edward

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About

This is a POSIX-compatible implementation of the standard Unix text editor ed(1). The implementation is written entirely in R7RS CHICKEN Scheme and provides a Scheme library interface for extending the editor with custom commands.

Status

I am presently not aware of any POSIX.1-2008 conformance issues. The library interface, for extending edward with custom commands, is usable but still very experimental and subject to change. Refer to the TODO.md file for more information.

Design

This implementation relies on parser combinators as well as hygienic Scheme macros to ease the implementation of ed commands. Each ed command is defined using a macro (i.e. similar to how one would normally define procedures in Scheme) and parsed through provided parser combinators. In a sense, thus employing language-oriented programming to implement ed commands and thereby making this ed implementation very hackable and easy to extend. The implementation is also split into a CHICKEN program and various library components which allows defining custom commands (refer to the library interface documentation below).

Installation

The program can be installed either using GNU make or chicken-install.

GNU make

Installation via GNU make does not require any CHICKEN configuration and should work out-of-the-box. As such, it is especially useful for packaging purposes. In order to build edward using GNU make run the following commands:

$ make

To install edward to system-wide directories run the following command:

$ make install

The GNU make installation method only installs the edward binary. It does not install the library interface. If you want to interact with the edward library you need to install edward via chicken-install.

chicken-install

Contrary to installation via GNU make, this installation method requires a properly configured CHICKEN toolchain. If CHICKEN has been configured correctly, run the following command to install both the library and the program component:

$ chicken-install

The edward binary will be added to a directory in your $PATH, furthermore the edward library will be available in your CHICKEN library path. More usage information for the edward library is provided below.

Tests

This repository contains both unit tests and integration tests. The latter require a reference implementation of a POSIX.1-2008 compatible ed implementation. Currently, GNU ed >= 1.20.2 is used for this purpose.

Both unit and integration tests can be run using:

$ make check

Optionally, tmux can be installed to also execute tests for edward’s end-of-file handling in an interactive environment.

Usage

For interactive usage I can highly recommend using this software in conjunction with a readline frontend such as rlwrap. This enables readline-like keybindings (e.g. Ctrl+A, Ctrl+W, …) as well as input history support.

Detailed usage instructions for the ed(1) text editor can be found in the POSIX documentation. Additionally, a nice introduction to the editor is provided in the book The Unix Programming Environment by Brian W. Kernighan and Rob Pike (Appendix 1: Editor Summary). In general, since ed is an old editor which hasn’t changed much in the past centuries, many historic documents explaining its usage are still applicable today. Most notably, volume 2A of the seventh edition of the UNIX Programmer’s Manual contains two documents which provide a beginner-friendly introduction to the editor: A Tutorial Introduction to the UNIX Text Editor and Advanced Editing on UNIX both written by Brian W. Kernighan.

Library Interface

Apart from an ed(1) implementation, edward also provides a library interface for extending the editor with custom commands. While the implementation provided here focuses solely on POSIX compatibility, extension to the POSIX standard can be supplied separately using the library interface. The edward library can be used by creating a custom CHICKEN Scheme program which imports the edward libraries, defines custom commands through provided hygienic macros, and executes (edward-main) to start the editor. For example, an edward variant which provides a pipe command for passing a range of lines through a filter can be implemented as follows:

(import (scheme base)
        (chicken process)
        (srfi 14)

        (edward cli)
        (edward util)
        (edward parse)
        (edward ed cmd)
        (edward ed addr)
        (edward ed posix)
        (edward ed editor))

;; Executor for the pipe command
(define (exec-pipe editor range cmd)
  (let-values (((in out _) (process cmd))
               ((lines) (editor-get-lines editor range)))
    (call-with-port
      out
      (lambda (port) (lines->port lines port)))
    (let ((recv (port->lines in)))
      (close-input-port in)
      (exec-delete editor range)
      (exec-insert editor (car range) (car recv)))))

;; Parser for the pipe command
(define-file-cmd (pipe exec-pipe (make-range))
  (parse-cmd-char #\|)
  (parse-token (char-set-complement (char-set #\newline))))

;; Start the editor
(edward-main)

Save this code in edward++.scm and compile it as follows:

$ csc -R r7rs edward++.scm

Drop the resulting edward++ binary somewhere in your $PATH and invoke it as usual. Naturally, it is possible to define multiple custom commands. Refer to lib/ed/posix.scm for the implementation of editor commands mandated by the POSIX standard. Additional commands are also available in a separate edward-contrib repository.

API Documentation

The API of the library interface is documented using Scheme source code comments. An HTML documentation can be generated from these comments using the scmdoc utility. More information on that is provided in doc/README.md.

The API documentation is available via: https://files.8pit.net/edward/latest/doc/

History

The existing GNU and BSD ed implementations are both derived from an implementation written by Andrew Moore in the early-to-mid 1990s. As such, they still share a lot of code and are potentially subject to the same bugs [1]. To the best of my knowledge, edward is the first ed implementation which strives to be fully POSIX compatible but is not derived from Andrew’s original implementation.

License

This program includes code from chibi-scheme (lib/parse/parse.scm) written by Alex Shinn and licensed under a BSD-style license. Furthermore, it includes a stripped-down and slightly modified version of the SRFI 214 reference implementation (lib/buffer/srfi214-minimal.scm) written by Adam Nelson under MIT. The program itself is licensed as follows:

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.