Thursday, May 26, 2005

Another Solution to the Fragile Base Class Problem

Here's a simple python implementation of the solution to the fragile base class problem described in Modular Reasoning in the Presence of Subtyping, and more accessibly in John Cowan's blog. It uses classes in place of "divisions".
class sgd(type):
def __init__(cls, name, bases, dct):
overridden_bases = [ base for attr in dct.keys() for base in cls.__mro__ if hasattr(base, attr) and not attr.startswith('__') ]
required_methods = [ (base,m) for base in overridden_bases for m in base.__dict__ if m not in cls.__dict__ and not m.startswith('__') ]
if required_methods:
raise "unimplemented methods:", required_methods

super(sgd, cls).__init__(name, bases, dct)

class sgd_class(object):
__metaclass__ = sgd

and then:
>>> class a(sgd_class):
... def f(self): pass
... def g(self): pass
>>> class b(a): pass
>>> class c(a):
... def f(self): pass
... def g(self): pass
>>> class d(a):
... def f(self): pass
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "", line 12, in __init__
if required_methods: raise "unimplemented methods:", required_methods
unimplemented methods:: [(<class '__main__.a'>, 'g')]

Monday, May 16, 2005

People Don't Read, and What To Do About It

Ever had someone flagrantly not read your email? Try putting the most important stuff at the top.

Not reading is a lot more prevalent than you think. Most people don't enjoy reading. Even those who do have too little time to read everything they'd like.

The solution is to write your emails in order of decreasing importance. Don't write in chronological order, or even in logical order. Employ the journalistic technique of "heads, decks, and leads" or headlines, sub-headlines, and leading paragraphs.

This is most important for messages to groups of people, and for documentation messages. In other situations, there isn't so much motivation to write more text than will be read.

(This is ironic as a blog post; those who see this post presumably do read. But this advice is for you! You don't realize how unusual you are.)

Friday, May 13, 2005

Exceptions Are Good For You

Languages with exceptions are good for you; they make you realize that your code can be interrupted any time.

#read user password without displaying on screen
os.system("stty -echo")
print "Password:",
password = raw_input()
os.system("stty echo")
What happens if the user changes her mind, and kills the program? All subsequent typing in the shell will be invisible! Checking for errors won't help here.

The only difference in modern languages is that the rest of a program may continue to run even after a function has been interrupted. This creates a new kind of obligation, but not a big one:
  1. you already have to worry about external state without exceptions (above example)
  2. you don't have to worry about local function state, because the exception causes it to be thrown out
  3. you do now have to worry about non-local state
Composable memory transactions would relieve us of even #3; an exception would roll back all the changes.

An effects system would at least catch the omission; it'd flag a function that makes multiple write calls to objects not mentioned in a finally cleanup block.

In languages without exceptions, you have to use a signal handler in the above example; in languages with exceptions, you can use a finally clause for the stty echo.

(In the spirit of the recent exception buzz (Joel Spolsky's popularization of Raymond Chen's rant, and GvR's "resource allocation" work)

Friday, May 06, 2005

Click For Each Page: ACM Queue

Some sites have an obnoxious policy of requiring you to page through their content by clicking "next", "next" a bunch of times. Presumably they do this to better gauge interest in the content; the user cared enough to click to the end of the article. (Web browsers don't naturally support reporting back to the server how far the user scrolls in a page or how long a user has a page open (and focused).) Or they could just be incompetent.

To get around this, you can often click their button "Print This Article" or equivalent.

For some sites, you may have to visit the last section and then click "Print This Article" there. Cute, eh? This is how I read ACM Queue.

Capacity For Self Deception

Someone should start a catalog of memes. It looks like this one started with "Our capacity for self-deception has no known limits." from Ascent of the Mountain, Flight of the Dove: An Invitation to Religious Studies by Michael Novak.

Though it's easier to demonstrate this for criminals, it also applies to the best of us.