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CHAPTER 2 GETTING STARTED WITH F# AND .NET
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let two = (printfn "Hello World"; 1+1) let four = two + two When executed, this code prints Hello World precisely once, when the right side of the definition of two is executed. F# doesn t have statements as such: the fragment (printfn "Hello World"; 1+1) is an expression, but when evaluated, the first part of the expression causes a side effect, and its result is discarded. It s also often convenient to use parentheses to delimit sequential code. The code from the script could in theory be parenthesized with a semicolon added to make the primitive constructs involved more apparent: (printfn "--> %d words in the text" numWords; printfn "--> %d duplicate words" numDups)
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Note The token ; is used to write sequential code within expressions, and ;; is used to terminate interactions
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with the F# Interactive session. Semicolons are optional when the individual fragments of your sequential code are placed on separate lines beginning at the same column position.
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Using .NET Libraries from F#
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The true value of F# lies not just in what you can do inside the language but also in what you can connect to outside the language. For example, F# doesn t come with a GUI library. Instead, F# is connected to .NET and via .NET to most of the significant programming technologies available on major computing platforms. You ve already seen one use of the .NET libraries, in the first function defined earlier: /// Split a string into words at spaces let splitAtSpaces (text: string) = text.Split ' ' |> Array.toList Here, text.Split is a call to a .NET library instance method called Split defined on all string objects. To emphasize this, the second sample uses two of the powerful libraries that come with the .NET Framework: System.Net and System.Windows.Forms. The full sample is in Listing 2-3 and is a script for use with F# Interactive. Listing 2-3. Using the .NET Framework Windows Forms and Networking Libraries from F# open System.Windows.Forms let form = new Form(Visible=true,TopMost=true,Text="Welcome to F#") let textB = new RichTextBox(Dock=DockStyle.Fill, Text="Here is some initial text") form.Controls.Add textB open System.IO open System.Net /// Get the contents of the URL via a web request
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CHAPTER 2 GETTING STARTED WITH F# AND .NET
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let http (url: string) = let req = System.Net.WebRequest.Create(url) let resp = req.GetResponse() let stream = resp.GetResponseStream() let reader = new StreamReader(stream) let html = reader.ReadToEnd() resp.Close() html let google = http "http://www.google.com" textB.Text <- http "http://news.bbc.co.uk" This example uses several important .NET libraries and helps you explore some interesting F# language constructs. The following sections walk you through this listing.
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The first thing you see in the sample is the use of open to access functionality from the namespace System.Windows.Forms: open System.Windows.Forms 7 discusses namespaces in more detail. The earlier declaration means you can access any content under this path without quoting the long path. If it didn t use open, you d have to write the following, which is obviously a little verbose: let form = new System.Windows.Forms.Form(Visible=true,TopMost=true, Text="Welcome to F#") You can also use open to access the contents of an F# module without using long paths. 7 discusses modules in more detail.
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More About open
Using open is an easy way to access the contents of namespaces and modules. However, there are some subtleties. For example, open doesn t actually load or reference a library instead, it reveals functionality from already-loaded libraries. You load libraries by referring to a particular DLL using #r in a script or -r as a command-line option. Libraries and namespaces are orthogonal concepts: multiple libraries can contribute functionality to the same namespace, and each library can contribute functionality to multiple namespaces. Often, one particular library contributes most of the functionality in a particular namespace. For example, most of the functionality in the System.Windows.Forms namespace comes from a library called System.Windows.Forms.dll. As it happens, this library is automatically referenced by F#, which is why you haven t needed an explicit reference to the library so far. You can place your code in a namespace by using a namespace declaration at the top of your file, as discussed further in 7. If two namespaces have types, subnamespaces, and/or modules with identical names, then when you open these, you can access the contents of both using the same shortened paths. For example, the namespace System contains a type String, and the namespace Microsoft.FSharp.Core contains a 20
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