A Lua REPL implemented in Lua

repl.lua

do
  local buffer = ''

  local function gather_results(success, ...)
    local n = select('#', ...)
    return success, { n = n, ... }
  end 

  local function print_results(results)
    local parts = {}

    for i = 1, results.n do
      parts[#parts + 1] = tostring(results[i])
    end 

    print(table.concat(parts, '  '))
  end 

  function evaluate_line(line)
    local chunk  = buffer .. line
    local f, err = loadstring('return ' .. chunk, 'REPL') -- first we prefix return

    if not f then
      f, err = loadstring(chunk, 'REPL') -- try again without return
    end 

    if f then
      buffer = ''
      local success, results = gather_results(xpcall(f, debug.traceback))

      if success then
        -- successful call
        if #results > 0 then
          print_results(results)
        end 
      else
        -- error
        print(results[1])
      end 
    else
      if err:match "'<eof>'$" then
        -- Lua expects some more input; stow it away for next time
        buffer = chunk .. '\n'
        return '>>'
      else
        print(err)
      end 
    end 

    return '>' 
  end 
end

function display_prompt(prompt)
  io.stdout:write(prompt .. ' ')
end

display_prompt '>' 
for line in io.stdin:lines() do
  local prompt = evaluate_line(line)
  display_prompt(prompt)
end

Your repl was useful for me on my side projects, thanks! 😄

Hint: Lua 5.2 changes deprecated loadstring and unpack, to load and table.unpack respectively.