This Week on Perl 6 (7 - 13 October 2001)

This Week In Perl 6 (7 - 13 Oct 2001)

Notes

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There were 419 messages across 92 threads, with 74 authors contributing.

NaN is Not a Number, and Not Not a Number

(61 posts) Tim Conrow pointed out an inconsistency between Perl 6's proposed semantics of NaN and the IEEE semantics found in other languages. (The subsequent debate consisted mostly of each side trying to convince the other how Wrong it was.)

The non-identity of NaN makes sense in strongly type languages where a numerical entity can only hold a number. There, the IEEE and mathematical notion of NaN isn't so much that it isn't a number, but that it is a number that just can't be represented. (As an integer, floating point number, or +- infinity.) In this context, there's an infinitely small probability that some non-representable number is numerically equivalent to some other non-representable number, so the non-identity makes sense.

However, for languages (such as Perl) where numerics aren't a strongly-typed unique entity, you've really two NaN properties to consider. You've got, as above, two Nans-That-Really-Are-Numbers-But-Can't-Be-Represented-As-Such, but then you've also got NaNs that aren't numbers because they simply aren't numbers.

In the latter case, it's less of the mathematical notion of NaN, with its associated non-identity, than with non-numeric strings having a generic notion of NaN. (That's "Not A Number", as in, "That's Not a Number, It's a String.") Given that notion, is makes sense for NaNs to have an identity. Two non-numeric strings are equally Not A Number.

The confusion, as you can see, is how to merge these two rather orthogonal definitions of What is Not a Number into one language, and discussion continues.

Numerical Strings

(14 posts) A spin-off of the above thread mused about extending some of Perl's numerical string recognition capabilities (such as 1_000_000 for one million) inside strings themselves, and perhaps extending them to handle standard metric suffixes. Several syntaxes and ramifications were discussed, and a couple apparently clean solutions proposed.

Reduction

(18 posts) John Williams asked whether the hyperoperator ( ^ ) would handle reduction:

I just read Apocalypse and Exegesis 3, and something stuck out at me because of its omission, namely using hyper operators for reduction.

$a ^+= @list; # should sum the elements of @list

Larry says @a ^+ 1 will replicate the scalar value for all a's, and Damian talks about doing summation with the reduce function without mentioning this compact syntax.

So am I overlooking some obvious reason why this won't work?

Damian Conway replied that no, it should indeed work, but would require the lvalue to force reduction, which reduce does not. (Without the lvalue, the scalar value would be extended to match the list.)

Parroty Bits

(16 posts) Dan Sugalski gave a quick rundown on Parrot Magic Cookies (PMCs) and how they work.

(18 posts) There was some debate about jump versus branch, and what each instruction is supposed to accomplish. Dan Sugalski:

Absolute addresses are, well, absolute addresses. Relative addresses are offsets from the current location.

I think the confusion's because the jump opcode's broken. When you say

jump 12

It should jump to absolute address 12, not 12 bytes/words/opcodes from the current position.

Subsequent discussion attempted to discern which absolute address - the absolute "virtual" address, similar to what the branch operator uses, or the real machine address of an operator. The current inclinations are for the latter, as that allows Parrot to jump between code segments.