How does something like ...sort_by(&:f) actually manage to function?

Rails
1

I’ve seen a claim on the web that &:f is just Ruby shorthand for &proc
{ |i| i.f }, but I’ve certainly never been able to get this &:f
notation to work in standard ruby.

Thanks,
Ken

2

It is not possible in “plain” ruby because Rails extends the Symbol
class with the possibility of converting it into a Proc (that’s what
happens when you precede something with a &). From the Rails source
code:

unless :to_proc.respond_to?(:to_proc)
class Symbol
# Turns the symbol into a simple proc, which is especially useful
for enumerations. Examples:
#
# # The same as people.collect { |p| p.name }
# people.collect(&:name)
#
# # The same as people.select { |p| p.manager? }.collect { |p|
p.salary }
# people.select(&:manager?).collect(&:salary)
def to_proc
Proc.new { |*args| args.shift.send(self, *args) }
end
end
end

And it does come very handy indeed.
Balint

On Feb 6, 1:07 am, Kenneth McDonald [email protected]

3

On 6 Feb 2009, at 00:13, [email protected] wrote:

It is not possible in “plain” ruby because Rails extends the Symbol
class with the possibility of converting it into a Proc (that’s what
happens when you precede something with a &). From the Rails source
code:

Although it’s worth noting that Symbol#to_proc is in newer versions of
ruby (1.9 and 1.8.7 IIRC)

Fred

4

Thank you!

Ken

5

OK, I see how this works now, but I can’t figure out how to get extra
args in
there, i.e. something like [1, 2, 3].collect(&:modulo, 2) doesn’t work
because
it’s improper Ruby syntax. I assume there’s a reason extra args are
allowed,
so could someone give a brief illustration of the calling convention?

Thanks in advance,
Ken

6

On 6 Feb 2009, at 15:58, Kenneth McDonald wrote:

OK, I see how this works now, but I can’t figure out how to get extra
args in
there, i.e. something like [1, 2, 3].collect(&:modulo, 2) doesn’t work
because
it’s improper Ruby syntax. I assume there’s a reason extra args are
allowed,
so could someone give a brief illustration of the calling convention?

I don’t think you can - you just have to use the longhand syntax in
those cases.

Fred

7

The first of the parameters passed to the block is the object that is
sent the message designated by the symbol. (Or in other words, the
method designated by the symbol is called on the object that is the
first parameter).

So people.collect(&:name) works because the collect iterator yields a
member of the people enumerable at each pass. Plus that object needs
to have name method. The important part to understand is that any
extra parameters the iterator yields to the block will be passed to
the method. In the above case, there is none, because collect only
yields one member, nothing more. I’ll give you an (very-convoluted)
example because it is hard to understand without one:

RND_TOP = 10
class Array
def select_with_random
selected = []
self.each do |e|
selected << e if yield e, rand(RND_TOP)
end
selected
end
end

numbers = Array.new(5) { rand(RND_TOP) }
above = numbers.select_with_random(&:>)
puts “numbers that are above another random number: #{above.inspect}}”

So I added a select iterator to the Fixnum class which not only yields
an element but also a random number. That random number is the extra
parameter that will be passed to the method designated by the symbol,
in this case, >. So iterating through random numbers, it will select
those that are bigger than another random number (no sense at all, I
know). The > method needs one parameter and it gets it.

I am sure that this can have actual, real world uses, too, I just
could not find out something realistic right now.

Balint

On Feb 6, 5:28 pm, Frederick C. [email protected]