This is pretty easy for the common use case (a (research) paper) about Haskell), but I had some more requirements.

• The code is split over several files
• I want to mix code fragments of those files
• I want effects, like uncovering code line by line

It took me a couple of hours to figure out a sensible approach, which I describe here. I’m sure there are more ways to do it, and I’d love to hear them!

Since our lhs2TeX activities will be spread over multiple files, we’ll want to make a simple formatting file to share. Let’s call it ‘talk.fmt’


%include polycode.fmt

% put formatting rules here


Then we set up our main LaTeX file. Because we might want to use some code examples in there that are not in the files, let’s make it an lhs-file: ‘talk.lhs’. In it’s preamble we define a couple of new LaTeX commands that are going to help us with making slides but referencing them in an arbitrary order.


\documentclass{beamer}

\end{document}


First, we have ‘\ignore’ to hide stuff from LaTeX that is only useful in the code and we don’t want to have in our presentation.

The ‘\defslide’ command takes two arguments, a name which is used as a label to reference to and the contents for the slide. The ‘<hidden>’ makes sure that these slides will not be rendered. In this case ‘hidden’ is not a special argument, just a non-existing ‘mode’.

To use a defined slide, we use ‘\slide’, which uses ‘\againslide’ to draw the slide, with all possible subframes (‘<1->’).

In the document body we include all the files which contain slides. Note that this should be .tex file, generated from our .lhs file. For example ‘Code.lhs’ could look like this.


%include talk.fmt

\defslide{thisisan}{
\frametitle{Just a normal frame}
\begin{code}
main :: IO ()
main = undefined
\end{code}}


I don’t uncourage anybody to mix styles like this, it’s just an a example, okay? What you also see here is the way to uncover a piece of code line by line. If you want to learn more about LaTeX Beamer I suggest you consult the fine manual.

For easy use, we can make a Makefile to tie everything up. We also use latexmk, a simple tool that automagically processes the LaTeX file as many times as necessary, which saves a ton of time.


default: talk.pdf

clean:
    latexmk -CA talk


If you want to try it for yourself just copy the above code to the mentioned filenames and run ‘make’ to get the pdf. (Provided you have all the necessary requirements.)

(I think I massaged all the code so that it gets through WordPress’ botching, but you’ll still need to replace the indenting spaces with tabs in the Makefile yourself.)

While in this example only one file with code is used, it easy to see that this way as many files as necessary can be used. As shown, the slides can be in arbitrary order and we can use nice LaTeX Beamer effects like uncovering. Hooray!

map () {
  if [ $# -le 1 ]; then
    return
  else
    local f=$1
    local x=$2
    shift 2
    local xs=$@ 

    $f $x 

    map "$f" $xs
  fi
}

I’m not going to explain everything, but note the ‘local’ commands to keep everything in function scope (to prevent strange bugs) and the quotes around $f in the recursive call to prevent a ‘function call’ of multiple words from being split.

Now you can do some completely nonsensical things, for which you really don’t need map, such as:

$ map touch aap noot mies

But also slightly more useful things such as

$ map "echo foo" aap noot mies
foo aap
foo noot
foo mies 

$ map "echo file:" `ls`
file: aap
file: noot
file: mies

To open up some more possibilities, I also defined a ‘rota’ function, with rotates the arguments of a command such that the last comes first.

rota () {
  local f=$1
  shift
  local args=($@)
  local idx=$(($#-1))
  local last=${args[$idx]}
  args[$idx]= 

  $f $last ${args[@]}
}

I’m using a array (yes, bash has arrays) to easily get the last element, as you can see

So, how about…

$ map "rota mv /tmp" aap noot mies

(Move the aap, noot and mies file to /tmp)

There are probably better (and more useful) examples of it’s usage, but you hopefully get the gist of it.

