Helper function for interactive Lua filters

interactive.lua

function interactive (it, env)
  local has_readline, RL = pcall(require, 'readline')

  if not has_readline then
    RL = {
      readline = function (prompt)
        io.stdout:write(prompt)
        return io.read()
      end
    }
  end

  -- setup environment in which the commands are run
  env = env or {}
  for k, v in pairs(_ENV) do
    env[k] = v
  end
  env.it = it

  local success, result
  local line

  while not env.STOP do
    line = RL.readline('pandoc> ')
    if not line or line == 'exit' or line == 'quit' then
      break
    end

    -- Try to interpret the input as an expression. If that fails,
    -- retry and treat it as a block.
    success, result = pcall(load('return ' .. line, '<repl>', 't', env))
    if not success then
      success, result = pcall(load(line, '<repl>', 't', env))
    end
    if success then
      io.stdout:write(tostring(result) .. '\n')
    else
      io.stderr:write(tostring(result) .. '\n')
    end
  end
  return env.it
end

Example use in pandoc Lua filter:

-- Ensure that, if the REPL is quit via `STOP = true`,
-- processing is skipped for **all remaining** `Str` elements.
local Str_env = {STOP = false}
Str = function (str) return interactive(str, Str_env) end
Pandoc = interactive