Module:Inspect

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inspect.lua

This library transforms any Lua value into a human-readable representation. It is especially useful for debugging errors in tables.

The objective here is human understanding (i.e. for debugging), not serialization or compactness.

Examples of use

inspect has the following declaration: local str = inspect(value, <options>).

value can be any Lua value.

inspect transforms simple types (like strings or numbers) into strings. 

assert(inspect(1) == "1")
assert(inspect("Hello") == '"Hello"')

Tables, on the other hand, are rendered in a way a human can read easily.

"Array-like" tables are rendered horizontally: 

assert(inspect({1,2,3,4}) == "{ 1, 2, 3, 4 }")

"Dictionary-like" tables are rendered with one element per line: 

assert(inspect({a=1,b=2}) == [[{
  a = 1,
  b = 2
}]])

The keys will be sorted alphanumerically when possible.

"Hybrid" tables will have the array part on the first line, and the dictionary part just below them: 

assert(inspect({1,2,3,b=2,a=1}) == [[{ 1, 2, 3,
  a = 1,
  b = 2
}]])

Subtables are indented with two spaces per level. 

assert(inspect({a={b=2}}) == [[{
  a = {
    b = 2
  }
}]])

Functions, userdata and any other custom types from Luajit are simply as <function x>, <userdata x>, etc.: 

assert(inspect({ f = print, ud = some_user_data, thread = a_thread} ) == [[{
  f = <function 1>,
  u = <userdata 1>,
  thread = <thread 1>
}]])

If the table has a metatable, inspect will include it at the end, in a special field called <metatable>: 

assert(inspect(setmetatable({a=1}, {b=2}) == [[{
  a = 1
  <metatable> = {
    b = 2
  }
}]]))

inspect can handle tables with loops inside them. It will print <id> right before the table is printed out the first time, and replace the whole table with <table id> from then on, preventing infinite loops. 

local a = {1, 2}
local b = {3, 4, a}
a[3] = b -- a references b, and b references a
assert(inspect(a) == "<1>{ 1, 2, { 3, 4, <table 1> } }")

Notice that since both a appears more than once in the expression, it is prefixed by <1> and replaced by <table 1> every time it appears later on.

Options

inspect has a second parameter, called options. It is not mandatory, but when it is provided, it must be a table.

options.depth

options.depth sets the maximum depth that will be printed out. When the max depth is reached, inspect will stop parsing tables and just return {...}: 

local t5 = {a = {b = {c = {d = {e = 5}}}}}

assert(inspect(t5, {depth = 4}) == [[{
  a = {
    b = {
      c = {
        d = {...}
      }
    }
  }
}]])

assert(inspect(t5, {depth = 2}) == [[{
  a = {
    b = {...}
  }
}]])

options.depth defaults to infinite (math.huge).

options.newline & options.indent

These are the strings used by inspect to respectively add a newline and indent each level of a table.

By default, options.newline is "\n" and options.indent is " " (two spaces). 

local t = {a={b=1}}

assert(inspect(t) == [[{
  a = {
    b = 1
  }
}]])

assert(inspect(t, {newline='@', indent="++"}), "{@++a = {@++++b = 1@++}@}"

options.process

options.process is a function which allow altering the passed object before transforming it into a string. A typical way to use it would be to remove certain values so that they don't appear at all.

options.process has the following signature: 

local processed_item = function(item, path)
  • item is either a key or a value on the table, or any of its subtables
  • path is an array-like table built with all the keys that have been used to reach item, from the root.
  • For values, it is just a regular list of keys. For example, to reach the 1 in {a = {b = 1}}, the path will be {'a', 'b'}
  • For keys, the special value inspect.KEY is inserted. For example, to reach the c in {a = {b = {c = 1}}}, the path will be {'a', 'b', 'c', inspect.KEY }
  • For metatables, the special value inspect.METATABLE is inserted. For {a = {b = 1}}}, the path {'a', {b = 1}, inspect.METATABLE} means "the metatable of the table {b = 1}".
  • processed_item is the value returned by options.process. If it is equal to item, then the inspected table will look unchanged. If it is different, then the table will look different; most notably, if it's nil, the item will dissapear on the inspected table.

Examples

Remove a particular metatable from the result: 

local t = {1,2,3}
local mt = {b = 2}
setmetatable(t, mt)

local remove_mt = function(item)
  if item ~= mt then return item end
end

-- mt does not appear
assert(inspect(t, {process = remove_mt}) == "{ 1, 2, 3 }")

The previous exaple only works for a particular metatable. If you want to make all metatables, you can use the path parameter to check wether the last element is inspect.METATABLE, and return nil instead of the item: 

local t, mt = ... -- (defined as before)

local remove_all_metatables = function(item, path)
  if path[#path] ~= inspect.METATABLE then return item end
end

assert(inspect(t, {process = remove_all_metatables}) == "{ 1, 2, 3 }")

Filter a value: 

local anonymize_password = function(item, path)
  if path[#path] == 'password' then return "XXXX" end
  return item
end

local info = {user = 'peter', password = 'secret'}

assert(inspect(info, {process = anonymize_password}) == [[{
  password = "XXXX",
  user     = "peter"
}]])

Invoking for other modules

In order to use inspect.lua for other Module usage, you have to reference it locally on top of your module. 

  local inspect = require( 'Module:Inspect' )

  local p = {}
  -- Rest of your module.

