Scheme was introduced in by Gerald J. Sussman and Guy L. Steele Jr., and was the first dialect of Lisp to fully support lexical scoping, first-class. us to author LENO material in the Scheme programming language. Thus, all source From M S W ord, PDF files have been generated using Adobe Acrobat. The Scheme Programming Language (4a Ed) - Ebook download as PDF File . pdf), Text File .txt) or read book online. manual programacion scheme.
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Rubik's cube solver using Scheme. Contribute to imaculate/Scheme development by creating an account on GitHub. The Scheme Programming Language, 4th Edition, by R. Kent Dybvig is available for purchase through all major online and retail bookstores directly from the. PDF/PS conversion by Costantino Pistagna. Page 2. Page 3. Chapter 1. Introduction. Scheme is a general-purpose computer programming language.
Simple Expressions Simple Recursion Following the list of auxiliary keywords is a sequence of one or more rules. As suggested above. When a procedure call is in tail position see below with respect to a lambda expression.
Readers new to programming should begin with an introductory text on programming. On the other hand. Preface 5. Features specific to particular implementations of Scheme are not included. This book is intended to provide an introduction to the Scheme programming language but not an introduction to programming in general. In particular. In its earliest form it was a small language intended primarily for research and teaching.
Steele Jr. It is not intended to supplant the Revised6 Report but rather to provide a more comprehensive introduction and reference manual for the language. Chapter 2 is an introduction to Scheme programming for the novice Scheme programmer that leads the reader through a series of examples. Many of the examples show how a standard Scheme syntactic form or procedure might be implemented. Readers unfamiliar with Scheme or Lisp should also consider reading The Little Schemer  to become familiar with the concepts of list processing and recursion.
The reader is expected to have had some experience programming and to be familiar with terms commonly associated with computers and programming languages. The artist controls only canvas size. Phil Dybvig. When this angle is small.
Pavel Curtis. Chapter 6. An online version of this book is available at http: John Zuckerman. Because the reference portion describes a number of aspects of the language not covered by the introductory chapters along with a number of interesting short examples. Bob Hieb. The page numbers appearing in the summary of forms and procedures and the italicized page numbers appearing in the index indicate the locations in the text where forms and procedures are defined.
They intersect with each other at a certain angle. They present each of Scheme's primitive procedures and syntactic forms in turn. Dan Friedman. For the cover illustration. John LaLonde. For instance. Many individuals contributed in one way or another to the preparation of one or more editions of this book. The lines are first steeped into various scalar fields that perturb their original straight shape.
The task of the artist is to introduce just enough chance at the right place so that the results are at the same time surprising. This is a game of uncertainty. The grids are regular. Matthias Felleisen. Carl Bruggeman. Joan Curry. Chapter 6 is lengthy. Oscar Waddell helped create the typesetting system used to format the printed and online versions of this book.
About the illustrations: Their tone and thickness vary slightly. Bob Prior. John Wait. Masks introduce holes in some layers. Bruce Duba. Chris Haynes. Chapter Chapter 7. A small amount of text and a few examples have been adapted from the Revised6 Report for this book.
Guy Steele. Eugene Kohlbecker. Chapter 5. Oscar Waddell. The summary of forms and index in the online edition include page numbers for the printed version and are thus useful as searchable indexes. Dave Plaisted. Mike Ashley. Chapter 12 contains a collection of example procedures. Finally and most importantly. They are not exactly equidistant. Chapter 4 describes operations for creating procedures and variable bindings. Susan Dybvig. Each line field is created by the composition of any number of grids of parallel lines.
Each of these programs demonstrates a particular set of features. Chapter 9. Frank Silbermann. John Simmons. Sam Daniel. The summary of forms and procedures is a useful first stop for programmers unsure of the structure of a syntactic form or the arguments expected by a primitive procedure.
Bob Burger. Many others have offered minor corrections and suggestions. Following Chapter 12 are bibliographical references. Chapter 8. All the images are created by a single Scheme program that makes most of the decisions. Simple atomic values.
