From: Jean-Hugues ROBERT <jean_hugues_robert@...>
Date: 2004-04-30T04:30:09+09:00
Subject: Re: coerce(), what protocol to implement it

At 03:45 30/04/2004 +0900, you wrote:
>Hi,
>
>I need to write a class whose instances can be on the left side of

Sorry, I meant on the right side. 38 and still I can figure it out ;-)

>a + operator. I am ready to implement whatever make the other side
>happy. But what is that I need to implement ?
>
>I am trying to understand coerce()
>
>Apparently when a receiver is not happy with the type of a parameter,
>it invokes coerce() on the parameter.
>if not_happy_with? param then x = param.coerce( self)
>
>coerce() then returns an array of two items. Both items can be new
>objects. However coerce() says that they are "compatible".
>
>Question:
>
>1) What happens if coerce() cannot produce such a result when there
>are no such "compatible" objects ? Apparently coerce() raises an
>ArgumentError exception. Right ?
>
>2) How can I make MyClass instances coercible ? That must be possible,
>because 1.coerce( "2") works, yet "2".coerce( 1) does not...
>
>3) Why does 1.coerce( "3") returns [3.0,1.0] instead of [1,3] while
>1.coerce( 3) returns [1,3] ?
>
>4) Can I redefine coerce() ? If so, when is it called ? (I tried,
>it is not called...)
>
>5) By defining to_str() I can have "xx" + MyClass.new(xx) work. What
>do I have to redefine to have 1 + MyClass.new(xx) work ?
>
>Google "ruby coerce" provided little help.
>
>Thanks in advance,
>
>Yours,
>
>Jean-Hugues
>
>
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Web:  http://hdl.handle.net/1030.37/1.1
Phone: +33 (0) 4 92 27 74 17