You can also use inspect to return data for the invoking frame. 

  function p.main( frame )
    return inspect( p.data )
  end


local inspect ={
  _VERSION = 'inspect.lua 3.1.0',
  _URL     = 'http://github.com/kikito/inspect.lua',
  _DESCRIPTION = 'human-readable representations of tables',
  _LICENSE = [[
    MIT LICENSE

    Copyright (c) 2013 Enrique García Cota

    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the
    "Software"), to deal in the Software without restriction, including
    without limitation the rights to use, copy, modify, merge, publish,
    distribute, sublicense, and/or sell copies of the Software, and to
    permit persons to whom the Software is furnished to do so, subject to
    the following conditions:

    The above copyright notice and this permission notice shall be included
    in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
    OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
    MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
    IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
    CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
    TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
    SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  ]]
}

local tostring = tostring

inspect.KEY       = setmetatable({}, {__tostring = function() return 'inspect.KEY' end})
inspect.METATABLE = setmetatable({}, {__tostring = function() return 'inspect.METATABLE' end})

-- Apostrophizes the string if it has quotes, but not aphostrophes
-- Otherwise, it returns a regular quoted string
local function smartQuote(str)
  if str:match('"') and not str:match("'") then
    return "'" .. str .. "'"
  end
  return '"' .. str:gsub('"', '\\"') .. '"'
end

-- \a => '\\a', \0 => '\\0', 31 => '\31'
local shortControlCharEscapes = {
  ["\a"] = "\\a",  ["\b"] = "\\b", ["\f"] = "\\f", ["\n"] = "\\n",
  ["\r"] = "\\r",  ["\t"] = "\\t", ["\v"] = "\\v"
}
local longControlCharEscapes = {} -- \a => nil, \0 => \000, 31 => \031
for i=0, 31 do
  local ch = string.char(i)
  if not shortControlCharEscapes[ch] then
    shortControlCharEscapes[ch] = "\\"..i
    longControlCharEscapes[ch]  = string.format("\\%03d", i)
  end
end

local function escape(str)
  return (str:gsub("\\", "\\\\")
             :gsub("(%c)%f[0-9]", longControlCharEscapes)
             :gsub("%c", shortControlCharEscapes))
end

local function isIdentifier(str)
  return type(str) == 'string' and str:match( "^[_%a][_%a%d]*$" )
end

local function isSequenceKey(k, sequenceLength)
  return type(k) == 'number'
     and 1 <= k
     and k <= sequenceLength
     and math.floor(k) == k
end

local defaultTypeOrders = {
  ['number']   = 1, ['boolean']  = 2, ['string'] = 3, ['table'] = 4,
  ['function'] = 5, ['userdata'] = 6, ['thread'] = 7
}

local function sortKeys(a, b)
  local ta, tb = type(a), type(b)

  -- strings and numbers are sorted numerically/alphabetically
  if ta == tb and (ta == 'string' or ta == 'number') then return a < b end

  local dta, dtb = defaultTypeOrders[ta], defaultTypeOrders[tb]
  -- Two default types are compared according to the defaultTypeOrders table
  if dta and dtb then return defaultTypeOrders[ta] < defaultTypeOrders[tb]
  elseif dta     then return true  -- default types before custom ones
  elseif dtb     then return false -- custom types after default ones
  end

  -- custom types are sorted out alphabetically
  return ta < tb
end

-- For implementation reasons, the behavior of rawlen & # is "undefined" when
-- tables aren't pure sequences. So we implement our own # operator.
local function getSequenceLength(t)
  local len = 1
  local v = rawget(t,len)
  while v ~= nil do
    len = len + 1
    v = rawget(t,len)
  end
  return len - 1
end

local function getNonSequentialKeys(t)
  local keys = {}
  local sequenceLength = getSequenceLength(t)
  for k,_ in pairs(t) do
    if not isSequenceKey(k, sequenceLength) then table.insert(keys, k) end
  end
  table.sort(keys, sortKeys)
  return keys, sequenceLength
end

local function getToStringResultSafely(t, mt)
  local __tostring = type(mt) == 'table' and rawget(mt, '__tostring')
  local str, ok
  if type(__tostring) == 'function' then
    ok, str = pcall(__tostring, t)
    str = ok and str or 'error: ' .. tostring(str)
  end
  if type(str) == 'string' and #str > 0 then return str end
end

local function countTableAppearances(t, tableAppearances)
  tableAppearances = tableAppearances or {}

