how to print barcode in c# net GETTING STARTED WITH F# AND .N ET in Font

Create PDF417 in Font GETTING STARTED WITH F# AND .N ET

CHAPTER 2 GETTING STARTED WITH F# AND .N ET
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Because let bindings are just one kind of expression, you can use them in a nested fashion. For example: let powerOfFourPlusTwoTimesSix n = let n3 = let n1 = n * n let n2 = n1 * n1 n2 + 2 let n4 = n3 * 6 n4 In the previous example, n1 and n2 are values defined locally by let bindings within the expression that defines n3. These local values are not available for use outside their scope. For example, the following code gives an error: let invalidFunction n = let n3 = let n1 = n + n let n2 = n1 * n1 n1 * n2 let n4 = n1 + n2 + n3 n4
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// Error! n3 is in scope, but n1 and n2 are not!
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Local scoping is used for many purposes in F# programming, especially to hide implementation details that you don t want revealed outside your functions or other objects. We cover this topic in more detail in 7.
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VALUES AND IMMUTABILITY
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In other languages, a local value is called a local variable. However, in F# you can t change the immediate value of locals after they ve been initialized, unless the local is explicitly marked as mutable, a topic we return to in 4. For this reason, F# programmers and the language specification tend to prefer the term value to variable. As you ll see in 4, data indirectly referenced by a local value can still be mutable even if the local value is not; for example, a local value that is a handle to a hash table cannot be changed to refer to a different table, but the contents of the table itself can be changed by invoking operations that add and remove elements from the table. However, many values and data structures in F# programming are completely immutable; in other words, neither the local value nor its contents can be changed through external mutation. These are usually just called immutable values. For example, all basic .NET types such as integers, strings, and System.DateTime values are immutable, and the F# library defines a range of immutable data structures such as Set and Map, based on binary trees. Immutable values bring many advantages. At first it might seem strange to define values you can t change. However, knowing a value is immutable means you rarely need to think about the object identity of these values you can pass them to routines and know they won t be mutated. You can also pass them between multiple threads without worrying about unsafe concurrent access to the values, discussed in 14. You can find out more about programming with immutable data structures at http://www.expert-fsharp.com/ Topics/FunctionalDataStructures.
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C HA PTER 2 GE TTIN G STARTED WITH F# AN D . NET
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Understanding Types
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F# is a typed language, so it s reasonable to ask what the type of wordCount is, and indeed F# Interactive has shown it already:
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val wordCount : string -> int * int
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This indicates that wordCount takes one argument of type string and returns int * int, which is F# s way of saying a pair of integers. The keyword val stands for value, and the symbol -> represents a function. No explicit type has been given in the program for wordCount or its argument text, because the full type for wordCount has been inferred from its definition. We discuss type inference further in the What Is Type Inference sidebar and in more detail in later chapters. Types are significant in both F# and .NET programming more generally for reasons that range from performance to coding productivity and interoperability. Types are used to help structure libraries, to guide the programmer through the complexity of an API and to place constraints on code to ensure it can be implemented efficiently. However, unlike many other typed languages, the type system of F# is both simple and powerful because it uses orthogonal, composable constructs such as tuples and functions to form succinct and descriptive types. Furthermore, type inference means you almost never have to write types in your program, though doing so can be useful. Table 2-1 shows some of the most important type constructors. We discuss all these types in more detail in 3 and 4. Table 2-1. Some Important Types, Type Constructors, and Their Corresponding Values
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