Posted by Tom Moertel
Fri, 22 May 2009 05:59:00 GMT
I know, the economy sucks, and everything is expensive these days. It’s even worse for you, a polyglot programmer with a serious programming-language obsession. You prowl Proggit, lounge at LtU, and occasionally step on over to Stack Overflow. But it’s just not enough. You need more. You need to hang out in meatspace with other fascinating programmers, diving into modern object systems, getting mechanical with crazy VMs, hacking on code like the wild code-hacking beast that you are.
Sure, it’s a nice dream and all, but how are you going to make it happen? And even if you could in theory make it happen, how could you afford to do it now, in this down economy?
Well, my friend, let me share a secret: You can make it happen. And you can afford it. Here’s how: Just be at the 10th Anniversary Yet Another Perl Conference. It’s day upon day upon day of jam-packed programming-language goodness of all sorts, not “just” Perl – and this year it’s the one conference you can afford.
Seriously, I did a little price-checking, and YAPC is about the most underpriced programming-fest on the planet:
| Conference |
Price |
| JavaOne |
$1,995 |
| RailsConf |
895 |
| PyCon |
450 |
| RubyConf |
200 |
| YAPC |
125 |
Wait, you’re not into Perl? No problem. The Perl community has always embraced diversity, and there’s a lot more than just Perl at YAPC. Check out the tag cloud for talks and you’ll see what I’m saying. At YAPC, the good stuff comes in enormous buckets, plenty for programming aficionados of all stripes. Here’s a taste:
- There’s the Parrot Virtual Machine Workshop. That’s right, YAPC comes front-loaded with a thunderous, 2-day, under-the-hood hackfest, all devoted to a modern VM – Parrot – that’s optimized for your favorite dynamic languages.
- You’re serious about keeping track of your code, right? Well, YAPC’s got talks about Git, too.
- Into objects? Then consider a barrel of talks about the Moose object system, which builds upon great ideas from Perl 6, CLOS (of LISP fame), Smalltalk, Java, BETA, OCaml, and Ruby – and sports roles, delegation, and a meta-object protocol.
- Are you into testing? Well, Perl culture is positively infected with testing. At YAPC|10, there are half a dozen talks about it. (Also: find out why you ought to be harnessing the power of TAP, even if you never program in Perl.)
- And, of course, there’s a little Haskell love, too: What Haskell did to my brain.
See, YAPC is for you.
Am I trying to persuade you to join us at YAPC? Yes. But I’m only doing it because I care about you. YAPC is a fascinating conference, packed with hackers from around the world, all eager to share interesting things, things many you would find delightful, if only you knew about them. So I’m letting you know about them, right now, so you don’t miss out.
Do yourself a favor. If you can figure out how to get your brain to Pittsburgh in the 4th week of June 2009 – yes, only 4 weeks away – then by all means register now for YAPC|10. It’s a great conference at a great price, and it’s something no discriminating hacker ought to be denied.
I hope to see you at YAPC|10.
Update: If any Haskellers are reading this and want to meet up at
YAPC, let me know. I’m trying to put together a
BOF session.
Posted in perl
Tags conferences, haskell, moose, parrot, perl, pittsburgh, programming, tap, yapc
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Posted by Tom Moertel
Mon, 25 Aug 2008 02:56:00 GMT
I finally got around to releasing PXSL Tools on Hackage. The package contains pxslcc, a preprocessor that converts Parsimonious XML Shorthand Language into XML, and supporting documentation.
If you want to hack on the Haskell sources, I’ve put the project on GitHub, too. See the pxsl-tools project page to browse the code, or just clone the repo and hack away:
$ git clone git://github.com/tmoertel/pxsl-tools.git
Tags git, haskell, pxsl
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Posted by Tom Moertel
Thu, 21 Aug 2008 01:50:00 GMT
Although at work I code mostly in Python – a language from which
lambda and map were nearly removed – I still find that functional-programming experience
has its benefits. One of the
“functional-programming dividends” I notice most often is insight
gained from considering problems from an algebraic perspective.
Recently, for example, I had a small parsing problem. I had to
write code that, given a simple grammar specification as input, emits
a regular expression that matches the language generated by the
grammar.
Here’s a simplified version of the problem. A grammar specification
is limited to a series of one or more atoms. For example, “a b c”
represents the atom “a”, followed by the atom “b”, followed by the
atom “c”. To generate the grammar, the series of atoms is interpreted
such that each atom (except the last) generates a production rule of
the following form:
atom_rule ::=
<the literal atom> (SPACE <the next rule> | NOTHING)
(SPACE represents literal white space and NOTHING represents an
empty string.) The grammar as a whole is rooted in the first atom’s
rule.
