Posted by Tom Moertel
Fri, 30 Mar 2007 04:34:00 GMT
How much content have I written for my blog? Let’s find out.
My blog runs on Typo, which is built upon
Ruby on Rails. Let’s fire up the Rails
console and gather a quick word count:
$ cd ~/blog
$ ruby script/console
Loading development environment.
>> require 'article'
=> true
>> Article.find(:all).inject(0) { |sum,a| sum +=
(a.body + a.extended.to_s).split(/\s+/).length }
=> 66665
So I have written about 66 kilo-words, which is entering novel
territory. Paperback-wise,
it’s about 190 pages.
All I need now is a villain and some cool cover art.
;-)
Posted in site news, rails, writing
Tags blog, rails, word_count, words, writing
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Posted by Tom Moertel
Wed, 28 Mar 2007 19:40:00 GMT
This article is part three in a series on introductory Haskell
programming. In the first
article,
we wrote a small program to recursively scan file-system directories
and print their contents as ASCII-art trees. In the second
article,
we refactored the program to make its logic more reusable by separating
the directory-scanning logic from the tree-printing logic. In this
article, we will address a shortcoming of the refactored version: It
must scan directory hierarchies completely before printing their
trees, i.e., it must scan and then print,
when doing both simultaneously is both more efficient and
more user friendly.
Recall from the previous article that our directory-printing program
is factored into three pieces of logic:
- fsTraverse, which traverses a file-system hierarchy and returns a tree data structure;
- showTree, which converts a tree into lovingly crafted ASCII art; and
- traverseAndPrint, which prints the tree for a file-system hierarchy by
using the first two pieces of logic.
The types of the functions are as follows:
fsTraverse :: Path -> DentName -> IO DirTree
showTree :: Tree String -> String
traverseAndPrint :: Path -> IO ()
Note that showTree is a pure function, but the other two return
IO actions that may have side effects.
Within traverseAndPrint, fsTraverse and showTree are combined
into a composite IO action by the =<< combinator:
putStr . showTree =<< fsTraverse root path
The sequencing semantics of Haskell’s IO monad forces all of the
effects of fsTraverse to complete before any following
effects can begin. To better understand these sequencing semantics,
let’s consider a simple example.
The IO-monad code,
can loosely be interpreted as running the action a, which forces
its side effects to occur, and then running the action b, which forces
its side effects to occur.
In reality, a and b are not actions. They are
functions. Like all Haskell functions, they are pure and have no side
effects. It’s just that a and b return values that
represent actions, and those actions may have side effects, and the
semantics of the IO monad guarantee the ordering of those effects
(should the actions end up being connected to the runtime’s
top-level IO action and executed). If you think that’s weird, hold
that thought. For now, all that’s important is that, if the composite
action represented by the expression
(a >> b) is executed, the
effects of a, regardless of how complex, will be executed
before the effects of b.
Thus if a represents building a tree by recursively scanning
a file-system hierarchy, the entire tree must be built before
b ever gets a chance to do its thing. For our particular
application, however, that particular sequencing is suboptimal. We
know from our earlier, monolithic implementation that the
file-system hierarchy can be scanned and printed simultaneously, which
is more efficient. Ideally, then, our refactored
implementation should be just as efficient.
In this article, we will look at one way to maintain the clean,
logical separation of the a part from the b part
while allowing the parts’ effects to be interleaved for efficiency.
We will use an extension to the Haskell language to make the
directory-scanning action lazy so that it builds the tree as the tree
is consumed.
Ready? Let’s dive in.
Read more...
Posted in haskell
Tags directory_tree_series, haskell, io, laziness, lazy, trees
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Posted by Tom Moertel
Tue, 27 Mar 2007 03:23:00 GMT
Recently, Reg Braithwaite wrote about the ad hominem
fallacy. His
article reminded me that debating – the art and science of
constructing sound arguments in the face of opposition – is a valuable
skill.
Though many online debates devolve into name calling and other
foolishness, most are rich opportunities to learn – if learning is
your goal. So make learning your goal.
Look at each debate you enter as a chance to discover something new.
When you participate, assume the other participants are good people,
who deserve an honest argument from you. If you learn the
fundamentals of logic and clear thinking, it’s easy to stay in the
debate, contribute, and increase your (and their) chances to learn.
Many people, however, overlook the opportunity to learn in order to
pursue the opportunity to win. What a mistake. If the price of
winning is ignorance, can you afford the purchase?
Therefore, when I debate, I make considerable efforts to be rational
and reasonable. Even so, it’s hard not to say the wrong thing when
a debate gets heated. To help keep me in the right frame of mind,
I use a simple, idealized debating model.
