emacs.d/clones/lisp/llthw.common-lisp.dev/1-02-12-format.html

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<title>A Brief Introduction to Format · Learn Lisp The Hard Way</title>
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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>
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TANSTAAFL
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The Hard Way is Easier
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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">
<a href="1-0-0-overview.html">
Grokking Lisp
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<ul class="articles">
<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>
<li class="chapter " data-level="3.1.1.2" data-path="1-01-02-repl.html">
<a href="1-01-02-repl.html">
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>
</li>
<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">
<a href="1-02-01-strings.html">
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>
<li class="chapter " data-level="3.1.2.3" data-path="1-02-03-unicode.html">
<a href="1-02-03-unicode.html">
Unicode and Strings
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</li>
<li class="chapter " data-level="3.1.2.4" data-path="1-02-04-chars.html">
<a href="1-02-04-chars.html">
Characters
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</li>
<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
</a>
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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">
<a href="1-02-11-princ.html">
Printing With princ
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</li>
<li class="chapter active" 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
</a>
</li>
<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
</a>
</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
</a>
</li>
<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
</a>
</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
</a>
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</ul>
</li>
<li class="chapter " data-level="3.1.3" data-path="1-03-0-getting-input-from-users.html">
<a href="1-03-0-getting-input-from-users.html">
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
</a>
</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
</a>
<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
</a>
</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>
</li>
<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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<a href="index.html" >A Brief Introduction to Format</a>
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<h1 id="exercise-1212">Exercise 1.2.12</h1>
<h2 id="a-brief-introduction-to-format">A Brief Introduction to Format</h2>
<p>Pretty much every programming language has some way to insert arbitrary data into a string. Lisp is no exception. One difference, perhaps, is that in Lisp, the <code>format</code> function is a part of the language standard, and must be fully supported for the implementation to call itself Common Lisp. In other languages, formatting strings is usually outside the scope of the core language, and only available through the standard library.</p>
<p>The <code>format</code> function in Lisp is so powerful, however, that I can only introduce you to a little bit of it right now. The full treatise on this surprising core ANSI Common Lisp functionality will appear in Chapter 1.18, once you have a better grasp on key features of the language.</p>
<h3 id="the-basics-of-format">The Basics of Format</h3>
<p>The <code>format</code> function uses a special type of string with its own internal syntax, called <em>control sequences</em>. These control sequences are made up of the <em>tilde</em> (<code>~</code>) character and the <em>control character</em> itself, and may also include certain infix parameters which customize the behavior of the control sequence.</p>
<p>For example, a <code>format</code> control sequence you&apos;ll use often is <code>~A</code>, the <em>aesthetic</em> control sequence. It consumes an argument, and inserts that argument into the string, printed the same way that the <code>princ</code> function would print it. <code>~S</code>, the <em>standard</em> control sequence, consumes an argument and prints it the way <code>prin1</code> would print it. And <code>~W</code>, the <em>write</em> control sequence, consumes an argument and prints it the way that <code>write</code> would print it without any of its keyword parameters changed.</p>
<p>What do I mean by &quot;consumes an argument&quot; though? Well, let me show you:</p>
<pre><code class="lang-lisp">(<span class="hljs-name">format</span> <span class="hljs-literal">nil</span> <span class="hljs-string">&quot;Hello, my name is ~A.&quot;</span> <span class="hljs-string">&quot;Colin&quot;</span>)
</code></pre>
<p>This expression calls the <code>format</code> function, tells it to <em>return</em> the formatted string by setting the output stream to <code>nil</code>, and creates a format string that consumes one argument which will be printed aesthetically. Following that is the one argument, a string with my name in it.</p>
<p>Now consider this example:</p>
<pre><code class="lang-lisp">(<span class="hljs-name">format</span> <span class="hljs-literal">nil</span> <span class="hljs-string">&quot;My name is ~A ~A.&quot;</span> <span class="hljs-string">&quot;Colin&quot;</span> <span class="hljs-string">&quot;Lupton&quot;</span>)
</code></pre>
<p>Now this example expects to consume <em>two</em> arguments. And those two arguments follow, which are now my first and last names as separate string objects.</p>
<p>Now try these out:</p>
<pre><code class="lang-lisp">(<span class="hljs-name">format</span> <span class="hljs-literal">t</span> <span class="hljs-string">&quot;Hello, ~A!&quot;</span> <span class="hljs-string">&quot;multiverse&quot;</span>)
(<span class="hljs-name">format</span> <span class="hljs-literal">t</span> <span class="hljs-string">&quot;Hello, ~S!&quot;</span> <span class="hljs-string">&quot;multiverse&quot;</span>)
(<span class="hljs-name">format</span> <span class="hljs-literal">t</span> <span class="hljs-string">&quot;Hello, ~W!&quot;</span> <span class="hljs-string">&quot;multiverse&quot;</span>)
</code></pre>
<h3 id="what-you-should-see">What You Should See</h3>
<p>You should notice right away a few differences between <code>format</code> and the standard Lisp printer functions. You should see something like this:</p>
<pre><code class="lang-lisp">* (format nil &quot;My name is ~A.&quot; &quot;Colin&quot;)
&quot;My name is Colin.&quot;
* (format nil &quot;My name is ~A ~A.&quot; &quot;Colin&quot; &quot;Lupton&quot;)
&quot;My name is Colin Lupton.&quot;
* (format t &quot;Hello, ~A!&quot; &quot;multiverse&quot;)
Hello, multiverse!
NIL
* (format t &quot;Hello, ~S!&quot; &quot;multiverse&quot;)
Hello, &quot;multiverse&quot;!
NIL
* (format t &quot;Hello, ~W!&quot; &quot;multiverse&quot;)
Hello, &quot;multiverse&quot;!
NIL
</code></pre>
<p>First up, notice that when you tell <code>format</code> to <em>return</em> the formatted string, you only get back one value now? The return value alone. But when you send the formatted string to <code>t</code>, which in the context of the <code>format</code> function&apos;s stream parameter is a shorthand for <code>*standard-output*</code>, you see that the formatted string is printed, and you get back a return value of <code>nil</code> instead of another string object.</p>
<p>Now notice that with the standard and write control sequences, the argument gets printed literally? That is, it includes the double-quote syntax that would tell the Lisp Reader that this is a string object, if you were to have Lisp try and read such a string---but since there&apos;s other junk in the string that Lisp wouldn&apos;t understand, it&apos;s not a good idea to try.</p>
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