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Using Anonymous Function Values
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Function values are so common in F# programming that it s convenient to define them without giving them names. Here is a simple example:
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CHAPTER 3 CREATING YOUR FIRST F# PROGRAM
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> let primes = [2; 3; 5; 7];; val primes : int list > let primeCubes = List.map (fun n -> n * n * n) primes;; val primeCubes: int list > primeCubes;; val it : int list = [8; 27; 125; 343] The definition of primeCubes uses the anonymous function value (fun n -> n * n * n). Such values are similar to function definitions but are unnamed and appear as an expression rather than as a let declaration. fun is a keyword meaning function, n represents the argument to the function, and n * n * n is the result of the function. The overall type of the anonymous function expression is int -> int. You could use this technique to avoid defining the intermediary function fetch in the earlier sample: let resultsOfFetch = List.map (fun url -> (url, http url)) sites You see anonymous functions throughout this book. Here is another example: > List.map (fun (_,p) -> String.length p) resultsOfFetch;; val it : int list = [3932; 2827 ] Here you see two things: The argument of the anonymous function is a tuple pattern. Using a tuple pattern automatically extracts the second element from each tuple and gives it the name p within the body of the anonymous function. Part of the tuple pattern is a wildcard pattern, indicated by an underscore. This indicates that you don t care what the first part of the tuple is; you re interested only in extracting the length from the second part of the pair.
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Computing with Aggregate Operators
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Functions such as List.map are called aggregate operators, and they re powerful constructs, especially when combined with the other features of F#. Here is a longer example that uses the aggregate operators Array.filter and List.map to count the number of URL links in an HTML page and then collects stats on a group of pages (this sample uses the function http defined in 2): let delimiters = [| ' '; '\n'; '\t'; '<'; '>'; '=' |] let getWords (s: string) = s.Split delimiters let getStats site = let url = "http://" + site let html = http url let hwords = html |> getWords let hrefs = html |> getWords |> Array.filter (fun s -> s = "href") (site,html.Length, hwords.Length, hrefs.Length)
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CHAPTER 3 CREATING YOUR FIRST F# PROGRAM
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Here, you use the function getStats with three web pages: > let sites = [ "www.live.com";"www.google.com";"search.yahoo.com" ];; val sites : string list > sites |> List.map getStats;; val it : (string * int * int * int) list = [("www.live.com", 7728, 1156, 10); ("www.google.com", 2685, 496, 14); ("search.yahoo.com", 10715, 1771, 38)] The function getStats computes the length of the HTML for the given web site, the number of words in the text of that HTML, and the approximate number of links on that page. The previous code sample extensively uses the |> operator to pipeline operations, discussed in the Pipelining with |> sidebar. The F# library design ensures that a common, consistent set of aggregate operations is defined for each structured type. Table 3-11 shows how the same design patterns occur for the map abstraction. Table 3-11. A Recurring Aggregate Operator Design Pattern from the F# Library
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List.map Array.map Option.map Seq.map
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Type
('T -> 'U) -> 'T list -> 'U list ('T -> 'U) -> 'T [] -> 'U [] ('T -> 'U) -> 'T option -> 'U option ('T -> 'U) -> seq<'T> -> seq<'U>
You may see types that define methods such as Map that provide a slightly more succinct way of transforming data. For example, this might allow you to write a transformation like so: site.Map getStats This may require the use of a type annotation. Both styles are used in F# programming, depending on the methods and properties available for a particular data type.
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