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+ + +Simply Scheme: Introducing Computer Science, Part 20: Sequential Programming + + +Simply Scheme: +Introducing Computer Science 2/e Copyright (C) 1999 MIT +

Part VI

+

Sequential Programming

+ +
+cover photo + +
Brian +Harvey
University of California, Berkeley +
Matthew +Wright
University of California, Santa Barbara +

+
Download PDF version +
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The three big ideas in this part are effect, sequence, and state. + +

Until now, we've been doing functional programming, where the focus is on +functions and their return values. Invoking a function is like asking a +question: "What's two plus two?" In this part of the book we're going to +talk about giving commands to the computer as well as asking it questions. +That is, we'll invoke procedures that tell Scheme to do something, +such as wash-the-dishes. (Unfortunately, the Scheme standard +leaves out this primitive.) Instead of merely computing a value, such a +procedure has an effect, an action that changes something. + +

Once we're thinking about actions, it's very natural to consider a sequence of actions. First cooking dinner, then eating, and then washing +the dishes is one sequence. First eating, then washing the dishes, and then +cooking is a much less sensible sequence. + +

Although these ideas of sequence and effect are coming near the end of our +book, they're the ideas with which almost every introduction to programming +begins. Most books compare a program to a recipe or a sequence of +instructions, along the lines of + +

to go-to-work
+  get-dressed
+  eat-breakfast
+  catch-the-bus
+
+ +

This sequential programming style is simple and natural, and it does +a good job of modeling computations in which the problem concerns +a sequence of events. If you're writing an airline reservation system, a +sequential program with reserve-seat and issue-ticket commands +makes sense. But if you want to know the acronym of a phrase, that's +not inherently sequential, and a question-asking approach is best. + +

Some actions that Scheme can take affect the "outside" world, such as +printing something on the computer screen. But Scheme can also carry out +internal actions, invisible outside the computer, but changing the +environment in which Scheme itself carries out computations. Defining a new +variable with define is an example; before the definition, Scheme +wouldn't understand what that name means, but once the definition has been +made, the name can be used in evaluating later expressions. Scheme's +knowledge about the leftover effects of past computations is called its state. The third big idea in this part of the book is that we can write +programs that maintain state information and use it to determine their +results. + +

Like sequence, the notion of state contradicts functional programming. +Earlier in the book, we emphasized that every time a function is invoked +with the same arguments, it must return the same value. But a procedure +whose returned value depends on state—on the past history of the +computation—might return a different value on each invocation, even with +identical arguments. + +

We'll explore several situations in which effects, +sequence, and state are useful: + +

•    Interactive, question-and-answer programs that involve keyboard input +while the computation is in progress; +
•    Programs that must read and write long-term data file storage; +
•    Computations that model an actual sequence of events in time +and use the state of the program to model information about the state of +the simulated events. + +

+After introducing Scheme's mechanisms for sequential programming, we'll use +those mechanisms to implement versions of two commonly used types of +business computer applications, a spreadsheet and a database program. + +

+ +

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+Brian Harvey, +bh@cs.berkeley.edu +
+ + -- cgit 1.4.1-2-gfad0