when i write method missing in Object class i'm getting the output different in each interface.
the code is
class Object
def method_missing(hgh)
puts self
end
end
when i use REPL like irb, i get
when i use the command line, i get no error, any reasons would be helpful, thanks in advance
The tl;dr answer is that both are correct. Just more stuff happen in the REPL.
When you run the code from the command line like:
ruby script.rb
All that happens is that it's evaluated.
Whereas REPLs like IRB read your input, evaluate it and print it in a loop.
In this case evaluating your code literally broke the REPL and resulted in the subsequent print failing.
Now you may be a bit confused by this. "There is a print in both cases, I use puts!". The print I'm referring to here is the result that gets visualised after each evaluation. In this case the method definition result (=> :method_missing).
It might not only be the printing itself. It can be the ton of other under the hood code that the REPL has to execute to keep state like "what code was defined on which line" and so on.
Think of what you just did - you made it so that every object has every method possible to return nil. That is not just for the code you write in the REPL. It's for the code of the REPL itself as well.
Related
When, during a pry debug session, I want to inspect step-by-step execution of FooClass.new.foo I'd do this in pry console
$ FooClass.new.foo #this gives me path and line of the method
break /path/to/foo_class.rb:LINE_WHERE_FOO_IS_DEFINED
FooClass.new.foo
This works but I need to look for the path, line and it leaves a breakpoint I sometimes have to remove.
There's a faster way:
break FooClass#foo
FooClass.new.foo
but it's still two steps, and the breakpoint stays.
Is there a way to do it in one command, like
step-into FooClass.new.foo
which would start a Pry subsession, enter the method execution and after it exits/finishes I'm back in the original session with no extra breakpoints left?
In other words: I'm in the middle of debugging, and see a method called few lines before (I can't step into it immediately). I don't want to put a binding.pry in the source (it may take a lot of time to start the debugging session again).
The problem has hidden complexity. For instance, one thing you can do is:
pry Foo.new.foo
or
Foo.new.foo.pry
This puts you into a binding on the return value of the function foo, but that is not what you want. It also shows there is complexity when your line of code Foo.new.foo runs, for a function to know what exactly to put a binding on.
So that's really the problem I feel; it would take understanding of the expression on the line to understand where to put a breakpoint. If you imagine it was working like the step function.
If your code Foo.new.foo was already a break point (e.g. you had a function with that code in and you put a breakpoint on a line like that) then you typed step, you would go into Foo.new Foo.initialize instance of foo foo.foo and so on. So, any code trying to do what you would like it to do would have a very hard time understanding where in the expression tree to stop and put your into pry.
This is why you have to tell pry where to stop using break in this context.
I'm wondering why the following tag methods produce different results:
Method 1:
def tag(html)
print "<#{html}>#{yield}</#{html}>"
end
Method 2:
def tag(html)
print "<#{html}>"
print yield
print "</#{html}>"
end
When I ran the following code in terminal using the above methods:
tag(:ul) do
tag(:li) { "It sparkles!" }
tag(:li) { "It shines!" }
tag(:li) { "It mesmerizes!" }
end
The first one gave me:
<li>It sparkles!</li><li>It shines!</li><li>It mesmerizes!</li><ul></ul>
The second one gave me:
<ul><li>It sparkles!</li><li>It shines!</li><li>It mesmerizes!</li></ul>
The second one is the output that I'm looking.
How come the first method prints 'yield' before it prints what comes before 'yield' in the string?
Just to echo #tadman's answer: order of evaluation AND inconsistency of api. Your block sometimes returns strings and sometimes prints strings as a side-effect.
print "<#{html}>"
print yield
print "</#{html}>"
Here you print, then yield. If the block returns a string (one of :li blocks), then it's printed right here. If it's a :ul block, then its side-effects happen (printing of li blocks) and nil is printed after that.
In the other case
print "<#{html}>#{yield}</#{html}>"
Ruby has to assemble one string to print. Which means yielding before any printing. Which means that side-effects happen before printing the opening <ul>. As the ul block returns nil, that's why it's printed empty at the end of the string (<ul></ul>).
Does it make sense?
The main problem is the order of operations. When you call print it will print immediately, there's no delay, which can cause problems.
In Ruby it's often easier to deal with code that returns strings rather than code that causes side-effects like printing. If they return strings you have control over where that output goes. If they print things immediately you need to be very careful about the order you call them in.
The way you're calling that code in the final assembly with the tag(:ul) call is actually going to be trouble. The second version of your method coincidentally orders things correctly.
It's not necessarily easy to fix this. If you return a string, then only the last string from your three tag calls will be used. If you print, you'll have to be sure you're using the second method to make it work.
Within the Rails system there's a way of capturing the output of these things for buffering purposes, but that's a very messy thing to try and do, it can get really confused when you try and handle all cases.
