When I typed the below in my IRB:
{}.class
#=> Hash #<~~ I got this one.
print {}.class
#NilClass
Why the above showing `NilClass`, but the below are not?
print [].class
#Array=> nil
print (1..2).class
#Range=> nil
print 2.class
#Fixnum=> nil
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because
print {}.class
is parsed as (print() {}).class
where {} is the closure block and not a Hash
--
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i mean the blocks you used in http://www.ruby-forum.com/topic/4411871
print thinks you want to call it with a block not with a hash
normally ruby functions when you add a block to them they ignored them
silency
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here I am making my confusions are more specific:
print(1..2).class
1..2=> NilClass
print (1..2).class
Range=> nil
Why the below 2 produces different output? How does `IRB` read the
above?
print.class
NoMethodError: undefined method `' for nil:NilClass
from (irb):5
from C:/Ruby193/bin/irb:12:in `<main>'
print .class
Array=> nil
How does `IRB` read the above?
print{}.class
=> NilClass
print {}.class
=> NilClass
Why here both lines giving the same outputs unlike the above 2 ?
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First of all thanks to all guys who has shown interest in my post.
Keeping in mind @Matthew's instructions I did the below. Hope now I am
able to clear my question once again to all the Experts out there.
C:\>irb --simple-prompt --noecho
{}.class
.class
(1..2).class
puts (1..2).class
Range
puts .class
Array
puts {}.class
print {}.class
p {}.class
Very strange behaviour `{}.class` is showing with every printing
statement. But which is not the case with `` and `(1..2)`. Now to see
the class `{}` I am requiring to add `()`. ~~~ actually my question was
why I need to do add `()` that with `{}` only, which is not needed with
`` and `(1..3)` ?
puts ({}.class)
Hash
p ({}.class)
Hash
print ({}.class)
Please let me know if you have any confusion to understand.
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BECAUSE { } are used for hashes AND blocks ... in this context you use
it as a block, not as a Hash ...
thats why
puts {:e=>4}
raises an SYNTAXERROR
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Humm,
Only within the `()` it is understood by IRB that i am feeding it a
Hash, otherwise error.
puts {:a => 4}
SyntaxError: (irb):17: syntax error, unexpected tASSOC, expecting '}'
puts {:a => 4}
^
from C:/Ruby193/bin/irb:12:in `<main>'
puts {:a => 4}.class
SyntaxError: (irb):18: syntax error, unexpected tASSOC, expecting '}'
puts {:a => 4}.class
^
from C:/Ruby193/bin/irb:12:in `<main>'
============================
puts ({:a => 4}.class)
Strange it is. would not it?
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Thanks for your re-explanation. This is how IRB works I understood.
print nil, {}
{}>>
?> ^C
print nil, {}.class
Now this thread can be closed.
Thanks to all guys who helped me to understand this `micro concept`. 
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Hans Mackowiak wrote in post #1101101:
Can you give me a bit knowledge about `closure block` concept. I didn't
here about it.
Hans Mackowiak wrote in post #1101107:
normally ruby functions when you add a block to them they ignored them
silency
I am thinking when IRB sees that `print {}.class`, Is there any chance
to treat it as (print {}).class #=> NilClass ?
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Please elaborate on the ways you have attempted to solve this problem yourself, as a basic courtesy to others on this list.
http://www.catb.org/esr/faqs/smart-questions.html#beprecise
Specifically: "Describe the diagnostic steps you took to try and pin down the problem yourself before you asked the question."
On 12/03/13 15:11, Love U Ruby wrote:
here I am making my confusions are more specific:
print(1..2).class
1..2=> NilClass
print (1..2).class
Range=> nil
Why the below 2 produces different output? How does `IRB` read the
above?print.class
NoMethodError: undefined method `' for nil:NilClass
from (irb):5
from C:/Ruby193/bin/irb:12:in `<main>'print .class
Array=> nil
How does `IRB` read the above?
print{}.class
=> NilClass
print {}.class
=> NilClass
Why here both lines giving the same outputs unlike the above 2 ?
You are messing up your code because you did not use parentheses,
leaving to ruby the task to guess what you wanted to do.
here I am making my confusions are more specific:
>> print(1..2).class
1..2=> NilClass
>> print (1..2).class
Range=> nil
(print(1..2)).class
1..2=> NilClass
print ((1..2).class)
Range=> nil
Why the below 2 produces different output? How does `IRB` read the
above?>> print.class
NoMethodError: undefined method `' for nil:NilClass
from (irb):5
from C:/Ruby193/bin/irb:12:in `<main>'
>> print .class
Array=> nilHow does `IRB` read the above?