An interpreter or compiler for Scheme can be quite small and potentially fast and highly reliable. Although some early Scheme systems were inefficient and slow. Chapter 1. These checks may be disabled in many implementations. These core forms. The storage required to hold the contents of an object is dynamically allocated as necessary and retained until no longer needed.
The relative inefficiency that sometimes remains results from run-time checks that support generic arithmetic and help programmers detect and correct various common programming errors. At the heart of the Scheme language is a small core of syntactic forms from which all other forms are built. Scheme programs are highly portable across versions of the same Scheme implementation on different machines.
It is a high-level language. Scheme supports many types of data values. These pointers remain behind the scenes. This is in contrast with many other languages where composite data values such as arrays are either statically allocated and never deallocated.
Scheme is a fairly simple language to learn. The extended syntactic forms and many primitive procedures can be defined in Scheme itself. Scheme is a challenging language to understand fully. While Scheme is often identified with symbolic applications. Scheme has been employed to write text editors. They are also portable across different implementations because of the efforts of a group of Scheme language designers who have published a series of reports.
Regardless of representation. Introduction 9. Scheme is a call-by-value language. Introduction Scheme is a general-purpose computer programming language. The most recent. In Scheme. Identifiers may be imported into a program or library or bound locally within a given block of code such as a library. Scheme also allows programmers to define new syntactic forms. Scheme implementations are required to implement tail calls as jumps gotos.
A tail call occurs when one procedure directly returns the result of invoking another procedure. That is. Block structure and lexical scoping help create programs that are modular. Continuations allow the implementation of complex control mechanisms including explicit backtracking.
Scheme supports the definition of arbitrary control structures with continuations. As with other procedures. In others.. As a result. It also facilitates the implementation of interpreters. Scheme procedures are not always named. This representation is the basis for the syntactic extension facilities provided by Scheme for the definition of new syntactic forms in terms of existing syntactic forms and procedures.
The scope of a binding is the block in which the bound identifier is visible minus any portions of the block in which the identifier is shadowed. Blocks may be nested. Certain variables local to the block are the parameters of the procedure. A continuation is a procedure that embodies the remainder of a program at a given point in the program. A continuation may be obtained at any time during the execution of a program.
Whenever it is invoked. Scheme programmers need master only simple procedure calls and recursion and need not be burdened with the usual assortment of looping constructs. A local binding is visible only lexically. An occurrence of an identifier of the same name outside this block refers to a different binding. Many algorithms are most elegantly or efficiently specified recursively. This does not mean. Efficient code for lexical scoping is possible because a compiler can determine before program evaluation the scope of all bindings and the binding to which each identifier reference resolves.
In some languages. In most languages. A special case of recursion. Scheme procedures may be recursive. For example. By default. Scheme variables and keywords are lexically scoped. To support lexical scoping.
These transformation procedures are themselves expressed in Scheme with the help of a convenient high-level pattern language that automates syntax checking. As with procedures in most other languages. Long identifiers are no substitute for comments. Common Lisp  and Scheme are both contemporary Lisp languages. Structured forms and list constants are enclosed within parentheses. Identifiers cannot start with an at sign and normally cannot start with any character that can start a number.
Common Lisp includes more specialized constructs. Scheme evolved from the Lisp language and is considered to be a dialect of Lisp.
Identifiers are delimited by whitespace. Lexical scoping and block structure are features taken from Algol 60 . Scheme Syntax Scheme programs are made up of keywords.
Matched sets of brackets [ ] may be used in place of parentheses and are often used to set off the subexpressions of certain standard syntactic forms for readability. Vectors are written similarly to lists. The remainder of this chapter describes Scheme's syntax and naming conventions and the typographical conventions used throughout this book.
The empty list is written. Common Lisp does not support continuations or require proper treatment of tail calls. Syntactic extensions are useful for defining new language constructs.
Common Lisp adopted lexical scoping and first-class procedures. A good rule is to use short identifiers when the scope of the identifier is small and longer identifiers when the scope is larger. Identifiers may be formed from letters. Like Scheme but unlike earlier Lisp languages. Section 1. There is no inherent limit on the length of a Scheme identifier. Scheme inherited from Lisp the treatment of values as first-class objects.
