From: Sam Roberts <sroberts@...>
Date: 2007-10-13T04:13:50+09:00
Subject: Re: StringIO is not IO?

On Sat, Oct 13, 2007 at 03:37:36AM +0900, Hongli Lai wrote:
> Eric Mahurin wrote:
> >Since StringIO doesn't use IO to base its implementation, it is not an IO.
> >
> >Why does it matter?  If you use standard duck typing, it shouldn't.
> >Class hierarchy doesn't matter with duck typing.  Classes are just an
> >implementation detail with duck typing.  You are best off not looking
> >#is_a?, #respond_to?, etc.  Just assume the object can do the methods
> >you'll be using and use.
> 
> Well, some of my code can write its output to a file (specified by a 
> filename string) or to an IO object. So my code first checks whether 
> output.is_a?(IO), which is not the case.
> 
> RMagick seems to do the same thing. I tried making RMagick write its 

So does RexML. You see same symptoms when you use open-uri, and then try
to pass the returned "not quite IO" object to RexML. Its unfortunate the
std libs aren't a better example of duck-typing. Search around, I think
_why and a few other blogs have talked about how difficult it is to do
streaming decoding of XML coming over HTTP with RexML because of this.

I think the idiomatic ruby way would be for objects that are "like" an
IO object to implement #to_io(), so libs could check for #to_io(). This
would be analogous to #to_str(), etc.

Sam