SRFI 122 draft 12 comments | Sudarshan S Chawathe | 25 Sep 2016 00:00 UTC |

Re: SRFI 122 draft 12 comments | Bradley Lucier | 26 Sep 2016 20:37 UTC |

array-fold-{left|right} and array-reverse | Bradley Lucier | 29 Sep 2016 18:15 UTC |

For draft 14 (Re: SRFI 122 draft 12 comments) | Sudarshan S Chawathe | 03 Dec 2016 22:06 UTC |

Re: For draft 14 (Re: SRFI 122 draft 12 comments) | Bradley Lucier | 09 Dec 2016 18:28 UTC |

For draft 14 (Re: SRFI 122 draft 12 comments)

*Sudarshan S Chawathe*03 Dec 2016 22:06 UTCSorry for being very late in following up on this message. I am including below some comments on the few items from the earlier message that still apply to the recent draft 14, dated 2016-11-28. (I'm also working through draft 14 more systematically and hope to post some (mostly minor) additional comments by tomorrow.) > > * array-permute: I don't think I understand the significance of the > > subtlety noted in the explanation (regarding permuting arguments, > > etc.). Is it a result of the convention used for permuting > > intervals, and similar to the "usual" confusion when multiplying > > permutations (cf. Knuth TAOCP 7.2.1.2."A general framework"), or > > something else? I suppose it doesn't really matter for the > > purpose of the SRFI, but I got the feeling I am missing some point > > here. > > Many mathematics textbooks specify a permutation by giving its range, > with the domain elements implicit, so the permutation function 1->2, > 2->1, 3->4, 4->5, and 5->3, or the set of ordered pairs (listed in order > of the first element of the pairs, i.e., the domain element) > $$ > \{(1,2),(2,1),(3,4),(4,5),(5,3))\} > $$ > is written as > $$ > (2 1 4 5 3). > $$ > > It didn't seem natural to me to work with permutations in this way, so > this SRFI we list the pairs in increasing order of the second elements > of the pairs (the range): > $$ > \{(2,1),(1,2),(5,3),(3,4),(4,5)\} > $$ > and then I specify the first elements, with the second elements implicit: > $$ > (2 1 5 3 4) > $$ > I didn't want to say too much about what I don't do, and rather spend > the time explaining what I *do* do. > > I can't find the word "framework" in TAOCP 7.2.1.2 from > > http://www.cs.utsa.edu/~wagner/knuth/fasc2b.pdf I think this is very similar to the issue addressed in Knuth's discussion of permutations. It appears on pages numbered (printed) 8 and 9 in the above copy (PDF page numbers 12, 13). Searching for 'framework' indeed gives no results; something to do with the PDF encoding perhaps. I am not sure if using the other convention for applying a permutation to a sequence (array/interval indices in the present case) would help any (probably not, as it would just reverse the problem). The main issue seems to be that the semantics of array-permute require the ability to 'apply' both the given permutation and its inverse, which I guess is a bit awkward implementation-wise. > > * (minor, formatting) Some (pseudo)code in the array-permute section > > seem to be missing 'pre' tags or some such. > > Yeah, sorry, I can't mix mathjax $\pi$ with <pre>, so I'm stuck with > this formatting. Still minor, of course, but for what it's worth, given the either-or situation, my preference would be formatting without mathjax. It's probably just a reflection of my difficulty with reading unindented code, though. > > * array-body: Is the returned object the same as the object created > > by an application of the maker argument to the storage-class of > > the array? (A clarification near its definition may help; I found > > things hard to follow.) > > It's *an* "object created by an application of the maker argument to the > storage-class of the array". But after a specialized array has been > curried, permuted, and translated, the array elements themselves might > occupy only a small subset of the body, and be stored in a rather > strange order. > > Do you think something needs to be said? I think so. The new draft (14) doesn't seem to explicitly define array-body explicitly at all, unless I missed it somehow. Sorry again for the very late follow-up, Regards, -chaw