Serpent

Lua serializer and pretty printer.

Features

• Human readable:
  • Provides single-line and multi-line output.
  • Nested tables are properly indented in the multi-line output.
  • Numerical keys are listed first.
  • Keys are (optionally) sorted alphanumerically.
  • Array part skips keys ({'a', 'b'} instead of {[1] = 'a', [2] = 'b'}).
  • nil values are included when expected ({1, nil, 3} instead of {1, [3]=3}).
  • Keys use short notation ({foo = 'foo'} instead of {['foo'] = 'foo'}).
  • Shared references and self-references are marked in the output.
• Machine readable: provides reliable deserialization using loadstring().
• Supports deeply nested tables.
• Supports tables with self-references.
• Keeps shared tables and functions shared after de/serialization.
• Supports function serialization using string.dump().
• Supports serialization of global functions.
• Supports __tostring and __serialize metamethods.
• Escapes new-line \010 and end-of-file control \026 characters in strings.
• Configurable with options and custom formatters.

Usage

local serpent = require("serpent")
local a = {1, nil, 3, x=1, ['true'] = 2, [not true]=3}
a[a] = a -- self-reference with a table as key and value

print(serpent.dump(a)) -- full serialization
print(serpent.line(a)) -- single line, no self-ref section
print(serpent.block(a)) -- multi-line indented, no self-ref section

local fun, err = loadstring(serpent.dump(a))
if err then error(err) end
local copy = fun()

Functions

Serpent provides three functions that are shortcuts to the same internal function, but set different options by default:

• dump(a[, {...}]) -- full serialization; sets name, compact and sparse options
• line(a[, {...}]) -- single line, no self-ref section; sets sortkeys and comment options
• block(a[, {...}]) -- multi-line indented, no self-ref section; sets indent, sortkeys, and comment options

Options

• name (string) -- name; triggers full serialization with self-ref section
• indent (string) -- indentation; triggers long multi-line output
• comment (true/false/maxlevel) -- provide stringified value in a comment (up to maxlevel of depth)
• sortkeys (true/false/function) -- sort keys
• sparse (true/false) -- force sparse encoding (no nil filling based on #t)
• compact (true/false) -- remove spaces
• fatal (true/false) -- raise fatal error on non-serilizable values
• nocode (true/false) -- disable bytecode serialization for easy comparison
• nohuge (true/false) -- disable checking numbers against undefined and huge values
• maxlevel (number) -- specify max level up to which to expand nested tables
• maxnum (number) -- specify max number of elements in a table
• valignore (table) -- allows to specify a list of values to ignore (as keys)
• keyallow (table) -- allows to specify the list of keys to be serialized. Any keys not in this list are not included in final output (as keys)
• valtypeignore (table) -- allows to specify a list of value types to ignore (as keys)
• custom (function) -- provide custom output for tables

These options can be provided as a second parameter to Serpent functions.

block(a, {fatal = true})
line(a, {nocode = true, valignore = {[arrayToIgnore] = true}})
function todiff(a) return dump(a, {nocode = true, indent = ' '}) end

Metatables with __tostring and __serialize methods

If a table or a userdata value has __tostring or __serialize method, the method will be used to serialize the value. If __serialize method is present, it will be called with the value as a parameter. if __serialize method is not present, but __tostring is, then tostring will be called with the value as a parameter. In both cases, the result will be serialized, so __serialize method can return a table, that will be serialize and replace the original value.

Sorting

A custom sort function can be provided to sort the contents of tables. The function takes 2 parameters, the first being the table (a list) with the keys, the second the original table. It should modify the first table in-place, and return nothing. For example, the following call will apply a sort function identical to the standard sort, except that it will not distinguish between lower- and uppercase.

local mysort  = function(k, o) -- k=keys, o=original table
  local maxn, to = 12, {number = 'a', string = 'b'}
  local function padnum(d) return ("%0"..maxn.."d"):format(d) end
  local sort = function(a,b)
    -- this -vvvvvvvvvv- is needed to sort array keys first
    return ((k[a] and 0 or to[type(a)] or 'z')..(tostring(a):gsub("%d+",padnum))):upper()
         < ((k[b] and 0 or to[type(b)] or 'z')..(tostring(b):gsub("%d+",padnum))):upper()
  end
  table.sort(k, sort)
end

local content = { some = 1, input = 2, To = 3, serialize = 4 }
local result = require('serpent').block(content, {sortkeys = mysort})