Thus the specification “a b c” represents the following grammar:
grammar ::= a_rule
a_rule ::= "a" (SPACE b_rule | NOTHING)
b_rule ::= "b" (SPACE c_rule | NOTHING)
c_rule ::= "c"
Note that the final atom’s production matches only the literal atom
itself: it has no following rule on which to chain.
The grammar, in turn, generates the following language:
a
a b
a b c
Thus, given the grammar specification “a b c”, my code had to produce
a regular expression that would match “a”, “a b”, or “a b c”.
At this point, please stop for a moment and think about this little
programming exercise. Do any solutions leap to mind? How would you
approach the problem? Form your opinions now, because I’m going to
ask you about them later. (If you’re feeling especially caffeinated, try
coding a solution before reading on.)
Read more...
Posted in functional programming
Tags folds, fp, haskell, python
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Posted by Tom Moertel
Tue, 18 Dec 2007 03:33:00 GMT
XML is fine for representing document-like things, but when it’s
twisted to represent build recipes, configuration files, and little
programming languages, it opens the gates to XML Hell. Once the
gates are opened, the demons of cargo-cult thinking are loosed upon
the world, where they are free to trick innocent programmers into
working with grotesquely twisted XML documents – something no human
mind was designed to comprehend. Ensnared, these programmers are
slowly drawn into the depths of XML Hell, from which their
lamentations echo across the
universe.
When the demons of cargo-cult thinking come for you, don’t be
ensnared! Instead, be prepared – with PXSL – the Parsimonious XML
Shorthand Language
(pronounced “pixel”).
What’s PXSL? It’s a luxurious, thermonuclear smoking jacket that you
can slip on using a convenient preprocessor. Use it whenever you see
grotesque XML on the horizon. Within PXSL’s plush (and stylish)
protection, you can create all the nasty, twisted XML that may be
demanded of you, but you need not descend into XML Hell to do it.
Instead, you can work from the comfort of a well-stocked lounge, where
clarity and conciseness are always on tap.
For example, here’s a snippet from an XSLT stylesheet, in the
original XML:
<xsl:template match="/">
<xsl:for-each select="//*/@src|//*/@href">
<xsl:value-of select="."/>
<xsl:text> </xsl:text>
</xsl:for-each>
</xsl:template>
And here’s the same snippet, written in PXSL:
template /
for-each //*/@src|//*/@href
value-of .
text << >>
Isn’t that refreshing?
Why PXSL?
There are lots of XML shorthands available. (The PXSL FAQ lists about ten of them.) So why choose
PXSL? Here’s why:
Also, PXSL is battle tested. It was first released in 2003 and has
been saving people from XML Hell since. People who try it seem to like it:
- I think PXSL could do wonders for soothing my irrational hatred for all things XML. —kowey
- Impressive… I converted some of my files from XML to PXSL and the readability was much improved. —chris
- Quite aside from the fact that XSLT is finally somewhat readable, the fact that you’ve added a serious macro system means that some serious scripting of XML can occur. I’m very impressed. —invisible
The next time you’re headed for XML Hell, why not give the venerable PXSL a try? You might just find that you like it, too.
This public service announcement was brought to you in celebration of
the 1.0 release of the pxsl-tools package. The PXSL-to-XML compiler
pxslcc is written in Haskell and uses the
cross-platform Haskell Cabal
build/package system to let you use PXSL just about anywhere.
Posted in programming
Tags haskell, pxsl, xml, xslt
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Posted by Tom Moertel
Mon, 10 Dec 2007 21:52:00 GMT
About three years ago, I switched to
Darcs
as my primary source-code management system. It was simple,
intuitive, and powerful, and it made managing my projects more fun and
less frustrating than any centralized VCS ever had. That it was
written in Haskell, one of my favorite programming languages, made
it even better. I was hooked.
Since then, the distributed SCM landscape has changed. Darcs hasn’t
improved much, but its competitors have made long strides, especially
Git and
Mercurial. Both
are crazy fast, vigorously developed, and widely used on large, highly
active real-world projects, such as the Linux kernel and Mozilla 2.
In comparison, Darcs has
stagnated.
When I started working for a new company recently, I had to consider
whether to advocate Darcs or something else. In the end, I decided
that Darcs would be a hard sell. Nobody else at the company uses
Haskell, and having to explain how to avoid the occasional corner
case
seemed liked a losing proposition.
After researching and playing around with Git and Mercurial, I settled
on Git. I like Git’s underlying hashed-blobs model better than
Mercurial’s revlogs, and Git seems to have slightly more development
momentum. Still, it was a close call. Either choice would have been
completely reasonable.
