Scheme Request for Implementation 250,
"Insertion-ordered hash tables",
by John Cowan,
is now available for discussion.

Its draft and an archive of the ongoing discussion are available at
<https://srfi.schemers.org/srfi-250/>.

You can join the discussion of the draft by filling out the
subscription form on that page.

You can contribute a message to the discussion by sending it to
<srfi-250@srfi.schemers.org>.

Here's the abstract:

This SRFI defines an interface to hash tables, which are widely
recognized as a fundamental data structure for a wide variety of
applications. A hash table is a data structure that:

- Is disjoint from all other types.
- Provides a mapping from objects known as keys to corresponding
objects known as values.
- Keys may be any Scheme objects in some kinds of hash tables, but
are restricted in other kinds.
- Values may be any Scheme objects.
- Provides an equality predicate which defines when a proposed key
is the same as an existing key. No table may contain more than one
value for a given key.
- Provides a hash function which maps a candidate key into a
non-negative exact integer.
- Supports mutation as the primary means of setting the contents of
a table.
- Provides key lookup and destructive update in (expected) amortized
constant time, provided that a satisfactory hash function is
available.
- Does not guarantee that whole-table operations work in the
presence of concurrent mutation of the whole hash table. (Values
may be safely mutated.)
- Unlike the hash tables of SRFI 125, which is the direct ancestor
of this specification, the hash tables described here are ordered
by insertion: that is, associations inserted earlier in the
history of the hash table appear earlier in the ordering. Advances
in the implementations of hash tables, as provided by C++, Python,
JavaScript, etc., make the provision of this new facility
practical. As a result, the hash tables of this SRFI do not
interoperate with the hash tables of SRFI 125, SRFI 126, or
existing R6RS implementations.

Regards,

SRFI Editor