While the two languages are similar. Scheme was the first Lisp dialect to adopt lexical scoping and block structure. Most large Scheme programs are built from a mix of syntactic extensions and procedure definitions. Scheme Syntax This is a change from previous versions of the Revised Report. Common Lisp's evaluation rules for procedures are different from the evaluation rules for other objects. Identifiers may be written in any mix of upper. Block comments are delimited by and pairs.
Multiple comment characters are often used to set off the latter kind of comment. Bytevectors are written as sequences of unsigned byte values exact integers in the range 0 through bracketed by vu8 and. Comments may appear on any line of a Scheme program. Since the number of whitespace characters spaces and newlines between expressions is not significant.
Comments explaining a particular Scheme expression are normally placed at the same indentation level as the expression. Predicates are procedures that return a true or false answer. The following procedures. Details of the syntax for each type of constant data are given in the individual sections of Chapter 6 and in the formal syntax of Scheme starting on page The names of some list procedures start with list-. Datum comments are typically used to comment out individual definitions or expressions.
Scheme conditional expressions actually treat f as false and all other objects as true. Procedures that perform input or output technically cause side effects. The following is a list of these naming conventions: Case is not important in the syntax of a number.
Scheme Naming Conventions Scheme's naming conventions are designed to provide a high degree of regularity. Case is important within character and string constants.
These include set! Scheme programs should be indented to show the structure of the code in a way that makes the code as readable as possible.. Strings are enclosed in double quotation marks. Comments explaining a procedure or group of procedures are normally placed before the procedures.. Scheme expressions may span several lines. Numbers may be written as integers. Datum comments may also be nested. A datum comment consists of a. Two other forms of comments are supported: Some Scheme values.
Scheme Naming Conventions. The boolean values representing true and false are written t and f. Typographical and Notational Conventions All Scheme objects are printed in a typewriter typeface. When a syntax violation is detected. Printing of this object is often suppressed by interactive Scheme systems. Italics are also used to set off technical terms the first time they appear. Some Scheme systems routinely use a special object to represent unspecified values.
The keyword or procedure name is given in typewriter font. Some standard expressions can evaluate to multiple values if one of their subexpressions evaluates to multiple values. In general. If the word "must" is used.
The typographical conventions used in this book are straightforward. Ellipses are used to specify zero or more occurrences of a subexpression or argument.
The same is true for syntax variables written in italics. If the word "should" is used. Do not count on these values being the same across implementations.
Syntax violations are detected prior to program execution. The phrase "syntax violation" is used to describe a situation in which a program is malformed. While most standard procedures return a single value. Typographical and Notational Conventions A standard procedure or syntactic form whose sole purpose is to perform some side effect is said to return unspecified. An italic typeface is used to set off syntax variables in the descriptions of syntactic forms and arguments in the descriptions of procedures.
Scheme expressions. When this situation can occur. This means that an implementation is free to return any number of values.
In the description of a syntactic form or procedure. This book uses the words "must" and "should" to describe program requirements. A syntax violation occurs if the structure of a syntactic form does not match its prototype. The remaining pieces of the syntax or arguments are shown in italics.
This includes syntactic keywords. Typographical and Notational Conventions. In most cases. In others. These abbreviations are explained at the start of the sections containing the affected entries.
Most Scheme implementations treat expressions loaded from a file the same as expressions typed at the keyboard. The simplest interaction with Scheme follows a "read-evaluate-print" cycle.
You will have learned the syntax of Scheme programs and how they are executed. They are explained in later sections of this chapter. The code is easy to read because the relationship between each expression and its subexpressions is clearly shown. Scheme sees each program as if it were on a single line.
After reading this chapter and working the exercises. Chapter 2. While Scheme provides various input and output procedures. The examples in this chapter and in the rest of the book follow a regular format. If you have access to an interactive Scheme system. Getting Started This chapter is an introduction to Scheme for programmers who are new to the language. In most Scheme systems.
