The choice of Scheme for beg c of programming that we pointed out at the beginning of theo hers is natural. First, the core of Scheme permits programmer focus on just those two elemer preface: programs as relations between quantities and evaluating programs for specific inputs Using just this core language, students can develop complete programs during the first session with a teacher Second, Scheme can easily be arranged as a tower of language levels. This property is crucial for beginners who make simple notational mistakes that generate obscure error messages about advanced features of a language. The result is often a wasteful search and a feeling of frustration on the student's part. To avoid this problem, our programming environment, DrScheme implements several carefully chosen sublanguages of Scheme. Based on this arrangement, the environment can signal error messages that are appropriate to a student's level of knowledge Better still, the layering of languages prevents many basic mistakes. We developed the layers and the protection modes by observing beginners for weeks in Rice's computer lab. As students learn more about programming and the language, the teacher can expose students to richer layers of the language, which allows students to write more interesting and more concise programs Third, the DrScheme programming environment offers a truly interactive evaluator. It consists of two windows: a Definitions window, where students define programs, and an Interactions window,which acts like a pocket calculator. Students can enter expressions into the latter,and DrScheme determines their values. In other words, computation starts with pocket-calculate arithmetic, which they know quite well, and quickly proceeds from there to calculations with structures,lists,and trees--the kinds of data that computer programs really manipulate Furthermore, an interactive mode of evaluation encourages students to experiment in all kinds of ways and thus stimulates their curiosity Finally, the use of an interactive evaluator with a rich data language permits students to focus on problem solving and program design activities The key improvement is that interactive evaluation renders a discussion of input and output operations(almost)superfluous. This has several consequences. First, input and output operations require memorization. Learning these things is tedious and boring. Conversely, students are better off learning problem-solving skills and using canned input and output support. Second, good text-oriented input requires deep programming skills, which are best acquired in a course on computational problem-solving Teaching bad text-oriented input is a waste of the teachers' and the students' time. Third, moderr software employs graphical user interfaces(GUD), which programmers design with editors and wizards" but not by hand. Again, students are best off learning to design the functions that are connected to rulers, buttons, text fields and so on, rather than memorizing the specific protocols that currently fashionable GUI libraries impose. In short, di ng input and output is a waste of valuable learning time during a first introduction to programming. If students decide to pursue programming in more depth, acquiring the necessary (Scheme) knowledge about input and output procedures is straightforward In summary, students can learn the core of Scheme in a couple of hours, yet the language is as powerful as a conventional programming language. As a result, students can focus immediately on the essence of programming, which greatly enhances their general problem-solving skills The parts of the book The book consists of eight parts and seven intermezzos. The parts focus on program design; the intermezzos introduce other topics concerning programming and computing. Figure 2 shows the -13- TEAM FLY PRESENTS-13- The choice of Scheme for beginners is natural. First, the core of Scheme permits programmers to focus on just those two elements of programming that we pointed out at the beginning of the preface: programs as relations between quantities and evaluating programs for specific inputs. Using just this core language, students can develop complete programs during the first session with a teacher. Second, Scheme can easily be arranged as a tower of language levels. This property is crucial for beginners who make simple notational mistakes that generate obscure error messages about advanced features of a language. The result is often a wasteful search and a feeling of frustration on the student's part. To avoid this problem, our programming environment, DrScheme, implements several carefully chosen sublanguages of Scheme. Based on this arrangement, the environment can signal error messages that are appropriate to a student's level of knowledge. Better still, the layering of languages prevents many basic mistakes. We developed the layers and the protection modes by observing beginners for weeks in Rice's computer lab. As students learn more about programming and the language, the teacher can expose students to richer layers of the language, which allows students to write more interesting and more concise programs. Third, the DrScheme programming environment offers a truly interactive evaluator. It consists of two windows: a Definitions window, where students define programs, and an Interactions window, which acts like a pocket calculator. Students can enter expressions into the latter, and DrScheme determines their values. In other words, computation starts with pocket-calculator arithmetic, which they know quite well, and quickly proceeds from there to calculations with structures, lists, and trees -- the kinds of data that computer programs really manipulate. Furthermore, an interactive mode of evaluation encourages students to experiment in all kinds of ways and thus stimulates their curiosity. Finally, the use of an interactive evaluator with a rich data language permits students to focus on problem solving and program design activities. The key improvement is that interactive evaluation renders a discussion of input and output operations (almost) superfluous. This has several consequences. First, input and output operations require memorization. Learning these things is tedious and boring. Conversely, students are better off learning problem-solving skills and using canned input and output support. Second, good text-oriented input requires deep programming skills, which are best acquired in a course on computational problem-solving. Teaching bad text-oriented input is a waste of the teachers' and the students' time. Third, modern software employs graphical user interfaces (GUI), which programmers design with editors and ``wizards'' but not by hand. Again, students are best off learning to design the functions that are connected to rulers, buttons, text fields and so on, rather than memorizing the specific protocols that currently fashionable GUI libraries impose. In short, discussing input and output is a waste of valuable learning time during a first introduction to programming. If students decide to pursue programming in more depth, acquiring the necessary (Scheme) knowledge about input and output procedures is straightforward. In summary, students can learn the core of Scheme in a couple of hours, yet the language is as powerful as a conventional programming language. As a result, students can focus immediately on the essence of programming, which greatly enhances their general problem-solving skills. The Parts of the Book The book consists of eight parts and seven intermezzos. The parts focus on program design; the intermezzos introduce other topics concerning programming and computing. Figure 2 shows the TEAMFLY TEAM FLY PRESENTS