{"id":1010,"date":"2014-01-02T15:16:55","date_gmt":"2014-01-02T15:16:55","guid":{"rendered":"http:\/\/putridparrot.com\/blog\/?p=1010"},"modified":"2014-01-02T15:16:55","modified_gmt":"2014-01-02T15:16:55","slug":"pattern-matching-in-f","status":"publish","type":"post","link":"https:\/\/putridparrot.com\/blog\/pattern-matching-in-f\/","title":{"rendered":"Pattern matching in F#"},"content":{"rendered":"

Pattern matching allows us to use a more powerful form of if..then..else or switch\/select case type statements.<\/p>\n

We declare a pattern matching implementation as follows<\/p>\n

\r\nlet option o =\r\n   match o with\r\n   | 1 -> "item 1"\r\n   | 2 -> "item 2"\r\n   | _ -> "unkown"\r\n\r\n\/\/ usage\r\nlet choice = option 2\r\n<\/pre>\n
The _ is used as a wildcard character, i.e. anything not matching the patterns above..<\/p>\n
We can also add matching along the lines of<\/p>\n
\r\nlet option o =\r\n   match o with\r\n   | o when o <= 0 -> failwith "value must be greater than 0"\r\n   \/\/ other patterns\r\n<\/pre>\n
You can also match to more than one pattern as per<\/p>\n
\r\nlet option o =\r\n   match o with\r\n   | 1 | 2 -> "one or two"\r\n   \/\/ other patterns\r\n<\/pre>\n
We can also pattern match on tuples for example<\/p>\n
\r\nlet options tuple =\r\nmatch tuple with\r\n   | "one", 1 -> "got one"\r\n   | "two", 2 -> "got two"\r\n   | _, _ -> "unknown"\r\n\r\n\/\/ usage\r\nlet k = options ("one", 1)\r\nprintfn "%s" <| k <\/pre>\n
Better still we can ignore part of a tuple<\/p>\n
 let options tuple =  match tuple with    | "one", _ -> "got one"\r\n   | _, 2 -> "got two"\r\n   | _, _ -> "unknown"\r\n\r\n\/\/ usage\r\nlet k = options ("?", 2)\r\nprintfn "%s" <| k <\/pre>\n
or we could use the following to denote tuples<\/p>\n
 let options (a, b) =     match (a, b) with    | "one", _ -> "got one"\r\n   | _, 2 -> "got two"\r\n   | _, _ -> "unknown"\r\n\r\nlet k = options ("?", 2)\r\nprintfn "%s" <| k\r\n<\/pre>\n
Type pattern matching<\/strong><\/p>\n
We can also use the pattern matching to match against the type of the value passed to it, for example<\/p>\n
\r\n<![CDATA[\r\nlet value = 3.12f\r\n\r\nlet y =\r\n   match box value with\r\n   | :? System.Int32 -> "integer"\r\n   | :? System.Single -> "float"\r\n   | :? System.String -> "string"\r\n   | _ -> "undefined"\r\n\r\nprintfn "%s" <| y\r\n]]>\r\n<\/pre>\n
In the above we need to use box<\/strong>, alternatively we can use the following<\/p>\n
\r\nlet y (o: obj) =\r\n   match o with\r\n   | :? System.Int32 -> "integer"\r\n   | :? System.Single -> "float"\r\n   | :? System.String -> "string"\r\n   | _ -> "undefined"\r\n\r\nprintfn "%s" <| string y\r\n<\/pre>\n
Note: Here, instead of creating a let statement to get the result from the match I’m supplying it directly to the printfn function. Interestingly whilst the return is a string and the printfn I set to output a string, at this point it’s a generic string and we need to cast it directly to a string (non-generic)<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"
Pattern matching allows us to use a more powerful form of if..then..else or switch\/select case type statements. We declare a pattern matching implementation as follows let option o = match o with | 1 -> "item 1" | 2 -> "item 2" | _ -> "unkown" \/\/ usage let choice = option 2 The _ […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[6],"tags":[],"class_list":["post-1010","post","type-post","status-publish","format-standard","hentry","category-f"],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/1010","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/comments?post=1010"}],"version-history":[{"count":19,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/1010\/revisions"}],"predecessor-version":[{"id":1086,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/1010\/revisions\/1086"}],"wp:attachment":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/media?parent=1010"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/categories?post=1010"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/tags?post=1010"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}