I recently decided to switch my attention from learning Racket to Chez Scheme. One of the reasons that I chose Racket was because of how easy it is to get up and running. Setting up a development environment for Chez requires jumping through a few more hoops. In this post, I document those hoops.

Chez Scheme

Installation

On macOS, I installed Chez with Homebrew.

$ brew install chezscheme

On Windows, I used the Windows installer.

So far, so good. Nothing tricky about installing Chez Scheme.

REPL

To launch the Chez REPL on macOS, open Terminal and type chez. On Windows, four Chez programs were installed (32- or 64-bit threaded and unthreaded). Open any of those programs to get a Chez REPL [1].

Test the REPL with simple expression.

> (+ 100 10 1)
111

The REPL has several nice features including:

• Navigate through previous expressions with the up and down arrow keys.
• Autocomplete functions and paths with TAB.
• Write and edit multi-line expressions.

> (define (example x y z)
    (if (> x 0)
        (+ y z)
        (- y z)))
> (example 1 2 3)
5

When navigating through previous expressions, only the first line of a multi-line expression is shown. To see (and edit) all lines, type CTRL+L. In the middle of an expression, RET creates a new line; to execute an expression from the middle of an expression, use CTRL+J.

Library Directory

Chez does not come with a package manager, but there are 3rd-party options, e.g., Akku. In this post, though, I will describe manual package management.

library-directories returns the directories where Chez looks for libraries.

> (library-directories)
(("." . "."))

The "." indicates that Chez is looking in the current directory [2]. If you are using a project-based workflow, then you could include your dependencies in the current directory, perhaps in a lib folder. For a 'global' approach, I created a library directory at /Users/username/scheme/lib on macOS, and at C:\scheme\lib on Windows.

Before we go over where to stash that directory information, let's cover library extensions.

> (library-extensions)
((".chezscheme.sls" . ".chezscheme.so")
  (".ss" . ".so")
  (".sls" . ".so")
  (".scm" . ".so")
  (".sch" . ".so"))

These are the file extensions that Chez uses when searching the library directories.

On macOS, you edit .zshrc to add information on library directories and extensions. From a Terminal window, open .zshrc with the Nano text editor.

$ nano .zshrc

These lines add a new directory to library-directories and a new extension to library-extensions.

export CHEZSCHEMELIBDIRS="/Users/username/scheme/lib:"
export CHEZSCHEMELIBEXTS=".sc::.so:"

The : at the end is used to indicate that the new entries should be appended to the existing entries. Remove the : to replace the default values with the new entries. After saving .zshrc, enter the following command in the Terminal.

$ source .zshrc

Now, from a Chez REPL, we can see the effect of our changes.

> (library-directories)
(("/Users/username/scheme/lib" . "/Users/username/scheme/lib")
  ("." . "."))
> (library-extensions)
((".chezscheme.sls" . ".chezscheme.so")
  (".sc" . ".so")
  (".ss" . ".so")
  (".sls" . ".so")
  (".scm" . ".so")
  (".sch" . ".so"))

If we have a library at /Users/username/scheme/lib/srfi/s1/lists.sls, then we import the library with (import (srfi s1 lists)), i.e., you pass the components of the path to import. If you can't import the library, look at the library call at the top of lists.sls, for example, because that will give you a clue of where the library expects to be placed in library-directories.

On Windows 10, type env in the search box in the task bar and open the program to Edit the system environment variables. Then click the Environment Variables button. Click the button to create a new system variable. Type CHEZSCHEMELIBDIRS and C:\scheme\lib; in the name and value fields, respectively. Click OK. Repeat the process using CHEZSCHEMELIBEXTS and .sc;;.so; in the name and value fields. The ; in the values fields has the same meaning as the : on macOS.

Emacs

Emacs is a versatile text editor and the default choice for Scheme programming.

Installation

On macOS, I installed Emacs with Homebrew.

$ brew install --cask emacs

On Windows, I installed MSYS2 and then ran the following command from within MSYS2 to install Emacs.

