Post History:

Animals Matter to Me

Analysis paralysis

I’m expecting to write two chain posts today. The next post will have some actual code, this one has some thoughts on the direction the code will be going until I learn more.

As someone who has spent most of their time using a Prolog interpreter that is not SWI-Prolog, I am mindful of how it can be frustrating to find some interesting looking project only to find that it relies on an SWI library which doesn’t have an equivalent available for another interpreter. While I do believe in using the best tool for the job, I would also like to produce a solution that people can use with the interpreter with which they are already comfortable and in which they have existing code.

For the first time I can finally see why people write code using an SWI library and never look back. So far, the HTTP library has provided some solution to all the basic things I’ve needed and I think I’ll be able to focus on the overall application design much sooner than I expected because I figured I would be dealing with all kinds of lower level HTTP issues. At the moment I have no idea how many websites are running on Prolog but this library really makes the web pretty accessible to any existing Prolog programmer if they care to take the time to write the HTML generating parts.

I am going to stick with the SWI-Prolog libraries for now, but my long term goal is still to provide a web solution compatible with more than just SWI-Prolog. I don’t think I could design a good interface from the ground up without being familiar with what’s already done anyway. Recently I was talking to Terry Swift, a core developer for XSB and he mentioned that some Prolog implementations were looking to have better library compatibility with each other, so that may include the HTTP library I’ve been using.


  • A way to use full HTML pages with some Prolog inside to add the dynamic parts, similar to how ERB is used in Rails. So far the page generating code has all been similar to Ruby’s Builder but with more punctuation, and it really feels like the entire page should not be generated by code. The Prosper framework may have had a solution for this, I will need to investigate.


Installing PlUnit on Ubuntu

Only a short update to the chain tonight because its getting late.

Before starting on the Prolog code for the site, I wanted to see if anyone in the Prolog community had documented how to do Test Driven Development (TDD) or Behavior Driven Development (BDD) in Prolog. I couldn’t even find some discussion on the subject let alone a how-to or tool set. For now, I will be on my own and figuring it out as I go.

The testing framework included with SWI-Prolog is called PlUnit. The Ubuntu SWI packages caused me to waste some more time today, as they don’t seem to include PlUnit, despite it being a package normally distributed with SWI. To use it, I had to get the current version directly from the SWI git respository. I kept the Ubuntu package-installed version of SWI and was able to manually install PlUnit next to it. Here are the steps for doing that.

Clone the SWI-Prolog git repository

git clone git://

Configure and install PlUnit

cd pl/packages/plunit
sudo make install

Note that there is no actual ‘make’ step. There is nothing to build, the files just need to be copied into the correct location.

Check installation

To check that it was installed properly, one can run the files from the PlUnit examples directory.

cd pl/packages/plunit/examples/ # in the cloned git repo
swipl -g "[simple],run_tests,halt."
 % simple compiled into ex_simple 0.01 sec, 108,664 bytes
 % PL-Unit: lists ........ done
 % All 8 tests passed


Apache reverse proxy to SWI-Prolog

It’s the second day of my chain, working towards having up and running a blog application written in Prolog itself. I spent some time tonight getting Apache to talk to directly to a demo app running on SWI-Prolog. While I consider myself capable around an Apache config, I still had to spend a good amount of time reading about proxy modules and options. It seemed that a simple “reverse proxy” is what I needed to start off with.

  • With a forward proxy the client knows which destination URL it wants and has been specifically configured to use a proxy server to get it.
  • With a reverse proxy, the client requests normal paths on the proxy server and a proxied result will be served to the client seamlessly. The client does not know that a proxy is involved, it all occurs on the server side.

Since the requests will be normal HTTP requests from anyone on the web, the reverse proxy is what I’m looking for. To setup the reverse proxy, I added the following lines to a virtual host configuration file:

ProxyRequests Off

<Proxy *>
  Order deny, allow
  Allow from all

ProxyPass /
ProxyPassReverse /

The Prolog application, a simple Hello World program that responds to HTTP requests, was listening on port 8000, hence the ProxyPass address. It didn’t work immediately. I started off with a 500 error, and the following line in the error log:

[warn] proxy: No protocol handler was valid for the URL /. If you
are using a DSO version of mod_proxy, make sure the proxy submodules 
are included in the configuration using LoadModule.

After some Googling, this was fixed by linking the proxy-http module file in /etc/apache2/mods-available to /etc/apache2/mods-enabled.

ln -s /etc/apache2/mods-available/proxy_http.load \

I’m sure eventually I will have some more complex proxy setup for load balancing, serving static files, etc. but this config gets me by for now. My next steps for tomorrow will be putting a small Prolog application serving some simple content on the site, and reading about SWI-Prolog’s unit testing features, which I didn’t get around to tonight.


