{"id":834,"date":"2009-07-01T16:02:25","date_gmt":"2009-07-01T14:02:25","guid":{"rendered":"http:\/\/www.navision-blog.de\/2009\/07\/01\/extensibility-of-functions-with-lambdas-in-f-and-c\/"},"modified":"2009-07-01T16:19:59","modified_gmt":"2009-07-01T14:19:59","slug":"extensibility-of-functions-with-lambdas-in-f-and-c","status":"publish","type":"post","link":"http:\/\/www.navision-blog.de\/blog\/2009\/07\/01\/extensibility-of-functions-with-lambdas-in-f-and-c\/","title":{"rendered":"Extensibility of functions with lambdas (in F# and C#)"},"content":{"rendered":"

One of the nice properties of functional programming languages is the easy extensibility of custom functions. Let\u2019s consider a simple F# function (from \u201cFAKE \u2013 F# Make\u201d<\/a>) for a recursive directory copy:<\/p>\n

open <\/span>System\nopen <\/span>System.IO\n\n\/\/\/ Copies a directory recursive\n\/\/\/ Thanks to Robert Pickering http:\/\/strangelights.com\/blog\/<\/a><\/span>\n\/\/\/  param target: target directory : string\n\/\/\/  param source: source directory : string\n<\/span>let <\/span>CopyDir target source =\n  Directory.GetFiles(source, "*.*"<\/span>, SearchOption.AllDirectories)\n    |> Seq.iter (fun <\/span>file -> \n      let <\/span>newFile = target + file.Remove(0, source.Length)\n      printf "%s => %s" <\/span>file newFile\n      Directory.CreateDirectory(Path.GetDirectoryName(newFile)) |> ignore\n      File.Copy(file, newFile, true<\/span>))<\/pre>\n
If we want to allow users to set custom file filters, we can add a third parameter:<\/p>\n
\/\/\/ Copies a directory recursive\n\/\/\/ and allows to filter the files<\/strong>\n\/\/\/ Thanks to Robert Pickering http:\/\/strangelights.com\/blog\/<\/span><\/span><\/span><\/a>\n\/\/\/  param target: target directory : string\n\/\/\/  param source: source directory : string\n\/\/\/  param filterFile: FilterFunction: string -> bool<\/strong>\n<\/span>let <\/span>CopyDirFiltered target source filterFile<\/strong> =\n  Directory.GetFiles(source, "*.*"<\/span>, SearchOption.AllDirectories)\n    |> Seq.filter filterFile<\/strong>\n    |> Seq.iter (fun <\/span>file -> \n      let <\/span>newFile = target + file.Remove(0, source.Length)\n      printfn "%s => %s" <\/span>file newFile\n      Directory.CreateDirectory(Path.GetDirectoryName(newFile)) |> ignore\n      File.Copy(file, newFile, true<\/span>))<\/pre>\n
Now we can define some filter functions:<\/p>\n
\/\/\/ Exclude SVN files (path with .svn)\n\/\/\/ excludeSVNFiles: string -> bool<\/strong> \n<\/span>let <\/span>excludeSVNFiles (path:string) = not <| path.Contains ".svn"\n\n<\/span>\/\/\/ Includes all files\n\/\/\/ allFiles: string -> bool<\/strong> <\/span>\n<\/span>let <\/span>allFiles (path:string) = true<\/span><\/pre>\n
Now it is possible to use CopyDirFiltered in the following ways:<\/p>\n
\/\/\/ Copies all files <=> same as CopyDir\n<\/span>CopyDirFiltered "C:\\\\target" "C:\\\\source" <\/span>allFiles\n\n\/\/\/ Copies all files except SVN files\n<\/span>CopyDirFiltered "C:\\\\target" "C:\\\\source" <\/span>excludeSVNFiles\n\n\/\/\/ Copies all files only if random number <> 2\n<\/span>let <\/span>r = new <\/span>Random()\nCopyDirFiltered "C:\\\\target" "C:\\\\source" <\/span>(fun <\/span>path -> <\/span>r.Next(5) <> 2)<\/pre>\n
Extensibility of functions in C#<\/h5>\n
Of course we can do the same thing in C# 3.0:<\/p>\n
