Expert F# 2.0


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WriteLine char value : unit'. In the previous sections, you got a taste of how you can use pattern matching with tuple, list, and option values. However, you can also use pattern matching in many other situations. When executed, the patterns of the rules are used one by one, and the first successful pattern match determines which result expression is used. The third pattern matches if agent is The last three rules all use wildcard patterns represented by the underscore character; these match all inputs.

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Patterns are a rich and powerful technique for simultaneous data analysis and decomposition. Table summarizes all the ways to form patterns in F ; many of these involve building up patterns from other patterns. The sections that follow look at some of these constructs, and Chapter 9 covers active patterns. Monday :?

What's new in F# 4.5

It then returns a pair made from the respective second parts of the tuples. The second rule is the mirror of this in case the values appeared in reverse order. The final cases of both of the previous examples use wildcard patterns to cover remaining cases. This makes the patterns exhaustive. Frequently, no wildcard is needed to ensure this, because for many input types F is able to determine whether the given set of rules is sufficient to cover all possibilities for the given shape of data.

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Chapter 4 discusses exceptions. F is frequently able to determine whether pattern-matching rules are redundant, such as if a rule can never be selected because previous rules subsume all such cases. In this case, a warning is given. F can often determine whether a match is exhaustive, and the use of wildcard patterns effectively disables this analysis for any particular pattern match. Guarding Rules and Combining Patterns Individual rules of a match can be guarded by a condition that is executed if the pattern itself succeeds.

The pattern "hi",v "high",v plays essentially the same role by matching pairs values where the left of the pair is "hi" or "high" and by binding the value v on either side of the pattern.

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Getting Started with Sequences Many programming tasks require the iteration, aggregation, and transformation of data streamed from various sources. One important and general way to code these tasks is in terms of values of the. NET type System. IEnumerable, which is typically abbreviated to seq in F code. A seq is a value that can be iterated, producing results of type type on demand.

Sequences are used to wrap collections, computations, and data streams and are frequently used to represent the results of database queries. The following sections present some simple examples of working with seq values.


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NET collection types: For example, System. SortedList is compatible with seq. NET languages. The difference can, for example, be detected by other. NET languages; because of this, a number of restrictions apply to type abbreviations. Defining Records The simplest concrete type definitions are records. One slight difference is that in the absence of further qualifying information, the type of the object being accessed is inferred from the record label.

This is based on the latest set of record labels in scope from record definitions and uses of open. The accesses in the third definition have been resolved using the default interpretation of record field labels in the absence of any other qualifying information. Cloning Records Records support a convenient syntax to clone all the values in the record, creating a new value with some values replaced.

Defining Discriminated Unions The second kind of concrete type definition discussed in this section is a discriminated union. You can also use either the contents field or the Value property to access the value of a reference cell.


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Which Data Structures Are Mutable? If a data structure can be mutated, then this is typically evident in the types of operations you can perform on that structure. In contrast, operations on immutable data structures typically return a new instance of the data structure when an operation such as add is performed. Example persistent data structures in the F library are F lists, options, tuples, and the types Microsoft.

Map and Microsoft. Chapter 7 looks at encapsulation in more detail, but one easy way to do this is to make data private to a function. Understanding Mutation and Identity F encourages the use of objects whose logical identity if any is based purely on the characteristics for example, fields and properties of the object. For example, the identity of a pair of integers 1,2 is determined by the two integers themselves; two tuple values both containing these two integers are for practical purposes identical.

This is because tuples are immutable and support structural equality, hashing, and comparison, discussed further in Chapters 5 and 8. Mutable reference cells are different; they can reveal their identities through aliasing and mutation. However, not all mutable values necessarily reveal their identity through mutation.

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For example, sometimes mutation is used just to bootstrap a value into its initial configuration, such as when connecting the nodes of a graph. These are relatively benign uses of mutation. Ultimately, you can detect whether two mutable values are the same object by using the function System.

You can also use this function on immutable values to detect whether two values are represented by the physically same value. But in this circumstance, the results returned by the function may change according to the optimization settings you apply to your F code. F also supports mutable locals that are implicitly dereferenced.


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In particular, unlike mutable references, mutable locals are guaranteed to be stack-allocated values, which is important in some situations because the. As a result, mutable locals may not be used in any inner lambda expressions or other closure constructs, with the exception of top-level mutable values, which can be used anywhere, and mutable fields of records and objects, which are associated with the heap allocated objects themselves.

You learn more about mutable object types in Chapter 6. Reference cells and types containing mutable fields can be used instead to make the existence of heapallocated imperative state obvious. Working with Arrays Mutable arrays are a key data structure used as a building block in many high-performance computing scenarios. Array, but you can access it with the short name Array.

Expert F# 2.0

Arrays are created either by using the creation functions in that module such as Array. Some useful methods are also contained in the System. Array class. Table shows some common functions from the Array module.

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OutOfMemoryException' was thrown. Arrays of other types are stored as an array of object references. Primitive types such as integers and floating-point numbers are all value types; many other. NET types are also value types. NET documentation indicates whether each type is a value type or not. Often, the word struct is used for value types.

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