Difference between puts a and puts "#{a}" - ruby

Question

I thought that doing puts #{a} would result in the same output as puts a, but found this not to be the case. Consider:
irb(main):001:0> a = [1,2]
=> [1, 2]
irb(main):002:0> puts a
1
2
=> nil
irb(main):003:0> puts "#{a}"
12
=> nil
irb(main):004:0>
In the above example it doesn't matter much, but it may matter when I want to print multiple variables on one line, such as (psudocode):
puts "There are #{a.size} items in the whitelist: #{a}"
Why is the output different here? Do they actually do different things, or have different semantics?

Answer 1

That's because "#{a}" calls the #to_s method on the expression.
So:
puts a # equivalent to, well, puts a
puts "#{a}" # equivalent to the next line
puts a.to_s
Update:
To elaborate, puts eventually calls #to_s but it adds logic in front of the actual output, including special handling for arrays. It just happens that Array#to_s doesn't use the same algorithm. (See puts docs here.) Here is exactly what it does...
rb_io_puts(int argc, VALUE *argv, VALUE out)
{
int i;
VALUE line;
/* if no argument given, print newline. */
if (argc == 0) {
rb_io_write(out, rb_default_rs);
return Qnil;
}
for (i=0; i<argc; i++) {
if (TYPE(argv[i]) == T_STRING) {
line = argv[i];
goto string;
}
line = rb_check_array_type(argv[i]);
if (!NIL_P(line)) {
rb_exec_recursive(io_puts_ary, line, out);
continue;
}
line = rb_obj_as_string(argv[i]);
string:
rb_io_write(out, line);
if (RSTRING_LEN(line) == 0 ||
!str_end_with_asciichar(line, '\n')) {
rb_io_write(out, rb_default_rs);
}
}
return Qnil;
}