Where possible create some kind of buffer these things can write to, then when everything's done write that out with print or whatever.
I have a need to decipher some Ruby code. Being a Python dev, I am having hard time making sense to some of the syntax.
I need to deal with some (mostly clean and readable) Sinatra code. I started with a Sinatra tutorial, and it looks something like this:
get '/' do
"Hello, World!"
end
Now, I know that in Ruby you don't need parentheses to call a function. So if I were to try to understand the above, I would say:
get is a function that takes as its first argument the route.
'/' is the first argument
do ... end block is an anonymous function
Please correct me if I am wrong above, and explain in detail anything I might be missing.
Also they say that Sinatra is a DSL -- does this mean that it is parsing some special syntax that is not official Ruby?
do ... end (or { ... }) is a block, a very important concept in Ruby. It was noticed that very often functions that take other functions as parameter (map, filter, grep, timeout...) very often accept a single function. So the Ruby designer decided to make a special syntax for it.
It is often said that in Ruby, everything is an object. This is not quite true: code is not an object. Code can be wrapped into an object. But Ruby blocks are pure code - not an object, not a first-order value at all. Blocks are a piece of code associated with a function call.
Your code snippet is equivalent to this:
self.get('/') do
return "Hello, World!"
end
The get method takes one parameter and a block; not two parameters. In a hypothetical example where get did take two parameters, we would have to write something like this:
get('/', lambda { "Hello, World" })
or
get('/', Proc.new { "Hello, World" })
but notice that the way we wrap code into objects involves calling methods lambda and Proc.new - and giving them a block (and zero parameters)!
There are many tutorials on "Ruby blocks", so I will not link any particular one.
Because of the block syntax, Ruby is very good at making dialects (still fully syntactic Ruby) that express certain concepts very neatly. Sinatra uses the get... "syntax" (but actually just a method call) to describe a web server; Rake uses task... "syntax" to describe build processes; RSpec, a testing framework, has its own DSL (that is still Ruby) that describes desired behaviours.
After some reading, I understood the code blocks.
Ruby code blocks are simple. They are 'closures'. There's two ways to write a block
do |x|
do_something(x)
end
{|x| do_something(x) }
The |x| is the argument that gets passed to the code within the block.
The crucial bit to grasping code blocks is to understand how they are used with methods.
In Ruby, methods are a bit different.
In addition to arguments, any method can accept a code block.
Code blocks are NOT arguments, but they are a separate entity that can be passed to a method along with arguments
A method can choose not to call the code block, in which case, any code block that was passed is ignored
If a method calls a code block, then it is necessary to pass it when calling the method, or otherwise Ruby will complain.
yield within a method executes the code block
For more on code blocks read this: http://mixandgo.com/blog/mastering-ruby-blocks-in-less-than-5-minutes
I am working with a custom testing framework and we are trying to expand some of the assert functionality to include a custom error message if the assert fails. The current assert is called like this:
assert_compare(first_term, :neq, second_term) do
puts 'foobar'
end
and we want something with the functionality of:
assert_compare(first_term, :neq, second_term, error_message) do
puts 'foobar'
end
so that if the block fails the error message will describe the failure. I think this is ugly, however, as the framework we are moving away from did this and i have to go through a lot of statements that look like:
assert.compare(variable_foo['ARRAY1'][2], variable_bar['ARRAY2'][2], 'This assert failed because someone did something unintelligent when writing the test. Probably me, since in am the one writing this really really long error statement on the same line so that you have to spend a quarter of your day scrolling to the side just to read it')
This type of method call makes it difficult to read, even when using a variable for the error message. I feel like a better way should be possible.
assert_compare(first_term, :neq, second_term) do
puts 'foobar'
end on_fail: 'This is a nice error message'
This, to me, is the best way to do it but i don't know how or if it is even possible to accomplish this in ruby.
The goal here is to make it as aesthetic as possible. Any suggestions?
You could make on_fail a method of whatever assert_compare returns and write
assert_compare(first_term, :neq, second_term) do
puts 'foobar'
end.on_fail: 'This is a nice error message'
In short, no. Methods in ruby take a block as the final parameter only. As Chuck mentioned you could attempt to make the on_fail method a method of whatever assert_compare returns and that is a good solution. The solution I've come up with is not what you are looking for, but it works:
def test block, param
block.call
puts param
end
test proc { puts "hello"}, "hi"
will result in
"hello"
"hi"
What I've done here is create a Proc (which is essentially a block) and then passed it as a regular parameter.
I had a test that did this:
expect(#parser.parse('adsadasdas')).to raise_error(Errno::ENOENT)
and it didn't work. I changed to:
expect { #parser.parse('adsadasdas') }.to raise_error(Errno::ENOENT)
And it worked.
When do we use curly braces and when do we use parentheses with expect?
