On 8/22/06, William C. [email protected] wrote:
I haven’t seen anyone mention it, and I think it’s important to know…
The name ‘Duck Typing’ comes from the saying “if it walks like a duck
and quacks like a duck, it must be a duck.”
(MonkeyTyping - The PEAK Developers' Center)
In short, if a type/class/whatever has all the methods of a duck (that
you intend to use), it’s a duck and can be used as a duck would be used,
regardless of what’s actually under the feathers.
Yes.
In other words, TCPSocket doesn’t just inherit from Socket, Object,
etc… It IS a Socket, it IS an Object, etc.
While this is true, it’s not a particularly good example to explain duck
typing.
Duck typing is not about what an object “is”, but what it can be used
for. Whether an object can be used for something depends on who is
using it.
Let’s look at DT’s example from the Pickaxe. Cutting the code down to
the bare essentials we’ve got:
def csv_from_row(op, row)
#code to compute res from row
op << res << CRLF
end
Now what kind of duck does op need to be? Simply an object which
accumulates objects via the << method.
His first use was:
result = “”
query.each_row {|row| csv_from_row(result, row)
http.write result
This works, Strings meet the requirements of the op parameter, but
they also have other characteristics. The important one here is the
subtle effect on GC. For a more detailed analysis of this see why’s
article http://whytheluckystiff.net/articles/theFullyUpturnedBin.html
So let’s try to find another object which we can put in the role of
the op parameter. Arrays also can be used. So we have
result = []
query.each_row {|row| csv_from_row(result, row)
http.write result
However we’ve now got a little problem in that although an array works
as the op parameter, http.write probably won’t like it. But that’s
easy to fix:
http.write result.join
Back when I was younger, and object[-oriented] programming was fairly
new, my friends and I spent a lot of time philosophizing about types
and objects. Although I don’t remember talking about ducks, we did
talk alot about the theater. The most common word we used to use for
what we now call a duck type was a role.
So when we duck type, we’re in a position similar to a director
casting for a role in a play. Sometimes, we find a great actor, but
we need to subtly adapt the script to fit his style. That’s what’s
happening in DT’s pickaxe example.
Another way of looking at this, is that duck typing is akin to the way
the American President is chosen. An assessment is performed on the
candidates, and then the country and world adapts.
Contrast this to the way things are done in picking the next British
monarch, or the type systems of C++ or Java. In both cases one has to
be born into the role, having the right genes via inheritance. Of
course in the first case, who gets the job doesn’t matter all that
much anymore.
–
Rick DeNatale
My blog on Ruby
http://talklikeaduck.denhaven2.com/