A chain of my own and making progress

Inspired by my colleague Chris Strom and his chain of self-improvement, I am starting one of my own. Chris described the chain this way:

The concept is that every day you produce something related to your craft. Each day you do this, you put an “X” through the day. After a little while, you have a chain of X’s and you begin to be offended at the idea of breaking the chain.

My chain will be getting up and running as a Prolog news, tutorial, and promotion site written in Prolog itself. After having this project on the back burner for a while I started working on it again just yesterday but without making much progress. The news is a bit better today.

SWI logo

Connect to Prolog using… Prolog

In general I want to keep my code for written in standard, portable Prolog which doesn’t depend on interpreter-specific libraries. That being said, it looks like SWI-Prolog has some pretty capable HTTP-related libraries that can act as the glue between my standard Prolog code and Apache.

There are many SWI-Prolog related packages in the Ubuntu repositories: the main interpreter itself, and then various libraries. Some of the packages might have missing dependencies because one can install the HTTP libraries without having the socket library installed, and the HTTP libraries failed to load without the socket library. This threw me off for a little while, since its not obvious which Ubuntu package includes the socket library (it’s swi-prolog-clib).

The command below will install SWI-Prolog and the socket and HTTP libraries. I then referred to the discussion and examples in the HTTP Support section of the SWI documentation.

sudo apt-get install swi-prolog swi-prolog-clib swi-prolog-http

For my next step, I am working on building the connecting layer using the HTTP library and investigating SWI’s unit testing support.

1 comment

The simplest thing that could possibly fail miserably

When I’m at a B’more on Rails meeting, I might do more listening than talking but just attending gets me excited to work on things, especially when I hear what others are working on. So when I got home from tonight’s meeting I thought I would spend some time on one of the programming tasks on my todo list.

A couple of months ago while waiting for the January B’more on Rails meeting to start, I noticed that was available and I thought it would be not only a good learning experience but also beneficial to the Prolog community to have a site dedicated to practical Prolog programming. I could have had the site running long ago, except that my big idea was that I wanted to write the blog app itself in Prolog. The most Prolog code people are usually exposed to is some simple example of unification, like:

parent(fred, bob).

father(X,Y) :- parent(X,Y), man(X), man(Y).
son(X,Y) :- father(Y,X).

and so starting off with an actual application that isn’t too complicated but is at least useful, like a blog, seemed like a good idea.

I originally looked for an existing framework for Prolog web applications. The only thing I found was something called Prosper, which looked promising but has a dependency on a Windows-specific C library. I decided not to spend the time to port this code since I had no clue whether the Prosper framework was even going to be useful for me.

My next thought was to write some simple Prolog application myself, and connect it to the web server via Rack and Passenger. The hurdle to doing that was finding a Ruby interface to SWI-Prolog, in which I planned to write the application. When I couldn’t find an interface, the work to write that code became a bigger job than writing the Prolog code itself.

The paths to my goal which I thought to be the shortest were growing longer in front of me and were filled with a lot of boring work before I could get to the part that interested me. So, today I decided to tackle the problem with the mindset of using “the simplest thing which could possibly work.” The first thing that came to mind was to use perl, which I knew had some existing modules for connecting to SWI-Prolog and would of course would very easily handle requests from Apache and passing them on to Prolog.

I tried installing two different SWI perl modules and neither one would build the C code they needed properly. After installing some missing libraries I eventually ran into more compilation errors. I was now faced with my “simplest thing” being another long boring road to reach even a small amount of satisfaction.

The final result is that is no more live than it was when I wasn’t working on it. I’ve seen some threads on “using Prolog from PHP” where by “using” they really just mean a system call to the Prolog interpreter and inserting the output into the PHP page. I could use the same approach with any language ruby, perl, etc. but it feels clunky and it is definitely ugly. However, it is now my most likely “simplest thing that could possibly work,” which I may use just to get the project off the ground while I figure out something better.


Isolate sign-up spam with OtherInbox

So many sites require signup and email verification that sometimes I find myself leaving a site I would have liked to try because OtherInbox logo I hate having yet another account and potentially more spam email from each site and their advertisers. For those sites that I really do want to sign up for, I’ve been using OtherInbox for my sign-up email address and keeping the spam isolated. OtherInbox provides a web-based email account that is specifically designed to help organize email from many senders and keep it out of your primary email, thus acting as your “other inbox”.

Here’s some of what makes OtherInbox great to use when creating accounts on new sites.

Invent new email addresses on the spot

An OtherInbox account gives you new email addresses without any setup. By default, any address can be created using the form sender @ For example, when I signed up for Technorati, I invented the address for my account. As soon as OtherInbox receives a message to a new email address, it will create a new mailbox for messages from that sender.

