Don't use unsafe T.head (and don't fail on empty links)

.gitignore

@@ -35,4 +35,5 @@ tags
 
 # JavaScript
 guidejs/node_modules/
+front/node_modules/
 static/js/

src/Guide/Diff/Tokenize.hs

@@ -62,10 +62,10 @@ consolidate s@("-":_) =
 consolidate (".":".":".":xs) = "..." : consolidate xs
 -- links
 consolidate s@("http":":":"/":"/":_) =
-  let (l, r) = span (\x -> x /= ")" && not (isSpace (T.head x))) s
+  let (l, r) = span (\x -> x /= ")" && not (startsWithSpace x)) s
   in  T.concat l : consolidate r
 consolidate s@("https":":":"/":"/":_) =
-  let (l, r) = span (\x -> x /= ")" && not (isSpace (T.head x))) s
+  let (l, r) = span (\x -> x /= ")" && not (startsWithSpace x)) s
   in  T.concat l : consolidate r
 consolidate ("(":"@":"hk":")":xs) = "(" : "@hk" : ")" : consolidate xs
 
@@ -88,3 +88,8 @@ op = over _1 mconcat . span (isOpToken . T.unpack)
   where
     isOpToken [c] = c `elem` (":!#$%&*+./<=>?@\\^|-~" :: String)
     isOpToken _   = False
+
+startsWithSpace :: Text -> Bool
+startsWithSpace s = case T.uncons s of
+  Nothing     -> False
+  Just (c, _) -> isSpace c

src/Guide/Markdown.hs

@@ -189,7 +189,7 @@ extractInlines = concatMap go
       CODE_BLOCK _ xs   -> [MD.Node Nothing (CODE xs) []]
 
 shortcutLinks :: MD.Node -> MD.Node
-shortcutLinks node@(MD.Node pos (LINK url title) ns) | '@' <- T.head url =
+shortcutLinks node@(MD.Node pos (LINK url title) ns) | "@" <- T.take 1 url =
   -- %20s are possibly introduced by cmark (Pandoc definitely adds them,
   -- no idea about cmark but better safe than sorry) and so they need to
   -- be converted back to spaces