Add OG types to page/post, add rendering in page head (#52)

src/MyWebLog/Template.fs

@@ -188,9 +188,35 @@ let parser =
     // Create various items in the page header based on the state of the page being generated
     it.RegisterEmptyTag("page_head",
         fun writer encoder context ->
-            let app = context.App
-            // let getBool name =
-            //     defaultArg (context.Environments[0].[name] |> Option.ofObj |> Option.map Convert.ToBoolean) false
+            let app     = context.App
+            let attrEnc = System.Web.HttpUtility.HtmlAttributeEncode
+
+            // OpenGraph tags
+            if app.IsPage || app.IsPost then
+                let writeOgProp (name, value) =
+                    writer.WriteLine $"""{s}<meta property=%s{name} content="{attrEnc value}">"""
+                writeOgProp ("og:title", if app.IsPage then app.Page.Title else app.Posts.Posts[0].Title)
+                writeOgProp ("og:site_name", app.WebLog.Name)
+                if app.IsPage then app.Page.Permalink else app.Posts.Posts[0].Permalink
+                |> Permalink
+                |> app.WebLog.AbsoluteUrl
+                |> function url -> writeOgProp ("og:url", url)
+                match if app.IsPage then app.Page.OpenGraph else app.Posts.Posts[0].OpenGraph with
+                | Some props ->
+                    writeOgProp ("og:type", string props.Type)
+                    props.Image.Properties |> Seq.iter writeOgProp
+                    match props.Description with Some desc -> writeOgProp ("og:description", desc) | None -> ()
+                    match props.Determiner  with Some det  -> writeOgProp ("og:determiner",  det)  | None -> ()
+                    match props.Locale      with Some loc  -> writeOgProp ("og:locale",      loc)  | None -> ()
+                    match props.LocaleAlternate with
+                    | Some alt -> alt |> List.iter (fun it -> writeOgProp ("og:locale:alternate", it))
+                    | None -> ()
+                    match props.Audio with Some audio -> audio.Properties |> Seq.iter writeOgProp | None -> ()
+                    match props.Video with Some video -> video.Properties |> Seq.iter writeOgProp | None -> ()
+                    match props.Other with
+                    | Some oth -> oth |> List.iter (fun it -> writeOgProp (it.Name, it.Value))
+                    | None -> ()
+                | None -> ()
 
             writer.WriteLine $"""{s}<meta name=generator content="{app.Generator}">"""