Goodbye irb, hello pry.

gistfile1.rb

pry(main)> @foo = []
=> []

pry(main)> show-doc @foo.index

From: array.c in Ruby Core (C Method):
Number of lines: 14

signature: index(*)

Returns the index of the first object in self such that is
== to obj. If a block is given instead of an
argument, returns first object for which block is true.
Returns nil if no match is found.
See also Array#rindex.

If neither block nor argument is given, an enumerator is returned instead.

   a = [ "a", "b", "c" ]
   a.index("b")        #=> 1
   a.index("z")        #=> nil
   a.index{|x|x=="b"}  #=> 1

This is an alias of #find_index.

pry(main)> show-method @foo.index

From: array.c in Ruby Core (C Method):
Number of lines: 24

static VALUE
rb_ary_index(int argc, VALUE *argv, VALUE ary)
{
    VALUE val;
    long i;

    if (argc == 0) {
    RETURN_ENUMERATOR(ary, 0, 0);
    for (i=0; i<RARRAY_LEN(ary); i++) {
        if (RTEST(rb_yield(RARRAY_PTR(ary)[i]))) {
        return LONG2NUM(i);
        }
    }
    return Qnil;
    }
    rb_scan_args(argc, argv, "1", &val);
    if (rb_block_given_p())
    rb_warn("given block not used");
    for (i=0; i<RARRAY_LEN(ary); i++) {
    if (rb_equal(RARRAY_PTR(ary)[i], val))
        return LONG2NUM(i);
    }
    return Qnil;
}