Fix README.md so that relative links from manual become absolute.

Previously they'd be broken links when viewed on GitHub or Hackage.
So we add the base URL for the pandoc manual.

2 files changed, 26 insertions, 16 deletions

diff --git a/README.md b/README.md
index 5959ba669..6bf495096 100644
--- a/README.md
+++ b/README.md
@@ -39,8 +39,9 @@ library. It can convert *from*
   - `gfm` ([GitHub-Flavored
     Markdown](https://help.github.com/articles/github-flavored-markdown/)),
     or the deprecated and less accurate `markdown_github`; use
-    [`markdown_github`](#markdown-variants) only if you need extensions
-    not supported in [`gfm`](#markdown-variants).
+    [`markdown_github`](https://pandoc.org/MANUAL.html#markdown-variants)
+    only if you need extensions not supported in
+    [`gfm`](https://pandoc.org/MANUAL.html#markdown-variants).
   - `haddock` ([Haddock
     markup](https://www.haskell.org/haddock/doc/html/ch03s08.html))
   - `html` ([HTML](http://www.w3.org/html/))
@@ -49,7 +50,8 @@ library. It can convert *from*
   - `jats` ([JATS](https://jats.nlm.nih.gov) XML)
   - `json` (JSON version of native AST)
   - `latex` ([LaTeX](http://latex-project.org))
-  - `markdown` ([Pandoc’s Markdown](#pandocs-markdown))
+  - `markdown` ([Pandoc’s
+    Markdown](https://pandoc.org/MANUAL.html#pandocs-markdown))
   - `markdown_mmd`
     ([MultiMarkdown](http://fletcherpenney.net/multimarkdown/))
   - `markdown_phpextra` ([PHP Markdown
@@ -98,8 +100,9 @@ It can convert *to*
   - `gfm` ([GitHub-Flavored
     Markdown](https://help.github.com/articles/github-flavored-markdown/)),
     or the deprecated and less accurate `markdown_github`; use
-    [`markdown_github`](#markdown-variants) only if you need extensions
-    not supported in [`gfm`](#markdown-variants).
+    [`markdown_github`](https://pandoc.org/MANUAL.html#markdown-variants)
+    only if you need extensions not supported in
+    [`gfm`](https://pandoc.org/MANUAL.html#markdown-variants).
   - `haddock` ([Haddock
     markup](https://www.haskell.org/haddock/doc/html/ch03s08.html))
   - `html` or `html5` ([HTML](http://www.w3.org/html/),
@@ -118,7 +121,8 @@ It can convert *to*
   - `latex` ([LaTeX](http://latex-project.org))
   - `man` ([roff
     man](http://man7.org/linux/man-pages/man7/groff_man.7.html))
-  - `markdown` ([Pandoc’s Markdown](#pandocs-markdown))
+  - `markdown` ([Pandoc’s
+    Markdown](https://pandoc.org/MANUAL.html#pandocs-markdown))
   - `markdown_mmd`
     ([MultiMarkdown](http://fletcherpenney.net/multimarkdown/))
   - `markdown_phpextra` ([PHP Markdown
@@ -163,7 +167,7 @@ It can convert *to*
   - `zimwiki` ([ZimWiki
     markup](http://zim-wiki.org/manual/Help/Wiki_Syntax.html))
   - the path of a custom lua writer, see [Custom
-    writers](#custom-writers) below
+    writers](https://pandoc.org/MANUAL.html#custom-writers) below
 
 </div>
 
diff --git a/tools/update-readme.lua b/tools/update-readme.lua
index eabaf2285..38662d627 100644
--- a/tools/update-readme.lua
+++ b/tools/update-readme.lua
@@ -8,15 +8,21 @@ f:close()
 result = {}
 
 function Div(elem)
-    local ident = elem.identifier or ""
-    local get = function(el)
-                    if el.identifier == ident then
-                        result = el
-                    end
-                end
-    if ident == 'input-formats' or ident == 'output-formats' then
-      pandoc.walk_block(pandoc.Div(mdoc.blocks), { Div = get })
-      return result
+  local ident = elem.identifier or ""
+  local fixrel = function(el)
+    if el.target:match("^#") then
+      el.target = "https://pandoc.org/MANUAL.html" .. el.target
     end
+    return el
+  end
+  local get = function(el)
+    if el.identifier == ident then
+      result = pandoc.walk_block(el, { Link = fixrel  })
+    end
+  end
+  if ident == 'input-formats' or ident == 'output-formats' then
+    pandoc.walk_block(pandoc.Div(mdoc.blocks), { Div = get })
+    return result
+  end
 end