\/\/\/ <summary>\n\/\/\/ <\/span>Copies a directory recursive\n<\/span>\/\/\/ <\/span>and allows to filter the files\n<\/span>\/\/\/ <\/summary>\n\/\/\/ <param name="target"><\/span>The target.<\/span><\/param>\n\/\/\/ <param name="source"><\/span>The source.<\/span><\/param>\n\/\/\/ <param name="fileFilter"><\/span>The file filter.<\/span><\/param>\n<\/span>public static void <\/span>CopyDirFiltered(string <\/span>target, string <\/span>source,\n                                   Func<\/span><string<\/span>, bool<\/span>> fileFilter)\n{\n    string<\/span>[] allFiles = Directory<\/span>.GetFiles(\n        source, "*.*"<\/span>, SearchOption<\/span>.AllDirectories);\n    foreach <\/span>(string <\/span>file in from <\/span>f in <\/span>allFiles\n                            where <\/span>fileFilter(f)\n                            select <\/span>f)\n    {\n        string <\/span>newFile = target + file.Remove(0, source.Length);\n        Console<\/span>.WriteLine("{0} => {1}"<\/span>, file, newFile);\n        Directory<\/span>.CreateDirectory(Path<\/span>.GetDirectoryName(newFile));\n        File<\/span>.Copy(file, newFile, true<\/span>);\n    }\n}<\/pre>\n
Now it is easy to use the C# function with lambdas:<\/p>\n
\n
\u201cA lambda expression is an anonymous function that can contain expressions and statements, and can be used to create delegates or expression tree types.\u201d<\/p>\n
[MSDN<\/a>]<\/p>\n<\/blockquote>\n
Func<\/span><string<\/span>, bool<\/span>> filterSVN = x => !x.Contains(".svn"<\/span>);\nFunc<\/span><string<\/span>, bool<\/span>> allFiles = x => true<\/span>;\n\n\/\/\/ <\/span>Copies all files <\/span><=> <\/span>same as CopyDir\n<\/span>CopyDirFiltered("C:\\\\target"<\/span>, "C:\\\\source"<\/span>, allFiles);\n\n\/\/\/ <\/span>Copies all files except SVN files\n<\/span>CopyDirFiltered("C:\\\\target"<\/span>, "C:\\\\source"<\/span>, filterSVN);\n\n\/\/\/ <\/span>Copies all files only if random number <\/span><> <\/span>2\n<\/span>var <\/span>r = new <\/span>Random<\/span>();\nCopyDirFiltered("C:\\\\target"<\/span>, "C:\\\\source"<\/span>, path => r.Next(5) != 2);<\/pre>\n
Keeping this simple technique in mind allows to create very flexible functions.<\/p>\n","protected":false},"excerpt":{"rendered":"
One of the nice properties of functional programming languages is the easy extensibility of custom functions. Let\u2019s consider a simple F# function (from \u201cFAKE \u2013 F# Make\u201d) for a recursive directory copy: open System open System.IO \/\/\/ Copies a directory recursive \/\/\/ Thanks to Robert Pickering http:\/\/strangelights.com\/blog\/ \/\/\/ param target: target directory : string \/\/\/ […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[23,448],"tags":[81,664,544,555],"_links":{"self":[{"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/posts\/834"}],"collection":[{"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/comments?post=834"}],"version-history":[{"count":9,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/posts\/834\/revisions"}],"predecessor-version":[{"id":843,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/posts\/834\/revisions\/843"}],"wp:attachment":[{"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/media?parent=834"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/categories?post=834"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/tags?post=834"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}