Book Image

F# 4.0 Design Patterns

By : Gene Belitski
Book Image

F# 4.0 Design Patterns

By: Gene Belitski

Overview of this book

Following design patterns is a well-known approach to writing better programs that captures and reuses high-level abstractions that are common in many applications. This book will encourage you to develop an idiomatic F# coding skillset by fully embracing the functional-first F# paradigm. It will also help you harness this powerful instrument to write succinct, bug-free, and cross-platform code. F# 4.0 Design Patterns will start off by helping you develop a functional way of thinking. We will show you how beneficial the functional-first paradigm is and how to use it to get the optimum results. The book will help you acquire the practical knowledge of the main functional design patterns, the relationship of which with the traditional Gang of Four set is not straightforward. We will take you through pattern matching, immutable data types, and sequences in F#. We will also uncover advanced functional patterns, look at polymorphic functions, typical data crunching techniques, adjusting code through augmentation, and generalization. Lastly, we will take a look at the advanced techniques to equip you with everything you need to write flawless code.
Table of Contents (20 chapters)
F# 4.0 Design Patterns
Credits
About the Author
Acknowledgements
About the Reviewer
www.PacktPub.com
Preface

The as pattern


Interestingly, a pattern case may have the as clause appended to it. This clause binds the matched value to a name that may be used within the corresponding result-expression of the match construction or elsewhere within a local context of an outer let binding. The following script demonstrates how flexible the as pattern can be (Ch4_3.fsx):

let verifyGuid g = 
  match System.Guid.TryParse g with 
  | (true,_ as r) -> sprintf "%s is a genuine GUID %A" g (snd r) 
  | (_,_ as r) -> sprintf "%s is a garbage GUID, defaults to %A" 
                        g (snd r);; 

In the first case, r is bound using as to the result of TryParse, which is the tuple, so the expression snd r yields the parsed GUID value.

In the second case, as bounds r to any tuple; however, it must be obvious from the match cases sequencing that this case matches the failed GUID parsing and the value of argument is a garbage.

The following screenshot reflects firing each of these using as binding match cases...