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Chapter 4
Extending DrScheme

DrScheme supports two forms of extension to the programming environment:

- A teachpack extends the set of procedures that are built into a language in DrScheme. For example, a teachpack might extend the Beginning Student language with a procedure for playing sounds.

  Teachpacks are particularly useful in a classroom setting, where an instructor can provide a teachpack that is designed for a specific exercise. To use the teachpack, each student must download the teachpack file and select it through the `Language``|``Add Teachpack...` menu item.

- A tool extends the set of utilities within the DrScheme environment. For example, DrScheme's `Check Syntax` button starts a syntax-checking tool, and the `Analyze` button starts the MrSpidey tool.

## 4.1 Teachpacks

Teachpacks are designed to supplement student programs with code that is beyond the teaching languages (Beginning Student, Intermediate Student, Advanced Student). For example, to enable students to play hangman, we supply a teachpack that

- implements the random choosing of a word

- maintains the state variable of how many guesses have gone wrong

- manages the GUI.

All these tasks are beyond students in the third week and/or impose memorization of currently useless knowledge on students. The essence of the hangman game, however, is not. The use of teachpacks enables the students to implement the interesting part of this exercise and still be able to enjoy today's graphics without the useless memorization.

A single Scheme source file defines a teachpack (although the file may access other files via `require`). The file must contain a module, according to the naming convention laid out in the MzScheme manual (the name of the file must be the name of the module, with an additional `.scm` or `.ss` extension on the filename). Each exported syntax definition or value definition from the module is provided as a new primitive form or primtive operation to the user, respectively.

As an example example, the following teachpack provides a lazy cons implementation. To test it, be sure to save it in a file named `lazycons.scm`.

    (module lazycons mzscheme
      (provide lcons lcar lcdr)

      (define-struct lcons (hd tl))

      (define-syntax (lcons stx)
        (syntax-case stx ()
          [(_ hd-exp tl-exp)
           (syntax (make-lcons
                     (delay hd-exp)
                     (delay tl-exp)))]))

      (define (lcar lcons) (force (lcons-hd lcons)))
      (define (lcdr lcons) (force (lcons-tl lcons))))

Then, in this program:

    (define (lmap f l)
      (lcons
       (f (lcar l))
       (lmap f (lcdr l))))

    (define all-nums (lcons 1 (lmap add1 all-nums)))

the list `all-nums` is bound to an infinite list of ascending numbers.

For more examples, see the `htdp` directory of the `teachpack` directory in the PLT installation.

## 4.2 Tools

A separate manual describes the mechanism for defining a tool. See PLT Tools: DrScheme Extension Manual.

## 4.3 Environment Variables

This section lists the environment variables that affect DrScheme's behavior. See the MzScheme manual for general information about environment variables.

- PLTNOTOOLS When this environment variable is set, DrScheme doesn't load any tools.

- PLTONLYTOOL When this environment variable is set, DrScheme only loads the tools in the collection named by the value of the environment variable.

- PLTDRCM When this environment variable is set, DrScheme installs the compilation manager before starting up, which means that the .zo files are automatically kept up to date, as DrScheme's (or a tools) source is modified.

- PLTDRDEBUG When this environment variable is set, DrScheme starts up with errortrace enabled. If the variable is set to profile, DrScheme also records profiling information about itself.