This question already has answers here:
Blocks and yields in Ruby
(10 answers)
Closed 9 years ago.
I keep writing the same pattern of code in Ruby, which seems like it would benefit from a 'do' style bit of code but I'm not sure how to write the method.
I keep doing this pattern of code, which starts and ends with the same lines of code...
x.increment!(:step_count) # same each time
# ...then some different code each
x.update_column(:step_description, "blerg message") # same each time
I feel it would benefit from a 'do' something that would look like this...
update_steps "blerg message" do
# ...then some different code each
end
And then inside the 'do' each time it does the common code.
How would I go about making a method where I can use a 'do'.
Thanks!
Edit: I think it's important to not close this because I didn't know to search for 'block' or 'yield'. People who may no know these terms may end up searching for 'do' instead.
Creating methods that accept a block is one of Ruby's most powerful features.
The common way to define such a method would be:
def foo(*args, &block)
# your code here
yield
# some more code
end
foo do
# This code runs when yield is called
end
There are a few things you should know about the above:
The &block parameter is not required. You can just use yield anyway. But there are a few reasons why you should add it to your method definition:
It makes it clear that your method accepts a block
The & basically transforms the block to a proc object. This could be handy since that way you can actually pass it around as a parameter to another method that accepts a block. You just need to re-apply the & to make it again a block.
Handling a proc object can be more powerful since you can also set its binding.
You can pass arguments to yield. The arguments you pass are the block local variables. For example in:
[1,2,3].each {|x| puts x}
yield is called with one of the array elements on every iteration. Calling yield with an argument is the same as block.call(a) where a is an argument.
If your method encounters a yield and there is no block given it will raise an exception. This might be correct in some cases. But if you want to have a different behavior if no block is given you can use the block_given? method to check it.
&block must be the last parameter in your method definition.
Pass it a block as an argument
def my_method(&block)
do_something_the_same
yield # calls whatever is inbetween "do" and "end"
end
Or you can also do this by calling block.call
def update_steps(&block)
block.call()
end
Related
I am new to Ruby and to Rails, and am trying to understand fully what I'm reading.
I am looking at some of the Rails source code, in this case action_controller/metal/instrumentation.rb.
def render(*args)
render_output = nil
self.view_runtime = cleanup_view_runtime do
Benchmark.ms { render_output = super }
end
render_output
end
I understand that *args is using the splat operator to collect the arguments together into an array. But after that, it stops making much sense to me.
I can't fathom why render_output is set to nil before being reassigned to equal super and then called with no arguments. I gather that some speedtest is being done, but coming from other languages I'd expect this to just be something more like Benchmark.ms(render_output) or perhaps Benchmark.start followed by render_output followed by Benchmark.end. I'm having a hard time following the way it works here.
But more importantly, I don't really follow why args isn't used again. Why bother defining a param that isn't used? And I mean, clearly it is getting used-- I just don't see how. There's some hidden mechanism here that I haven't learned about yet.
In this context, it is important to note how super works, because in some cases it passes implicitly arguments and you might not expect that.
When you have method like
def method(argument)
super
end
then super is calling the overridden implementation of method implicitly with the exact same arguments as the current method was called. That means in this example super will actually call super(argument).
Of course, you can still define a method call that explicitly sends other arguments to the original implementation, like in this example:
def method(argument)
super(argument + 1)
end
Another important edge-case is when you want to explicitly call super without any arguments although the current method was called with arguments then you need to be very explicit like this
def method(argument)
super() # note the empty parentheses
end
Let me try to describe you what I think this code does.
*args*
is using the splat operator to collect the arguments together into an array
that is totally correct, however they don't use it, and if you will go to master branch, they just changed it to *. Asking why it is defined and not used, I think that's question about bad design. They should have called it _args or at least like it is now just single splat *.
render_output is set to nil because of scopes, it has to be explicitly defined out block, lambda, proc in order to store value in it, otherwise its visibility will be locked only to those lambda, proc, block execution. Refer to this article
Benchmark.start. Blocks are great ruby construction. You are totally correct that speedtest is done, we can see it is just decorator for benchmark library.
source.
