Hello

Returns the number of arguments required by the block. If the block

takes no arguments, returns -1. If it takes one argument, returns -2.

Otherwise, returns a positive argument count unless the last argument is

prefixed with *, in which case the argument count is negated. The number

of required arguments is anInteger for positive values, and ( anInteger

+1).abs otherwise.

Proc.new {||}.arity » 0

Proc.new {|a|}.arity » -1 ### <<< error?

Proc.new {|a,b|}.arity » 2

Proc.new {|a,b,c|}.arity » 3

Proc.new {|*a|}.arity » -1

Proc.new {|a,*b|}.arity » -2

I would say it’s a typo

Regards, Daniel