The example programs are formatted in a style that "looks nice" and conveys the structure of the program. With an interactive Scheme system. The important thing is to establish one style and keep to it. Try typing "Hi Mom! Section 2. The system should respond with "Hi Mom! You will get more from this chapter if you are sitting in front of an interactive Scheme system. When your program starts getting longer.
Interacting with Scheme Most Scheme systems provide an interactive programming environment that simplifies program development and experimentation. Preparing your program in a file has several advantages: You can define a procedure and apply it to arguments to see how it works. You can even type in an entire program consisting of a set of procedure definitions and test it without leaving the system. Getting Started The value of the expression is given after the. Scheme ignores indentation and line breaks.
A program often called a read-evaluate-print loop. An expression you might type from your keyboard is given first. One of the simplest Scheme expressions is a string constant. This frees you to concentrate on writing your program without worrying about how its results will be displayed. All structured forms are enclosed in parentheses and written in prefix notation.
If you left off a closing parenthesis or double quote. We say more about the expressions that make up this definition later in this chapter. Here are a few more expressions to try. Try using square. Scheme expressions may span more than one line. If you left off a single quote in the last expression.
For now it suffices to say that define establishes variable bindings. The Scheme system knows when it has an entire expression by matching double quotes and parentheses. Interacting with Scheme. Let's assume you call the file "reciprocal. Just try again. Return to Scheme and try loading your file with the procedure load. You can try to figure out on your own what they mean or wait to find out later in the chapter. As you can see. Note the form of these expressions. The expressions below are called procedure applications.
Simple Expressions The simplest Scheme expressions are constant data objects. Scheme supports other object types. This regularity simplifies the syntax of expressions.
We saw some examples of strings and numbers in the preceding section. Numbers are constants. Any procedure application. Throughout this chapter. Simple Expressions Inexact numbers are usually represented internally using IEEE standard floating-point representations. Exact integers and rational numbers have arbitrary precision.
Each procedure accepts two numeric arguments. If you enter a number. We can thus nest applications of the arithmetic procedures given above to evaluate more complicated formulas. Whenever you try one of the examples in the text. Procedure applications may be nested. Scheme echoes it back to you. Let's discuss numbers in a little more detail. In an interactive Scheme system.
The following examples show that Scheme supports several types of numbers. Simple Expressions. This answer is not good enough. Because quote is required fairly frequently in Scheme code.
The answer is that we must tell Scheme explicitly to treat a list as data rather than as a procedure application. In many languages. We will see more of them later in this chapter.
We do this with quote. While we will not discuss them in this chapter. Try the following expression with and without the quote wrapper. In some cases. A quote expression is not a procedure application. When we evaluate the mathematical expression 1. You might notice that lists look just like procedure applications and wonder how Scheme tells them apart.
The list of numbers 1 2 3 4 5 could hardly be confused with a procedure application. Scheme recognizes a single quotation mark ' preceding an expression as an abbreviation for quote. Not all quote expressions involve lists. Simple numeric objects are sufficient for many tasks.
Each syntactic form is evaluated differently. Lists are written as sequences of objects surrounded by parentheses. Symbols and variables in Scheme are similar to symbols and variables in mathematical expressions and equations. Try entering the above expressions without the quote. Scheme supports several other syntactic forms in addition to procedure applications and quote expressions. Lists need not contain only one type of object.
Now might be a good time to turn to Section 6. We often say an object is quoted when it is enclosed in a quote expression. It is an entirely different syntactic form. Lists may be nested may contain other lists. Scheme supports many other arithmetic procedures. Now let's discuss some Scheme procedures for manipulating lists. The names "car" and "cdr" are derived from operations supported by the first computer on which a Lisp language was implemented.
The procedure cons constructs lists. Creating a new list by adding an Section 2. The cdr of a list with one element is. This is demonstrated by the Scheme interpreter given in Section It takes two arguments.
You might wonder why applications and variables share notations with lists and symbols. Many people believe this to be one of the most important features of Scheme. The second argument is usually a list. While symbols are commonly used to represent variables in symbolic representations of equations or programs. There are two basic procedures for taking lists apart: Each requires a nonempty list as its argument.