You are wondering why we cannot just pass it as Benchmark.ms(render_output), that's because what will be given to benchmark ms function? It will be given result, like <div> my html </div. And how we can measure this string result - no how. That's why we calling super right in this block, we want to access parent class function and wrap it inside block, so we are not calling it, we just construct it, and it will be called inside benchmark lib, and measured execution like
class Benchmark
...
def realtime # :yield:
r0 = Process.clock_gettime(Process::CLOCK_MONOTONIC)
yield
Process.clock_gettime(Process::CLOCK_MONOTONIC) - r0
end
...
end
So here we can count realtime of function execution, this is the code from original library
I am struggling with Ruby do/end blocks.
For instance, I have the following example:
::ApplicationController.class_eval do
def close_sdb_connection
puts "Closing sdb connection."
end
end
What this piece of code does? Why we need blocks? What would be the equivalent code without the do/end blocks?
do and end are reserved words. They have no generic higher purpose (at least until Matz goes nuts like Larry Wall and says they do). In your sample code, the external do ... end means the code block – same as { ... }. The internal end serves as the right delimiter of the def ... end method definition.
In the context of class_eval, the code sample in the OP does exactly the same thing as:
class ::ApplicationController
def close_sdb_connection
puts "Closing sdb connection."
end
end
However, block statements, such as Foo.class_eval do ... end significantly differ from non-block statements, such as class Foo; ... end, as the blocks are closures over local variables.
It is interesting to note, that not each do ... end indicates a block.
In general, a do ... end or { ... } block is used to put code inside that is not to be evaluated in the same way as the rest of the code. In particular, a block is evaluated under a certain binding (i.e., a certain assignment of local variables and the self keyword) at a certain timing for a certain number of times, all of which is controlled by the method that the block is attached to.
In this particular case, the block is evaluated with a binding different from the outside. Particularly, the code inside would be evaluated as if it were inside the class body of the receiver ::ApplicationController.
This question already has an answer here:
naked asterisk as parameter in method definition: def f(*)
(1 answer)
Closed 10 years ago.
When I was reading Rails code, I found this
def save(*)
create_or_update || raise(RecordNotSaved)
end
What does the * do? :O
I know what happens when we use it like *args, but in this case, it's just plain *.
ref https://github.com/rails/rails/blob/master/activerecord/lib/active_record/persistence.rb#L119
It means the same thing as it does when used with a parameter name: gobble up all the remaining arguments. Except, since there is no name to bind them to, the arguments are inaccessible. In other words: it takes any number of arguments but ignores them all.
Note that there actually is one way to use the arguments: when you call super without an argument list, the arguments get forwarded as-is to the superclass method.
In this specific case, save doesn't take any arguments. That's what happens with a naked splat. But, as you may be aware, calling save on an ActiveRecord model accepts options because this method gets overridden by ActiveRecord::Validations here:
https://github.com/rails/rails/blob/v3.1.3/activerecord/lib/active_record/validations.rb#L47
# The validation process on save can be skipped by passing <tt>:validate => false</tt>. The regular Base#save method is
# replaced with this when the validations module is mixed in, which it is by default.
def save(options={})
perform_validations(options) ? super : false
end
I see ruby code that looks like the following. It seems to be some sort of idiom for creating configuration or settings but I don't really understand it. Also, how would the Application.configure part of this code look?
MyApp::Application.configure do
config.something = foo
config.....
config....
.
config....
end
First of all, that configuration way is not specific to Ruby ; it's the applications (or libraries, gems) that choose to use it or not.
To explain you what does that code do, I'll take your snippet as an example:
MyApp::Application.configure do
config.something = foo
end
Here, you are calling MyApp::Application.configure method, with no parameter. After the call, you're giving it a block.
You can think of blocks as a piece of code that you can use however you want.
They can be written in one single line or many:
{ puts 'hello' }
{ |param| puts param } # with passing it a param
# or
do |param|
puts param
end
(remember my_array.each do ... end? It's a block you pass it. ;) )
Now, that block would be called inside the configure method thanks to yield.
yield uses (or executes) the instructions of the block that has been passed to the method.
