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+<HEAD>
+<TITLE>Simply Scheme part VI introduction</TITLE>
+</HEAD>
+<BODY>
+<CITE>Simply Scheme</CITE> 2/e Copyright (C) 1999 MIT
+<H1>Sequential Programming</H1>
+
+<TABLE><TR><TD>
+<P><IMG SRC="simply.jpg" ALT="cover photo">
+<TD valign="center">
+<CITE><A HREF="http://www.cs.berkeley.edu/~bh/">Brian
+Harvey</A><BR><A HREF="http://www.cnmat.berkeley.edu/~matt">Matthew
+Wright</A><BR>University of California, Berkeley</CITE>
+<BR><BR><A HREF="http://www-mitpress.mit.edu/book-home.tcl?isbn=0262082810">MIT
+Press web page for Simply Scheme</A>
+</TABLE>
+
+<P><A HREF="simply-toc.html">(back to Table of Contents)</A>
+
+<HR>
+
+<P>The three big ideas in this part are <EM>effect, sequence,</EM> and <EM>state.</EM>
+
+<P>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 <EM>do</EM> something,
+such as <CODE>wash-the-dishes</CODE>.  (Unfortunately, the Scheme standard
+leaves out this primitive.)  Instead of merely computing a value, such a
+procedure has an <EM>effect,</EM> an action that changes something.
+
+<P>Once we're thinking about actions, it's very natural to consider
+a <EM>sequence</EM> 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.
+
+<P>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
+
+<PRE>
+to go-to-work
+  get-dressed
+  eat-breakfast
+  catch-the-bus
+</PRE>
+
+<P>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 <CODE>reserve-seat</CODE> and <CODE>issue-ticket</CODE> 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.
+
+<P>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 <CODE>define</CODE> 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 <EM>state.</EM> 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.
+
+<P>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.
+
+<P>We'll explore several situations in which effects,
+sequence, and state are useful:
+
+<UL>
+<LI>Interactive, question-and-answer programs that involve keyboard input
+while the computation is in progress;
+<LI>Programs that must read and write long-term data file storage;
+<LI>Computations that <EM>model</EM> an actual sequence of events in time
+and use the state of the program to model information about the state of
+the simulated events.
+</UL>
+
+<P>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.
+
+<P><A HREF="simply-toc.html">(back to Table of Contents)</A>
+</BODY>
+</HTML>