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<li class="header">LEARN LISP THE HARD WAY</li>
<li class="chapter " data-level="1.1" data-path="./">
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Second Draft (in-progress)
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CHANGELOG
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TODO
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<li class="header">PREFACE</li>
<li class="chapter " data-level="2.1" data-path="preface.html">
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TANSTAAFL
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<li class="chapter " data-level="2.2" data-path="preface-part-two.html">
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The Hard Way is Easier
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<li class="chapter " data-level="2.3" data-path="preface-part-three.html">
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Who Is This Book For?
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<li class="chapter " data-level="2.4" data-path="introduction.html">
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Lisp: A Future History
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Acknowledgements
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<li class="header">PART ONE</li>
<li class="chapter " data-level="3.1" data-path="1-0-0-overview.html">
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Grokking Lisp
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<li class="chapter " data-level="3.1.1" data-path="1-01-00-lisp-bootcamp.html">
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Common Lisp Bootcamp
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<li class="chapter " data-level="3.1.1.1" data-path="1-01-01-syntax-overview.html">
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Syntax Overview in 5 Minutes
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<li class="chapter active" data-level="3.1.1.2" data-path="1-01-02-repl.html">
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The REPL
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<li class="chapter " data-level="3.1.1.3" data-path="1-01-03-expressions.html">
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Expressions, Parentheses, and Return Values
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<li class="chapter " data-level="3.1.1.4" data-path="1-01-04-lists-cons-cells.html">
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Lists, Cons-Cells, and Memory
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<li class="chapter " data-level="3.1.1.5" data-path="1-01-05-symbols.html">
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Symbols and Namespaces
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<li class="chapter " data-level="3.1.1.6" data-path="1-01-06-prefix-notation.html">
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Prefix Notation
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<li class="chapter " data-level="3.1.1.7" data-path="1-01-07-style-guide.html">
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Common Lisp Style Guide
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<li class="chapter " data-level="3.1.1.8" data-path="1-01-08-configuration.html">
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Configuring Your Development Environment
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</ul>
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<li class="chapter " data-level="3.1.2" data-path="1-02-00-input-output.html">
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Printing, Streams, and Strings
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<li class="chapter " data-level="3.1.2.1" data-path="1-02-01-strings.html">
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Strings
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<li class="chapter " data-level="3.1.2.2" data-path="1-02-02-more-strings.html">
<a href="1-02-02-more-strings.html">
More Strings
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<li class="chapter " data-level="3.1.2.3" data-path="1-02-03-unicode.html">
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Unicode and Strings
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</li>
<li class="chapter " data-level="3.1.2.4" data-path="1-02-04-chars.html">
