Hi all,
I'm a 'ruby nubie' and otherwise unfamiliar with the ':eval' technique
it employs.
Are there any decent reference guides to it's use?
Alternatively, can anyone give me a quick explanation of what, when and
how to use it?
This for example, from an ajax scaffold im working on, makes little
sense to me (it doesn't help that it's also Saturday afternoonr!)
:eval => "detail.bookings.collect{ |booking| booking.date }.join(', ')"
Many thanks,
Jon.
There's actually several different things going on, so I'll give my best explanation one by one.
First, names prefixed with a colon, like :eval, are symbols. If you're familiar with Lisp, then that's good, because they're quite similar to Lisp symbols. If you're not familiar with symbols in Lisp, then a quick search of the mailing list archives should provide plenty of explanations for symbols. For now, though, you can consider them as a literal value, a little like an integer (e.g., 12) but without all the arithmetic, basically, they have a unique value in and of themselves, and can be compared (:foo == :foo, :foo != :bar).
Next, you've got a hash literal - hash literals don't always need the enclosing {}. The symbol is being used as a key in the hash, which is a very common use of symbols. For example:
p :foo => "bar", :moose => "squirrel"
{:foo=>"bar", :moose=>"squirrel"}
So what you have is a hash whose key is the symbol :eval, and whose value is a string of Ruby code that looks a lot like something to do with ActiveRecord. Perhaps the key to the puzzle is that there's an eval method in Ruby which takes a string and interprets it as Ruby code, for example:
eval('p :foo => "bar", :moose => "squirrel"')
Will get the same result as the example above.
What I suspect is going on is that the hash is being used to (perhaps optionally) provide a snippet of Ruby code to be executed later in the program.
Symbols and hash literals creep up all over the place in Ruby code, so there's no particular advice I could give about when it's appropriate to use them. Eval is somewhat trickier - it and its cousins instance_eval and class_eval which are used extensively in metaprogramming in Ruby, and it's obviously handy to be able to pass code snippets around as strings in some tasks. They do have their pitfalls, though. Again, a search of the mailing list should give interesting discussions about when where and how to use eval.
Hope that's of some help.
matthew smillie.
On Aug 19, 2006, at 13:40, Jon wrote:
Hi all,
I'm a 'ruby nubie' and otherwise unfamiliar with the ':eval' technique
it employs.Are there any decent reference guides to it's use?
Alternatively, can anyone give me a quick explanation of what, when and
how to use it?This for example, from an ajax scaffold im working on, makes little
sense to me (it doesn't help that it's also Saturday afternoonr!):eval => "detail.bookings.collect{ |booking| booking.date }.join(', ')"
Jon wrote:
Hi all,
I'm a 'ruby nubie' and otherwise unfamiliar with the ':eval' technique
it employs.Are there any decent reference guides to it's use?
Alternatively, can anyone give me a quick explanation of what, when and
how to use it?This for example, from an ajax scaffold im working on, makes little
sense to me (it doesn't help that it's also Saturday afternoonr!):eval => "detail.bookings.collect{ |booking| booking.date }.join(', ')"
Many thanks,
Jon.
You use eval with ajaxscaffold when you want to supply more complicated formula to display a cell value.
In simpler case you just use field name e.g.
AjaxScaffold::ScaffoldColumn.new(self, { :name => "some_field",:label => "some_label".r})
Then ajaxscaffold can use just send method to obtain field value.
When you use associations then case is more complicated:
AjaxScaffold::ScaffoldColumn.new(self, { :name => "some_field_id",:label => "some_label".r, :eval => "some_model.some_field.some_property" , :filter => 'some_filter'})
and ajax scaffold must use eval to obtain a value (it is slower, but in ruby 1.8.5 seems to be quite fast)
the "detail.bookings.collect{ |booking| booking.date }.join(', ')" is quite complex so eval must be used
look for <% column_value = eval(scaffold_column.eval) rescue %>
in your generated ajaxscaffold code.
In simpler cases like "some_model.some_field.some_property" you could do a send trick:
def send_eval s
s.split('.').inject(self){|a,b| a.send(b.to_sym)}
end
but it seems that ruby's eval is a lot faster:
class A
attr_accessor :a
def x
"foo"
end
end
class B
attr_accessor :a
def initialize
@a=A.new
end
end
class C
attr_accessor :b
def initialize
@b=B.new
end
end
$c=C.new
def c
$c
end
def send_eval s
s.split('.').inject(self){|a,b| a.send(b.to_sym)}
end
s="c.b.a.x"
n = 10000
require 'benchmark'
Benchmark.bm do |x|
x.report("eval") { n.times{ eval s } }
x.report("send_eval") { n.times{ send_eval s } }
end
here are the results:
ruby 1.8.4:
user system total real
eval 0.219000 0.000000 0.219000 ( 0.219000)
send_eval 0.343000 0.000000 0.343000 ( 0.359000)
ruby 1.8.5 preview4
user system total real
eval 0.109000 0.000000 0.109000 ( 0.117000)
send_eval 0.375000 0.000000 0.375000 ( 0.374000)
lopex
Thanks guys, some outstandingly useful and pertinent advice.
I know it's a common complaint with Ruby, but is there no online or
offline documentation regarding eval?
Cheers again,
Jon.
This is a very good description of symbols:
http://onestepback.org/index.cgi/Tech/Ruby/SymbolsAreNotImmutableStrings.red
James Edward Gray II
On Aug 19, 2006, at 9:57 AM, Matthew Smillie wrote:
First, names prefixed with a colon, like :eval, are symbols. If you're familiar with Lisp, then that's good, because they're quite similar to Lisp symbols. If you're not familiar with symbols in Lisp, then a quick search of the mailing list archives should provide plenty of explanations for symbols.
Jon wrote:
Thanks guys, some outstandingly useful and pertinent advice.
I know it's a common complaint with Ruby, but is there no online or
offline documentation regarding eval?
Check http://ruby-doc.org/core/classes/Kernel.src/M003216.html\. No idea what context your string's getting evaluated in, though... That'd be ajaxscaffold-specific.
--
Alex
The book Ruby for Rails has a section that explains and compares the various forms of evaluating Ruby source code (eval(), instance_eval(), class_eval()/module_eval()). It's very well done.
James Edward Gray II
On Aug 19, 2006, at 11:00 AM, Jon wrote:
I know it's a common complaint with Ruby, but is there no online or
offline documentation regarding eval?