I wrote a bit about function objects here. However, if you don't buy that the persistent state of function objects provides something that anonymous functions cannot, how about this: readability. In some cases.
Anonymous functions are boss and cool, and extremely common in idiomatic Ruby. However, in some cases they can get a little... esoteric. Consider:
people.sort do |lhs, rhs|
lhs, rhs = rhs, lhs if ascending?
result = lhs.name <=> rhs.name
if result == 0
result = lhs.date_of_birth <=> rhs.date_of_birth
end
# etc...
end
Sometimes, anonymity isn't the answer. Consider:
class ByNameAscending
def self.to_proc
Proc.new { |lhs, rhs| rhs.name <=> lhs.name }
end
end
This allows you to write this:
people.sort(&ByNameAscending)
Or, to push the example to the extreme:
class SortOrder
def initialize(direction = :descending)
@direction = direction
end
def by(attribute)
attributes << attribute
self
end
alias_method :and, :by
def to_proc
Proc.new do |lhs, rhs|
lhs, rhs = rhs, lhs if ascending?
return lhs <=> rhs if attributes.empty?
attributes.each do |attribute|
result = lhs.send(attribute) <=> rhs.send(attribute)
return result if result != 0
end
0
end
end
private
def attributes
@attributes ||= []
end
def ascending?
@direction == :ascending
end
end
def ascending; SortOrder.new(:ascending); end
Which gives us:
people.sort(&ascending.by(:name).and(:date_of_birth))
A DSL for generating sort order function objects. It could be useful.
i really like the syntax of people.sort(&ascending.by(:name).and(:date_of_birth))
Did you consider using sort_by? It would be faster and easier to write.
Very cool
You're absolutely right that #sort_by would simplify this function. To be honest, I tried to come up with an example with a fair bit of complexity in the proc in order to illustrate the readability improvements of using a function object, and it ended up being a bit contrived. A function that does something to circumvent #<=> would have been better, although I can't think of anything even vaguely reasonable at the moment.
And, thinking about it just for a moment, I can't think of a good way to make the syntax read as nicely with #sort_by.
I'm bad at examples, but hopefully the general idea makes sense. I'm always interested to know of any real-world instances where this sort of thing turned up useful.
Or
instead of
It demonstrates that you can sort on multiple attributes by slapping them in an array.
It's probably worse than the original, but it is one line of code. :)