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Characters
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<li class="chapter " data-level="3.1.2.5" data-path="1-02-05-more-chars.html">
<a href="1-02-05-more-chars.html">
More Characters
</a>
</li>
<li class="chapter " data-level="3.1.2.6" data-path="1-02-06-char-codes.html">
<a href="1-02-06-char-codes.html">
Character Codes
</a>
</li>
<li class="chapter " data-level="3.1.2.7" data-path="1-02-07-strings-from-chars.html">
<a href="1-02-07-strings-from-chars.html">
Strings from Chars
</a>
</li>
<li class="chapter " data-level="3.1.2.8" data-path="1-02-08-printing.html">
<a href="1-02-08-printing.html">
Printing
</a>
</li>
<li class="chapter " data-level="3.1.2.9" data-path="1-02-09-more-printing.html">
<a href="1-02-09-more-printing.html">
More Printing
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<li class="chapter " data-level="3.1.2.10" data-path="1-02-10-prin1.html">
<a href="1-02-10-prin1.html">
Printing With prin1
</a>
</li>
<li class="chapter " data-level="3.1.2.11" data-path="1-02-11-princ.html">
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Printing With princ
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<li class="chapter " data-level="3.1.2.12" data-path="1-02-12-format.html">
<a href="1-02-12-format.html">
A Brief Introduction to Format
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<li class="chapter " data-level="3.1.2.13" data-path="1-02-13-more-format.html">
<a href="1-02-13-more-format.html">
A Little Bit More on Format
</a>
</li>
<li class="chapter " data-level="3.1.2.14" data-path="1-02-14-pathnames.html">
<a href="1-02-14-pathnames.html">
Pathnames
</a>
</li>
<li class="chapter " data-level="3.1.2.15" data-path="1-02-15-streams.html">
<a href="1-02-15-streams.html">
Streams
</a>
</li>
<li class="chapter " data-level="3.1.2.16" data-path="1-02-16-file-streams.html">
<a href="1-02-16-file-streams.html">
File Streams
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</li>
<li class="chapter " data-level="3.1.2.17" data-path="1-02-17-binary-streams.html">
<a href="1-02-17-binary-streams.html">
Binary Streams
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<li class="chapter " data-level="3.1.2.18" data-path="1-02-18-prompting-users.html">
<a href="1-02-18-prompting-users.html">
Prompting Users
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</li>
<li class="chapter " data-level="3.1.2.19" data-path="1-02-19-pretty-printing.html">
<a href="1-02-19-pretty-printing.html">
Pretty-Printing
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</ul>
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<li class="chapter " data-level="3.1.3" data-path="1-03-0-getting-input-from-users.html">
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Extra Credit: Getting Input from Users
</a>
</li>
<li class="chapter " data-level="3.1.4" data-path="1-04-0-lists.html">
<a href="1-04-0-lists.html">
Lists and List-Operations
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</li>
<li class="chapter " data-level="3.1.5" data-path="1-05-0-lookups-trees.html">
<a href="1-05-0-lookups-trees.html">
Extra Credit: Look-up Lists and Trees
</a>
</li>
<li class="chapter " data-level="3.1.6" data-path="1-06-0-math.html">
<a href="1-06-0-math.html">
Numbers and Math
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<ul class="articles">
<li class="chapter " data-level="3.1.6.1" data-path="1-06-01-integers.html">
<a href="1-06-01-integers.html">
Integers
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</li>
<li class="chapter " data-level="3.1.6.2" data-path="1-06-02-more-integers.html">
<a href="1-06-02-more-integers.html">
More Integers
</a>
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<li class="chapter " data-level="3.1.6.3" data-path="1-06-03-hexadecimal-notation.html">
<a href="1-06-03-hexadecimal-notation.html">
Hexadecimal Integer Notation
</a>
</li>
<li class="chapter " data-level="3.1.6.4" data-path="1-06-04-octal-notation.html">
<a href="1-06-04-octal-notation.html">
Octal Integer Notation
</a>
</li>
<li class="chapter " data-level="3.1.6.5" data-path="1-06-05-binary-notation.html">
<a href="1-06-05-binary-notation.html">
Binary Integer Notation
</a>
</li>
<li class="chapter " data-level="3.1.6.6" data-path="1-06-06-ratios.html">