Missing Darcs
When I started using Git on real projects, the one thing I really
missed was the ability to easily amend earlier patches, something
Darcs made trivial. Let me
explain. The typical development workflow goes something like this:
- Checkout copy of upstream code base.
- Implement feature X.
- Commit.
- Implement independent feature Y.
- Commit.
- Implement independent feature Z.
- Commit.
- Push new features back upstream.
Now, what really happens is that when I’m implementing Y or Z,
I’ll realize that I made a mistake in X. The trick is then
fixing X so that my fix is part of the changeset/patch for X that
ultimately gets pushed upstream in the last step. That way, the
upstream folks will see only a single, clean patch for feature X – not
a mishmash of patches that together represent X.
In Darcs, amending the original patch is easy because its patch theory
lets me tweak the patch for X independently of the other patches.
Darcs will simply ask me which patch I want to amend, and I’ll select
the orignal patch for X:
$ emacs # fix X
$ darcs amend-record # amend original patch for X
Mon Dec 10 14:43:13 EST 2007 Tom Moertel <tom@moertel.com>
* Implemented Z
Shall I amend this patch? [yNvpq], or ? for help: n
Mon Dec 10 14:42:12 EST 2007 Tom Moertel <tom@moertel.com>
* Implemented Y
Shall I amend this patch? [yNvpq], or ? for help: n
Mon Dec 10 14:41:46 EST 2007 Tom Moertel <tom@moertel.com>
* Implemented X
Shall I amend this patch? [yNvpq], or ? for help: y
hunk ./x 1
-X1
+X2
Shall I add this change? (1/?) [ynWsfqadjkc], or ? for help: y
Finished amending patch:
Mon Dec 10 14:43:25 EST 2007 Tom Moertel <tom@moertel.com>
* Implemented X
That’s it. The exact same process will work regardless of when I
realize I need to fix X: before I start Y, while I’m implementing Y,
after I’ve committed Y, while I’m working on Z, or after I’ve committed
Z.
Learning to love Git
With Git, however, I can amend a commit only if I haven’t committed anything else before making my fix. In Git’s mind, Y depends on X, and Z
depends on Y, even if they really are independent of one another.
So if I commit the original patch for X and then immediately realize I
need to make a fix, before I start working on Y or Z, it’s easy:
$ emacs # implement X
$ git commit -m 'Implemented X'
# discover problem in X
$ emacs # fix X
$ git commit --amend # amend original patch
More typically, it’s only while I’m working on Y that I’ll
realize I need to fix X. Then it’s more complicated
to amend the original commit:
$ emacs # implement X
$ git commit -m 'Implemented X'
$ emacs # start working on Y
# discover problem in X
$ git stash # stash away half-completed work on Y
$ emacs # fix X
$ git commit --amend # amend original patch for X
$ git stash apply # restore work on Y
$ emacs # continue working on Y
While not as convenient as Darcs’s workflow, it’s perfectly workable.
Now let’s consider another fairly typical case: I commit X and Y and
then start working on Z before I notice the problem in X. I used to
think that Git couldn’t handle this case, but it can, thanks to
git rebase --interactive:
$ emacs # implement X
$ git commit -m 'Implemented X'
$ emacs # implement Y
$ git commit -m 'Implemented Y'
$ emacs # start working on Z
# discover problem in X
$ git stash # stash away half-completed work on Z
$ emacs # fix X
$ git commit -m 'Fixed X'
$ git rebase --interactive HEAD~3 # see comments below
$ git stash apply # restore work on Z
$ emacs # continue working on Z
The
git rebase --interactive command is
powerful. What the
command does, as called in the snippet above, is invoke my editor of
choice on a text file describing the last 3 commits (that’s the
HEAD~3 part):
# Rebasing 3ad99a7..b9a8405 onto 3ad99a7
#
# Commands:
# pick = use commit
# edit = use commit, but stop for amending
# squash = use commit, but meld into previous commit
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
pick 0885540 Implemented X
pick 320b115 Implemented Y
pick b9a8405 Fixed X
I can then edit the file to reorder, merge (squash), and/or remove
the commits. In this example, I want to merge the fix for X into
the original commit that implemented X. So I edit the file like so:
pick 0885540 Implemented X
squash b9a8405 Fixed X
pick 320b115 Implemented Y
Then I save the file, at which point Git takes over and makes the
requested changes, merging the fix for X into the
original commit for X. Now the log shows the original implementation
and fix as one commit:
$ git log
commit f387d650976246c0854d028b040cca40e542be56
Author: Tom Moertel <tom@moertel.com>
Date: Mon Dec 10 15:11:26 2007 -0500
Implemented Y
commit 82a1c849ffd1bd688d5bc9d99be0e63548a89c4c
Author: Tom Moertel <tom@moertel.com>
Date: Mon Dec 10 15:13:03 2007 -0500
Implemented X
Fixed X
commit 3ad99a7ef537b7ae99e435e0d2b4b0d03de92c65
Author: Tom Moertel <tom@moertel.com>
Date: Mon Dec 10 15:11:14 2007 -0500
Initial checkin
Once I figured out how to use git rebase --interactive, I stopped
missing Darcs and started loving Git.
