20 November, 2012


(Sorry I'm making up the code in this post rather than actually distilling down the real implementations - it probably doesn't run but you can get the idea)

A few times recently I've wanted to output column-like data from haskell: HTML tables in two cases, and CSV in another.

In these two cases, I wanted column headings (<th> tags in the HTML case; and a CSV heading line in the CSV case.

Previously I've written code that looks roughly like:

mapM_ putHeading ["heading1","heading2","heading3"]
forM_ rows $ \(entry1, entry2, entry3) -> do
  putCell entry1
  putCell entry2
  putCell entry3

The annoyance here was that nothing ties together the headings and the data values: although in the case of two or three columns, it is relatively simple to see the correspondence, it was getting hard in some wider cases.

Vaguely inspired by lenses, I rewrote some of this code to look like this:

cols = [("heading 1", \(entry1, _, _) -> entry1),
        ("heading 2", \(_, entry2, _) -> entry2),
        ("heading 3", \(_, _, entry3) -> entry3)
mapM_ putHeading (map fst cols)
forM_ rows $ \row -> forM_ cols \col -> (putCell ((snd col) row))

What this does is package up the column heading and the code to generate (for each row) the appropriate content. This makes it easier (I hope) to keep the headings and the data aligned. Also, all the boilerplate that you don't see here (putHeading and putCell disguise it) can be shared, with only a new cols defined for each different table.

13 November, 2012


One of the first cool things you encounter in functional programming is map.

Say you have a function length :: String -> Int which gives the length of a string:

> length "hello"
(in real haskell, thats not actually the type of length but its close enough for now)

Now you can apply that to a list of strings, like this:

> map length ["hello","goodbye"]

For most of I've thought of this as meaning "apply the function length to each element of the list ["hello", "goodbye"].

But theres a slightly different interpretation thats a bit more "functional" feeling, that I've come across recently.

Consider only applying the first argument to map (you can do that in haskell...):

map length
Whats the type of this expression? It is [String] -> [Int]. So what its done is converted a function from string to int, into a new function from lists-of-strings to lists-of-ints.
And now we have that function that converts a list of strings to lists of ints, we can apply it to a list of strings:
> (map length) ["hello","goodbye"]

So the different reading that I see now is "lift this function to work on lists", first, followed by application to a list.

The same new intuition applies to functors in general and fmap, and its from thinking more about category theory that this view of things starts to appeal to me.

06 November, 2012

direct.uk consultation notes

Nominet, the authority for the .uk top level domain has a consultation on opening up registration of second level domains, such as example.uk. (At present, people generally can register third level domains such as example.co.uk, example.org.uk, example.ac.uk or example.gov.uk)
Tomorrow I'm going to a round table session (will King Arthur be there?), and so in preparation I made some notes on my response to their consulation document.My notes are here.

05 November, 2012

why didn't I do this before?

I finally got round to making a shell helper, sr:

echo ssh+screen reattach to $1
ssh -tv $1 screen -x

Oh the hours I've wasted over the last 10 years on those very keystrokes...