<a href="1-06-06-ratios.html">
Ratios and Rational Numbers
</a>
</li>
<li class="chapter " data-level="3.1.6.7" data-path="1-06-07-floating-point.html">
<a href="1-06-07-floating-point.html">
Floating-point Numbers
</a>
</li>
<li class="chapter " data-level="3.1.6.8" data-path="1-06-08-constants.html">
<a href="1-06-08-constants.html">
Numeric Constants
</a>
</li>
<li class="chapter " data-level="3.1.6.9" data-path="1-06-09-complex-numbers.html">
<a href="1-06-09-complex-numbers.html">
Complex Numbers
</a>
</li>
<li class="chapter " data-level="3.1.6.10" data-path="1-06-10-arithmetic.html">
<a href="1-06-10-arithmetic.html">
Arithmetic
</a>
</li>
<li class="chapter " data-level="3.1.6.11" data-path="1-06-11-more-arithmetic.html">
<a href="1-06-11-more-arithmetic.html">
More Arithmetic
</a>
</li>
<li class="chapter " data-level="3.1.6.12" data-path="1-06-12-even-more-arithmetic.html">
<a href="1-06-12-even-more-arithmetic.html">
Even More Arithmetic
</a>
</li>
<li class="chapter " data-level="3.1.6.13" data-path="1-06-13-exponents.html">
<a href="1-06-13-exponents.html">
Exponents
</a>
</li>
<li class="chapter " data-level="3.1.6.14" data-path="1-06-14-logarithms.html">
<a href="1-06-14-logarithms.html">
Logarithms
</a>
</li>
<li class="chapter " data-level="3.1.6.15" data-path="1-06-15-trigonometry.html">
<a href="1-06-15-trigonometry.html">
Trigonometry
</a>
</li>
<li class="chapter " data-level="3.1.6.16" data-path="1-06-16-psuedorandom-numbers.html">
<a href="1-06-16-psuedorandom-numbers.html">
Pseudo-Random Numbers
</a>
</li>
</ul>
</li>
<li class="chapter " data-level="3.1.7" data-path="1-07-0-arrays.html">
<a href="1-07-0-arrays.html">
Extra Credit: Arrays and Vectors
</a>
</li>
<li class="chapter " data-level="3.1.8" data-path="1-08-0-variables.html">
<a href="1-08-0-variables.html">
Variables, Parameters, and Constants
</a>
</li>
<li class="chapter " data-level="3.1.9" data-path="1-09-0-closures.html">
<a href="1-09-0-closures.html">
Extra Credit: Closures
</a>
</li>
<li class="chapter " data-level="3.1.10" data-path="1-10-0-functions.html">
<a href="1-10-0-functions.html">
Functions and Macros
</a>
</li>
<li class="chapter " data-level="3.1.11" data-path="1-11-0-text-adventure.html">
<a href="1-11-0-text-adventure.html">
Extra Credit: A Simple Text Adventure
</a>
</li>
<li class="chapter " data-level="3.1.12" data-path="1-12-0-namespaces.html">
<a href="1-12-0-namespaces.html">
Namespaces, Symbols, Packages, and Systems
</a>
</li>
<li class="chapter " data-level="3.1.13" data-path="1-13-0-simple-web-app.html">
<a href="1-13-0-simple-web-app.html">
Extra Credit: A Simple Web Application
</a>
</li>
<li class="chapter " data-level="3.1.14" data-path="1-14-0-conditionals.html">
<a href="1-14-0-conditionals.html">
Conditionals
</a>
</li>
<li class="chapter " data-level="3.1.15" data-path="1-15-0-command-line-utility.html">
<a href="1-15-0-command-line-utility.html">
Extra Credit: Command-Line Utilities
</a>
</li>
<li class="chapter " data-level="3.1.16" data-path="1-16-0-map-loop.html">
<a href="1-16-0-map-loop.html">
Mapping and Looping
</a>
</li>
<li class="chapter " data-level="3.1.17" data-path="1-17-0-iterate.html">
<a href="1-17-0-iterate.html">
Extra Credit: Revisiting Loops with Iterate
</a>
</li>
<li class="chapter " data-level="3.1.18" data-path="1-18-0-format.html">
<a href="1-18-0-format.html">
Format Strings
</a>
</li>
<li class="chapter " data-level="3.1.19" data-path="1-19-0-dsl.html">
<a href="1-19-0-dsl.html">
Extra Credit: Domain Specific Languages
</a>
</li>
<li class="chapter " data-level="3.1.20" data-path="1-20-0-review.html">
<a href="1-20-0-review.html">
Part One in Review
</a>
</li>
</ul>
</li>
<li class="header">PART TWO</li>
<li class="chapter " data-level="4.1" data-path="2-0-0-overview.html">
<a href="2-0-0-overview.html">
The Suffusion of Blue
</a>
<ul class="articles">
<li class="chapter " data-level="4.1.1" data-path="2-01-0-programming-paradigms.html">
<a href="2-01-0-programming-paradigms.html">
Programming Paradigms
</a>
</li>
<li class="chapter " data-level="4.1.2" data-path="2-02-0-regex.html">
<a href="2-02-0-regex.html">
Extra Credit: Regular Expressions
</a>
</li>
<li class="chapter " data-level="4.1.3" data-path="2-03-0-objects-control.html">
<a href="2-03-0-objects-control.html">
Objects and Control Structures
</a>
