掌握F#编程：打造高效、可靠的.NET应用

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xiv About the author

Book forum

Purchase of Get Programming with F# includes free access to a private web forum run by

Manning Publications where you can make comments about the book, ask technical

questions, and receive help from the author and from other users. To access the forum,

go to www.manning.com/books/get-programming-with-f-sharp. You can also learn

more about Manning's forums and the rules of conduct at https://forums.manning.com/

forums/about.

Manning’s commitment to our readers is to provide a venue where a meaningful dia-

logue between individual readers and between readers and the author can take place. It

is not a commitment to any specific amount of participation on the part of the author,

whose contribution to the forum remains voluntary (and unpaid). We suggest you try

asking the author some challenging questions lest his interest stray! The forum and the

archives of previous discussions will be accessible from the publisher’s website as long

as the book is in print.

About the author

ISAAC ABRAHAM is an F# MVP and has been a .NET developer since

.NET 1.0, with an interest in cloud computing and distributed data

problems. He nowadays lives in both the UK and Germany, and is

the director of Compositional IT. He specializes in consultancy,

training and development, and helping customers adopt high-

quality, functional-first solutions on the .NET platform.

Isaac focuses on practical software development techniques and

practices that can deliver good return quickly. He believes that

although an understanding of, for example, functional program-

ming underpinnings is important, these lessons need to be delivered in a way that

quickly illustrates value and benefit to students.

2 Introduction Welcome to Get Programming with F#!

What is F#, and why does it matter?

Let’s first start at the beginning and discuss at a relatively high level some of the key

points of what F# is (and isn’t!). Then we’ll discuss the key question that you probably

have on the tip of your tongue: why should you spend time learning a new language

with a new way of doing things, when you could be productively bashing out code right

now in C# or VB?

What is F#?

What’s this whole buzz about F#? F# is a language that’s rapidly growing in popularity

and attracting developers to be used in a variety of domains, not just from within the

.NET ecosystem but also from without.

Let’s start by quoting directly from the fsharp.org website:

F# is a mature, open source, cross-platform, functional-first programming language. It empowers

users and organizations to tackle complex computing problems with simple, maintainable, and

robust code.

Let’s discuss a few points mentioned in that quote:

 Mature—F# is a mature language with an established community. Based on the

ML family of programming languages, and now at version 4, it’s been a first-class

citizen of Visual Studio since Visual Studio 2010 (though you may not even have

been aware of it) and runs on the .NET Framework, which itself is over 15 years

old now. In terms of the risk of using F#, you don’t have to be concerned about

things such as sourcing package dependencies or that you won’t be able to find

resources or help online.

 Open source—F# as a project is entirely open source and has been for several years

(way before Microsoft’s recent public shift toward open source). The compiler is

open source, anyone can submit changes to the compiler and language (subject to

approval of course!), and it runs not only on Windows but also on Mono, with

support for Microsoft’s new .NET Core well on the way at the time of writing.

 Functional-first—F# is prescriptive in the sense that the language encourages us

but doesn’t force us as developers to write code in a functional programming (FP)

style. We’ll describe those features in more detail shortly, and how they affect the

way we code, but for now just know that it’s an alternative way to model and

solve problems compared to the object-oriented (OO) paradigm that you’re used

to. But as F# runs on the .NET Framework (an inherently OO framework), F#

needs to support OO features. So, you can also consume and create classes and

interfaces just as you can with C#; it’s simply that it’s not the idiomatic way to

3What is F#, and why does it matter?

write F#. In this book, we’ll be effectively ignoring the OO side of F#—which

would mostly just be about learning a new syntax—and focusing on the FP side,

which is much more interesting.

Another point that the preceding quote alludes to is that F# helps us focus on delivering

business logic and ultimately business value, instead of having to focus on complex

design patterns, class hierarchies, and so on in order to achieve quality. This is partly

because F# contains a powerful compiler that does a lot of the heavy lifting for us, which

can lead to more succinct solutions than you might normally achieve in C#. But in addi-

tion, the FP paradigm (in conjunction with F#) emphasizes composing small pieces of

functionality together to naturally build more powerful abstractions rather than

through designing large class hierarchies up front and building downward.

What is functional programming?

To explain this a little more, let’s quickly define FP within the context of this book. Just

as with OO design, defining exactly what FP is can be subjective; many languages that

support FP to some extent will have some features that others do not. At the end of the

day, you can think of FP as a sliding scale of language features that encourage a particu-

lar style of programming; I suggest that any FP language should have good support for

a few core fundamentals:

 Immutability—The ability to create values that can never be changed in their life-

time, which leads to a clear separation of data and functionality (unlike the OO

world, which merges both state and behavior into classes).

 Expressions—The notion that every operation in your program has a tangible out-

put that can be reasoned about.

 Functions as values—The ability to easily create, use, and share functions as a unit

of abstraction and composition within a system.

There’s certainly more to FP than just that, and many languages have features such as

sum types, pattern matching, type classes, and the often-dreaded monad; we’ll deal

with some of these features throughout this book, but the preceding three are the key

ones that we’ll really emphasize. Remember that FP is just another way to model solu-

tions to programming problems; there’s no real magic involved, nor do you need to be a

mathematical genius.

F# alongside other programming languages

Figure 1 illustrates where I believe F# fits in within the spectrum of other mainstream

programming languages.

F# and C#—two sides of the same coin

Let’s start by looking at F# which, as already stated, is a functional-first language. It’s

clearly on the FP side of the axis, yet it’s not on the far right. Why? Well, F# makes it easy

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