I wrote about this model six years ago on Kuro5hin, but it’s worth
revisiting. The model is not magic, and I doubt it’s
novel, but it has helped me. Maybe it can help you, too.
Here it is:
- The motivation for debating is to arrive at a better understanding of reality (i.e., the truth).
- All participants share this motivation.
- All participants are intelligent, rational human beings, each fully capable of drawing logical conclusions from facts.
- The reason for disagreements is not because participants want to disagree but rather because their understandings of the facts differ.
- Therefore, the objective of debating is to share information until the participants can bring their understandings of the facts into alignment, which will allow for agreement or at least consensus.
I know that the model and reality part ways at the outset. When I
debate, however, I pretend the model is reality. I do this is
because it allows me to participate earnestly. Forcing myself to make
meaningful contributions increases the chance that debates will end
in somebody learning something useful.
Nevertheless, debates often go wrong. That’s the second reason I use
the model. It gives me something to compare real debates with so that
problems are easy to spot and classify. If a key participant in a
debate makes personal attacks or refuses to accept demonstrated facts,
for example, the problem is easy to see and classify: It is a debate
killer. Time to move on.
Another tool that has helped me stay on track is D. Q. McInerny’s
wonderful introduction to logic, Being Logical: A Guide to Good
Thinking.
This short book, inspired by Strunk and White’s classic, tiny text on
writing, The Elements of
Style,
introduces the foundations of logic, explains how to construct sound
arguments, and prepares you to recognize and avoid illogical thinking
(fallacies). The book is a pleasure to read and makes a handy reference (I
keep mine within arm’s reach). If you need a quick refresher on clear
thinking, add this delightful book to your toolbox.
Don’t forget: Every debate is an opportunity to learn. So when
debating, make learning your goal. And if you learn to debate, you
will have an easier time debating and learning.
Debate to learn. Learn to debate.
Posted in interesting stuff
Tags arguments, debating, learning, life, logic, people, thinking
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Posted by Tom Moertel
Fri, 09 Mar 2007 22:40:00 GMT
If you want to get more out of IMDB movie ratings, check out my
IMDB Movie Rating Decoder Ring, now updated with fresher data (as of 2 March 2007).
Posted in statistics
Tags data, decoder_rinng, imdb, movies, ratings, stars, statistics
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Posted by Tom Moertel
Wed, 07 Mar 2007 21:04:00 GMT
In my previous article on writing a simple directory-tree printer in
Haskell,
I wrote a small program to recursively scan file-system
directories and print their contents as ASCII-art trees. The
program made for an approachable example of how to use Haskell for
“imperative” tasks, but it has a few problems.
First, the directory-scanning logic and tree-printing logic are
intertwined. Neither is reusable. Second, both bits of logic are
rigid, specialized for this particular task. Even if you could
reuse them, you wouldn’t want to.
In this article, the second in a series, we will explore ways to make
our original code more reusable. We will separate the directory
scanning from the tree printing, harness the power of some old
friends from Haskell’s libraries, and think about the costs
and benefits of our changes.
The plan
Recall our original directory tree–listing solution, the
core of which I will reprint below:
tlist path =
visit (if "/" `isPrefixOf` path then "" else ".") "" "" "" path
visit path leader tie arm node = do
putStrLn (leader ++ arm ++ tie ++ node)
visitChildren (path ++ "/" ++ node) (leader ++ extension)
where
extension = case arm of "" -> ""; "`" -> " "; _ -> "| "
visitChildren path leader =
whenM (doesDirectoryExist path) $ do
contents <- getDirectoryContents path
`catch` (\e -> return [show e])
let visibles = sort . filter (`notElem` [".", ".."]) $ contents
arms = replicate (length visibles 1) "|" ++ ["`"]
zipWithM_ (visit path leader "-- ") arms visibles
The tlist function kicks off the process for a particular
file-system path, handing off to visit which recursively descends the
directory tree from the root node. The visit function calls
visitChildren to expand the subtree, if any, for each node visited.
The visitChildren function, in turn, calls back to visit to
repeat the process for each child in the subtree.
In effect, we are traversing the tree rooted at path, printing each
node in passing.
To separate the traversal part from the printing part, we will
introduce a tree data structure. The file system–traversal code
will emit a tree, and the tree-showing code will consume a tree. We
will rewrite our old tlist function, which we might as well rename to
the more descriptive traverseAndPrint, to glue the two pieces
together with the tree serving as glue:
traverseAndPrint :: Path -> IO ()
traverseAndPrint path = do
tree <- fsTraverse root path
putStrLn (showTree tree)
where
root = if "/" `isPrefixOf` path then "" else "."
That’s the plan. Now let’s carry it out.
Read more...
Posted in haskell
Tags directory_tree_series, haskell, io, refactoring, trees
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