I'm writing a Lua filter for Pandoc to deduce the HTML pagetitle from the first h1 heading of the input document. Other than the print() function, io.stderr.write does not append a newline. I found in the answer to NewLine(\n) alternative in Lua? that [[ and ]] can be used to create multi-line string literals, but surprisingly this does not work with strings that contain nothing but a line break.

local pagetitle

function Header(header)
  if not pagetitle and header.level == 1 then
    pagetitle = pandoc.utils.stringify(header)
  end
end

-- capture current line break:
local EOL = [[
]]

function Meta(meta)
  if not meta.pagetitle then
    if pagetitle then
      meta.pagetitle = pagetitle
    else
      io.stderr:write(
        'WARNING: h1 missing, defaulting HTML pagetitle to input filename.', EOL)
      meta.pagetitle = PANDOC_STATE.input_files[1]
    end
    return meta
  end
end

Maybe that's just a Lua bug. On the other hand, it didn't look right from the start, although the idea seemed quite clever at first.

What would you suggest instead?

I'm writing a Lua filter for Pandoc to deduce the HTML pagetitle from the first h1 heading of the input document. Other than the print() function, io.stderr.write does not append a newline. I found in the answer to NewLine(\n) alternative in Lua? that [[ and ]] can be used to create multi-line string literals, but surprisingly this does not work with strings that contain nothing but a line break.

local pagetitle

function Header(header)
  if not pagetitle and header.level == 1 then
    pagetitle = pandoc.utils.stringify(header)
  end
end

-- capture current line break:
local EOL = [[
]]

function Meta(meta)
  if not meta.pagetitle then
    if pagetitle then
      meta.pagetitle = pagetitle
    else
      io.stderr:write(
        'WARNING: h1 missing, defaulting HTML pagetitle to input filename.', EOL)
      meta.pagetitle = PANDOC_STATE.input_files[1]
    end
    return meta
  end
end

Maybe that's just a Lua bug. On the other hand, it didn't look right from the start, although the idea seemed quite clever at first.

What would you suggest instead?

• lua
• cross-platform
• line-breaks
• lua-5.4

1 Answer 1

Use the string.format() function to add the newline as \n, also make print warnings a function to be future-safe:

local pagetitle

local function print_warning(msg)
  io.stderr:write(string.format('WARNING: %s\n', msg))
end

function Header(header)
  if not pagetitle and header.level == 1 then
    pagetitle = pandoc.utils.stringify(header)
  end
end

function Meta(meta)
  if not meta.pagetitle then
    if pagetitle then
      meta.pagetitle = pagetitle
    else
      print_warning('h1 missing, defaulting HTML pagetitle to input filename.')
      meta.pagetitle = PANDOC_STATE.input_files[1]
    end
    return meta
  end
end

While writing the question, I figured out an answer myself after finding a matching programming idom.