I know Ruby supports a suffix if like:
number = -42 if opposite
but what's the purpose of this? Why would it be used in place of the prefix if statement?
The suffix-style if and unless can also be good for "guard clauses", in the form of:
return if ...
return unless ...
Here's an example:
# suffix-style
def save
return false if invalid?
# go for it
true
end
Versus:
# indented style
def save
if valid?
# go for it
true
else
false
end
end
In the second example, the entire implementation of the method has to be shifted over by one indent due to the valid? check, and we need an extra else clause. With the suffix style, the invalid? check is considered an edge case that we handle and then bail out, and the rest of the method doesn't need an indent or an else clause.
This is sometimes called a "guard clause" and is recommended by the Ruby Style Guide.
It can make the code easier to read in some cases. I find this to be true especially in the case of unless, where you have some action you usually want to perform:
number = -42 unless some_unusual_circumstance_holds
Once you have it for unless, for symmetry it makes sense to support it for if as well.
number = -42 if opposite
is the same as
if opposite
number = -42
end
Some people prefer the one-liner for readability reasons. Imagine a line like:
process_payment if order_fulfilled?
Doesn't that read nice?
Postfix style does not have the else section. It is useful when you only want to do something with one of the two cases divided by the condition and don't want to mess with the other case.
It's the same as prefix but shorter. The only reason is to save vertical space in the text editor.
Related
I've got constructs like the following that Rubocop complains about that I'm trying to appease
unless foo && !foo.empty? ...
(item exists and is not empty)
and
bar && !bar.positive?
(item exists and is not positive)
In both cases, Rubocop throws a Style/SafeNavigation and says I should use &. instead. However, there does not seem to be a way to write it.
In both cases part of the problem is that there is no concise way of expressing the opposite function - there is no "not empty" for strings, and no "negative or zero" for numbers. Writing bar&.negative? returns the opposite result for zero, for example.
In the other case, writing unless !foo&.empty? has Rubocop complain about unless+a negation, but again there's no way to rewrite it as an if foo&. with any operation that exists.
(btw this is straight Ruby, no Rails, so I don't have blank? and present?)
This:
next unless foo && !foo.empty?
Could be replaced with:
next if foo.nil? || foo.empty?
And similarly, this:
next unless bar && !bar.positive?
Could be replaced with:
next if bar.nil? || bar.positive?
The real code smell here, in my opinion, was the next unless ... not. The double negative indicates that there's probably a cleaner way to write it.
This is an oversight in RuboCop, because the suggested correction can not be guaranteed to have the same semantics. It will be fixed in the upcoming 0.50.1 patch, and the cop will ignore cases with negation.
In the meanwhile, if you're dealing with conditions where your variable could be nil, one option is to use the fact that nil responds to the coercion methods. Example, assuming foo is either an array or nil:
# With nil check
unless foo && !foo.empty?
# With array coercion
if foo.to_a.empty?
I have to use String.scan function, which returns empty array if there is no match.
I wanted to assign a variable with the scan function and check it there is a match, but unfortunately I cannot do that because it won't return nil or false on no match.
I wanted to do this (1 line):
if ip = str.scan(/\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}/)
...
#use ip
end
but because it won't return nil on no match I must do:
ip_match = str.scan(/\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}/)
unless ip_match.empty?
#use ip
end
Is there some more elegant way to write this - to be able to do assignment and empty check at the same time or some other way to beautify the code?
Thanks
Since scan returns an array, and even if you are sure there would be only one result, you could do this.
str.scan(/\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}/).each do |ip|
#use ip
end
There's a difference between elegant and cryptic or "concise".
In Perl you'll often see people write something equivalent to:
if (!(ip = str.scan(/\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}/)).empty?)
It's a bit more concise, terse, tight, whatever you want to call it. It also leads to maintenance issues because of the = (equate) vs. what should normally be an equality test. If the code is passed to someone who doesn't understand the logic, they might mistakenly "correct" that, and then break the code.
In Ruby it's idiomatic to not use equate in a conditional test, because of the maintenance issue, and instead use the assignment followed by a test. It's clearer code.
Personally, I prefer to not use unless in that sort of situation. It's an ongoing discussion whether unless helps generate more understandable code; I prefer if (!ip_match.empty?) because it reads more like we'd normally talk -- I seldom start a statement with unless in conversation. Your mileage might vary.
I would preferably do something like this using String helper match
ip_validator = /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/
# match return nil if no match
if str.match ip_validator
# blah blah blah.....
end
help me keep code dry and clean.
May be this is not the most elegant , looking for others if any :)
Your ip_validator regex seems to be week check this out Rails 3: Validate IP String
For instance, this takes 4 lines which is too much space for such a simple operation:
if something_is_true
puts 'error'
return
end
this one is a one-liner but looks awkward.
if something_is_true; puts 'error'; return; end
Can we do something like
# it would be great if this would work because it is short and readable
puts 'error' and return if something_is_true
I'm not sure why you think space is at such a premium that your original code is "too much." Give your code room to breathe, and let it take the space it needs. Getting too tricky in order to "save space" is a false economy. The most important thing is that your code be readable and understandable. Your original code looks great to me.
I agree with #NedBatchelder that your original code is probably best. Others have pointed out that in your particular example, you can use return puts 'error'.
