Nov 16 meeting notes

Kevin Reid kpreid at switchb.org
Thu Nov 24 06:23:43 PST 2011


On Nov 23, 2011, at 13:27, David Herman wrote:

> An additional point about consistency: many scripting languages talk about the "length" of a table data structure because they actually expose the representation of a table as an array or list.

I dispute your examples, as follows:

> IIRC Perl/PHP associative arrays,

In Perl, _hashes_ are different builtin objects from _arrays_; they merely have useful conversion behavior when used in _list context_ (which is a call-semantics issue unrelated to data structures). The way to get the number of elements is to call 'keys %myhash' in scalar context. 'length %myhash' will do something useless. I'm not familiar with PHP.

> Python dicts,

Again, different builtin objects. If they have any array nature, it is not exposed to the programmer; myDict[key] gets you a lookup by hash key, not array indexing.

> Lisp alists,

An alist in Lisp is not a distinctly typed data structure at all; it is only and entirely a particular application of linked lists (which are an application of cons cells (pairs/tuples)) for which there are utility functions in the standard library. A (Common) Lisp programmer looking for the size of an alist would indeed (length my-alist), but if they had a hash table it would be (hash-table-count my-ht). (In CL, the length function applies to all _sequences_, but a hash table is not a sequence; there are no functions generic over all kinds of collections.)

> and Ruby hashes are all transparently built from arrays or lists. But Map and Set aren't arrays, they're their own data structure. So IMO it makes sense to name their methods on their on terms, not on the terms of a different data structure.


(For what it's worth, I agree with the previous suggestion to make .length used only for things which have [0], [1], [2], …; and that if it is instead a collection with .get() and .put()/.set() or similar then .size() or .count() makes sense.)

-- 
Kevin Reid                                  <http://switchb.org/kpreid/>



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