Tuesday, 31 March 2009

Autovivification in Perl 5

On the Perl 5 Porters mailing list, Graham Barr points at an interesting bit of history.

The question discussed is autovivification of non-existent hash elements when used in an lvalue context. Usually, they are autovivified: (this is with 5.10.0)

$ perl -E '1 for $x{a}; say for keys %x'
$ perl -E 'map $_, $x{a}; say for keys %x'

But not when used as subroutine arguments:

$ perl -E 'sub f{} f($x{a}); say for keys %x; say "END"'

Graham says that sub args were special cased out during 5.005 development, for unclear reasons.

A small bit of inconsistency is that hash slices are autovivified in sub calls:

$ perl -E 'sub f{} f(@x{a}); say for keys %x'

We have no good overview of the current state of autovivification behaviour in Perl 5. It's known that it can change with strictures. The current perltodo notes: Make all autovivification consistent w.r.t LVALUE/RVALUE and strict/no strict. (It's a known bug that strictures can affect autovivification; I wonder whether taintedness can affect this too.)

A good first step towards consistency would be to produce a matrix of autovivification for a bunch of data types (array elements, hash elements, hash slices, tied or not) and a bunch of operators that take lvalues (map, grep, foreach, sub call, chained -> dereferences, etc.) with and without strictures/taint. Then, that could be turned into a regression test, and finally we could tweak the behaviour in 5.12 to make it more consistent.

Want to help?

Friday, 27 March 2009

Polybius at the funeral

Plutarch, in his Life of Philopoemen, mentions that a young man named Polybius was carrying the urn of the general:

They burnt his body, and put the ashes into an urn, and then marched homeward, not as in an ordinary march, but with a kind of solemn pomp, half triumph, half funeral, crowns of victory on their heads, and tears in their eyes, and their captive enemies in fetters by them. Polybius, the general's son, carried the urn, so covered with garlands and ribbons as scarcely to be visible; and the noblest of the Achaeans accompanied him.

Polybius, is, of course, the great historian, and also one of the major sources of Plutarch for the second Punic war and the conquest of the Greece by Rome. But Plutarch does not mention that. He just expects his reader to know who he's talking about.

Do we read here, between the lines, Plutarch's secret regret of not having lived in interesting times -- of not having something original to write on, and of being a mere compiler? Was Plutarch dreaming of being a Thucydides or a Polybius, who, like Clausewitz twenty centuries afterwards, were unlucky officers before becoming great historians?

Or maybe Plutarch, who was a subject of the Roman Caesar, but still Greek and proud of it, didn't want to insist on, but rather allude to, the image of the historian of the downfall of Greece, in his youth, taking part in the funeral of the man who was nicknamed, by the Romans themselves, "the last of the Greeks".

Thursday, 26 March 2009

Sons of a snake

Both Alexander the Great and P. Cornelius Scipio Africanus the Elder were said to be sons of Zeus (respectively Jupiter), the god having taken the shape of a giant snake to impregnate their mothers. It's difficult to judge which part Alexander and Scipio themselves had in the fabrication of those legends. According to Plutarch, Alexander asserted his divine parenthood when he was talking with Asians and Egyptians, but not with Macedonians and Greeks. Scipio never asserted it, but never negated it either. Moreover, Scipio was familiar with the Greek culture, so he might have just copied Alexander's legend for his own political reasons. Also, Alexander's legend might not be completely unrelated to the legend of Buddha's birth, notably the part where his mother dreams about being pregnant of a powerful animal.

The theme of the great general son of a god in the Indo-European history and mythology would be interesting to explore. I don't see what role does the snake symbol play in this system, but it reminds me a bit of the Celtic Melusine.