(print).class
NoMethodError: undefined method `' for nil:NilClass
print (.class)
Array=> nil
Think about what's going on when you are using or not a space character
>> print{}.class
=> NilClass
>> print {}.class
=> NilClass
This one is a bit tricker but as you are already be told, the {} is taken
as a block and not as a Hash. So both are interpreted in the same way.
(print{}).class
=> NilClass
(print {}).class
=> NilClass
Fortunately you won't have to do this often, won't you ?
Anyway what you probably wanted to do is this:
print({}.class)
Hash=> nil
On Tue, Mar 12, 2013 at 01:41:09PM +0900, Love U Ruby wrote:
Why here both lines giving the same outputs unlike the above 2 ?
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Note that one of my points was that the following are the same:
print {}
print do
end
See also:
5.times { }
5.times do
end
But this is different:
print nil, {}
Also, ruby method calls have optional parentheses:
3.inspect
3.inspect()
foo
foo()
print 'x'
print( 'x' )
The interpreter always reads curlies {} as a block unless it makes no sense
to do so. Parens and commas make it clear it's a variable (and thus a Hash).
Sent from my phone, so excuse the typos.
Among the repliers there hasn't been any confusion at all.
You're question had been answered in the very first reply
(and additionally several times in later posts) so please
don't ask the same question five times but instead try to
read the answers more carefully.
To clarify (hopefully) even more: this has _nothing_ to do with IRB.
It's the Ruby _parser_ that needs and follows rules on how to
treat ambiguous expressions (like any parser does).
A different example: 2 + 3 * 4
This is parsed as 2 + (3 * 4) according to the rules for
operator precedence. If you want to enforce a different
interpretation of this expression, you need to make that clear
to the parser by adding some parentheses, e.g. (2 + 3) * 4
Same for your question: if you want {} to be interpreted as
a hash you need to make that clear to the parser.
Am 12.03.2013 08:55, schrieb Love U Ruby:
Please let me know if you have any confusion to understand.
I'm pretty sure you want: print( {}.class )
But that's not what you asked for.
This is pretty simple syntax stuff.
On 12 March 2013 03:29, Love U Ruby <lists@ruby-forum.com> wrote:
Hans Mackowiak wrote in post #1101107:
> normally ruby functions when you add a block to them they ignored them
> silencyI am thinking when IRB sees that `print {}.class`, Is there any chance
to treat it as (print {}).class #=> NilClass ?
--
Matthew Kerwin, B.Sc (CompSci) (Hons)
http://matthew.kerwin.net.au/
Stop using irb. Write a .rb file and run it. Then you will (hopefully) see
that:
1. print needs a "\n" to push output to the next line
print 'h'
print 'i'
print "p\ncat"
2. anything that starts with"#=>" isn't _real_ output; it's stuff irb does
x = 3 # no output
2b) those things are the _VALUE OF THE PREVIOUS EXPRESSION_
puts '#=> ' + x.inspect
3. `print` has a _value_ of nil
x = print('') # x is now nil
puts '#=> ' + x.inspect # see?
3b) `foo.class` has a value of the object that foo is a new one of
foo = Object.new
foo2 = foo.class.new
4. these are the same:
print {}
print do
end
4b) print {} outputs nothing, and still has a value of nil
4c) nil.class has a value of NilClass
Sent from my phone, so excuse the typos.
unknown wrote in post #1101202:
Thank you very much to you for your effort to explain me the topic in
different way.
A different example: 2 + 3 * 4
This is parsed as 2 + (3 * 4) according to the rules for
operator precedence. If you want to enforce a different
interpretation of this expression, you need to make that clear
to the parser by adding some parentheses, e.g. (2 + 3) * 4Same for your question: if you want {} to be interpreted as
a hash you need to make that clear to the parser.
The whole topic is understood by me.
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Matthew Kerwin wrote in post #1101134:
I'm pretty sure you want: print( {}.class )
But that's not what you asked for.This is pretty simple syntax stuff.
See when typed in IRB the below it gives
{}.class
#=> Hash
And Hans said IRB treat `print {}.class` as `(print {}).class` which
causes the output as `Nilclass`.
But I was thinking (print {}).class also results `NilClass`. So which is
the one IRB actually recognized to give the output `NilClass`.
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