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HIRE_DATE = 02/05/1994 00:00:00 DEPT_NO = 622 JOB_CODE = Eng JOB_GRADE = 5 JOB_COUNTRY = USA SALARY = 32000 FULL_NAME = Guckenheimer, Mark You will observe that very little changes were needed to convert the SQL Server AdventureWorks contact table example given earlier in the chapter to an example that executed a query against the Firebird employee example database.

Language-Integrated Query (LINQ) is the next generation of .NET data access technology. It borrows heavily from functional programming, so it fits very nicely with F#.

I Note All examples in this section and other sections about LINQ are based on the Community Technology

Preview of May 2006, the Microsoft .NET LINQ Preview (May 2006), and the F# LINQ bindings that match this release. If you use the examples with later versions of LINQ, you will have to make changes to the code.

At its heart, LINQ is a set of libraries for manipulating collections that implement the IEnumerable<T> interface, a lot like F# s Seq module, which was discussed in 7. The idea is that you can use this library to query any in-memory collection, whether the data comes from a database, an XML file, or just objects returned from another API. Although the concepts implemented in the LINQ library will be familiar to you by now, they follow a slightly different naming convention that is based on SQL. For instance, the equivalent of Seq.map is called Sequence.Select, and the equivalent Seq.filter is called Sequence.Where. The next example shows how to use this library. The first step is to import the methods exposed by the LINQ library into a more usable form; this is how to do that: #light #I "C:\Program Files\LINQ Preview\Bin";; #r "System.Query.dll";; open System.Query open System.Reflection

// define easier access to LINQ methods let select f s = Sequence.Select(s, new Func<_,_>(f)) let where f s = Sequence.Where(s, new Func<_,_>(f)) let groupBy f s = Sequence.GroupBy(s, new Func<_,_>(f)) let orderBy f s = Sequence.OrderBy(s, new Func<_,_>(f)) let count s = Sequence.Count(s) Once these functions have been imported, they can easily be applied, typically using the pipe forward operator. The following example demonstrates this. It uses the LINQ library to query the string class and group the overloads of its nonstatic methods together. // query string methods using functions let namesByFunction = (type string).GetMethods() |> where (fun m -> not m.IsStatic) |> groupBy (fun m -> m.Name) |> select (fun m -> m.Key, count m) |> orderBy (fun (_, m) -> m) namesByFunction |> Seq.iter (fun (name, count) -> printfn "%s - %i" name count) The results are as follows: ToLowerInvariant - 1 TrimEnd - 1 GetHashCode - 1 TrimStart - 1 GetEnumerator - 1 GetType - 1 GetTypeCode - 1 ToUpperInvariant - 1 Clone - 1 CopyTo - 1 get_Length - 1 Insert - 1 get_Chars - 1 PadLeft - 2 CompareTo - 2 PadRight - 2 ToUpper - 2 ToLower - 2 ToString - 2 Trim - 2 Remove - 2 ToCharArray - 2 Substring - 2 IsNormalized - 2

Normalize - 2 Replace - 2 IndexOfAny - 3 EndsWith - 3 Equals - 3 StartsWith - 3 LastIndexOfAny - 3 Split - 6 LastIndexOf - 9 IndexOf - 9

The goal of LINQ to XML is to provide an XML object model that works well with LINQ s functional style of programming. Table 9-4 summarizes the important classes within this namespace. Table 9-4. A Summary of the Classes Provided by LINQ to XML

This class provides the functionality for XML nodes that can contain other nodes. This class represents the XML document as a whole. This class represents an element in the XML document, that is, a regular XML node that can be a tag, <myTag />, or can possibly contain other tags or an attribute, such as myAttribute="myVal". This class represents a document type tag. This class represents a processing instruction, which is a tag of the form < name instruction >. This class represents text contained within the XML document. This class represents the name of a tag or an attribute.