLinkedArrayList which is the one that matters most.

[codeboost]

LGTMT 👍

[radarroark]

Cool, I have no more changes to add so feel free to merge