</li>
<li class="chapter " data-level="4.1.4" data-path="2-04-0-data-persistence.html">
<a href="2-04-0-data-persistence.html">
Extra Credit: Persistence and Databases
</a>
</li>
<li class="chapter " data-level="4.1.5" data-path="2-05-0-extended-types.html">
<a href="2-05-0-extended-types.html">
Extended Types
</a>
</li>
<li class="chapter " data-level="4.1.6" data-path="2-06-0-threads-memos-parallel.html">
<a href="2-06-0-threads-memos-parallel.html">
Extra Credit: Concurrency and Memoization
</a>
</li>
<li class="chapter " data-level="4.1.7" data-path="2-07-0-logic-and-more-math.html">
<a href="2-07-0-logic-and-more-math.html">
Logic and Advanced Math
</a>
</li>
<li class="chapter " data-level="4.1.8" data-path="2-08-0-number-theory.html">
<a href="2-08-0-number-theory.html">
Extra Credit: Number Theory
</a>
</li>
<li class="chapter " data-level="4.1.9" data-path="2-09-0-binary-octets-bits.html">
<a href="2-09-0-binary-octets-bits.html">
Binary Streams, Octet-Vectors, and Bit-Vectors
</a>
</li>
<li class="chapter " data-level="4.1.10" data-path="2-10-0-improved-text-adventure-engine.html">
<a href="2-10-0-improved-text-adventure-engine.html">
Extra Credit: An Improved Text Adventure Engine
</a>
</li>
<li class="chapter " data-level="4.1.11" data-path="2-11-0-conditions.html">
<a href="2-11-0-conditions.html">
Conditions and Error Handling
</a>
</li>
<li class="chapter " data-level="4.1.12" data-path="2-12-0-2d-game.html">
<a href="2-12-0-2d-game.html">
Extra Credit: Write a 2D Game
</a>
</li>
<li class="chapter " data-level="4.1.13" data-path="2-13-0-compiler.html">
<a href="2-13-0-compiler.html">
The Compiler
</a>
</li>
<li class="chapter " data-level="4.1.14" data-path="2-14-0-tree-shaker.html">
<a href="2-14-0-tree-shaker.html">
Extra Credit: Write a Tree-Shaker
</a>
</li>
<li class="chapter " data-level="4.1.15" data-path="2-15-0-docs-and-inspection.html">
<a href="2-15-0-docs-and-inspection.html">
Documentation and Inspection
</a>
</li>
<li class="chapter " data-level="4.1.16" data-path="2-16-0-foreign-libs.html">
<a href="2-16-0-foreign-libs.html">
Extra Credit: Foreign Libraries in Lisp
</a>
</li>
<li class="chapter " data-level="4.1.17" data-path="2-17-0-debugging-testing.html">
<a href="2-17-0-debugging-testing.html">
Debugging and Unit Testing
</a>
</li>
<li class="chapter " data-level="4.1.18" data-path="2-18-0-ffi.html">
<a href="2-18-0-ffi.html">
Extra Credit: Write a Foreign Function Interface
</a>
</li>
<li class="chapter " data-level="4.1.19" data-path="2-19-0-essential-libs.html">
<a href="2-19-0-essential-libs.html">
Essential Lisp Libraries
</a>
</li>
<li class="chapter " data-level="4.1.20" data-path="2-20-0-packaging-libs.html">
<a href="2-20-0-packaging-libs.html">
Extra Credit: Packaging Lisp Libraries
</a>
</li>
<li class="chapter " data-level="4.1.21" data-path="2-21-0-review.html">
<a href="2-21-0-review.html">
Detailed Syntax Review
</a>
</li>
</ul>
</li>
<li class="header">PART THREE</li>
<li class="chapter " data-level="5.1" data-path="3-00-00-overview.html">
<a href="3-00-00-overview.html">
Lisp So(u)rcery
</a>
<ul class="articles">
<li class="chapter " data-level="5.1.1" data-path="3-01-00-web-apps.html">
<a href="3-01-00-web-apps.html">
Real-world Web Apps
</a>
</li>
<li class="chapter " data-level="5.1.2" data-path="3-02-00-typesetting.html">
<a href="3-02-00-typesetting.html">
Typesetting
</a>
</li>
<li class="chapter " data-level="5.1.3" data-path="3-03-00-mobile.html">
<a href="3-03-00-mobile.html">
Native Mobile Applications
</a>
</li>
<li class="chapter " data-level="5.1.4" data-path="3-04-00-gui.html">
<a href="3-04-00-gui.html">
Cross-platform Desktop Applications
</a>
</li>
<li class="chapter " data-level="5.1.5" data-path="3-05-00-system-utils.html">
<a href="3-05-00-system-utils.html">
Drivers, Daemons, and System-Utilities
</a>
</li>
<li class="chapter " data-level="5.1.6" data-path="3-06-00-reverse-engineering.html">
<a href="3-06-00-reverse-engineering.html">
Reverse Engineering
</a>
</li>
<li class="chapter " data-level="5.1.7" data-path="3-07-00-graphics.html">
<a href="3-07-00-graphics.html">
Graphics Rendering
</a>
</li>
<li class="chapter " data-level="5.1.8" data-path="3-08-00-gaming.html">
<a href="3-08-00-gaming.html">
OpenGL, SDL, and 3D Game Development
</a>
</li>
<li class="chapter " data-level="5.1.9" data-path="3-09-00-audio.html">
