One of the annoyances of F#, well it is when you come from C# (or the likes), is that to use a function or type, the function or type needs to have been declared before it’s used. Obviously this is a problem if a type references another type which itself references the first type – but ofcourse they F# guys have a way to handle this and it’s called mutual type recursion.<\/p>\n
Let’s look at an example of what I’m talking about and at the solution<\/p>\n
\r\n[<Measure>\r\ntype g = \r\n static member toKilograms value : float<g> = value \/ 1000.0<g\/kg>\r\nand [<Measure> kg = \r\n static member toGrams value : float<kg> = value * 1000.0<g\/kg>\r\n<\/pre>\nThe above is taken from my previous post on units of measure.<\/p>\n
So in the above code we have type g returning a unit of measure type kg and type kg returns a unit of measure type g. Obviously g cannot use kg as it’s not declared before type g, but it can in the above example by using the and<\/strong> keyword. This can be thought of, as telling the compiler to wait until it’s read all the “chained” types in before evaluating them.<\/p>\n
The same technique is used for creating recursive functions which might call other functions which might call the previous function, i.e. circular function calls.<\/p>\n
Here’s some code taken from Microsoft’s MSDN site (referenced below) as this is a better example than any I’ve come up with to demonstrate this<\/p>\n
\r\nlet rec Even x =\r\n if x = 0 then true \r\n else Odd (x - 1)\r\nand Odd x =\r\n if x = 1 then true \r\n else Even (x - 1)\r\n<\/pre>\nSo in this code we can see that the function Even is marked as rec<\/strong> so it’s recursive, it may call the function Odd which in turn may call function Even. Again we chain these functions together using the and<\/strong> keyword.<\/p>\n
References<\/strong><\/p>\n
Mutually Recursive Functions<\/a>
\nMutually Recursive Types<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"One of the annoyances of F#, well it is when you come from C# (or the likes), is that to use a function or type, the function or type needs to have been declared before it’s used. Obviously this is a problem if a type references another type which itself references the first type – […]<\/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-2845","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\/2845","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=2845"}],"version-history":[{"count":5,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/2845\/revisions"}],"predecessor-version":[{"id":2932,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/2845\/revisions\/2932"}],"wp:attachment":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/media?parent=2845"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/categories?post=2845"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/tags?post=2845"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}