Formatters

Serpent supports a way to provide a custom formatter that allows to fully customize the output. For example, the following call will apply `Foo{bar} notation to its output (used by Metalua to display ASTs):

print((require "serpent").block(ast, {comment = false, custom =
  function(tag,head,body,tail)
    local out = head..body..tail
    if tag:find('^lineinfo') then
      out = out:gsub("\n%s+", "") -- collapse lineinfo to one line
    elseif tag == '' then
      body = body:gsub('%s*lineinfo = [^\n]+', '')
      local _,_,atag = body:find('tag = "(%w+)"%s*$')
      if atag then
        out = "`"..atag..head.. body:gsub('%s*tag = "%w+"%s*$', '')..tail
        out = out:gsub("\n%s+", ""):gsub(",}","}")
      else out = head..body..tail end
    end
    return tag..out
  end}))

Limitations

• Doesn't handle userdata (except filehandles in io.* table).
• Threads, function upvalues/environments, and metatables are not serialized.

Performance

A simple performance test against serialize.lua from metalua, pretty.write from Penlight, and tserialize.lua from lua-nucleo is included in t/bench.lua.

These are the results from one of the runs:

• nucleo (1000): 0.256s
• metalua (1000): 0.177s
• serpent (1000): 0.22s
• serpent (1000): 0.161s -- no comments, no string escapes, no math.huge check
• penlight (1000): 0.132s

Serpent does additional processing to escape \010 and \026 characters in strings (to address http://lua-users.org/lists/lua-l/2007-07/msg00362.html, which is already fixed in Lua 5.2) and to check all numbers for math.huge. The seconds number excludes this processing to put it on an equal footing with other modules that skip these checks (nucleo still checks for math.huge).

Author

Paul Kulchenko (paul@kulchenko.com)

License

See LICENSE file.

History

v0.25 (Sep 29 2013)

• Added maxnum option to limit the number of elements in tables.
• Optimized processing of tables with numeric indexes.

v0.24 (Jun 12 2013)

• Fixed an issue with missing numerical keys (fixes #8).
• Fixed an issue with luaffi that returns getmetatable(ffi.C) as true.

v0.23 (Mar 24 2013)

• Added support for cdata type in LuaJIT (thanks to Evan).
• Added comment to indicate incomplete output.
• Added support for metatables with __serialize method.
• Added handling of metatables with __tostring method.
• Fixed an issue with having too many locals in self-reference section.
• Fixed emitting premature circular reference in self-reference section, which caused invalid serialization.
• Modified the sort function signature to also pass the original table, so not only keys are available when sorting, but also the values in the original table.

v0.22 (Jan 15 2013)

• Added ability to process __tostring results that may be non-string values.

v0.21 (Jan 08 2013)

• Added keyallow and valtypeignore options (thanks to Jess Telford).
• Renamed ignore to valignore.

v0.19 (Nov 16 2012)

• Fixed an issue with serializing shared functions as keys.
• Added serialization of metatables using __tostring (when present).

v0.18 (Sep 13 2012)

• Fixed an issue with serializing data structures with circular references that require emitting temporary variables.
• Fixed an issue with serializing keys pointing to shared references.
• Improved overall serialization logic to inline values when possible.

v0.17 (Sep 12 2012)

• Fixed an issue with serializing userdata that doesn't provide tostring().

v0.16 (Aug 28 2012)

• Removed confusing --err comment from serialized results.
• Added a short comment to serialized functions when the body is skipped.

v0.15 (Jun 17 2012)

• Added ignore option to allow ignoring table values.
• Added comment=num option to set the max level up to which add comments.
• Changed all comments (except math.huge) to be controlled by comment option.

v0.14 (Jun 13 2012)

• Fixed an issue with string keys with numeric values ['3'] getting mixed with real numeric keys (only with sortkeys option set to true).
• Fixed an issue with negative and real value numeric keys being misplaced.

v0.13 (Jun 13 2012)

• Added maxlevel option.
• Fixed key sorting such that true and 'true' are always sorted in the same order (for a more stable output).
• Removed addresses from names of temporary variables (for stable output).

v0.12 (Jun 12 2012)

• Added options to configure serialization process.
• Added goto to the list of keywords for Lua 5.2.
• Changed interface to dump/line/block methods.
• Changed math.huge to 1/0 for better portability.
• Replaced \010 with \n for better readability.

v0.10 (Jun 03 2012)

• First public release.