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Using Lazy Sequences from External Sources
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Sequences are frequently used to represent the process of streaming data from an external source, such as from a database query or from a computer s file system. For example, the following recursive function constructs a seq<string> that represents the process of recursively reading the names of all the files under a given path. The return types of Directory.GetFiles and Directory.GetDirectories are string[]; and as noted earlier, this type is always compatible with seq<string>: open System.IO let rec allFiles dir = Seq.append (dir |> Directory.GetFiles) (dir |> Directory.GetDirectories |> Seq.map allFiles |> Seq.concat) This gives the following type: val allFiles : string -> seq<string> Here is an example of the function being used on one of our machines: > allFiles @"c:\WINDOWS\system32";; val it : seq<string> = = seq ["c:\\WINDOWS\\system32\\$winnt$.inf"; "c:\\WINDOWS\\system32\\12520437.cpx"; "c:\\WINDOWS\\system32\\12520850.cpx"; "c:\\WINDOWS\\system32\\6to4svc.dll"; ...] The allFiles function is interesting partly because it shows many aspects of F# working seamlessly together: Functions as values: The function allFiles is recursive and is used as a first-class function value within its own definition. Pipelining: The pipelining operator |> provides a natural way to present the transformations applied to each subdirectory name.
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CHAPTER 3 CREATING YOUR FIRST F# PROGRAM
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Type inference: Type inference computes all types in the obvious way, without any type annotations. NET interoperability: The System.IO.Directory operations provide intuitive primitives, which can then be incorporated in powerful ways using succinct F# programs. Laziness where needed: The function Seq.map applies the argument function lazily (on demand), which means subdirectories aren t read until required.
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One subtlety with programming with on-demand or lazy values like sequences is that side effects such as reading and writing from an external store shouldn t in general happen until the lazy sequence value is consumed. For example, the previous allFiles function reads the top-level directory C:\ as soon as allFiles is applied to its argument. This may not be appropriate if the contents of C:\ are changing. You can delay the computation of the sequence by using the library function Seq.delay or by using a sequence expression, covered in the next section, where delays are inserted automatically by the F# compiler.
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Using Sequence Expressions
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Aggregate operators are a powerful way of working with seq<type> values. However, F# also provides a convenient and compact syntax called sequence expressions for specifying sequence values that can be built using operations such as choose, map, filter, and concat. You can also use sequence expressions to specify the shapes of lists and arrays. It s valuable to learn how to use sequence expressions: They re a compact way of specifying interesting data and generative processes. They re used to specify database queries when using data-access layers such as Microsoft s Language Integrated Queries (LINQ). See 15 for examples of using sequence expressions this way. They re one particular use of computation expressions, a more general concept that has several uses in F# programming. 9 discusses computation expressions, and we show how to use them for asynchronous and parallel programming in 13.
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The simplest form of a sequence expression is seq { for value in expr .. expr -> expr }. Here, -> should be read yield. This is a shorthand way of writing Seq.map over a range expression. For example, you can generate an enumeration of numbers and their squares as follows: > let squares = seq { for i in 0 .. 10 -> (i,i*i) };; val squares : seq<int * int> > squares;; val it : seq<int * int> = [ (0,0); (1,1); (2,4); (3,9); ... ] The more complete form of this construct is seq { for pattern in seq -> expression }. The pattern allows you to decompose the values yielded by the input enumerable. For example, you can consume the elements of squares using the pattern (i,iSquared):
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