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Not All Side Effects Are Equal
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It s often helpful to use the weakest set of side effects necessary to achieve your programming task and at least be aware when you re using strong side effects:
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CHAPTER 4 INTRODUCING IMPERATIVE PROGRAMMING
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Weak side effects are effectively benign given the assumptions you re making about your application. For example, writing to a log file is very useful and is essentially benign (if the log file can t grow arbitrarily large and crash your machine!). Similarly, reading data from a stable, unchanging file store on a local disk is effectively treating the disk as an extension of read-only memory, so reading these files is a weak form of side effect that isn t difficult to incorporate into your programs. Strong side effects have a much more corrosive effect on the correctness and operational properties of your program. For example, blocking network I/O is a relatively strong side effect by any measure. Performing blocking network I/O in the middle of a library routine can have the effect of destroying the responsiveness of a GUI application, at least if the routine is invoked by the GUI thread of an application. Any constructs that perform synchronization between threads are also a major source of strong side effects.
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Whether a particular side effect is stronger or weaker depends very much on your application and whether the consequences of the side effect are sufficiently isolated and separated from other entities. Strong side effects can and should be used freely in the outer shell of an application or when you re scripting with F# Interactive; otherwise, not much can be achieved. When you re writing larger pieces of code, you should write your application and libraries in such a way that most of your code either doesn t use strong side effects or at least makes it obvious when these side effects are being used. Threads and concurrency are commonly used to mediate problems associated with strong side effects; 14 covers these issues in more depth.
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Avoid Combining Imperative Programming and Laziness
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It s generally thought to be bad style to combine delayed computations (that is, laziness) and side effects. This isn t entirely true; for example, it s reasonable to set up a read from a file system as a lazy computation using sequences. However, it s relatively easy to make mistakes in this sort of programming. For example, consider the following code: open System.IO let reader1, reader2 = let reader = new StreamReader(File.OpenRead("test.txt")) let firstReader() = reader.ReadLine() let secondReader() = reader.ReadLine() // Note: we close the stream reader here! // But we are returning function values which use the reader // This is very bad! reader.Close() firstReader, secondReader // Note: stream reader is now closed! The next line will fail! let firstLine = reader1() let secondLine = reader2() firstLine, secondLine This code is wrong because the StreamReader object reader is used after the point indicated by the comment. The returned function values are then called, and they try to read from the captured variable reader. Function values are just one example of delayed computations: other examples are lazy values, sequences, and any objects that perform computations on demand. Be careful not to build delayed
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CHAPTER 4 INTRODUCING IMPERATIVE PROGRAMMING
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objects such as reader that represent handles to transient, disposable resources, unless those objects are used in a way that respects the lifetime of that resource. The previous code can be corrected to avoid using laziness in combination with a transient resource: open System.IO let line1, line2 = let reader = new StreamReader(File.OpenRead("test.txt")) let firstLine = reader.ReadLine() let secondLine = reader.ReadLine() reader.Close() firstLine, secondLine Another technique uses language and/or library constructs that tie the lifetime of an object to some larger object. For example, you can use a use binding within a sequence expression, which augments the sequence object with the code needed to clean up the resource when iteration is finished or terminates. This technique is discussed further in 8 and shown by example here: let reader = seq { use reader = new StreamReader(File.OpenRead("test.txt")) while not reader.EndOfStream do yield reader.ReadLine() } The general lesson is to try to keep your core application pure. Use both delayed computations (laziness) and imperative programming (side effects) where appropriate, but be careful about using them together.
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In this chapter, you learned how to do imperative programming in F#, from some of the basic mutable data structures such as reference cells to working with side effects such as exceptions and I/O. You also looked at some general principles for avoiding the need for imperative programming and isolating your uses of side effects. The next chapter returns to some of the building blocks of both functional and imperative programming in F#, with a deeper look at types, type inference, and generics.
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