Posted in programming
Tags darcs, dvcs, git, haskell, scm
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Posted by Tom Moertel
Mon, 19 Nov 2007 18:07:00 GMT
Issue 9 of The Monad.Reader is hot off the presses! The issue focuses on three Google-Summer-of-Code projects for Haskell: Cabal configurations, Darcs’s Patch Theory, and the typechecker-framework TaiChi. Good stuff.
I know what I’ll be reading for lunch today.
Posted in haskell
Tags haskell, tmr
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Posted by Tom Moertel
Sat, 08 Sep 2007 00:19:00 GMT
I just released an updated version of cabal2rpm, a small program (written in Perl) that creates RPM spec files from Cabal package descriptions. RPM is the software-packaging format used by several popular Linux distributions, including Red Hat and Fedora. Cabal is the packaging format used by the Haskell community to distribute software written in Haskell.
Bryan O’Sullivan’s cabal-rpm also creates spec files from Cabal packages. Unlike cabal2rpm, it is written in Haskell and directly interfaces with the Cabal libraries. Long term, it is the way to go. For now, however, cabal2rpm may be more convenient because it works out of the box. (To use cabal-rpm, you’ll first need to install the just-tagged Cabal 1.2.0 library, not yet in wide distribution.)
Posted in haskell
Tags cabal, cabal2rpm, haskell
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Posted by Tom Moertel
Sat, 01 Sep 2007 19:39:00 GMT
Via Reddit I found Mark Nelson’s post about a recent word puzzle from NPR’s
Weekend Edition:
Take the names of two U.S. States, mix them all together, then rearrange the letters to form the names of two other U.S. States. What states are these?
The puzzle is fairly straightforward to solve by hand (think about
it), but let’s write a program to solve it. That will give us a convenient
excuse to discuss a super-handy function I use all the time:
clusterBy. In Haskell, it looks like this:
import Control.Arrow ((&&&))
import qualified Data.Map as M
clusterBy :: Ord b => (a -> b) -> [a] -> [[a]]
clusterBy f = M.elems . M.map reverse . M.fromListWith (++)
. map (f &&& return)
What clusterBy does is group a list of values by their signatures,
as computed by a given signature function f, and returns
the groups in order of ascending signature. For example, we
can cluster the words “the tan ant gets some fat” by length, by
first letter, or by last letter just by changing the
signature function we give to clusterBy:
*Main> let antwords = words "the tan ant gets some fat"
*Main> clusterBy length antwords
[["the","tan","ant","fat"],["gets","some"]]
*Main> clusterBy head antwords
[["ant"],["fat"],["gets"],["some"],["the","tan"]]
*Main> clusterBy last antwords
[["the","some"],["tan"],["gets"],["ant","fat"]]
If we use sort as the signature function, we can find anagrams:
*Main> clusterBy sort antwords
[["fat"],["tan","ant"],["gets"],["the"],["some"]]
And that brings us back to the original puzzle. To find the solution,
we must consider each unique pair of state names to form a “word” and
find the anagrams among a list of such “words.”
Assuming we are given
a list of state names on standard input, one state per line, we can
write the shell of our solution as follows:
main = mapM_ print . solve . lines =<< getContents
The shell delegates the real work to solve. It’s job is to
compute the unique, 2-state combinations from the original
list of states, and then find the anagrams among these combinations.
As before, finding the anagrams is simply a matter of calling
clusterBy with the right signature function. We also filter
out the trivial results, which are not valid solutions:
solve = filter ((>1) . length) . clusterBy signature . ucombos
ucombos xs = [[x,y] | x <- xs, y <- xs, x < y]
signature = sort . filter isAlpha . concat
That’s it. Now we can solve the puzzle by feeding our program a list of states:
$ runhaskell anagrams2.hs < states.txt
[["NORTH CAROLINA","SOUTH DAKOTA"],
["NORTH DAKOTA","SOUTH CAROLINA"]]
What a handy little function, that clusterBy.
Posted in programming
Tags clusterby, functions, haskell, hof, puzzles
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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
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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