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Defining New Exception Types
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F# lets you define new kinds of exception objects that carry data in a conveniently accessible form. For example, here is a declaration of a new class of exceptions and a function that wraps http with a filter that catches particular cases: exception BlockedURL of string let http2 url = if url = "http://www.kaos.org" then raise(BlockedURL(url)) else http url You can extract the information from F# exception values, again using pattern matching: > try raise(BlockedURL("http://www.kaos.org")) with | BlockedURL(url) -> printf "blocked! url = '%s'\n" url;; blocked! url = 'http://www.kaos.org' Exception values are always subtypes of the F# type exn, an abbreviation for the .NET type System.Exception. The declaration exception BlockedURL of string is shorthand for defining a new F# class type BlockedURLException, which is a subtype of System.Exception. Exception types can also be defined explicitly by defining new object types. s 5 and 6 look more closely at object types and subtyping. Table 4-3 summarizes the exception-related language and library constructs.
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CHAPTER 4 INTRODUCING IMPERATIVE PROGRAMMING
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Table 4-3. Exception-Related Language and Library Constructs
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Example Code
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raise expr failwith expr try expr with rules try expr finally expr
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F# library function F# library function F# expression F# expression
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Raises the given exception Raises an Exception exception Catches expressions matching the pattern rules Executes the finally expression both when the computation is successful and when an exception is raised A rule matching the given .NET exception type A rule matching the given .NET exception type and naming it as its stronger type A rule matching the given data-carrying F# exception A rule matching any exception, binding the name exn to the exception object value A rule matching the exception under the given condition, binding the name exn to the exception object value
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| : ArgumentException -> | : ArgumentException as e -> | Failure(msg) -> expr | exn -> expr
F# pattern rule F# pattern rule F# pattern rule F# pattern rule
| exn when expr -> expr
F# pattern rule
Having an Effect: Basic I/O
Imperative programming and input/output are closely related topics. The following sections show some very simple I/O techniques using F# and .NET libraries.
Very Simple I/O: Reading and Writing Files
The .NET types System.IO.File and System.IO.Directory contain a number of simple functions to make working with files easy. For example, here s a way to output lines of text to a file: > open System.IO;; > File.WriteAllLines("test.txt", [| "This is a test file."; "It is easy to read." |]);; val it : unit = ()
CHAPTER 4 INTRODUCING IMPERATIVE PROGRAMMING
Many simple file-processing tasks require reading all the lines of a file. You can do this by reading all the lines in one action as an array using System.IO.File.ReadAllLines: > open System.IO;; > File.ReadAllLines("test.txt");; val it : string [] = [| "This is a test file."; "It is easy to read." |]
If necessary, the entire file can be read as a single string using System.IO.File.ReadAllText: > File.ReadAllText("test.txt");; val it : string = "This is a test file.\r\nIt is easy to read\r\n" You can also use the results of System.IO.File.ReadAllLines as part of a list or sequence defined using a sequence expression: > [ for line in File.ReadAllLines("test.txt") do let words = line.Split [| ' ' |] if words.Length > 3 && words.[2] = "easy" then yield line ];; val it : string list = [| "It is easy to read." |]
.NET I/O via Streams
The .NET namespace System.IO contains the primary .NET types for reading/writing bytes and text to/from data sources. The primary output constructs in this namespace are as follows: System.IO.BinaryWriter: Writes primitive data types as binary values. Create using new BinaryWriter(stream). You can create output streams using File.Create(filename). System.IO.StreamWriter: Writes textual strings and characters to a stream. The text is encoded according to a particular Unicode encoding. Create by using new StreamWriter(stream) and its variants or by using File.CreateText(filename). System.IO.StringWriter: Writes textual strings to a StringBuilder, which eventually can be used to generate a string.
Here is a simple example of using System.IO.File.CreateText to create a StreamWriter and write two strings: > let outp = File.CreateText("playlist.txt");; val outp : StreamWriter > outp.WriteLine("Enchanted");; val it : unit = ()
CHAPTER 4 INTRODUCING IMPERATIVE PROGRAMMING
> outp.WriteLine("Put your records on");; val it : unit = () > outp.Close(); These are the primary input constructs in the System.IO namespace: System.IO.BinaryReader: Reads primitive data types as binary values. When reading the binary data as a string, it interprets the bytes according to a particular Unicode encoding. Create using new BinaryReader(stream). System.IO.StreamReader: Reads a stream as textual strings and characters. The bytes are decoded to strings according to a particular Unicode encoding. Create by using new StreamReader(stream) and its variants or by using File.OpenText(filename). System.IO.StringReader: Reads a string as textual strings and characters.
Here is a simple example of using System.IO.File.OpenText to create a StreamReader and read two strings: > let inp = File.OpenText("playlist.txt");; val inp : StreamReader > inp.ReadLine();; val it : string = "Enchanted" > inp.ReadLine();; val it : string = "Put your records on" > inp.Close();; val it : unit = ()
Tip Whenever you create objects such as a StreamReader that have a Close or Dispose operation or that
implement the IDisposable interface, you should consider how to eventually close or otherwise dispose of the resource. We discuss this later in this chapter and in 8.
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