[Python-ideas] allow `lambda' to be spelled λ

[Stephan Houben]

Hi list,

Here is my speculative language idea for Python:

Allow the following alternative spelling of the keyword `lambda':

λ

(That is "Unicode Character 'GREEK SMALL LETTER LAMDA' (U+03BB).")

Background:

I have been using the Vim "conceal" functionality with a rule which visually
replaces lambda with λ when editing Python files. I find this a great
improvement in
readability since λ is visually less distracting while still quite
distinctive.
(The fact that λ is syntax-colored as a keyword also helps with this.)

However, at the moment the nice syntax is lost when looking at the file
through another editor or viewer.
Therefore I would really like this to be an official part of the Python
syntax.

I know people have been clamoring for shorter lambda-syntax in the past, I
think this is
a nice minimal extension.


Example code:

  lst.sort(key=lambda x: x.lookup_first_name())
  lst.sort(key=λ x: x.lookup_first_name())

  # Church numerals
  zero = λ f: λ x: x
  one = λ f: λ x: f(x)
  two = λ f: λ x: f(f(x))

(Yes, Python is my favorite Scheme dialect. Why did you ask?)

Note that a number of other languages already allow this. (Racket, Haskell).

You can judge the aesthetics of this on your own code with the following
sed command.

  sed 's/\<lambda\>/λ/g'

Advantages:

* The lambda keyword is quite long and distracts from the "meat" of the
lambda expression.
  Replacing it by a single-character keyword improves readability.

* The resulting code resembles more closely mathematical notation (in
particular, lambda-calculus notation),
  so it brings Python closer to being "executable pseudo-code".

* The alternative spelling λ/lambda is quite intuitive (at least to anybody
who knows Greek letters.)


Disadvantages:

For your convenience already noticed here:

* Introducing λ is introducing TIMTOWTDI.

* Hard to type with certain editors.
  But note that the old syntax is still available.
  Easy to fix by upgrading to VIM ;-)

* Will turn a pre-existing legal identifier λ into a keyword.
  So backward-incompatible.

Needless to say, my personal opinion is that the advantages outweigh the
disadvantages. ;-)

Greetings,

Stephan
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