The shared notation allows Scheme programs to be represented as Scheme data. Just as quoting a list tells Scheme to treat a parenthesized form as a list rather than as a procedure application.
Numbers and strings may be quoted. The cdr of the last pair in a proper list is the empty list. An improper list is printed in dotted-pair notation. Use your Scheme system to verify your answers. Even pairs whose cdrs are lists can be written in dotted-pair notation. Exercise 2. Notice the word "usually" in the description of cons's second argument.
This might be a good time to turn to that section and familiarize yourself with the other procedures given there. More formally. The procedure cons actually builds pairs. A list is a sequence of pairs. Does your explanation cover the last example in Exercise 2. Determine which compositions of car and cdr applied to a b c d yield b. Determine all legal compositions of car and cdr applied to a b c d.
In fact. This need not be the case. A Scheme evaluator is free to evaluate the expressions in any orderleft to right. The value of a quote expression of the form quote object is simply object.
Applying the addition procedure to 3 and 4 yields 7. Multiplying 7 by 2 we get By applying this process at each level. Before we go on to more syntactic forms and procedures.
Constant objects. Evaluating Scheme Expressions. One possibility is the following. While procedure is often a variable that names a particular procedure. The evaluation of a quote expression is more similar to the evaluation of constant objects..
The second point is that procedure is evaluated in the same way as arg1. You have probably also worked out in your mind a rule for evaluating procedure applications of the form procedure arg Section 3. This rule works for procedure applications but not for quote expressions because the subexpressions of a procedure application are evaluated.
The remaining syntactic forms are syntactic extensions defined. We will discuss the remaining core syntactic forms and a few syntactic extensions in the remaining sections of this chapter. Evaluating Scheme Expressions Let's turn to a discussion of how Scheme evaluates the expressions you type. We have already established the rules for constant objects such as strings and numbers: Since expressions in the first position of a procedure application are evaluated no differently from other expressions.
We say the variables are bound to the values by the let.. An open bracket must be matched by a close bracket. The following examples demonstrate how to do this using Scheme's let syntactic form. A let expression is often used to simplify an expression that would contain two identical subexpressions. Variables and Let Expressions Suppose expr is a Scheme expression that contains a variable var. We use brackets for let and.. The general form of a let expression is let var expr. Doing so also ensures that the value of the common subexpression is computed only once..
We refer to variables bound by let as let-bound variables. Variables and Let Expressions It is possible to nest let expressions. Variables and Let Expressions. The region where a variable binding is visible is called its scope. It appears within the body of both let expressions. This form of scoping is referred to as lexical scoping. Do not perform any algebraic simplifications. The expression above could be rewritten so that the variable bound by the inner let is new-x.
Only the inner binding for x is visible. The scope of the first x in the example above is the body of the outer let expression minus the body of the inner let expression. The inner binding for x is said to shadow the outer binding. A let-bound variable is visible everywhere within the body of its let expression except where it is shadowed.
Shadowing may be avoided by choosing different names for variables. Explain how you derived this value. A procedure is just as much an object as a number. The variables var. Verify that the value of your expression is the same as that of the original expression. Where x is bound to the value of.. When the body of the let is complicated.
In each case we need a different let expression.
It does not have any meaningful printed representation as far as Scheme is concerned. The argument values. Lambda Expressions. A variable that occurs free in a lambda expression should be bound.
As with let expressions. In this case. The procedure expects its actual parameter to be a number. Because procedures are objects. The variable y does not occur free in the lambda expression since it is bound by the lambda expression.
The most common operation to perform on a procedure is to apply it to one or more values. The variable x is said to occur free in the lambda expression or to be a free variable of the lambda expression.
The Scheme Programming Language.. At least n actual parameters must be supplied. This is true even if another binding for x is visible where the procedure is applied The formal parameter specification can be in any of the following three forms: The third case is a hybrid of the first two cases See Section 3.
In the first case. Let's consider a few examples to help clarify the more general syntax of lambda expressions.. As mentioned above. The variables var1. In the second and third cases. In the second. Lambda Expressions These arguments are automatically formed into a list to which the variable x is bound.
