You are previewing Introducing Erlang.

Introducing Erlang

Cover of Introducing Erlang by Simon St. Laurent Published by O'Reilly Media, Inc.
  1. Introducing Erlang
  2. Preface
    1. Who This Book Is For
    2. Who This Book Is Not For
    3. What This Book Will Do For You
    4. How This Book Works
    5. Why I Wrote This Book
    6. Other Resources
    7. Are You Sure You Want Erlang?
    8. Erlang Will Change You
    9. Conventions Used in This Book
    10. A Note on Erlang Syntax
    11. Using Code Examples
    12. Help This Book Grow
    13. Please Use It For Good
    14. Safari® Books Online
    15. How to Contact Us
    16. Acknowledgments
  3. 1. Getting Comfortable
    1. Installation
    2. Firing It Up
    3. First Steps: The Shell
      1. Moving through Text
      2. Moving through History
      3. Moving through Files
    4. Doing Something
    5. Calling Functions
    6. Numbers in Erlang
    7. Working with Variables in the Shell
      1. Seeing Your Bound Variables
      2. Clearing Bound Variables in the Shell
  4. 2. Functions and Modules
    1. Fun with fun
    2. Defining Modules
      1. From Module to Fun
      2. Functions and Variable Scope
      3. Module Directives
    3. Documenting Code
      1. Documenting Modules
      2. Documenting Functions
      3. Documenting Your Application
  5. 3. Atoms, Tuples, and Pattern Matching
    1. Atoms
    2. Pattern Matching with Atoms
    3. Atomic Booleans
    4. Guards
    5. Underscoring That You Don’t Care
    6. Adding Structure: Tuples
      1. Pattern Matching with Tuples
      2. Processing Tuples
  6. 4. Logic and Recursion
    1. Logic Inside of Functions
      1. Evaluating Cases
      2. If This, Then That
      3. Variable Assignment in case and if Constructs
    2. The Gentlest Side Effect: io:format
    3. Simple Recursion
      1. Counting Down
      2. Counting Up
      3. Recursing with Return Values
  7. 5. Communicating with Humans
    1. Strings
    2. Asking Users for Information
      1. Gathering Terms
      2. Gathering Characters
      3. Reading Lines of Text
  8. 6. Lists
    1. List Basics
    2. Splitting Lists into Heads and Tails
    3. Processing List Content
    4. Creating Lists with Heads and Tails
    5. Mixing Lists and Tuples
    6. Building a List of Lists
  9. 7. Higher-Order Functions and List Comprehensions
    1. Simple Higher-Order Functions
    2. Creating New Lists with Higher-Order Functions
      1. Reporting on a List
      2. Running List Values Through a Function
      3. Filtering List Values
    3. Beyond List Comprehensions
      1. Testing Lists
      2. Splitting Lists
      3. Folding Lists
  10. 8. Playing with Processes
    1. The Shell Is a Process
    2. Spawning Processes from Modules
    3. Lightweight Processes
    4. Registering a Process
    5. When Processes Break
    6. Processes Talking Amongst Themselves
    7. Watching Your Processes
    8. Breaking Things and Linking Processes
  11. 9. Exceptions, Errors, and Debugging
    1. Flavors of Errors
    2. Catching Runtime Errors as They Happen
    3. Raising Exceptions with throw
    4. Logging Progress and Failure
    5. Debugging through a GUI
    6. Tracing Messages
    7. Watching Function Calls
  12. 10. Storing Structured Data
    1. From Tuples to Records
      1. Setting Up Records
      2. Creating and Reading Records
      3. Using Records in Functions and Modules
    2. Storing Records in Erlang Term Storage
      1. Creating and Populating a Table
      2. Simple Queries
      3. A Key Feature: Overwriting Values
      4. ETS Tables and Processes
      5. Next Steps
    3. Storing Records in Mnesia
      1. Starting up Mnesia
      2. Creating Tables
      3. Reading Data
      4. Query List Comprehensions
  13. 11. Getting Started with OTP
    1. Creating Services with gen_server
    2. A Simple Supervisor
    3. Packaging an Application
  14. 12. Next Steps Through Erlang
    1. Moving Beyond the Shell
    2. Distributed Computing
    3. Processing Binary Data
    4. Input and Output
    5. Testing, Analyzing, and Refactoring
    6. Networking and the Web
    7. Data Storage
    8. Extending Erlang
    9. Languages Built on Erlang
    10. Community
    11. Sharing the Gospel of Erlang
  15. A. An Erlang Parts Catalog
    1. Shell Commands
    2. Reserved Words
    3. Operators
    4. Guard Components
    5. Common Functions
    6. Strings and Formatting
    7. Data Types for Documentation and Analysis
  16. B. OTP Templates
  17. Colophon
  18. Copyright

Chapter 9. Exceptions, Errors, and Debugging

“Let it crash” is a brilliant insight, but one whose application you probably want to control. While it’s possible to write code that constantly breaks and recovers, it can be easier to write and maintain code that explicitly handles failure where it happens. However you choose to deal with errors, you’ll definitely want to be able to track them down in your application.

Flavors of Errors

As you’ve already seen, some kinds of errors will keep Erlang from compiling your code, and the compiler will also give you warnings about potential issues, like variables that are declared but never used. Two other kinds of errors are common: runtime errors, which turn up when code is operating and can actually halt a function or process, and logic errors, which may not kill your program but can cause deeper headaches.

Logic errors are often the trickiest to diagnose, requiring careful thought and perhaps some time with the debugger, log files, or a test suite. Simple mathematical errors can take a lot of work to untangle. Sometimes issues are related to timing, when the sequence of operations isn’t what you expect. In severe cases, race conditions can create deadlocks and halting, but more mild cases can produce bad results and confusion.

Runtime errors can also be annoying, but they are much more manageable. In some ways you can see handling runtime errors as part of the logic of your program, though you don’t want to get carried away. In Erlang, unlike ...

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