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[forwarded on behalf of Oleg] Hello! I'm sorry I'm late to the party, which has almost finished. It looks like a great party. The following comments apply to the latest draft http://sgmiller.org/srfi-44.html "As standard Scheme lacks the ability to automatically dispatch on different function behaviors based on the types of arguments, it difficult to devise a portable implementation of this SRFI which allows arbitrary future collections. It is expected that Scheme system implementors will take advantage of generic function or object-oriented features of their systems to implement this SRFI efficiently." I wonder why a module system was not mentioned at all? Quite often people use a particular standard collection as an implementation vehicle for their advanced data structure. A collection will be encapsulated into the data structure. For example, a graph data structure may use a dictionary to store adjacency information for a node (or to store a cost value for a pair of nodes). In that case, most of the implementation code is monomorphic with respect to the collection type and no run-time dispatch is required. Given an advanced module system, I can import a particular dictionary implementation (e.g., associative lists) with renaming make-alist => make-mydict alist-get => mydict-get etc. In my own code I would use the names make-mydict, mydict-get, etc. exclusively. If at a later time I decide to use a different dictionary implementation, I would only need to change the module declaration statement. I have also taken a look at the Edison: http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/hfl/hfl/edison/ in particular, Coll/Coll/Collection.hs The latter gave a few suggestions. > procedure: % value ... => % > Constructs a % collection with the zero or more values provided > as its initial contents. Should we add that if the collection is a set, it is unspecified which element is kept in case of duplicates? Or should duplicates be outlawed? [Edison takes the former approach]. > procedure: *-add bag value => * > procedure: *-add![!] bag value => * > Adds a single value to a bag. > procedure: *-add-all dest-bag source-bag => * > procedure: *-add-all![!] dest-bag source-bag => * > Adds all the values in the source bag to the destination bag. Edison says: insert [our add] keeps the new element in case of duplicates, but insertSeq [our add-all] keeps an unspecified element. Edison defines the procedure count, for the occurrence count of an element in a collection. Sure, count may be expensive for some collections. For sets, the occurrence count is defined too (but required to return either 0 or 1). Edison also defines a procedure > instanceName :: c -> String > -- the name of the module implementing c which can be useful, for example, for generating error messages. The procedure is useful regardless of a particular module or OO system. For ordered collections, Edison defines procedures to view, insert or delete the maximum and the minimum elements. I don't know how useful this may be (many advanced collections do allow efficient implementations of such operations -- especially for collections that are used for priority queues or dequeues). It goes without saying that many operations in Edison -- like union or intersection of two collections -- are perhaps too high-order or too laborous or too infrequent to warrant their specification in SRFI-44. It's better to stick to the bare minimum and provide extensions only for very frequent operations. > procedure: *-ref sequence integer => boolean > Returns the value stored in the sequence at the integer index > provided. It is an error to reference an index outside the range of > the sequence. The return type can't be boolean. Right? BTW, often a function *-ref-with-default is surprisingly useful. The function takes an extra argument -- the default value -- and returns literally (in the sense of eq?) that argument in case the lookup failed. Edison's sequences, http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/hfl/hfl/edison/Seq/Sequence.hs are also interesting, in particular procedures lview, rview, inBounds, adjust and zippers. However, one may also leave them out for SRFIs to come. > (eq? <input collection> (update-procedure <input collection>)) will > always return a non-false value. Perhaps this statement is too conservative? R5RS requires that eq? (as well as eqv? and equal?) return a boolean value. Therefore, the only non-false return value of eq? is #t. It might be a good idea to say that explicitly. > The collection fold procedure then invokes the folding function on a > value of the collection and the seeds, and receives from the folding > function a proceed value followed by a like number of seed return > values. If the termination value is non-false, Perhaps you meant "the proceed value" rather than a termination value. Otherwise, "termination value" is not mentioned anywhere else in the document. > In order to ensure a consistent ordering in the collection, the > ordering function must return the same result for like inputs over > time. In other words, an ordering function must be a pure function, a mapping of its arguments. One might add that an implementation of a collection may assume (and (equal? x x1) (equal? y y1)) ==> (equal? (ordering-function x y) (ordering-function x1 y1)) Should the same requirement be applied to a equivalence function as well? Cheers, Oleg
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