  if type(t) == 'table' then
    if not tableAppearances[t] then
      tableAppearances[t] = 1
      for k,v in pairs(t) do
        countTableAppearances(k, tableAppearances)
        countTableAppearances(v, tableAppearances)
      end
      countTableAppearances(getmetatable(t), tableAppearances)
    else
      tableAppearances[t] = tableAppearances[t] + 1
    end
  end

  return tableAppearances
end

local copySequence = function(s)
  local copy, len = {}, #s
  for i=1, len do copy[i] = s[i] end
  return copy, len
end

local function makePath(path, ...)
  local keys = {...}
  local newPath, len = copySequence(path)
  for i=1, #keys do
    newPath[len + i] = keys[i]
  end
  return newPath
end

local function processRecursive(process, item, path, visited)

    if item == nil then return nil end
    if visited[item] then return visited[item] end

    local processed = process(item, path)
    if type(processed) == 'table' then
      local processedCopy = {}
      visited[item] = processedCopy
      local processedKey

      for k,v in pairs(processed) do
        processedKey = processRecursive(process, k, makePath(path, k, inspect.KEY), visited)
        if processedKey ~= nil then
          processedCopy[processedKey] = processRecursive(process, v, makePath(path, processedKey), visited)
        end
      end

      local mt  = processRecursive(process, getmetatable(processed), makePath(path, inspect.METATABLE), visited)
      setmetatable(processedCopy, mt)
      processed = processedCopy
    end
    return processed
end



-------------------------------------------------------------------

local Inspector = {}
local Inspector_mt = {__index = Inspector}

function Inspector:puts(...)
  local args   = {...}
  local buffer = self.buffer
  local len    = #buffer
  for i=1, #args do
    len = len + 1
    buffer[len] = args[i]
  end
end

function Inspector:down(f)
  self.level = self.level + 1
  f()
  self.level = self.level - 1
end

function Inspector:tabify()
  self:puts(self.newline, string.rep(self.indent, self.level))
end

function Inspector:alreadyVisited(v)
  return self.ids[v] ~= nil
end

function Inspector:getId(v)
  local id = self.ids[v]
  if not id then
    local tv = type(v)
    id              = (self.maxIds[tv] or 0) + 1
    self.maxIds[tv] = id
    self.ids[v]     = id
  end
  return tostring(id)
end

function Inspector:putKey(k)
  if isIdentifier(k) then return self:puts(k) end
  self:puts("[")
  self:putValue(k)
  self:puts("]")
end

function Inspector:putTable(t)
  if t == inspect.KEY or t == inspect.METATABLE then
    self:puts(tostring(t))
  elseif self:alreadyVisited(t) then
    self:puts('<table ', self:getId(t), '>')
  elseif self.level >= self.depth then
    self:puts('{...}')
  else
    if self.tableAppearances[t] > 1 then self:puts('<', self:getId(t), '>') end

    local nonSequentialKeys, sequenceLength = getNonSequentialKeys(t)
    local mt                = getmetatable(t)
    local toStringResult    = getToStringResultSafely(t, mt)

    self:puts('{')
    self:down(function()
      if toStringResult then
        self:puts(' -- ', escape(toStringResult))
        if sequenceLength >= 1 then self:tabify() end
      end

      local count = 0
      for i=1, sequenceLength do
        if count > 0 then self:puts(',') end
        self:puts(' ')
        self:putValue(t[i])
        count = count + 1
      end

      for _,k in ipairs(nonSequentialKeys) do
        if count > 0 then self:puts(',') end
        self:tabify()
        self:putKey(k)
        self:puts(' = ')
        self:putValue(t[k])
        count = count + 1
      end

      if mt then
        if count > 0 then self:puts(',') end
        self:tabify()
        self:puts('<metatable> = ')
        self:putValue(mt)
      end
    end)

    if #nonSequentialKeys > 0 or mt then -- result is multi-lined. Justify closing }
      self:tabify()
    elseif sequenceLength > 0 then -- array tables have one extra space before closing }
      self:puts(' ')
    end

    self:puts('}')
  end
end

function Inspector:putValue(v)
  local tv = type(v)

  if tv == 'string' then
    self:puts(smartQuote(escape(v)))
  elseif tv == 'number' or tv == 'boolean' or tv == 'nil' then
    self:puts(tostring(v))
  elseif tv == 'table' then
    self:putTable(v)
  else
    self:puts('<',tv,' ',self:getId(v),'>')
  end
end

-------------------------------------------------------------------

function inspect.inspect(root, options)
  options       = options or {}

  local depth   = options.depth   or math.huge
  local newline = options.newline or '\n'
  local indent  = options.indent  or '  '
  local process = options.process

  if process then
    root = processRecursive(process, root, {}, {})
  end

  local inspector = setmetatable({
    depth            = depth,
    level            = 0,
    buffer           = {},
    ids              = {},
    maxIds           = {},
    newline          = newline,
    indent           = indent,
    tableAppearances = countTableAppearances(root)
  }, Inspector_mt)

  inspector:putValue(root)

  return table.concat(inspector.buffer)
end

setmetatable(inspect, { __call = function(_, ...) return inspect.inspect(...) end })

return inspect
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