Fixed pipe tables -- headerless tables are not allowed.

GFM and PHP Markdown Extra pipe tables require headers.
Previously pandoc allowed pipe tables not to include headers,
and produced headerless pipe tables in Markdown output, but this
was based on a misconception about pipe table syntax. This
commit fixes this.

Note:  If you have been using headerless pipe tables, this may
cause existing tables to break.

Closes #1996.

README

@@ -1827,8 +1827,8 @@ Pipe tables look like this:
 The syntax is [the same as in PHP markdown extra].  The beginning and
 ending pipe characters are optional, but pipes are required between all
 columns.  The colons indicate column alignment as shown.  The header
-can be omitted, but the horizontal line must still be included, as
-it defines column alignments.
+cannot be omitted.  To simulate a headerless table, include a header
+with blank cells.
 
 Since the pipes indicate column boundaries, columns need not be vertically
 aligned, as they are in the above example.  So, this is a perfectly

src/Text/Pandoc/Readers/Markdown.hs

@@ -1287,11 +1287,9 @@ pipeBreak = try $ do
 
 pipeTable :: MarkdownParser ([Alignment], [Double], F [Blocks], F [[Blocks]])
 pipeTable = try $ do
-  (heads,aligns) <- try ( pipeBreak >>= \als ->
-                     return (return $ replicate (length als) mempty, als))
-                  <|> ( pipeTableRow >>= \row -> pipeBreak >>= \als ->
-
-                          return (row, als) )
+  (heads,aligns) <- pipeTableRow >>= \row ->
+                    pipeBreak >>= \als ->
+                    return (row, als)
   lines' <- sequence <$> many1 pipeTableRow
   let widths = replicate (length aligns) 0.0
   return $ (aligns, widths, heads, lines')

src/Text/Pandoc/Writers/Markdown.hs

@@ -508,7 +508,12 @@ pipeTable headless aligns rawHeaders rawRows = do
                              AlignCenter  -> ':':replicate w '-' ++ ":"
                              AlignRight   -> replicate (w + 1) '-' ++ ":"
                              AlignDefault -> replicate (w + 2) '-'
-  let header = if headless then empty else torow rawHeaders
+  -- note:  pipe tables can't completely lack a
+  -- header; for a headerless table, we need a header of empty cells.
+  -- see jgm/pandoc#1996.
+  let header = if headless
+                  then torow (replicate (length aligns) empty)
+                  else torow rawHeaders
   let border = nowrap $ text "|" <> hcat (intersperse (text "|") $
                         map toborder $ zip aligns widths) <> text "|"
   let body   = vcat $ map torow rawRows

tests/pipe-tables.txt

@@ -1,7 +1,7 @@
 Simplest table without caption:
 
 | Default1 | Default2 | Default3 | 
-|----------|----------|----------|
+ |----------|----------|----------|
 |12|12|12|
 |123|123|123|
 |1|1|1|
@@ -27,6 +27,7 @@ Simple table without caption:
 
 Headerless table without caption:
 
+|       |      |        |
 |------:|:-----|:------:|
 |12|12|12|
 |123|123|123|
@@ -48,5 +49,6 @@ One-column:
 
 Header-less one-column:
 
+|   |
 |:-:|
 |hi|