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4.1 Teachpacks
4.2 Environment Variables

4 Extending DrScheme

DrScheme supports two forms of extension to the programming environment:

4.1 Teachpacks

Teachpacks are designed to supplement student programs with code that cannot be expressed in a teaching language. For example, to enable students to play hangman, we supply a teachpack that

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 (see Modules). Each exported syntax definition or value definition from the module is provided as a new primitive form or primitive operation to the user, respectively.

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

  #lang scheme
  (provide (rename-out [:lcons lcons]) lcar lcdr)
  (define-struct lcons (hd tl))
  (define-syntax (:lcons stx)
    (syntax-case stx ()
      [(_ hd-exp tl-exp)
                 (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)
     (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" sub-collection in the "teachpack" collection of the PLT installation.

4.2 Environment Variables

Several environment variables can affect DrScheme’s behavior: