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Recrusively printing table items

Matias Guijarro wrote:

I would like to change the default behaviour of Lua when representing a table through the tostring() function ; I would prefer to have something more like in Python for example (instead of having "table: 0x8062ac0", I would like to see the contents, for example { 1, 2, 3 }).

So first of all I tried to set a new metatable for tables, As Shmuel pointed out, table metatables are per-table, so they don't really work for what you want to do.

Lua 5.1.1  Copyright (C) 1994-2006 Lua.org, PUC-Rio
> Tbl = setmetatable({}, {__tostring = function(t)
>>                                        return "Hello!"
>>                                      end})
> print(Tbl) -- Works for this table,
Hello!
> print({}) -- but doesn't work for other tables.
table: 0x493378

A way that will work is to simply replace the tostring function itself. Replace it with a function that calls your table-to-string function for tables, and calls the original tostring for everything else. Roberto's Programming in Lua book explains how to write the table-to-string function. (The first edition is available online: http://www.lua.org/pil/12.1.html.) As it happens, I happened to write an implementation of just the functionality you're looking for a few days ago. It is below, but I encourage you to try whipping together at least a simple version of your own before looking at mine. - Aaron

This script makes tostring convert tables to a representation of their contents.

 





  

    
  

  

The real tostring:





  
local _tostring = tostring






  

    
  

  

Characters that have non-numeric backslash-escaped versions:





  
local BsChars = {
  ["\a"] = "\\a",
  ["\b"] = "\\b",
  ["\f"] = "\\f",
  ["\n"] = "\\n",
  ["\r"] = "\\r",
  ["\t"] = "\\t",
  ["\v"] = "\\v",
  ["\""] = "\\\"",
  ["\\"] = "\\\\"}






  

    
  

  

Is Str an "escapeable" character (a non-printing character other than
space, a backslash, or a double quote)?





  
local function IsEscapeable(Char)
  return string.find(Char, "[^%w%p]") -- Non-alphanumeric, non-punct.
    and Char ~= " " -- Don't count spaces.
    or string.find(Char, '[\\"]') -- A backslash or quote.
end






  

    
  

  

Converts an "escapeable" character (a non-printing character,
backslash, or double quote) to its backslash-escaped version; the
second argument is used so that numeric character codes can have one
or two digits unless three are necessary, which means that the
returned value may represent both the character in question and the
digit after it:





  
local function EscapeableToEscaped(Char, FollowingDigit)
  if IsEscapeable(Char) then
    local Format = FollowingDigit == "" and "\\%d" or "\\%03d" .. FollowingDigit
    return BsChars[Char]
      or string.format(Format, string.byte(Char))
  else
    return Char .. FollowingDigit
  end
end






  

    
  

  

Quotes a string in a Lua- and human-readable way.  (This is a
replacement for string.format's q  placeholder, whose result
isn't always human readable.)





  
local function StrToStr(Str)
  return '"' .. string.gsub(Str, "(.)(%d?)", EscapeableToEscaped) .. '"'
end






  

    
  

  

Quote a string into lua form (including the non-printable characters from
0-31, and from 127-255).





  
local function quote(_)
  local fmt = string.format
  local _ = fmt("%q", _)
  _ = _)
  _ = string.gsub(_, "\\\n", "\\n")
  _ = _ = string.gsub(_, "[%z\1-\31,\127-\255]", function (x)
  return fmt("\\%03d",string.byte(x))
  end)
  return _
end

StrToStr = quote






  

    
  

  

Lua keywords:





  
local Keywords = {["and"] = true, ["break"] = true, ["do"] = true,
  ["else"] = true, ["elseif"] = true, ["end"] = true, ["false"] = true,
  ["for"] = true, ["function"] = true, ["if"] = true, ["in"] = true,
  ["local"] = true, ["nil"] = true, ["not"] = true, ["or"] = true,
  ["repeat"] = true, ["return"] = true, ["then"] = true,
  ["true"] = true, ["until"] = true, ["while"] = true}






  

    
  

  

Is Str an identifier?





  
local function IsIdent(Str)
  return not Keywords[Str] and string.find(Str, "^[%a_][%w_]*$")
end






  

    
  

  

Converts a non-table to a Lua- and human-readable string:





  
local function ScalarToStr(Val)
  local Ret
  local Type = type(Val)
  if Type == "string" then
    Ret = StrToStr(Val)
  elseif Type == "function" or Type == "userdata" or Type == "thread" then
    Ret = "<" .. __tostring(Val) .. ">"
  else
    Ret = _tostring(Val)
  end 
  return Ret
end

local function private(str)
  return type(str) == "string" and str:sub(1,1) == "_" end






  

    
  

  

Converts a table to a Lua- and human-readable string.





  
local function TblToStr(Tbl, Seen)
  Seen = Seen or {}
  local Ret = {}
  if not Seen[Tbl] then
    Seen[Tbl] = true
    local LastArrayKey = 0
    for Key, Val in pairs(Tbl) do
      if not private(Key) then
        if type(Key) == "table" then
          Key = "[" .. TblToStr(Key, Seen) .. "]"
        elseif not IsIdent(Key) then
          if type(Key) == "number" and Key == LastArrayKey + 1 then






  

    
  

  

Don't mess with Key if it's an array key.





  
            LastArrayKey = Key
          else
            Key = "[" .. ScalarToStr(Key) .. "]"
          end
        end
        if type(Val) == "table" then
          Val = TblToStr(Val, Seen)
        else
          Val = ScalarToStr(Val)
        end
        Ret[#Ret + 1] =
          (type(Key) == "string"
            and (Key .. "= ") -- Explicit key.
            or "") -- Implicit array key.
          .. Val
      end
    end
    Ret = "{" .. table.concat(Ret, ", ") .. "}"
  else
    Ret = "<cycle to " .. __tostring(Tbl) .. ">"
  end
  return Ret
end






  

    
  

  

A replacement for tostring that prints tables in Lua- and
human-readable format:





  
function tostring(Val)
  return type(Val) == "table"
    and TblToStr(Val)
    or _tostring(Val)
end







  

    
  

  

A test function:





  
local function _tostringTest()
  local Fnc = table.sort --function() end
  local Tbl = {}
  Tbl[Tbl] = Tbl
  local Tbls = {
    {},
    {1, true, 3},
    {Foo = "Bar"},
    {["*Foo*"] = "Bar"},
    {Fnc},
    {[{}] = {}},
    {1, 2, 3, {4, 5}},
    Tbl,
  }
  local Strs = {
    "{}",
    "{1, true, 3}",
    '{Foo= "Bar"}',
    '{["*Foo*"]= "Bar"}',
    '{<' .. __tostring(Fnc) .. '>}',
    "{[{}]= {}}",
    "{1, 2, 3, {4, 5}}",
    "{[<cycle to " .. __tostring(Tbl) .. ">]= <cycle to "
      .. __tostring(Tbl) .. ">}",
  }
  for I, Tbl in ipairs(Tbls) do
    print(tostring(Tbl))
    assert(tostring(Tbl) == Strs[I],
      "Assertion failed on " .. __tostring(tostring(Tbl)))
  end
end

return {tests=_tostringTest}