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► | 4 | Extending Dr Scheme

4.1 | Teachpacks
4.2 | Environment Variables

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4.1`` Teachpacks
4.2`` Environment Variables

Version: 4.1.4
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### 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.

  See Teachpacks for information in creating teachpacks.

- A tool extends the set of utilities within the DrScheme environment. For example, DrScheme’s Check Syntax button starts a syntax-checking tool. For information on creating tools, see Plugins: Extending DrScheme.

#### 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

- implements the random choosing of a word,

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

- 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 (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)
#'(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" sub-collection in the "teachpack" collection of the PLT installation.

#### 4.2`` Environment Variables

Several environment variables can affect DrScheme’s behavior:

- 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. If the variable is bound to a parenthesized list of collections, only the tools in those collections are loaded (The contents of the environment variable are read and expected to be a single symbol or a list of symbols).

- 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.

  If the variable is set to trace then the compilation manager’s output is traced, using the manager-trace-handler procedure.

- 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.

- PLTDRBREAK : When this environment variable is set, DrScheme creates a window with a break button, during startup. Clicking the button breaks DrScheme’s eventspace’s main thread. This works well in combination with PLTDRDEBUG since the source locations are reported for the breaks.

- PLTDRTESTS : When this environment variable is set, DrScheme installs a special button in the button bar that starts the test suite. (The test suite is available only in the source distribution.)

- PLTSTRINGCONSTANTS : When this environment variable is set, DrScheme prints out the string constants that have not yet been translated. If it is set to a particular language (corresponding to one of the files in "string-constants" collection) it only shows the unset string constants matching that language.

  This environment variable must be set when ".zo" files are made. To ensure that you see its output properly, run setup-plt with the -c flag, set the environment variable, and then run setup-plt again.

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