Example: Let's define a method with a yield inside of it:
def hello
puts "Hello #{yield}"
end
If you call this method, you'd get a 'hello': no block given (yield) (LocalJumpError)'.
You need to pass it a block: hello { :Samy }.
The result would then be Hello Samy. As you can see, it simply used what was in the block passed to the method.
That's exactly what's happening in the Rails configuration code. You simply set config.something (config is a method) to some value, and that same config.something = foo is execute inside configure.
You can learn more about yield and blocks here, and on this great book.
The part from "do" until "end" is called a block, and is getting passed to the configure class method on Application. (all ruby methods can accept arguments and a block)
so the Application.configure method is creating a configuration object with a set of defaults, and then calling the block. The block is then setting the values you see, having the effect of overriding them.
It's then setting that configuration object as a class variable (like a global variable) so that other classes can use the configuration object later in the application lifecycle.
Hope that simplified description helps!
I have two examples that give the same result.
With block:
def self.do_something(object_id)
self.with_params(object_id) do |params|
some_stuff(params)
end
end
def self.with_params(object_id, &block)
find_object_by_id
calculate_params_hash
block.call(params_hash)
end
and with method:
def self.do_something(object_id)
some_stuff(self.get_params(object_id))
end
def self.get_params(object_id)
find_object_by_id
calculate_params_hash
params_hash
end
The second solution seems more straightforward, but I found some usages of the first one in our application code. My question is: in which situation the first one is recommended? What are the pros and cons of each one?
Normally people use blocks when they want to run a piece of code inside of another piece of code. Examples:
DB.with_shard_for_user(user_id) do |db|
# perform operations on a user's shard
end # shard is reverted back to original value
File.new(filename) do |file|
# work with file
end # file is closed automatically
User.transaction do
# run some operations as a single transaction
end
These blocks are closed on their lexical context (they capture variables from where the block is declared, and carry them over to the place when blocks are called).
Schematic structure of a method that accepts a block.
def transaction
open_transaction # pre- part
yield if block_given? # run provided code
commit_transaction # post- part
rescue
rollback_transaction # handle problems
end
In your first example, use of a block is probably unjustified (IMHO). Too complex for no apparent reason.
The main difference between a block and function as per your example is that the block runs within the context of the calling function.
So if your example was as:
def self.do_something(object_id)
x = "boogy on"
self.with_params(object_id) do |params|
some_stuff(params)
puts x
end
end
The code within the block can access the variable x that was defined outside the block. This is called a closure. You couldn't do this if you were just calling a function as per your second example.
Another interesting thing about blocks is they can affect the control flow of the outer function. So it is possible to do :
def self.do_something(object_id)
self.with_params(object_id) do |params|
if some_stuff(params)
return
end
end
# This wont get printed if some_stuff returns true.
puts "porkleworkle"
end
If the some_stuff call within the block returns a true value, the block will return. This will return out of the block and out of the dosomething method. porkleworkle would not get output.
In your examples you don't rely on either of these, so using function calls is probably much cleaner.
However, there are many situations where using blocks to allow you to take advantage of these things is invaluable.
When you call with_params(), you are not only sending in data, you are also providing some code to run. See if different blocks are sent into the with_params() call:
...
self.with_params(object_id) do |params|
some_other_stuff()
some_stuff(params)
end
...
and somewhere else:
...
self.with_params(object_id) do |params|
even_more_stuff(params)
end
...
If the blocks are all the same or with_params() is just called from one place then you might consider eliminating the blocks.
To sum up: use blocks if you want to pass into the method different bits of code(blocks) as well as data: hey with_params, take this data(object_id) and, by the way, run this code(block) while you're at it.
BTW you are doing different things in the two examples: with_params() returns
some_stuff(params_hash)
after evaluating the block. And get_params() just returns
params_hash
A block totally rely on your code, but a function has its own code.
So, if your code varies situation by situation, use block.
If not, build a function and use it as a block box.