Still, for the sake of learning, you can group multiple statements with parentheses, and therefore use a small number of statements in places where you could otherwise use only one. You said:
# it would be great if this would work because it is short and readable
puts 'error' and return if something_is_true
You can do this with:
(puts 'error'; return) if something_is_true
This is kind of awful but I think it will work because puts returns nil:
puts 'error' || return if something_else
In this specific case, return without a value will return nil; as this happens to also be the return value of puts, you can get the same effect with just:
return puts "error" if something_else
Someday you will probably care less about how many cycles you can spend in a single line of code. I would use the if-end block because it is simple, clear, and... you know, that's what it's there for.
I'd recommend never to use ; to join statements, it tends to be unreadable. But still there are other approaches, two ideas: first one, join error(msg, return_value = nil) and return:
return(error("Message")) if something_is_true
return(error("Message", value_to_be_returned)) if something_is_true
Second one, in Ruby it's idiomatic to signal problems using exceptions, so you can write this perfectly idiomatic one-liner:
raise MyException.new("human explanation of the error") if condition
The same idea used in an assignment:
link = doc.at_css(".content a.link") or raise MyException.new("msg")
I apologize if this question is answered somewhere but I'm not positive I'm phrasing it right for Google, and I haven't seen it in any style guides.
Since Ruby has multiple ways to show negativity in a conditional, what is the preferred way to write a conditional that is checking that one part is true and one part is false? Example:
if array && !array.include?('Bob')
#do stuff!
But you could also say:
if array
#do stuff! unless array.include?('Bob')
or:
if array && not array.include?('Bob')
#do stuff
or:
if !array.nil? && !array.include?('Bob')
or a wacky double unless:
unless array.nil?
#do stuff unless array.include?('Bob')
And several others. Any idea which is considered the most Rubyish? Sources to back your opinion up?
As far as documented guidelines, the only thing that i can think of is the Google guide that admonishes "don't use and and or; always use && and || instead.".
Other than that, it somewhat depends on the context. If all you have is code to be executed if one condition is true and the other false, then I would definitely put them in a single if with && !:
if array && !array.include?('Bob')
#do stuff!
On the other hand, you might have additonal code that gets executed if the first condition is true even if the second one is also true; in that case, the nested unless or if makes sense:
if array
do stuff! unless array.include? 'Bob'
do other stuff anyway
end
Does not improve readability, but still interesting.
If you use Rails you can use ActiveSupport try extension like this:
if array.try(:include?, 'Bob')
# Do stuff
In order to write more concisely, rather than do this:
test_value = method_call_that_might_return_nil()
if test_value
do_something_with test_value
end
I've been assigning in the conditional:
if test_value = method_call_that_might_return_nil()
do_something_with test_value
end
Is this bad style? The still-more-concise syntax:
do_something_with test_value if test_value = method_call_that_might_return_nil()
is not allowed, as discussed in another SO question, and will remain that way in 1.9, according to Matz (http://redmine.ruby-lang.org/issues/show/1141).
Given the possible confusion of assignment and comparison, does this make it too hard to read the code?
It is GOOD style to use assignments in conditionals. If you do so, wrap the condition in parentheses.
# bad (+ a warning)
if v = array.grep(/foo/)
do_something(v)
# some code
end
# good (MRI would still complain, but RuboCop won't)
if (v = array.grep(/foo/))
do_something(v)
# some code
end
# good
v = array.grep(/foo/)
if v
do_something(v)
# some code
end
See the community style guide for more information
One somewhat widespread idiom is to use and, which would look something like this:
tmp = method_call_that_might_return_nil and do_something_with tmp
Another possibility would be to call #nil? explicitly, that way the intent becomes a little bit clearer; in particular it is really obvious that you actually meant to assign instead of compare:
unless (tmp = method_call_that_might_return_nil).nil?
do_something_with tmp
end
Concise code is not necessarily better code. Concision is useful when it improves the communication of intended code behavior from author to future maintainers. I think enough of us come from backgrounds in which we've had accidental assignments in if blocks (when we meant to have an equality comparison) that we prefer styles in which it's absolutely clear that assignment is meant, rather than comparison. The .nil? idiom already mentioned has that property, and I'd consider it cleaner than having the bare assignment inside the if condition. Really, though, I don't see the harm in having the extra line of code for the assignment.
The functional-programming way to do this is to use andand. It's a readable way of chaining method calls so that a nil in the middle stops the chain. So your example would be something like:
method_call_that_might_return_nil.andand.tap {|obj| do_something_with obj}
## or, in the common case: ##
method_call_that_might_return_nil.andand.do_something
Yeah, I would say it's bad style due to the possible confusion between assignment and comparison. It's only one more line to assign and then test, and it avoids having someone in the future think that was a bug and patch it to use == instead.
C programmers do this a lot. I don't see a problem with it in Ruby either so long as it's clear what's happening.
I think it's fine. Aversion to assignment in a condition comes from knowing that a missed key stroke when typing == turns a comparison into an unintended assignment. A stylistic prohibition on using assignment in a condition makes such accidents stand out like to the eye (and sometimes to the language, as in C, where many compilers can be made to emit a warning if they encounter an assignment in a condition). On the other hand, tests also make such accidents stand out. If your code is well covered by tests, you can consider discarding such prohibitions.
Due to the warning, performing the assignment in the if clause has a quite pungent smell. If you do have an else case to handle then the case ... in ... pattern matching can offer something:
case method_call_that_might_return_nil
in nil
# handle nil case
in test_value # pattern match to a new variable
# handle non-nil case with return value of method assigned to test_value
end
Or...
case method_call_that_might_return_nil
in test_value if test_value # pattern match to a new variable if not nil
# handle non-nil case with return value of method assigned to test_value
else
# handle nil case
end