From: Sam Roberts <sroberts@...>
Date: 2003-02-10T04:04:18+09:00
Subject: Operator reordering, good idea? (was Re: ruby-dev summary 19457-19539)

Hi, I'm just a beginner buy program, but...

Quoteing ksaito@uranus.dti.ne.jp, on Mon, Feb 10, 2003 at 01:26:17AM +0900:
> [ruby-dev:19457] equality between "a" and Exception.new("a")
> 
>  Akira Tanaka asked if it is intended that '==' doesn't have commutativity.

I think anybody who has programmed with ruby for more than a few hours has
to know that

  lhs == rhs

is the same as:

  lhs.==(rhs)

or they won't understand most things about the language.

So, == is obviously not commutative. Neither is any other method.

  lhs.do_to rhs

If lhs doesn't have a method "do_to", but rhs does, ruby could reorder
this to

  rhs.do_to lhs

but doesn't.

>  This behavior comes from the equality checking mechanism of String.
>  For example, the following two expressions doesn't return same value.
> 
>    (a) "a" == Exception.new("a")
>    (b) Exception.new("a") == "a"
> 
>  (a) returns true and (b) returns false. It is because Exception.new("a")
>  is automatically transformed to a String object "a" via to_str method in
>  case of (a).  Matz changed a method for checking equality of strings as
>  follows.
> 
>    In case of a == b and b is not a String object, the expression is true
>    if b has a method to_str and b == a is true.

If I understand this correctly, this is unpredictable and surprising to me.

For me, one test of sensibility would be the question: how do I get this
"commutative" behaviour for my own classes? Its easy to define a method
"+" that converts its arg to some type if necessary, but how do I get
ruby to reorder a "+" so that if my class is on the right, it gets
put on the left?

For example:

  irb(main):018:0> "a" + 5
  TypeError: failed to convert Fixnum into String
          from (irb):18:in `+'
          from (irb):18
  irb(main):019:0> class Fixnum
  irb(main):020:1> def to_str
  irb(main):021:2> self.to_s
  irb(main):022:2> end
  irb(main):023:1> end
  nil
  irb(main):024:0> "a" + 5
  "a5"
  irb(main):025:0> 5 + "a"
  TypeError: String can't be coerced into Fixnum
          from (irb):25:in `+'
          from (irb):25

Will this change apply to String.+, or just String.==?

Obviously, this is just my opinion.

Thanks for such a fun language,
Sam