This code is just for fun (I don’t expect I’m ever going to use it, even), so it’s nowhere near being robust. There are a lot of (possible) bugs concerning spaces and undetected grouping of words (with double quotes). Use at your own risk

I think the result is something that has a bit of a learning curve and is therefore less suitable for ‘one-time’ usage. On the other hand, because it’s more of a picture and not a bunch of numbers I think it’s much easier to learn the timetable by heart for regular users.

For specification I wanted a small and simple language and preferably not start with writing a parser, so I decided to hitch a ride with Haskell first.

I’d love to hear your opinion, both a first reaction and after experimenting with it.

» Download it

(view readme or view haskell module)


1
1 1
2 1
1 2 1 1
1 1 1 2 2 1


Once I solved the puzzle, I wanted to see if I could come up with a short function that generates this sequence. This is my haskell version:


next [] = []
next (x:xs) = [length start + 1,x] ++ next end
where (start, end) = span (== x) xs


You can call it like this:

main = mapM (putStrLn . unwords . map show) (iterate next [1])


Update: Martijn told me about some background behind this sequence. It’s called the Look and Say Sequence, and it’s a quite interesting sequence indeed.

Vim is a great editor and OS X a lovely operating system, but unfortunately, they do not go really well together.  No wonder requests for better OS X integration are at the top of the requested features list.

But so far the Mac Vim community doesn’t seem to have the manpower to get cranking on these issues. Luckily there’s Jjgod Jiang working on a Summer Of Code project to build a Cocoa GUI for vim.

As avid Vim users we’d like to contribute something as well. So we took the week off and started hacking (with just a grain of C knowlegde and no prior Mac API experience) Vim to implement the ODB Editor Suite which will make it possible to use Vim from a whole bunch of FTP applications. We’re making progress and hope to ‘release’ a first version patch soon.

If time allows we’ll also try to develop a simple ‘Edit in Vim’ button that can be used from text widgets in any application.


#!/bin/bash
echo "cd `pwd`" > ~/.bash_last_dir
echo "rm ~/.bash_last_dir" >> ~/.bash_last_dir
echo "touch ~/.bash_last_dir" >> ~/.bash_last_dir
open -a Terminal .


If you include the following line at the end of your ~/.bash_login, you’re all set:


source ~/.bash_last_dir


Make sure the ~/bin directory is in your path, and type “dup” in your Terminal to duplicate it.

I created a small tool which recursively searches for the (partial) filename you provide in the current directory. If it finds only one match, it will launch vim for that file. If it finds more matches, it will show you a list of files to choose from. It will remember your choice, and assign a weight to that file, so that it will bubble to the top of the list next time.

Currently the weight is assigned to the relative filepath. This means that you can easily switch to another branch of your project if it has the same structure. It also means that you always have to run the program from the root of your project.

As a small bonus I added a feature where you can specify the command to run. By default it is vim, but if you provide a different command as the second argument, it will launch that command. For example, you can run it as “v myfile less”, which will open the selected file in less instead of vim.

I hope you enjoy it, let me know if you do. Here’s the link: v.rb. Save the file somewhere in your path.

For example:

$ irb --simple-prompt
>> [1,2,3].zip [4,5,6], [7,8,9]
=> [[1, 4, 7], [2, 5, 8], [3, 6, 9]]


The night after I did this little experiment I suddenly realized: this is a transpose function! I quickly got out of bed to document my little discovery:


def transpose(matrix)
matrix[0].zip *matrix[1..-1]
end


It’s the little things that make me like this language so much.

Update: I forgot the asterisk before matrix[1..-1] in the above method.

sum = 0 ;
$stdin.each do |line|
sum+=line.to_i
end
puts sum


Pretty straightforward. I’d rather had written the script in Haskell, but I figured I needed to get things done. This evening I tried the same thing in Haskell, but to I was surprised: it was even quicker and more straightforward than the Ruby version! Here it is:

main = interact $ show . sum . map read . lines


If you’d like more of these tiny tools, see Haskell Unix Tools.

