Re: StringIO is not IO?

On 10/12/07, Hongli Lai <[e-mail]> 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.

I suggest that you either use another way to figure out how you should
treat an argument (like number of arguments or a flag) or don't use
"multi-methods" (split the single method into multiple so that you
don't have to look at the "type" to determine what to do).
Multi-methods really aren't a good thing in ruby and clash with the
concept of duck-typing.

> RMagick seems to do the same thing. I tried making RMagick write its
> output to a StringIO, but that failed because it thought it isn't an IO
> object. I spent quite some time on debugging these two problems, and I
> guess that am not the first one.

It sounds like it has the same problem that your code does.  If you
don't do duck-typing you'll restrict flexibility.

> Finally, when one thinks about it logically, StringIO *should* be an IO.
> There's even the word 'IO' in its class name. So why not do an 'include
> IO' in StringIO? That would solve a lot of problems and make the class
> hierarchy more logical.

IO is a class.  You can't include a class.  Even if you could, you
wouldn't want StringIO to have all of the data-structures needed to
implement IO.

To get what you want, you'd really IO to be based on an abstract class
that had no implementation (like Fixnum and Bignum are based on
Integer which has not implementation to be used directly).

Again, your best bet is to just steer clear of #is_a?, #respond_to?,
etc.  If you want these to make multi-methods, just say no - use
multiple methods instead.

Of course the ruby core itself doesn't follow the duck typing
philosophy all the time (actually most of the time it doesn't).  Take
Regexp for example.  Regexp is completely tied to String.  You can't
make something that looks like a String (like say the opposite of
StringIO - an IOString, or a rope - tree structure for a string) and
have it work with Regexp.