rococo - then rubble » 2007

Pretty colors: Syntax Highlighting in Mercurial

Sunday, November 18th, 2007

I recently switched my personal revision control system from subversion to mercurial. One of the great things about mercurial is the built-in web interface, but I missed the syntax highlighting that’s available in interfaces such as ViewVC.

I’ve written a mercurial extension that applies pygments code highlighting. This extension is now available in the main mercurial repo.

To enable it, install pygments and add the following entry to hgrc:

[extension]
hgext.highlight =

An example of the output.

Thanks to micha who wrote an initial patch.

Posted in Mercurial, Python | No Comments »

Python λ Shorthand

Saturday, November 10th, 2007

In Python a lambda expression (anonymous function) is created with the lambda keyword:

map(lambda x: x+1, [1,2,3])

Some Scheme interpreters such as Dr. Scheme allow the λ symbol (U+03BB) as a shorthand for lambda. I started wondering what this would look like in Python. For example:

map(λ x: x+1, [1,2,3])

Or for a slightly more gratuitously complex example, a recursive factorial with the Y combinator:

Y = (λ X:
         (λ p:
              X(λ arg: p(p)(arg)))
     ((λ p:
           X(λ arg: p(p)(arg)))))

F = (λ f: (λ x: (x*f(x-1) if x > 0 else 1)))

fact = Y(F)
fact(5) => 120

The interpreter changes to make this work this are quite simple. All it requires is small change to the grammar, an update to the syntax checker, and a hack in the parser generator to treat characters with the high-bit set as though they were in the alpha class.

There are a few cases where the shorthand is beneficial. For example, compare:

[i for i in lst if i > 42]

to:

filter(λ x: x > 42, lst)

While list comprehensions are the standard python way to accomplish this, I do like how the predicate comes first in the latter version.

Also, pretty much all of the functions in the itertools module could work nicely with the shorthand form:

itertools.groupby(lst, λ x: x.somevalue)

Even so this is probably isn’t something that would be worth adding to the language. Lambda expressions are rarely useful in Python so a shorthand form is not going at add much benefit compared to the added language complexity. Not to mention the need for a UTF-8 aware editor and terminal.

Using It

Since λ probably doesn’t appear on your keyboard a certain amount of editor configuration is required. The easiest way in emacs is to define a key binding that inserts the character:

(global-set-key "\C-c\C-l" (lambda () (interactive) (insert "λ")) )

Alternatively, abbrev-mode can insert this character whenever the word lambda is typed:

(define-abbrev-table 'global-abbrev-table
  '(("lambda" "λ" nil 0)))

To build an interpreter with this enabled you’ll need this patch applied to a recentish checkout of python 3k. I don’t know if it would work on earlier versions.

Update: On reddit, gwern pointed out that it’s better to translate from lambda -> λ in the editor display rather than embed this symbol in the text. There is (of course) elisp to do this.

Posted in Python | No Comments »