Sort Gotcha

sort-gotcha.md

Sort Gotcha

> <matthew mark luke john>.unique(as => *.chars).sort(by => *.chars)
(mark matthew)

Ahh good, my code works... except it's not really sorting by chars here because unlike unique, sort takes a code block not a named parameter. However there is no error to indicate this is not DWIM. It just silently sorts Naturally.

If your data happens to sort Naturally in the same order your by option would have sorted it - and you don't notice - you now have a bug laying dormant in your code that could go unnoticed indefinitely.

Sort should either handle this syntax, or otherwise throw up a warning;

I brought this up on #perl6 on 2016-04-10 and 2016-04-11 but no solution proposed.

This has confused another person here

moritz said

back in the days, we allowed calling positional arguments by name
during these bad old days, calling it with by => ... would have worked

Maybe unique needs a multi that accepts a positional &as, considering it's used way more than :&with, and it would make it more consistent with other List-y methods like sort, map, grep, classify, etc.

> < alpha beta gamma delta epsilon >.unique(as => *.chars)
(alpha beta epsilon)
> < alpha beta gamma delta epsilon >.unique(*.chars)
Cannot resolve caller unique(List: WhateverCode); none of these signatures match:
    ($: *%_)
    ($: :&as!, :&with!, *%_)
    ($: :&as!, *%_)
    ($: :&with!, *%_)
  in block <unit> at <unknown file> line 1