What does it mean for argument handling to be "rigid"?

Question

Taken from the documentation for Proc#lambda?:

Returns true for a Proc object for which argument handling is rigid. Such procs are typically generated by lambda.

What is "rigid argument" handling?

Answer 1

Lambdas will raise an ArgumentError if passed the wrong number of arguments, Proc.new won't.

Example:

lam = ->(x){ "OK" }
lam.lambda? # => true
lam.call # => ArgumentError
lam.call(1) # => OK

proc = Proc.new { |x| "OK" }
proc.lambda? # => false
proc.call # => OK
proc.call(1) # => OK
proc.call(1,2,3,4,5,6,7,8,9) # => OK