$ pacman -S mingw-w64-x86_64-emacs

On macOS, I open Emacs via the icon in my applications folder. On Windows, I launch Emacs by typing emacs in the MSYS2 console.

Basic Usage

The power of Emacs is in the keyboard shortcuts and customization. When you are browsing info on Emacs, you will see shorthand for referring to keyboard combinations, e.g., C-x C-f corresponds to CTRL+X followed by CTRL+F. The other important key is the meta key with M as the shorthand. On my MacBook, the meta key is option. On Windows, the default meta key is ALT. Similar to .zshrc, Emacs can be customized through commands saved in the .emacs file.

Geiser

Geiser is a package that provides the ability to run several different Scheme implementations from within Emacs. We can install Geiser through MELPA.

Open Emacs, enter C-x C-f to find a file, and type .emacs at the prompt. On macOS, I added the following to .emacs.

(require 'package)
(add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/") t)
(package-initialize)

On Windows, I added the more extensive code provided on MELPA's Getting Started page.

Save .emacs and restart Emacs. Then type M-x followed by package-refresh-contents. If that is successful, you will see the message Package refresh done in the minibuffer. To install Geiser, type M-x and then package-install. In response to the Install package: prompt, type geiser-chez and hit return. You also need to add (require 'geiser-chez) to the .emacs file.

To customize Geiser, I used the menu options rather than directly editing the .emacs file. Choose Options/Customize Emacs/Specific Group... and type geiser at the prompt. Click on Geiser Chez and change the name of the binary to chez (macOS only).

The Chez REPL is launched through Emacs with M-x followed by geiser-chez. You can navigate through the previous expressions with ESC+P and ESC+N. Multi-line expressions, autocomplete, and syntax highlighting are also supported.

Library Directory

Apparently, the changes that we made to .zshrc on macOS and the environment variables on Windows to point Chez to libraries and extensions are not picked up by the Chez REPL as used by Geiser. We need to add a couple of lines to .emacs.

On macOS...

(setenv "CHEZSCHEMELIBDIRS" "/Users/username/scheme/lib:")
(setenv "CHEZSCHEMELIBEXTS" ".sc::.so:")

On Windows...

(setenv "CHEZSCHEMELIBDIRS" "C:\\scheme\\lib;")
(setenv "CHEZSCHEMELIBEXTS" ".sc;;.so;")

UPDATE (2019-08-20): In Emacs, I eventually noticed that there is an option to highlight matching parantheses, which I find very helpful. Select Options/Highlight Matching Parantheses and then Options/Save Options. I've also started using company-mode for text completion. I was also pleased to discover that reindenting lines in Emacs is as simple as selecting the section to indent and pressing TAB.

UPDATE (2019-12-04): Add the following lines to your .emacs file for scheme-mode to recognize the .sls file extension that is used with scheme code.

(add-to-list 'auto-mode-alist '("\\.sls\\'" . scheme-mode))

UPDATE (2019-12-13): In addition to using TAB to reindent lines in Emacs, my other most used keyboard shortcuts are for executing, commenting, and selecting code. To execute the code in an s-expression, place your cursor at the end of the s-expression and type C-x C-e. If the executed code displays any output, it will be shown in the minibuffer and not the REPL [3]. To evaluate several s-expressions, highlight the region and type C-c C-r. To select an s-expression, place your cursor at the beginning of the s-expression and type M-C-space (where space is the space bar). I've done a lot of fumbling around trying to select s-expressions by dragging the cursor with the mouse so I'm excited to recently discover this last one.

[1] Alternatively, add one of the four versions of Chez Scheme to the Windows path environment variable by typing env in the search box in the task bar and then opening the program to Edit the system environment variables. Then click the Environment Variables button. Select the row for Path and then click on the Edit... button. Then click the New button and add the path. For the 64-bit threaded version, use C:\Program Files\Chez Scheme 9.5\bin\ta6nt. Now, you can launch Chez Scheme from the command prompt with start scheme and run Scheme programs with start scheme path\to\myschemefile.ss.

[2] You can find the current directory by running current-directory.

[3] If you want the output of the code displayed in the REPL, you will have to copy and paste it to the REPL.