# Strange behaviour if exploding parameters **URL:** \ **Category:** Ruby\ **Created:** [June 16, 2010, 4:01pm UTC](https://www.ruby-forum.com/t/strange-behaviour-if-exploding-parameters/190131 "2010-06-16T16:01:04Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![juuser](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/5f8ce5/32.png) [@juuser](https://www.ruby-forum.com/u/juuser)\ **Post date:** [June 16, 2010, 4:01pm UTC](https://www.ruby-forum.com/t/strange-behaviour-if-exploding-parameters/190131/1 "2010-06-16T16:01:04Z")
Hello! I’ve just happened to find strange behaviour when trying to explode parameters. Why doesn’t it work? irb(main):007:0\> lambda {|\*parameters| \*parameters.class} SyntaxError: compile error (irb):7: syntax error, unexpected ‘}’, expecting tCOLON2 or ‘[’ or ‘.’ from (irb):7 from :0 class is Array if 0 or more parameters given irb(main):009:0\> l = lambda {|\*parameters| p \*parameters.class} =\> #Proc:0x04b7e510@:9(irb) irb(main):010:0\> l[] Array =\> nil class is Array if some parameters given irb(main):012:0\> l = lambda {|\*parameters| x = \*parameters; p x.class} =\> #Proc:0x04b2c880@:12(irb) irb(main):013:0\> l[1,2] Array =\> nil class is NilClass instead of Array? irb(main):014:0\> l[] NilClass =\> nil Any ideas why class is NilClass instead of Array if \*parameters is assigned to local variable? I’d expect it to be always an Array and if no parameters given then an empty Array. Best Regards, Jarmo ---
**Author:** ![juuser](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/5f8ce5/32.png) [@juuser](https://www.ruby-forum.com/u/juuser)\ **Post date:** [June 16, 2010, 5:58pm UTC](https://www.ruby-forum.com/t/strange-behaviour-if-exploding-parameters/190131/2 "2010-06-16T17:58:15Z")
Jarmo P. wrote: > Why doesn’t it work? > irb(main):007:0\> lambda {|\*parameters| \*parameters.class} > SyntaxError: compile error > (irb):7: syntax error, unexpected ‘}’, expecting tCOLON2 or ‘[’ or ‘.’ > from (irb):7 > from :0 Your problem is basically this: > > a = [1,2,3] > > =\> [1, 2, 3] > > > \*a.class > > SyntaxError: compile error > > (irb):11: syntax error, unexpected ‘\n’, expecting tCOLON2 or ‘[’ or ‘.’ > > from (irb):11 > > from :0 > > > a.class > > =\> Array So I think you want: irb(main):002:0\> lambda {|\*parameters| parameters.class} =\> #Proc:0xb744ca98@:2(irb) > class is Array if 0 or more parameters given > irb(main):009:0\> l = lambda {|\*parameters| p \*parameters.class} > =\> #Proc:0x04b7e510@:9(irb) > irb(main):010:0\> l[] > Array > =\> nil So now you are demonstrating: > > p \*a.class > > Array > > =\> nil This is because “p” is a method, and so you can use splat to expand its arguments. I think what you’ve written is equivalent to: irb(main):023:0\> p(\*(a.class)) Array =\> nil Strangely, \* works on a non-array here. Possibly you are relying on implicit to\_a: > > a.class.to\_a > > (irb):24: warning: default `to\_a’ will be obsolete > > =\> [Array] > class is Array if some parameters given > irb(main):012:0\> l = lambda {|\*parameters| x = \*parameters; p x.class} > =\> #Proc:0x04b2c880@:12(irb) > irb(main):013:0\> l[1,2] > Array > =\> nil > > class is NilClass instead of Array? > irb(main):014:0\> l[] > NilClass > =\> nil Of course, same as this: irb(main):025:0\> x = \*[] =\> nil You’re basically doing a multiple-assignment with zero values on the right-hand side, so it gets nil. Similarly: > > a,b,c = 1,2 > > =\> [1, 2] > > > c > > =\> nil > Any ideas why class is NilClass instead of Array if \*parameters is > assigned to local variable? I’d expect it to be always an Array and if > no parameters given then an empty Array. You need to look at the value ‘parameters’, and that’s what you’ll see. If you take the parameters array and splat it with \*, then you’re asking for ruby to do more. ---
**Author:** ![juuser](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/5f8ce5/32.png) [@juuser](https://www.ruby-forum.com/u/juuser)\ **Post date:** [June 17, 2010, 4:22pm UTC](https://www.ruby-forum.com/t/strange-behaviour-if-exploding-parameters/190131/3 "2010-06-17T16:22:35Z")
On Wed, Jun 16, 2010 at 9:01 AM, Jarmo P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > ``` > from :0 > > ``` > > class is Array if some parameters given > =\> nil Hi, I think you might be a bit confused about how \* works in your param list. It will aggregate all the params into a single array: lambda { |\*params| params }.call 1 , 2 , 3 # =\> [1, 2, 3] So you can call class on it lambda { |\*params| params.class }.call 1 , 2 , 3 # =\> Array return is a method invocation (at least in my mind) so you can “explode” the brackets here too, and it becomes like “return 1 , 2 , 3” which is syntactically the same as “call 1 , 2 , 3” lambda { |\*params| return \*params }.call 1 , 2 , 3 # =\> [1, 2, 3] But there are two things that don’t really make sense. “1 , 2 , 3” all by itself is not valid Ruby. Leaving it on its own line like this lambda { |\*params| \*params }.call 1 , 2 , 3 # =\> syntax error Is like saying this lambda { |\*params| 1 , 2 , 3 }.call 1 , 2 , 3 # =\> syntax error Since that is not valid, then what does it mean to say \*params.class ? Well, first of all, the .class is evaluated first (not sure how to say that, “higher binding” maybe?) The class is an Array. so this becomes “\*Array” which is probably not what you want, and only works, again, when passing it to return. lambda { |\*params| return \*params.class }.call 1 , 2 , 3 # =\> Array lambda { |\*params| \*params.class }.call 1 , 2 , 3 # =\> syntax error So, in summary, I think this is what you were looking for: lambda { |\*params| params.class }.call 1 , 2 , 3 # =\> Array ---
**Author:** ![juuser](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/5f8ce5/32.png) [@juuser](https://www.ruby-forum.com/u/juuser)\ **Post date:** [June 17, 2010, 4:40pm UTC](https://www.ruby-forum.com/t/strange-behaviour-if-exploding-parameters/190131/4 "2010-06-17T16:40:19Z")
Brian and Josh - thank You both for these fine answers. I got my understanding now about what’s happening behind the scenes. Jarmo