From: Jean-Hugues ROBERT <jean_hugues_robert@...>
Date: 2004-04-30T03:45:40+09:00
Subject: coerce(), what protocol to implement it

Hi,

I need to write a class whose instances can be on the left side of
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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