In response to OP's comment, I've edited and completely rewritten my answer. I realize that my original answer was oversimplified, so much so that it could be considered incorrect.
Your question was actually addressed somewhat by this other StackOverflow question.
One poster, Peter Alfvin, makes a good point when he says:
As for rules, you pass a block or a Proc if you're trying to test
behavior (e.g. raising errors, changing some value). Otherwise, you
pass a "conventional" argument, in which case the value of that
argument is what is tested.
The reason you're encountering the phenomenon you're seeing has to do with the raising of errors. When you pass #parser.parse('adsadasdas') as an argument (use parentheses) to expect, you are essentially telling ruby:
Evaluate #parser.parse('adsadasdas') first.
Take the result and pass this to expect.
expect should see if this result matches my expectation (that is, that Errno:ENOENT will be raised).
But, what happens is: when ruby evaluates #parser.parse('adsadasdas'), an error is raised right then and there. Ruby doesn't even get a chance to pass the result on to expect. (For all we care, you could have passed #parser.parse('adsadasdas') as an argument to any function... like multiply() or capitalize()) The error is raised, and expect never even gets a chance to do its work.
But when you pass #parser.parse('adsadasdas') as a proc (a code block) to expect using curly braces, what you are telling ruby is this:
expect, get ready to do some work.
expect, I would like you to keep track of what happens as we evaluate #parser.parse('adsadasdas').
Ok, expect, did the code block that was just evaluated raise a Errno:ENOENT error? I was expecting that it would.
When you pass a code block to expect, you are telling expect that you want it to examine the resulting behavior, the changes, made by your code block's execution, and then to let you know if it meets up to the expectations that you provide it.
When you pass an argument to expect, you are telling ruby to evaluate that argument to come to some value before expect even gets involved, and then you are passing that value to expect to see if it meets up to some expectation.
TL;DR: use expect(exp) to specify something about the value of exp and use expect { exp } to specify a side effect that occurs when exp is executed.
Let's unpack this a bit. Most of RSpec's matchers are value matchers. They match (or not) against any ruby object. In contrast, a handful of RSpec's matchers can only be matched against a block, because they have to observe the block while it's running in order to operate properly. These matchers concern side effects that take place (or not) while the block executes. The matcher would have no way to tell if the named side effect had occurred unless it is passed a block to execute. Let's consider the built-in block matchers (as of RSpec 3.1) one-by-one:
raise_error
Consider that one can return an exception from a method, and that is different than raising the exception. Raising an exception is a side effect, and can only be observed by the matcher by it executing the block with an appropriate rescue clause. Thus, this matcher must receive a block to work properly.
throw_symbol
Throwing symbols is similar to raising errors -- it causes a stack jump and is a side effect that can only be observed by running a block inside an appropriate catch block.
change
Mutation to state is a side effect. The matcher can only tell if there was a change to some state by checking the state before hand, running the block, then checking the state after.
output
I/O is a side effect. For the output matcher to work, it has to replace the appropriate stream ($stdout or $stderr) with a new StringIO, execute the block, restore the stream to its original value, and then check the contents of theStringIO`.
yield_control/yield_with_args/yield_with_no_args/yield_with_successive_args
These matchers are a bit different. Yielding isn't really a side effect (it's really just syntactic sugar for calling another function provided by the caller), but yielding can't be observed by looking at the return value of the expression. For the yield matchers to work, they provide a probe object that you pass on to the method-under-test as a block using the &probe syntax:
expect { |probe| [1, 2, 3].each(&probe) }.to yield_with_successive_args(1, 2, 3)
What do all these matchers have in common? None of them can work on simple ruby values. Instead, they all have to wrap a block in an appropriate context (i.e. rescuing, catching or checking before/after values).
Note that in RSpec 3, we added some logic to provide users clear errors when they use the wrong expect form with a given matcher. However, in the specific case of expect(do_something).to raise_error, there's nothing we can do to provide you a clear explanation there -- if do_something raises an error (as you expect it to...), then the error is raised before ruby evaluates the to argument (the raise_error matcher) so RSpec has no way to check with the matcher to see if supports value or block expectations.
in short:
use curly-brace (a block): when you want to test the behavior
use parenthesis when you want to test the returned value
worth reading: As for rules, you pass a block or a Proc if you're trying to test behavior (e.g. raising errors, changing some value). Otherwise, you pass a "conventional" argument, in which case the value of that argument is what is tested. - from this answer
In the test written with parentheses, the code is executed normally, including all normal error handling. The curly-brace syntax defines a block object upon which you can place the expectation. It encapsulates the code you expect to be broken and allows rspec to catch the error and provide its own handling (in this case, a successful test).
You can think of it this way as well: with the parentheses, the code is executed before being passed to the expect method, but with the block, expect will run the code itself.