# Re: assert\_raise question

**URL:** <https://www.ruby-forum.com/t/re-assert-raise-question/147020>\
**Category:** Ruby\
**Created:** [August 27, 2008, 4:57pm UTC](https://www.ruby-forum.com/t/re-assert-raise-question/147020 "2008-08-27T16:57:11Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![stickstone](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@stickstone](https://www.ruby-forum.com/u/stickstone)\
**Post date:** [August 27, 2008, 4:57pm UTC](https://www.ruby-forum.com/t/re-assert-raise-question/147020/1 "2008-08-27T16:57:11Z")

</div>

> def test\_raise  
> assert\_raise ArgumentError {raise ArgumentError.new(“basic argument issue”)}  
> end
> 
> Now there’s suddenly an Exception: undefined method ‘ArgumentError’…  
> I was more than a little surprised to discover that the parens make a  
> difference here? Can anyone explain to me _why_ there’s a difference?

I believe that x y z is parsed as x(y(z)):

irb(main):013:0\> puts ArgumentError  
ArgumentError  
=\> nil  
irb(main):014:0\> puts ArgumentError 123  
(irb):14: warning: parenthesize argument(s) for future version  
NoMethodError: undefined method `ArgumentError' for main:Object from (irb):14 from :0 irb(main):015:0> puts ArgumentError(123) NoMethodError: undefined method`ArgumentError’ for main:Object  
from (irb):15  
from :0  
irb(main):016:0\>

and x y {block} is parsed as x(y() {block}), i.e. y is forced to be a  
method  
call as that’s the only meaningful thing next to a block.

irb(main):017:0\> def Wibble(\*args,&blk)  
irb(main):018:1\> puts “Args: #{args.inspect}”  
irb(main):019:1\> puts “With block” if blk  
irb(main):020:1\> end  
=\> nil  
irb(main):021:0\> puts Wibble { raise “hello” }  
Args: []  
With block  
nil  
=\> nil  
irb(main):022:0\> puts Wibble Wibble { raise “hello” }  
(irb):22: warning: parenthesize argument(s) for future version  
Args: []  
With block  
Args: [nil]  
nil  
=\> nil

There’s also the ambiguity between hash literal and block to consider:

irb(main):027:0\> puts { :one =\> 2 }  
SyntaxError: compile error  
(irb):27: parse error, unexpected tASSOC, expecting ‘}’  
puts { :one =\> 2 }  
^  
from (irb):27  
from :0  
irb(main):028:0\> puts({ :one =\> 2 })  
one2  
=\> nil  
irb(main):029:0\> puts 123, { :one =\> 2 }  
123  
one2  
=\> nil  
irb(main):030:0\> puts { raise }

=\> nil  
irb(main):031:0\> puts :one =\> 2  
one2  
=\> nil

B.

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<div class="post-metadata">

**Author:** ![stickstone](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6f9a4e/32.png) [@stickstone](https://www.ruby-forum.com/u/stickstone)\
**Post date:** [August 27, 2008, 5:33pm UTC](https://www.ruby-forum.com/t/re-assert-raise-question/147020/2 "2008-08-27T17:33:14Z")

</div>

Brian C. wrote:

> > def test\_raise  
> > assert\_raise ArgumentError {raise ArgumentError.new(“basic argument issue”)}  
> > end
> > 
> > Now there’s suddenly an Exception: undefined method ‘ArgumentError’…  
> > I was more than a little surprised to discover that the parens make a  
> > difference here? Can anyone explain to me _why_ there’s a difference?
> 
> I believe that x y z is parsed as x(y(z)):
> 
> irb(main):013:0\> puts ArgumentError  
> ArgumentError  
> =\> nil  
> irb(main):014:0\> puts ArgumentError 123  
> (irb):14: warning: parenthesize argument(s) for future version  
> NoMethodError: undefined method `ArgumentError' for main:Object from (irb):14 from :0 irb(main):015:0> puts ArgumentError(123) NoMethodError: undefined method`ArgumentError’ for main:Object  
> from (irb):15  
> from :0  
> irb(main):016:0\>
> 
> and x y {block} is parsed as x(y() {block}), i.e. y is forced to be a  
> method  
> call as that’s the only meaningful thing next to a block.

[snip]

Well, now that you explained it, it makes perfect sense! Thanks.

I guess in the future I’ll double-check a little more carefully that my  
code is doing what I expect when I leave out parens.

Doug