arity

Returns the number of arguments that would not be ignored. If the block is declared to take no arguments, returns 0. If the block is known to take exactly n arguments, returns n. If the block has optional arguments, return -n-1, where n is the number of mandatory arguments. A proc with no argument declarations is the same a block declaring || as its arguments.

proc {}.arity          #=>  0
proc {||}.arity        #=>  0
proc {|a|}.arity       #=>  1
proc {|a,b|}.arity     #=>  2
proc {|a,b,c|}.arity   #=>  3
proc {|*a|}.arity      #=> -1
proc {|a,*b|}.arity    #=> -2
proc {|a,*b, c|}.arity #=> -3

proc   { |x = 0| }.arity       #=> 0
lambda { |a = 0| }.arity       #=> -1
proc   { |x=0, y| }.arity      #=> 1
lambda { |x=0, y| }.arity      #=> -2
proc   { |x=0, y=0| }.arity    #=> 0
lambda { |x=0, y=0| }.arity    #=> -1
proc   { |x, y=0| }.arity      #=> 1
lambda { |x, y=0| }.arity      #=> -2
proc   { |(x, y), z=0| }.arity #=> 1
lambda { |(x, y), z=0| }.arity #=> -2