<a href="3-09-00-audio.html">
Audio Generation and Manipulation
</a>
</li>
<li class="chapter " data-level="5.1.10" data-path="3-10-00-data.html">
<a href="3-10-00-data.html">
Data Aggregation and Analysis
</a>
</li>
<li class="chapter " data-level="5.1.11" data-path="3-11-00-cryptosec.html">
<a href="3-11-00-cryptosec.html">
Cryptography and Security
</a>
</li>
<li class="chapter " data-level="5.1.12" data-path="3-12-00-fintech.html">
<a href="3-12-00-fintech.html">
Financial Software and Crypto-Currencies
</a>
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<h1 id="exercise-112">Exercise 1.1.2</h1>
<h2 id="the-repl">The REPL</h2>
<p>The Lisp REPL---a READ-EVAL-PRINT Loop---is the core of the interactive approach to Lisp development. You type in an expression, the expression is <em>read</em>, <em>evaluated</em>, the return result is <em>printed</em>, and then it waits for more input. The REPL is marked by a prompt, like the command line for your operating system, and it is the main interface to your current running Lisp image. Depending on the implementation of Lisp you are using, you may see different prompts. You can also customize your prompt, but the means of doing so are implementation-dependent.</p>
<p>Typically, imagining that the underscore character below is your blinking cursor, you&apos;ll see something to the effect of:</p>
<pre><code class="lang-lisp">CL-USER&gt; _
</code></pre>
<p>But in SBCL, the default prompt is much less informative:</p>
<pre><code class="lang-lisp">* _
</code></pre>
<p>For convenience sake, when you should enter a piece of code in the REPL for immediate evaluation, each line of code will be marked by the SBCL prompt character. When you should enter all the code given in a file, there will be no prompt character preceding each line, and you will be provided with a file name to enter it in.</p>
<p>Remember, you don&apos;t type in the prompt character <code>*</code> at the REPL; even if your prompt looks different, the <code>*</code> <em>represents</em> all Common Lisp REPL prompts. For example, you&apos;ll often see code examples like this:</p>
<pre><code class="lang-lisp">* (<span class="hljs-name">+</span> <span class="hljs-number">1</span> <span class="hljs-number">1</span>)
<span class="hljs-number">2</span>
* (<span class="hljs-name">apply</span> #&apos;+ <span class="hljs-number">1</span> <span class="hljs-number">2</span> <span class="hljs-number">3</span> <span class="hljs-number">4</span> &apos;(<span class="hljs-number">5</span> <span class="hljs-number">6</span>))
<span class="hljs-number">21</span>
</code></pre>
<p>The example above means, you first enter the expression <code>(+ 1 1)</code> in the REPL; evaluating it returns the result <code>2</code>, exactly what you would expect to get back for 1 + 1. Then, you get back to the prompt, and can enter the expression, <code>(apply #&apos;+ 1 2 3 4 &apos;(5 6))</code>, which returns <code>21</code>. If you&apos;re using SBCL from the command-line, this shows a direct transcript of the session---extra line-break between the evaluated expression and return result and all. If you are using Emacs+SLIME, you won&apos;t see that extra line-break.</p>
<p>A lot of Lisp literature and reference works explicitly mark the return value with an ASCII-formatted fat arrow: <code>=&gt;</code>. This can make spotting the return value of a form much easier in examples. We will adopt this convention herein as well, when it is necessary for visual clarity. In such a case, the examples above would then look like this:</p>
<pre><code class="lang-lisp">* (<span class="hljs-name">+</span> <span class="hljs-number">1</span> <span class="hljs-number">1</span>)
=&gt; <span class="hljs-number">2</span>
* (<span class="hljs-name">apply</span> #&apos;+ <span class="hljs-number">1</span> <span class="hljs-number">2</span> <span class="hljs-number">3</span> <span class="hljs-number">4</span> &apos;(<span class="hljs-number">5</span> <span class="hljs-number">6</span>))
=&gt; <span class="hljs-number">21</span>
</code></pre>
<p>In most exercises, we will separate the code examples into two copies: first, just the code you are meant to type, and then again with the correct return results for you to compare your work.</p>
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