The procedure named h is similar but separates out the second argument. In the first example. While f accepts any number of arguments. The value of the procedure named g in the third example is a list whose first element is the first argument and whose second element is a list containing the remaining arguments.
Scheme provides the abbreviations cadr and cddr for the compositions of car with cdr and cdr with cdr. Given only the simple tools you have read about up to this point. Top-Level Definitions If you completed the exercises from the last section. As suggested above. What you need is a top-level definition. Let's establish a top-level definition of the double-any procedure of the last section.
Top-Level Definitions The variables bound by let and lambda expressions are not visible outside the bodies of these expressions. Suppose you have created an object. We can use double-any as if it were a primitive procedure. Top-level definitions. They are easily defined as follows.
Although it is shorter. Top-Level Definitions. A definition of the form define var0 lambda var1. This syntax is often referred to. The exact syntax depends upon the format of the lambda expression's formal parameter specifier. Top-level definitions make it easier for us to experiment with a procedure interactively because we need not retype the procedure each time it is used..
Let's try defining a somewhat more complicated variation of double-any.. This allows you to define procedures in any order you please. The system should not. The procedure returned by doubler accepts one argument. This is especially useful when you are trying to organize a file full of procedure definitions in a way that makes your program more readable. Let's give proc2 a top-level definition and try proc1. It is necessary when two procedures defined at top level depend upon each other.
What happens if you attempt to use a variable that is not bound by a let or lambda expression and that does not have a top-level definition?
Try using the variable i-am-not-defined to see what happens. The following should not cause an exception. We can define. Conditional Expressions So far we have considered expressions that perform a given task unconditionally.
Define each of these with the compose procedure of the preceding exercise. Suppose that we wish to write the procedure abs. Write the procedure compose. The most straightforward way to write abs is to determine whether the argument is negative and if so negate it. In the expression above. If its argument x is negative.
The procedure abs could be written in a variety of other ways. Any of the following are valid definitions of abs. Use compose to define cadr and cddr.
The fourth treats zero separately. The third asks if n is greater than zero or n is equal to zero. Were if a procedure. The syntactic form or operates in a manner similar to if. Why is if a syntactic form and not a procedure? In order to answer this. In case a. Our definition of reciprocal avoids this problem by testing for zero before dividing. The general form of an or expression is or expr. This clarification is necessary because there are many possible true values.
If there are no subexpressions. The second asks if n is not less than zero. Like quote. To be more precise.. The fifth is somewhat tricky. Conditional Expressions In the case where there are no subexpressions. The following procedure. The predicate null? Common Lisp.. Not all predicates require numeric arguments.
The names of most predicates end with a question mark? The value of the and expression is the value of the last subexpression evaluated. Only f is considered false. Conditional Expressions. A predicate is a procedure that answers a specific question about its arguments and returns one of the two values t or f. Using and.
Scheme also provides a set of type predicates that return true or false depending on the type of the object. Two pairs are not the same by eqv? If the two arguments are equivalent. The predicate pair?. This book is intended to provide an introduction to the Scheme programming language but not an introduction to programming in general.
The reader is expected to have had some experience programming and to be familiar with terms commonly associated with computers and programming languages. Readers unfamiliar with Scheme or Lisp should also consider reading The Little Schemer to become familiar with the concepts of list processing and recursion.
Readers new to programming should begin with an introductory text on programming. Scheme has been standardized both formally and informally.
A related series of reports, the "Revised Reports on the Algorithmic Language Scheme", document an evolving informal standard that most implementations support. The current report in this series is the "Revised Report on the Algorithmic Language Scheme", which was completed in This book covers the language of the Revised6 Report.
It is not intended to supplant the Revised6 Report but rather to provide a more comprehensive introduction and reference manual for the language, with more explanatory text and examples, suitable more for users than for implementors. Features specific to particular implementations of Scheme are not included.
On the other hand, no book on Scheme would be complete without some coverage of the interactive top level, since nearly every Scheme system supports interactive use in one form or another, even though the behavior is not standardized by the Revised6 Report.
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