Markdown: Link reference definitions (#63)

core/Parsers_Markdown.fs

@@ -163,6 +163,41 @@ let private footnote : TryNewParser =
   }
 
 
+/// Link reference definition. "Wraps" a single paragraph
+let linkRefDef : TryNewParser =
+  fun ctx ->
+  let rxLabel = mdMarker @"\[\s*\S.*?\]:\s*"
+
+  let rec wrapFLR : FirstLineRes -> FirstLineRes = function
+  | Pending r -> Pending (wrapResultParser nlpWrapper r)
+  | Finished r -> Finished (wrapResultParser (fun _ -> Some flpWrapper) r)
+
+  and flpWrapper line : FirstLineRes =
+    match tryMatch rxLabel line with
+    | Some m -> onMatch line m
+    | None ->
+        match tryParsers paraTerminators ctx line with
+        | Some flr -> flr
+        | None -> defaultPara ctx line |> wrapFLR
+
+  and nlpWrapper paraParser line : NextLineRes =
+    match tryMatch rxLabel line with
+    | Some m -> FinishedOnPrev <| Some (onMatch line m )
+    | None ->
+        match tryParsers paraTerminators ctx line with
+        | None ->
+            match paraParser line with
+            | ThisLine flr -> ThisLine (wrapFLR flr)
+            | FinishedOnPrev r -> FinishedOnPrev r
+        | someParser -> FinishedOnPrev someParser
+
+  and onMatch line (m: string[]) : FirstLineRes =
+    let prefixFn = blankOut' line.split (indentWidth line) 0 "    "
+    defaultPara ctx (trimWhitespace line) |> wrapFLR |> wrapPrefixFn prefixFn
+
+  fun line -> tryMatch rxLabel line <|>> onMatch line
+
+
 /// List item container
 let private listItem : TryNewParser =
   fun ctx ->
@@ -192,7 +227,7 @@ let private paraTerminators : List<TryNewParser> =
 
 /// list of all blocks except default paragraph
 let private contentBlocks : List<TryNewParser> =
-  paraTerminators @ [indentedCode]
+  paraTerminators @ [linkRefDef; indentedCode]
 
 
 /// Finds a new block, checking all options and falling back on default paragraph in

docs/content-types/markdown/linkrefdefs.md

@@ -0,0 +1,51 @@
+# Markdown: Link reference definitions
+
+> language: "markdown"
+
+This doesn't precisely follow the [link reference definition
+specification](https://spec.commonmark.org/current/#link-reference-definitions), but
+should be a "good enough" implementation
+
+A link reference definition, like all markdown paragraphs, can be indented up to 3 spaces.
+It has the pattern `[<label>]:` where `<label>` is characters containing at least 1
+non-whitespace character.
+
+If a single long line is wrapped, created lines are indented 4 spaces, to match
+[footnotes](footnotes.md).
+
+    [link]: /url "link title"          ->     [link]: /url    ¦
+                    ¦                             "link title"¦
+
+
+Link reference definitions do **not** interrupt a paragraph; ie. there must be a blank
+line between one and a normal paragraph. If there's no blank line it'll be treated as part
+of the paragraph.
+
+    paragraph        ¦                        paragraph text   ¦
+    text [link]      ¦                 ->     [link] [link]:   ¦
+    [link]: some_url ¦                        some_url         ¦
+
+
+LDRs consist of just a single paragraph, and no blank line is required between multiple
+LRDs.
+
+    text [one]         ¦                      text [one] text    ¦
+    text [two]         ¦                      [two]              ¦
+                       ¦               ->                        ¦
+    [one]: url1 description                   [one]: url1        ¦
+      [two]: url2      ¦                          description    ¦
+                                                [two]: url2      ¦
+
+
+LRDs are terminated by any of the normal paragraph-interrupting blocks, like fenced code
+block or list item.
+
+    [one]:    ¦                               [one]: url¦
+     url      ¦                               - list    ¦
+    - list item                                 item    ¦
+              ¦                                         ¦
+    [two]:    ¦                        ->     [two]: url¦
+        url   ¦                               ``` js    ¦
+    ``` js    ¦                               let i;    ¦
+    let i;    ¦                               ```       ¦
+    ```       ¦                                         ¦