Another possible trap: %h<a b>:exists

[AlexDaniel]

Not sure if this is a big enough trap, but it just got me, so…

I had code like this:

my %h = <a 1 b 2>;
say ‘hello’ if %h<a>:exists;
say ‘world’ if %h<x>:exists;

Output:

hello

Then I decided to check for more than one element, and using slices is what came to my mind immediately:

my %h = <a 1 b 2>;
say ‘hello’ if %h<a b>:exists;
say ‘world’ if %h<x y>:exists;

Output:

hello
world

Not exactly what I hoped for!

Thinking about it, it makes sense. Still, perhaps it is worth to mention something like this as one of the traps (are there any other traps similar to this one?).

[trosel]

@AlexDaniel Why does that make sense? What happened exactly?

[b2gills]

%h<x y>:exists

returns a list of 2 elements, and that list is True

So you really need something like this

say ‘world’ if all %h<x y>:exists;

[AlexDaniel]

Yes, depending on what you want exactly, you should use any or all.

The fact that it returns the element itself (and not a list with one element) is a possible source for other traps. It makes sense because this is what you mean in most cases, but when you start generalizing you stumble upon completely different behavior.