Verification link emails still come to you directly

Whenever OtherInbox receives an email from a sender it has not seen before, it forwards this first message to your primary email account. Since these emails often contain a “Click here to verify your account” link, no extra effort is required to verify these new accounts. After that first email comes to you, OtherInbox will keep any future messages and not tell you about each one.

OtherInbox New Sender email

Automatic message organization

OtherInbox has a nice interface for separating messages from each sender and indicates how many messages are unread from each sender as well. This happens automatically just by inventing a new email address and receiving an email at that address.

OtherInbox mailbox interface

I have 1 OtherInbox beta invitation available so let me know in the comments (include an email address) if you’d like to give it a try.


The three faces of uploaded files in Rails

When using a file_field to upload files to a Rails app, the parameter containing the uploaded file can be one of three possible classes:

  • If a user submits the form without selecting a file for the upload input field, the parameter will be an empty String.
  • If the uploaded file is small, less than about 20 KB, the parameter will be an instance of ActionController::UploadedStringIO.
  • If the file is larger, it will be saved as a temporary file and the parameter will be an instance of ActionController::UploadedTempfile.

It is possible to deal with objects of all three types without explicitly checking the class of the parameter. All three will respond to blank? but only the empty string (no file selected) will return true. Note that a zero length file will return false to blank?. Both types of file data have read and size methods and they do what one would expect.

One method to use with care is local_path. Although both uploaded data types respond to it, UploadedTempfile will return the path to the temporary file created on the server, and UploadedStringIO will return nil because no file was created in the filesystem. Some code that was incorrectly relying on a local_path being present caused the problem I encountered at work which made me aware of this issue.

Another way to deal with this is to let a plugin handle it. Plugins like attachment_fu deal with the different classes as part of their normal handling of uploaded files.

The solution for the particular piece of code we were dealing with was to change from attempting to explicitly open a temp file to using the blank? and read methods to get the uploaded data, regardless of type.


Using Google Effectively

It was suggested to me that I should share my thoughts on how I search effectively using Google. This turned out to be harder than I expected since I haven’t spent much time thinking about what I do, I’ve just been doing it. I will probably add more posts on this subject as I realize things I’ve omitted.

The key to finding things quickly with Google is to start with a half decent query and then refine it, possibly many times. Google provides some simple but important ways of searching and taking advantage of them all is important.

A good result rarely appears after the second page of results

If you don’t find what you are looking for early in the results, you need to refine your query. My first step for doing so is just adding another term. As a simple example, if I’m searching for something related to “Rails” and starting getting train references, sticking “Ruby” at the end of the query will usually solve that. As I look at result pages I try to find other terms which might be helpful in further narrowing down the results.

Try to imagine context

It can be difficult, but the better you can determine context from the two line page snippet that Google provides, the fewer irrelevant links you’ll need to click on. I scan the page titles first, then read the page snippets for the ones that sound relevant and click on the most promising one.

Put exact phrases in double quotes

Phrases in double quotes will be matched exactly, whereas an unquoted list of words will be matched anywhere in a page. This is especially useful for matching error messages exactly and eliminating pages which have partial matches but not exact ones.

Exclude things standing in your way

If you are seeing results about something unrelated to what you are searching for, clearing them out of the way might uncover what you really want. Do this by excluding search terms with a minus in front of them. Usually explicitly excluding one general, but irrelevant, term that you are seeing will remove many of those bad results and give you something new.

Let Google teach you

The Advanced Search page has a great feature that, like many things Google, stands out as being different. Like typical advanced search pages, it helps you build a query based on the different options available. The unique part is that it shows you the search query it is building at the top of the page, so that you can learn how to perform a similar search in the future from the Google main page, or your browser’s search box. Learning the various operators available to you is important as well.

Refine many times if necessary

I might change my search query five times or more while looking for something specific. I think that quickly evaluating the quality of what is returned and adjusting accordingly is my best strategy for tracking down exactly what I want.

I rarely give up on a search without finding something useful, so hopefully these suggestions will help others improve their searches. Please share any tips of your own in the comments!

1 comment

Worst commit message ever

Commit Message: Fixed it.


Trac bookmark shortcuts in Firefox

If I know the number of a Trac ticket I want to see, or if I am interested in a revision I see in an svn blame, then I usually want to go directly to that page in Trac as quickly as possible. To do this I use Firefox bookmarks with keywords.

When I want to see a particular ticket, just type “ticket 12345” in the address bar and Firefox expands that to I use the ‘changeset’ keyword for source revisions so I type “changeset 52312” which gets expanded to

To do this yourself, create two new bookmarks in Firefox with the following parameters.

  • Name: Trac ticket shortcut
    • Location:
    • keyword: ticket
  • Name: Trac changeset shortcut
    • Location:
    • keyword: changeset

Of course the fields can be adjusted to suit personal preferences. This will work for other ticket tracking systems as well, just adjust the location to match the URL your site uses.