Chapter 2

Interactive Environment

2.1  Introduction

DrScheme provides an interactive environment. The upper, or drschemedefinitions windowDefinitions, window is a full-featured, graphical editor with Emacs-like key bindings. The lower, or drschemeinteractions windowInteractions, window implements a read-eval-print loop (REPL).

The Execute button loads the contents of the Definitions window into the Interactions window.

2.2  The Transparent REPL

The REPL in DrScheme is transparent (as described in the DrScheme paper). This avoids subtle history-induced errors and misunderstandings. DrScheme's REPL avoids these problems in two ways:

• Every time the user clicks Execute, DrScheme erases all internal state associated with the REPL.

• DrScheme warns users when they make changes, do not click Execute, and evaluate expressions in the REPL.