From: Love U Ruby <lists@...>
Date: 2013-03-17T22:48:11+09:00
Subject: Confusion with `nil` output from the method Symbol#<=>

Symbol#<=> ->
http://www.ruby-doc.org/core-2.0/Symbol.html#method-i-3C-3D-3E

simply says: Compares symbol with other_symbol after calling to_s on
each of the symbols. Returns -1, 0, +1 or nil depending on whether
symbol is less than, equal to, or greater than other_symbol. **`nil` is
returned if the two values are incomparable.**


I was trying to understand how `Symbol#<=>` works when returning `nil`.
Doing so I played with the code:

    >> :x.to_s
    => "x"
    >> 'x'.to_s
    => "x"

From the above `IRB` code I thought the return value will be `0`. But
the actual is `nil`. As doc says that before using `<=>` operator `to_s`
is applied both the `RHO` and `LHO`. But here the below code is not
supporting that principle, seems to me.

    >> :x <=> "x"
    #=> nil

So I tried to see the source code,to answer myself:

    static VALUE
    sym_cmp(VALUE sym, VALUE other)
    {
        if (!SYMBOL_P(other)) {
            return Qnil;
        }
        return rb_str_cmp_m(rb_sym_to_s(sym), rb_sym_to_s(other));
    }

Looking at the source code it is clear that if `RHO` is not the object
of class `Symbol`, `nil` will be returned. Let's see the something
more in IRB:

    >> "x" <=> :x
    #=> nil

Again `nil`. The source code saying that
`rb_str_cmp_m(rb_sym_to_s(sym),rb_sym_to_s(other))` will be executed
now. So now I went to the see the source code of STRING.C
->(https://github.com/ruby/ruby/blob/trunk/string.c). So we are
basically pasing the `rb_str_cmp_m(???,"x")`. Now the I found from
github:(`?` means don't know what value)

    rb_str_cmp_m(VALUE str1, VALUE str2)
    {
        int result;

        if (!RB_TYPE_P(str2, T_STRING)) {
    VALUE tmp = rb_check_funcall(str2, rb_intern("to_str"), 0, 0);
    if (RB_TYPE_P(tmp, T_STRING)) {
    result = rb_str_cmp(str1, tmp);
    }
    else {
    return rb_invcmp(str1, str2);
    }
        }
        else {
    result = rb_str_cmp(str1, str2);
        }
        return INT2FIX(result);
    }

But the above code I couldn't understand.But I beleieve it has the
answer how `nil` is producing when `LHO` is not the object of class
`Symbol`.

Can anyone help me here to understand how the `nil` is coming when `LHO`
is not `sym`?

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