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Postings from December 2019
Dave Mitchell wrote: > copy ro # no P6 equivalent, and not very useful? Slightly useful: unlike a ro alias, its value couldn't change in a way that the sub doesn't know about. >Placeholder direct alias parameters are forbidden. There's no (*$, *@). I don't see a good reason to forbid these. >Now for the bikeshedding about what syntax to use for direct aliasing. >Zefram tentatively proposed using $x\, whereas I propose *$x. I'm fairly happy with the prefix "*". I do find it clearer than the postfix "\". However, I don't find any of your specific arguments for it meritorious. In particular: >* By having a single syntactical slot to indicate aliasing, that slot can > populated with only one of two chars to indicate which type of aliasing > is wanted (\$x and *$x). With two slots there's ambiguity: what does > \$x\ mean? "\$x\" would of course be prohibited, just as "*\$x" or "\*$x" would be prohibited. I don't find either syntax for both kinds of aliasing at once to be more or less inviting than the other. With "\*$x" actually being legal (and useful) expression syntax, it really doesn't look as though "\" or "*" syntactically precludes the other. >* It has a loose mnemonic association with aliasing via typeglob > assignment: *foo = ....;. I find this association to be an argument *against* prefix "*". The usage you're proposing has nothing at all to do with typeglobs, so the association with the glob sigil is misleading. It also might interfere a bit with any future work to process actual typeglobs in signatures. (It's quite feasible to add a facility for lexical glob names in the future, and there's motivation in that I/O handles are very often wrapped in globs.) >* It has a loose association with Perl 6's array flattening syntax, *@a, This association, too, is misleading, because the facility is semantically very different. >There is the question of default behaviour. In Perl 5 currently the >default is to copy, while in Perl 6 the default is a read-only alias. The default should remain copying. Read-only aliasing is nicer behaviour, but a Perl 5 :ro alias (as you described it) is not, because of the inability to then pass the variable as a subroutine argument. Writable aliasing would also be surprising behaviour, because it is customary to copy arguments, and therefore writing to argument-derived variables normally doesn't write to the arguments. >possibility is to make aliasing the default in the presence of :ro. Too confusing. Until now, ":ro" has been an attribute that gets orthogonally composed with other signature features. Don't break the orthogonality. -zefram