crystal-lang
Allow array destructuring to produce nil values
Didn't know how best to phrase this. Currently if you want to destructure an array you can do so like this:
arr = [1, 2, 3]
one, two, three = arr
and it works fine. However, a problem is posed when you want to destructure an array that might be empty, or might have less values than you're trying to assign. This happens a lot with String.split. For instance:
time = "1/2/30"
days, hours, minutes = time.split('/') # This will work
time = "1/2"
days, hours, minutes = time.split('/') # This produces an Overflow Error
Maybe there's something I'm missing?
@watzon a minor correction, it produces an Index out of bounds (IndexError) and not an Overflow Error
I propose this gets re-opened. The original issue got a little off topic with workaround suggestions.
The core issue brought up here is that split cannot be reliably used with multi-assign. I just ran into this myself and trying to determine the cause of the error was difficult.
As it stands, x, y = str.split(“ “) may or may not cause a runtime error.
I agree. This is something I deal with all the time and it would be great to have a solution.
ary = value.split x, y = ary[0], ary[1]?
So this is already possible, just not a one liner. Like most things :-)
The one-liner for a split between two components is x, _, y = str.partition(" "). It doesn't even need to allocate an array.
That doesn't work for more than two components, though. Maybe that could be improved. But I think the huge problem with that is that all values become nilable. So you may get a convenient destructuring line, but then you have to deal with nilable types afterwards. And that should be avoided. Especially considering there are means to ensure the arity matches.
I think the huge problem with that is that all values become nilable. So you may get a convenient destructuring line, but then you have to deal with nilable types afterwards
That's the whole point of this issue. That's the feature, not the problem.
Yes, it's a feature when you can't be sure of the number of elements. But when you've validated that, it's a problem.
It would be really annoying if the destructured variables would be nilable in this example:
time = "1/2/30"
time_components = time.split('/')
raise "Invalid format" unless time_components.size == 3
days, hours, minutes = time_components
Maybe I didn't make myself clear, but the idea is not to change how days, hours, minutes = time_components work but, as I proposed in #2617 to be able to translate some to []?.
Your code snippet could be:
time = "1/2/30"
days, hours?, minutes? = time.split('/', 3)
raise "Invalid format" unless hours && minutes
I like that proposal @asterite. It still allows for the expressive one-liner but adds some caution for the developer.
I understand the overall desire to avoid introducing nils everywhere, but this kind of usecase is more along the lines of pattern matching. Looking through it from that lens I think makes it ok to introduce nils and forcing the developer to check. This already can return nil for example:
"foo".match(/bar/) # => nil
If this change is something people don't want, though, I would propose perhaps dis-allowing multi-assignment for something like split.
I'm implementing something like the Redis protocol and so I have a lot of strings like "CMD arg1 arg2". It was working great for commands I tested with arguments, but as soon as I introduced the first command without args I got Index out of bounds (IndexError).
To be honest, I don't think we'll do anything about this. You get an index error whole changing code and that's totally fine.
I'm implementing something like the Redis protocol and so I have a lot of strings like "CMD arg1 arg2".
String#split etc. are convenient but not very great for performance-sensitive use cases. For that an efficient custom parser would do much better.
With #10410 and #11145, the situation will be as follows:
days, hours, minutes = "1/2/30".split('/') # okay
days, hours, minutes = "1/2".split('/') # Unhandled exception: Multiple assignment count mismatch
days, hours, minutes = "1/2/3/4".split('/') # Unhandled exception: Multiple assignment count mismatch
days, *hours_minutes = "1/2/30".split('/') # okay, hours_minutes == %w(2 30)
days, *hours_minutes = "1/2".split('/') # okay, hours_minutes == %w(2)
days, *hours_minutes = "1".split('/') # okay, hours_minutes == [] of String
days, *hours_minutes = "1/2/3/4".split('/') # okay, hours_minutes == %w(2 3 4)
# extra trailing elements are ignored, similar to current behaviour
# (these assignments wouldn't call `#[](3..)` on the intermediate result)
days, hours, minutes, *_ = "1/2/30".split('/') # okay
days, hours, minutes, *_ = "1/2".split('/') # Unhandled exception: Multiple assignment count mismatch
days, hours, minutes, *_ = "1/2/3/4".split('/') # okay
I faced this problem:
Writing this:
array = [] of String
*first_elements, last_element = array
p first_elements
p last_element
Will raise an exception:
Unhandled exception: Index out of bounds (IndexError)
from /usr/lib/crystal/indexable.cr:73:20 in '[]'
from eval:1:5 in '__crystal_main'
from /usr/lib/crystal/crystal/main.cr:115:5 in 'main_user_code'
from /usr/lib/crystal/crystal/main.cr:101:7 in 'main'
from /usr/lib/crystal/crystal/main.cr:127:3 in 'main'
from /usr/lib/libc.so.6 in '??'
from /usr/lib/libc.so.6 in '__libc_start_main'
from /home/crystal/.cache/crystal/crystal-run-eval.tmp in '_start'
from ???
The expected result is:
[]
nil
The workaround to solve is:
array = [] of String
first_elements = array[0..-2]? # => []
last_element = array[-1]? # => nil
Maybe the splat assignment should handle this case for the user.
This issue has been mentioned on Crystal Forum. There might be relevant details there:
https://forum.crystal-lang.org/t/multiple-assignment/6943/2
