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Still on David's issues; SRFI 44



Now that I've cooled off a bit, let's try something destructive.

Whether SRFI 69 should be about general mappings or hash tables, has
been IMO adequately addressed; if there are suggestions about how to
make it clearer in the abstract that it _is_ about hash tables, they are
welcome.

What I haven't been adequately addressing is the neglection of SRFI 44.
This is mainly because I have no motivation for criticising an
already-finalised SRFI; I was hoping I could just go ahead and work this
SRFI to be good, unhindered by SRFI 44.  It's possible that some other
people feel the same way, I don't know.

But because David seems to be very concerned about the neglection of
SRFI 44, I probably have to tell why I'm not heeding its call.
Personally, I can't see this as such a serious matter as I don't think
an SRFI being _finalised_ is reason enough to obey it.  The "real" test
for an SRFI is whether it will ever get added in mainstream
implementations, not whether it is able to go through peer review.  Peer
review is only a means to get it good enough to appeal to implementors.

Now, let's get to the point.  The first objection I have to SRFI 44 is
that it's an example of biting more than one can chew.  Standardising
naming conventions for a data structure requires, in my opinion, a lot
of everyday work with that data structure.  SRFI 44 not only tries to
address lots and lots of data structures at once, it also tries to
address forthcoming collections whose characteristics are unknown.
There is very little hope that it will provide an optimal naming scheme
or taxonomy for those data structures.

Nor is this kind of "abstraction first" approach needed.  Being an SRFI
about _lists_ did not prevent SRFI 1 from affecting SRFI 69's naming
conventions.  Of course, all the SRFI's for concrete data types will
look for an example in the already-existing data types.  This kind of
careful work that balances similarity with already existing data types
and special needs of the new data type is what will most probably make
the proper API for a wide variety of data structures.

The second (or third) objection is that SRFI 44 itself is confused about
its emphasis.  It addresses two relatively orthogonal issues: the naming
schemes of concrete data types, and how these concrete data types are
arranged in a type hierarchy (and how that type hierarchy is
implemented).  The second one is alien in a language like Scheme, which
has always been about concrete data types, and brings us all the
problems of taxonomy.  Similarly to Aristotle's ontological
categorisation, SRFI 44 has a weird division of genera against
qualities: for example, sequence is a subtype of bag, but pure
mutability and directionality are properties separate from the
hierarchy.  Some properties are, however, dependent on others, such as
every ordered collection is a directional collection.  There are also
other attributes which receive a mention but are not separately listed
as "attributes", such as immutability.

The subtype hierarchy is also flawed: for example, the SRFI states that
there is collection-fold-keys-left, but the procedure is only defined
for sequences and maps.  Ergo, set is-not-a collection.

The common thing about these problems is that they are both problems
very generally found in object oriented design: premature abstraction,
and arbitrariness in domain analysis.  And I think these problems,
together with the vastness of the scope of SRFI 44, contribute to the
problems in SRFI 44's mapping API.  For instance, sequences could be a
subtype of map, with very limited domain.  But they are not, and so we
have *-get instead of *-ref and *-put! instead of *-set!.  (SRFI 69
originally had both naming schemes, but *-get, *-put! was downvoted, and
IMO, righteously so.)

Now, on to concrete criticism.  What would SRFI 44 API of hash tables
look like?

Hash tables would have two supertypes: map and collection.  Collection
operations would forget about the keys.  No indexed collection could be
a subtype of a hash table, and no keyed collection could be a subtype of
an indexed collection.

There would be an asymmetry between items in a set and keys in a map:
even though these two are very similar, the former are treated as
"values" while the latter are treated as "keys".  Consequently,
operations for these have different names.

Hash tables could have a value equivalence function which would be never
used, but the user could not supply a hash function.  This implies that
appropriate hash functions could be guaranteed only up to some specific
coarseness (as with SRFI 69 defaults), and beyond that, hash tables
could not guarantee their complexity constraints.  In fact, every time
the user supplied an unrecognised key equivalence function, the
implementation would have to use a constant hash function (like (lambda
(x) 0) for example) to guarantee correctness, destroying all the value
of hash tables.

Hash tables would lack an operation that folds with both keys and
values, and hash-table->alist.  Generally, _all_ access to the actual
associations of the hash table would be indirect.

hash-table-update! would not be there, forcing people to look up the key
twice when they update the value.

Hash tables should be able to implement -delete-from! for a bag datatype
that does not yet exist.

Despite all this, I've tried to craft SRFI 69 so that it does not
_prevent_ a SRFI 44 style API to hash tables.  The only unclear point,
which David diligently dug out, is hash-table-equivalence-function.  It
can be that systems implementing both SRFI 44 and SRFI 69 will have to
add the (meaningless) item equivalence function to the hash table
implementation data structure _just in case_ somebody asks for it.  But
I hope the rest of the world can safely just not bother.

Panu

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