Build a Text Editor in Lisp - Part 1

Tagged as lisp, part 1, text editor

Written on 2019-08-10

Introduction

In this series of posts, we'll go over building a text editor in Lisp, based on Antirez' great work on the minimal editor, Kilo, and as an adaptation of the brilliant C-based Snaptoken tutorials.

Setup

I'll be using SBCL and Quicklisp on Linux, so I cannot guarantee all of this will work correctly on Windows or Mac. Platform-specific things will need to be adapted for your chosen OS. I'll try to point these out when I'm aware of them.

We'll start by creating our Quicklisp project.
Navigate to your Quicklisp local-projects directory - for me, this is ~/home/quicklisp/local-projects - and create a new directory for the project (mine is called "tex").
In there we'll create two files: tex.asd and tex.lisp.

tex.asd Will contain our project specification and build information, currently as follows:

(asdf:defsystem #:tex
  :components ((:file "tex")))

tex.lisp For now will contain our basic setup code to make sure everything builds and runs smoothly before we start writing any real code.

(defpackage #:tex
  (:use :cl)
  (:export main))
  
(in-package :tex)
  
(defun main ()
  (format t "OK~%"))

NOTE: Of course, anywhere you see tex, you'll want to replace this with your chosen project name.

We define our tex package and export its main function, so we can call it from slime or elsewhere later* and use it as the build entry point in a later step, also.
We declare that everything below the (in-package :tex) line, is read into the tex package and so will need to be called with the syntax (tex:main).
Lastly, we create our main function and use format to print "OK".

format uses directives in the control string to interpolate variables, as substitutes for common escaped characters such as newline (\n), and for various other text formatting-related helper shortcuts. Here, we use the ~% directive for its newline purpose. Had we wanted to substitute a variable or other string into the format, we could write something like (format t "Hello ~a~%" "world!") which would substitute the ~a for "world!" and the ~% will give us the newline, as expected.

*Parts of this fail to work in slime, for reasons that will become clear in the next part, but for now this will work fine.

We can, now, take this opportunity to run and check that everything works fine. In your slime buffer, or SBCL REPL, enter:

(ql:quickload :tex)
(tex:main)

Where :tex is the your chosen project name.
Your output should look like this:

CL-USER> (ql:quickload :tex)
To load "tex":
  Load 1 ASDF system:
    tex
; Loading "tex"

(:TEX)
CL-USER> (tex:main)
OK
NIL
CL-USER> 

Providing this went smoothly and your output is similar to mine, we're ready to move on to the next step!

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