Rakudo: Smartmatch with Any and Pair tries to call a non-existent method on Any

[Altai-man]

The Problem

> Any ~~ (10 => 'ten');
No such method '10' for invocant of type 'Any'
  in block <unit> at <unknown file> line 1

Also https://colabti.org/irclogger/irclogger_log/perl6?date=2018-12-02#l617

Expected Behavior

> Any ~~ (10 => 'ten');
False

Actual Behavior

No such method '10', which is... Odd.

Environment

• Operating system: Linux miskatonic 4.18.19_1 #1 SMP PREEMPT Wed Nov 14 23:03:54 UTC 2018 x86_64 GNU/Linux
• Compiler version (perl6 -v):

➜  ~ perl6 -v
This is Rakudo version 2018.10 built on MoarVM version 2018.10
implementing Perl 6.c.

[jstuder-gh]

This looks to be proper functionality to me, although really very awkward in this explicit key => pair situation.

As seen here, when smartmatching an object to a Pair it will call the "key" method and compare truthiness of the method's result with that of the Pair's value.

multi method ACCEPTS(Pair:D: Mu $other) {
    $other."$!key"().Bool === $!value.Bool
}

It's really useful when used with an implicit colon-pair.

'/path/to/my/file.ext'.IO ~~ :e

OR

my @a = lazy 1, 2, 3;
say @a ~~ :lazy   # OUTPUT: True
say @a ~~ :!lazy  # OUTPUT: False

In these explicit pair instances it definitely comes across as awkward and a bit confusing, I think. Maybe this is a case where a more awesome error message would be a good idea? I'm not sure what it would say exactly though.

[zoffixznet]

One argument: in the language, we do follow the principle that smartmatching is never meant to throw and this behaviour clearly violates it.

Another argument: since the behaviour is defined in terms of calling a method, I think returning any value, and especially False is a sizeable footgun. Above, $file ~~ :e was chosen as an example. Imagine a code, where this check guards some serious operation, such as purging some data, except you have a bug in your code and $file isn't an IO::Path object, but just a Str, which does NOT have method .e. Currently, such a bug will be quickly discovered, but if we'd return False instead, the check will silently fail and the logic will proceed on the assumption the file doesn't exist.

---

more awesome error message

👍

[Altai-man]

although really very awkward in this explicit key => pair situation

Well, it is a golfed example. Real example is that I have a hash of pairs and when get something, I want to check if it was in this Hash or not, or I just got Any. The issue is that it can be not only pair, but a type object too, so I cannot use .defined, because such values would fail. So I was going to match Any ~~ $maybe-pair-or-a-type-object to check. I already rewrote it with :exists check though.

Now I see why it tries a call, and I have never used ~~ :e like checks before, yet it looks usable. Searching if method exists on left side would be an expensive operation, I guess.

There is no issue from me now, so it can be even closed, I think, it is my own fault to be trying to avoid :exists usage.

[jonathanstowe]

Still a less than awesome error message, Is it worth catching the "No such method" and printing something a bit more self-explanatory?

[lizmat]

Added improved error message with b6c8f5ed19 , so closing this ticket.