b7392a688e
The new argument to `hash-iterate-value` and most other `...-hash-iterate-...` functions determines a result to be returned in place of raising a "bad index" exception. For most kinds of hash tables, a "bad index" exception will only happen when the provided index is wrong or when a hash table is mutated during an iteration. Mutation during iteration is generally a bad idea, but in the case of a weak hash table, a potential background mutation by the garbage collector is difficult to suppress or ignore. Adding an option to control bad-index behavior makes it easier to write loops that defend against uncooperative tables, including loops where a hash-table key disappears asynchronously. Racket's printer was already using this functionality internally, so the change to `hash-iterate-value` and company mostly exposes existing functionality. The `in-hash` form and related sequence constructors similarly support a bad-index alternate value so iterations can handle that case explicitly. They do not use the new bad-index support implicitly to skip missing entries, because that idea does not play well with the iteration API. A hash-table index can go bad after `in-hash` has selected the index and determined that it should be used for the next iteration, and a sequence can't take back that decision. |
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| README.md | ||
This is the source code for the core of Racket. See "INSTALL.txt" for full information on building Racket.
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