peteg's blog 2007-09-15-UnicodeNumerals.autumn

Unicode Numerals.

OK, take a deep breath. Look at this page and tell me the world isn't crazy.

Say you want to talk to the world in Unicode, but you want to do it quickly. Well, obviously you're going to draft C's atoi and friends to convert numerals to your internal integer type, right? That's great in theory, but when your code is running on someone else's webserver that you know little about, things might get a little tricky.

Haskell's FFI specifies that the functions in the CString module are subject to the current locale, which renders them unpredictable on the hitherto mentioned webserver. I can imagine a numeral encoding that e.g. strtol_l understands with the locale setting of today that it fails to understand tomorrow. I don't think there are enough manpages in all the world to clarify this problem.

Solution? Use integers only for internal purposes, like user identifiers, render them in ASCII, and use Unicode strings for everything else. Don't use the CString module, carefully unpack UTF-8 ByteStrings into Haskell Strings, and don't expect warp speed. If you're (cough) putting this stuff in a library, hope like hell your users don't try anything too weird.

One day someone will resolve all the issues of implementing a proper Unicode I/O layer, and I will thank them for it.

Proving insertion sort correct.

Fun and games. All I want to do is convince Isabelle of the proposition that, for a given list of objects, there is a permutation that is sorted according to a given order. In case anyone thinks this is completely trivial, I ask that you try to show that a recursive insertion sort does it before reading this cheat sheet. Perhaps merge sort is easier. Anyway, it is annoying that there appear to be no results on sorting sequences in Isabelle's library.

Trees ain't trees.

Any idiot knows a tree has a branching structure, or at least that's what we've been telling the first years since time immemorial. There was a proper CS tree at UNSW back before I got there that the internet doesn't appear to remember so well. This photo of the "right way up" fig is from Geoff Whale's data structures textbook, and carries the following attribution:

Evidence that trees sometimes really are as shown in data structure textbooks. Sculpture using the medium of a dead fig tree at UNSW. Photo by Russell Bastock.

I reproduce it here without permission. That must be the Applied Science building in the background and my guess is that it sat at the eastern end of where the Red Centre is now.

I spotted this purported "tree" on Centennial Avenue, near Avoca St (which runs between Queen and Centennial parks) several months ago but have only now got around to photographing it. I grant that it appears to be a DAG, and is therefore for most purposes a tree.