Currently, I’m following a course on Advanced Functional Programming at the Utrecht University. One of the advanced topics tought in that course is about Arrows. Arrows are an abstraction of computations, just like Monads, but even more general and pretty nice to define stream functions with (useful for defining logical circuits, for example). I’m still trying to get my head fully around it, but so far I understand it enough to add a new part to the evolution (a missing link!): the factorial function defined with arrows.

I will present the code below and sparsely comment on it. Most of the code is actually from the Advanced Functional Programming lecture.

-- We need this library
import Control.Arrow        

-- Define a type for stream functions, from one list to another
newtype SF a b = SF { runSF :: [a] -> [b] }      

-- Define the Arrow functions for the SF type.  Once you've
-- defined these three, you'll get a lot of other functions for
-- free.
instance Arrow SF where
  arr f = SF (map f)
  SF f >>> SF g = SF (f >>> g)
  first (SF f) = SF (unzip >>> first f >>> uncurry zip)      

-- arr   lifts a function to the arrow domain
-- >>>   combines (connects) two arrow functions
-- first only applies the function to the first part from a
--       tuple      

-- See the http://www.haskell.org/arrows for more
-- information and some nice graphics, which will help
-- understanding.

Okay, so far, so good. By defining the ‘first’ and ‘>>>’ functions, we get an ‘&&&’ combinator for free, which we will use in our final definition. For example: ‘f &&& g’ can be understood as accepting and input x, then applying both f and g to it, resulting in the output of a tuple of the results. In short, the type of ‘&&&’ is ‘Arrow arr => arr a b -> arr a c -> arr a (b, c)’.

-- The type of loop is (ArrowLoop arr) => arr (a, c) (b, c) ->
-- arr a b.  A loop makes a function from 'a' to 'b', while
-- 'looping' a 'c'.
instance ArrowLoop SF where
  loop (SF f) = SF $ \\as ->
    let (bs,cs) = unzip (f (zip as (stream cs)))
    in bs      

-- Note in the definition that cs is defined recursive(!)  This
-- is really where the usefulness of Haskell's lazyness kicks
-- in.  Although, because zip and unzip are both strict, we
-- have to help it a little, by defining a helper function
-- stream.      

stream :: [a] -> [a]
stream ~(x:xs) = x:stream xs      

-- The ~ in the pattern makes the pattern matching irrefutable,
-- which means that the pattern will not be interpreted but
-- assumed to be correct.

Well, with that out of the way we can get to the good stuff. We just need two simple helpers to modify the stream.

-- 'Delays' a stream by prepending an element.  This can be
-- used to initialize a stream _before_ the input is read.
-- We're going to need this to use that nasty loop above.
-- Remember, the c was defined recursive, without
-- initialization.
delay :: a -> SF a a
delay x = SF (x:)      

-- Takes an input and muliplies it.  It 'simply' lifts an
-- uncurried (*) to the arrow domain.
-- Example (try it yourself): mul (6,7) = 42
mul :: Arrow arr => arr (Integer, Integer) Integer
mul = arr (uncurry (*))

Okay, now, the basic idea is to supply a stream function with the list of integers and then all we have to do is multiply the result of the first item with the second and multiply that result with the third, etc. Aha, that’s easy, we can just use the loop for that. The only thing we need is in initialization value to start the multiplying with. For this we use the delay. Schematicly, our plan looks like the following picture.

You can clearly see how the loop works while the input and the output of the whole is still a single stream. The dot in the image where the output of ‘mul’ splits is actually the ‘&&&’ combinator. The code then looks like this.

facSF :: SF Integer Integer
facSF = loop (mul >>> (arr id &&& delay 1))

Pretty clean, eh? Only thing left is a nice wrapper to run this thing.

-- Using Integers so that for large numbers (and factorials
-- tend to get really large fast) it still produces nice
-- output.  Try 'fac 420' for example.
fac :: Integer -> Integer
fac x = runSF facSF [1..x] !! fromInteger (x - 1)

This concludes our fun with Arrows. Viva la evolucion!