instance method

Exception#cause

() -> ::Exception?

cause → exception or nil

static VALUE
exc_cause(VALUE exc)
{
    return rb_attr_get(exc, id_cause);
}

Returns the previous value of global variable $!, which may be nil (see Global Variables):

Example

begin
  raise('Boom 0')
rescue => x0
  puts "Exception: #{x0};  $!: #{$!};  cause: #{x0.cause.inspect}."
  begin
    raise('Boom 1')
  rescue => x1
    puts "Exception: #{x1};  $!: #{$!};  cause: #{x1.cause}."
    begin
      raise('Boom 2')
    rescue => x2
      puts "Exception: #{x2};  $!: #{$!};  cause: #{x2.cause}."
    end
  end
end

Output:

Exception: Boom 0;  $!: Boom 0;  cause: nil.
Exception: Boom 1;  $!: Boom 1;  cause: Boom 0.
Exception: Boom 2;  $!: Boom 2;  cause: Boom 1.

See also #backtrace_locations #detailed_message #backtrace #exception also in 4.1 · 3.4 · 3.3 · 3.2 · 3.1 · 3.0

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