Implementing a Lisp

To extend my knowledge and to have a better understanding of the Lisp's foundations, I implemented a simple Lisp in C. It supports symbols, integers, a few primitives, anonymous functions, closures, macros and garbage collection. The implementation follows the KISS principles and is around 1400 lines of code.


Discussion: http://news.ycombinator.com/item?id=3919685
Code: https://github.com/kototama/kml

Emacs key bindings for Lisp programming

I finished reading Learning GNU Emacs, Third Edition this week. The edition is from 2004 but the book has aged well. All essential concepts, modes and key bindings are presented. The Emacs documentation system is also explained, which is useful to know if you want to further extend your knowledge once you read the book. One chapter is an introduction to Emacs programming with Emacs Lisp. It gives the basis to start and explains how to implement a simple major mode. Of course since 2004, the Emacs ecosystem has changed so you won't find something on Magit mode (for Git) or on the Emacs Lisp Package Archive but if you are not an expert Emacs user, you will learn from reading it.

One chapter is on programming modes and Lisp key bindings are presented. Even if you don't use paredit (and really you should - just take a few minutes to learn its key binding) many commands are available to work with S-expressions. Here is one table of the book that illustrates them:

Structurally editing S-expressions gives an intense satisfaction and will make you regret Lisp syntax whenever you have to program with an another language family.

js2-mode fun

In Emacs with the JavaScript js2-mode...

No side effects? How bad can THAT be :-) ?

Updating Clojure namespaces

Continuing my experimentations with Emacs Lisp, I created a function that automatically updates the namespace of a Clojure buffer in conformity with its pathname. This is useful after renaming a file, since changing a file pathname requires to change its namespace, which is sometimes annoying to do by hand.
Once added to your .emacs file, you can call it with:

M-x clojure-update-ns

It assumes clojure-mode is already loaded.

Earmuffs and variables

According to the Clojure coding standards for libraries, variables names should be written with *earmuffs* only when they are intended to be rebind.

Here and there I forget this rule and use earmuffs most of the time, so I decided to create an Emacs Lisp function to add/remove earmuffs to variable:





Add it to your .emacs file and then place your cursor on a variable and call it with

M-x earmuffy

to add earmuffs, or type

C-u M-x earmuffy

to remove them.

Dear language designers, do not forget Ada

These last days I took some time to refresh my old memories of the Ada language. While Ada, like all languages, has defaults on its own I'm still very impressed by some functionalities it offers and that current mainstream languages still don't have. Most of these functionalities are there since the first standardized version in 1983 (since then the language standard was extended in 1995, 2005 and the next version is expected for 2012).

The first impressive future is about primitive types. Here are some of the types defined in Ada and their C equivalent (taken from [1]):

Ada Type	Description	C Equivalent
Character	A single character	char
Integer	An integer (32-bit) number	int
Natural	Zero or positive integer	-
Positive	A positive integer	-
Long_Integer	A big integer (same as long in Gcc)	long (same as int in Gcc)
Long_Long_Integer	A really big (64-bit) integer	long long
Short_Integer	A small (16-bit) integer	short
Short_Short_Integer	A really small (8-bit) integer	char
Float	A real number	float
Long_Float	A big real number	double
Long_Long_Float	A really big real number	long double
Short_Float	A smaller real number	?
Fixed	A fixed-point real number	-
String	An Ada fixed-length string	char array

Boolean types are also provided. New types are defined and type safety is guaranteed. Here we define a new type based on a Long_Float and a variable of this type:

type Speed is new Long_Float;

Speedy_Gonzales_Speed : Speed;

Since Speed is a new type, it is not a Long_Float and thus assignment from a Long_Float value to a Speed variable is forbidden:

X : Long_float := 300.0; 
Speedy_Gonzales_Speed := X;

The compiler effectively gives an error:

expected type "Speed" defined at line 6
found type "Standard.Long_Float"

When it makes sense, we can define a subtype of another primitive type. Assignments between types and their subtypes is allowed. Here we define the type Degree which is a subtype of Float:

subtype Degree is Long_Float;

Oven_Temperature : Degree;
Y : Long_Float := 255.0;

Oven_Temperature := Y;

Types can be constrained with a range definition (taken from [2]):

type Degrees is new Float range -273.15 .. Float'Last;

This allow a Degrees variable to range from -273.15 (absolute zero) to the last value allowed by a Float.

This strong typing provides safety and contrasts with what is seen in current mainstream programming languages. Languages like Java don't provide rich primitive types (there are for instance no unsigned integer in Java!) and require the programmer to define its own cumbersome classes if he wants new types (which poses readability problems if the types are new number types and the language does not support operator overloading). Some languages like C don't even bother so much about type conversion: a float can be assigned to an integer. This leads to solutions which are far from being satisfying (see for instance C FAQ - round).

A lot of other possibilities are offered to the Ada programmer, for instance decimal types can be defined with their precision. The precision is then guaranteed by the compiler:

type money is delta 0.01 digits 18;

Ada also supports multidimensional arrays, bit-level memory access, definition of memory pool, concurrency programming in a task-oriented way, object-oriented programming, generic packages etc.

At another level Scheme and Common Lisp supports a numeric towel. For example, 3 is an integer. Therefore 3 is also a rational, a real, and a complex (example from the Scheme standard).

;; in Scheme:
> (rational? (/ 1 5))
#t
> (rational? 3)
#t
> (real? 3)
#t

While this discussion may not make so much sense for dynamic languages such as Clojure, I think new languages being designed, which are not dynamically typed, could really benefit of having such rich primitives types. A complex type systems like the one from ML or Haskell is not needed for this purpose and primitives types is the most basic and most used feature of a programming language, so dear languages designers, next time, have a small thought for Ada.

[1] http://www.pegasoft.ca/resources/boblap/book.html
[2] http://en.wikibooks.org/wiki/Ada_Programming

Fed up with typing ns declarations?

While watching the demonstration video of the Play framework, I saw that the developer had some nice templates for its TextMate editor. A few Google searches tells us that it is also possible to have the same functionality in Emacs with the YASnippet template system.

Typing all these long namespaces declarations in Clojure is quickly boring when you create a lot of files, so why not create a template for that?

After having installed YASnippet, create a clojure-mode directory inside the yasnippet/snippets/text-mode directory and create a file named ns with this content:

(ns `(let* ((nsname '())
        (dirs (split-string (buffer-file-name) "/"))
        (aftersrc nil))
     (dolist (dir dirs)
        (if aftersrc
            (progn
                (setq nsname (cons dir nsname))
                (setq nsname (cons "." nsname)))
             (when (or (string= dir "src") (string= dir "test"))
                (setq aftersrc t))))
     (when nsname
       (replace-regexp-in-string "_" "-" (substring (apply 'concat (reverse nsname))  0 -5))))`
     (:use $1)
     (:require ))

Now, when inside a Clojure buffer type "ns" and TAB to complete ; if you are for instance in the src/mylib/utils/swing_stuff.clj buffer, this will be expanded into the following text:

(ns mylib.utils.swing-stuff
  (:use )
  (:require ))

Isn't that handy?

Note: this is my first hack with Emacs Lisp, and now that it is working I'm publishing it without any further improvements, so be indulgent regarding the implementation!