clojure - Clojure 中的 seq 函数注意事项

Question

在 clojure 的 seq 函数的文档字符串中，它提到:

Note that seqs cache values, thus seq should not be used on any Iterable whose iterator repeatedly returns the same mutable object.

这句话是什么意思？为什么要强调同一个可变对象？

Answer 1

评论是added later并提到这个ticket :

Some Java libraries return iterators that return the same mutable object on every call:

• Hadoop ReduceContextImpl$ValueIterator
• Mahout DenseVector$AllIterator/NonDefaultIterator
• LensKit FastIterators

While careful usage of seq or iterator-seq over these iterators worked in the past, that is no longer true as of the changes in CLJ-1669 - iterator-seq now produces a chunked sequence. Because next() is called 32 times on the iterator before the first value can be retrieved from the seq, and the same mutable object is returned every time, code on iterators like this now receives different (incorrect) results.

Approach: Sequences cache values and are thus incompatible with holding mutable and mutating Java objects. We will add some clarification about this to seq and iterator-seq docstrings. For those iterators above, it is recommended to either process those iterators in a loop/recur or to wrap them in a lazy-seq that transforms each re-returned mutable object into a proper value prior to caching.