Feature #12350: Introduce Array#find! that raises an error if element not found

• Status: Open
• Assignee: -
• Target version: -

[ka8725 (Andrey Koleshko)]

It would be great to have some function (like Hash#fetch) for Array that raises an exception if element not found by passed block in find.

For example:

[].find! { |el| el == 1 }
=> ElementError: element not found

I just noticed that there are many cases when it's needed to raise an exception (because this is exceptional situation) if an element not found.

Currently I just define methods like this one:

def find_price!(size)
  found_price = prices.find { |p| p.key == size }
  fail "Price for #{size} not found" unless found_price
  found_price
end

[ka8725 (Andrey Koleshko)]

• Description updated (diff)

[zverok (Victor Shepelev)]

I'm pretty unhappy that community/style guidelines are banning the straightforward and readable solution:

prices.find { |p| p.key == size } or fail "Price for #{size} not found"

[ka8725 (Andrey Koleshko)]

Victor Shepelev wrote:

I'm pretty unhappy that community/style guidelines are banning the straightforward and readable solution:

prices.find { |p| p.key == size } or fail "Price for #{size} not found"

I'm not aware about the banning. Could you elaborate what is bad in your example? Basically I don't see anything bad in your code except a little bit verbose like in mine solution.

[zverok (Victor Shepelev)]

Could you elaborate what is bad in your example? Basically I don't see anything bad in your code except a little bit verbose like in mine solution.

I don't know! The most popular and widely accepted Ruby Style Guide just bans them: https://github.com/bbatsov/ruby-style-guide#no-and-or-or

And when I've tried to rant about how and/or are useful, it all went sideways: https://www.reddit.com/r/ruby/comments/478a17/rant_on_good_ruby_idiom_andor_operators/

[nobu (Nobuyoshi Nakada)]

Victor Shepelev wrote:

Could you elaborate what is bad in your example? Basically I don't see anything bad in your code except a little bit verbose like in mine solution.

I don't know! The most popular and widely accepted Ruby Style Guide just bans them: https://github.com/bbatsov/ruby-style-guide#no-and-or-or

I don't think it makes a sense, however, you can use || instead if you don't like or of course.

prices.find { |p| p.key == size } || raise "Price for #{size} not found"

And it feels better to me because of the error message.

[cremno (cremno phobia)]

You can pass a callable object to Enumerable#find:

def find_price!(size)
  prices.find(->{ fail "Price for #{size} not found" }){ |p| p.key == size }
end

[dsferreira (Daniel Ferreira)]

Documentation can be improved to use example of callable object don't you think?
I'm happy to create a PR with it.

[matz (Yukihiro Matsumoto)]

I am not against adding a method that raises an exception, but find! is not a proper name for it.
Besides that, we can raise an exception for non-existing values by the current behavior.
So let us start improving documentation. Daniel, could you create an PR?

Matz.