etf

A Lua library for encoding and decoding the Erlang External Term Format.

Tested on Lua 5.1 - 5.4 and LuaJIT.

By default, this decodes and encodes with logic similar to erlpack, but decoders and encoders have options to customize how values are processed.

LICENSE

MIT (see file LICENSE).

Some files are third-party and contain their own licensing:

• csrc/thirdparty/bigint/bigint.h: Zero-clause BSD, details in source file.
• csrc/thirdparty/miniz/miniz.h: MIT license, details in csrc/thirdparty/miniz/LICENSE.

Decoding

local decoder = etf.decoder(options) -- create a decoder
local decoded = decoder:decode('\131\116\0\0\0\2\100\0\1\97\97\1\100\0\1\98\97\2')

-- decoded will be a table like:
   { a = 1, b = 2 }

options is an optional table with the following keys, all optional:

• use_integer - set to true to decode all integers as etf.integer userdata.
• use_float - set to true to decode all floats as etf.float userdata.
• version - specify the Erlang Term Format version you wish to decode. As far as I can tell, 131 is the only version in existence.
• atom_map - customize how Atom types are decoded. This can be a table, or a function that accepts a string (representing the atom name) and a boolean (true if the atom is a map key, false otherwise).

Here's how various Erlang types are mapped to Lua by default:

Supported	Erlang Type	Lua Type
[ ]	ATOM_CACHE_REF
[x]	ZLIB	(automatically decompressed and decoded)
[x]	SMALL_INTEGER_EXT	number
[x]	INTEGER_EXT	number or etf.integer (based on value)
[x]	FLOAT_EXT	number
[x]	PORT_EXT	table
[x]	NEW_PORT_EXT	table
[x]	V4_PORT_EXT	table
[x]	PID_EXT	table
[x]	NEW_PID_EXT	table
[x]	SMALL_TUPLE_EXT	table
[x]	LARGE_TUPLE_EXT	table
[x]	MAP_EXT	table
[x]	NIL_EXT	table (empty)
[x]	STRING_EXT	string
[x]	LIST_EXT	table
[x]	BINARY_EXT	string
[x]	SMALL_BIG_EXT	number or etf.integer
[x]	LARGE_BIG_EXT	number or etf.integer
[x]	REFERENCE_EXT	table
[x]	NEW_REFERENCE_EXT	table
[x]	NEWER_REFERENCE_EXT	table
[x]	FUN_EXT	table
[x]	NEW_FUN_EXT	table
[x]	EXPORT_EXT	table
[x]	BIT_BINARY_EXT	string
[x]	NEW_FLOAT_EXT	number
[x]	ATOM_UTF8_EXT	string or boolean or etf.null
[x]	SMALL_ATOM_UTF8_EXT	string or boolean or etf.null
[x]	ATOM_EXT	string or boolean or etf.null
[x]	SMALL_ATOM_EXT	string or boolean or etf.null

Decoding Details

Integers

etf will figure out the maximum and minimum integer values that can be safely handled by Lua at run-time. When any integer is decoded, it will use Lua's number type if possible, and a etf.integer userdata if it's outside the safe range.

You can opt to have all integers be returned as etf.integer userdatas. The benefit of this is all values will use the same type. On Lua 5.2 and later, the etf.integer userdatas can be compared to regular Lua numbers, but on Lua 5.1 you can only compare etf.integer values with other etf.integer values.

To enable this, create the decoder with the use_integer option set to true:

local etf = require'etf'
local decoder = etf.decoder({use_integer = true })
local val = decoder:decode('\131\97\1') -- returns a integer
print(debug.getmetatable(val).__name)
-- prints "etf.integer"

Atoms

Erlang supports a concept of "atoms" which doesn't completely translate to Lua.

In Erlang, one can create a map like:

Map = #{ a => 1, b => false, c => hello }

In that example, a, b, false, and hello are all atoms. They're essentially small strings that can be used for map keys, enums, etc.

Note that Erlang doesn't have a boolean type. false is just another atom.

By default, atoms are decoded with the following logic:

• If the atom is a map key (like a and b in the example), it's decoded as a string.
• If the atom is a value (like false and hello in the example, then:
  • Atom true is decoded as Lua's boolean true.
  • Atom false is decoded as Lua's boolean false.
  • Atom nil is decoded as etf.null, which is an atom userdata.
  • Anything else is decoded as a string.

This is meant to be compatible with erlpack, and to make decoded data as easy to handle as possible.

If your application has other atoms that need to be translated into values, you can specify the atom_map parameter. This can be a table with string keys, or a function. The function should accept a string parameter representing the atom name, and a boolean representing if the atom is a map key or not.

The default logic can be represented as:

local function atom_map(str, is_key)
  if is_key then return str end
  if str == 'true' then
    return true
  elseif str == 'false' then
    return false
  elseif str == 'nil' then
    return etf.null
  end
  return str
end

If for example, you wanted to keep string keys but keep the values as atoms:

local function atom_map(str, is_key)
  if is_key then return str end
  return etf.atom(str)
end

Tuples and Lists

SMALL_TUPLE_EXT, LARGE_TUPLE_EXT, LIST_EXT, and NIL_EXT will be decoded into array-like tables (all keys are integers, they're consecutive, and they start at 1).

The table will have a metatable set to indicate the original type - etf.tuple_mt for tuples, and etf.list_mt for lists.

Maps

MAP_EXT will be decoded into a Lua table. By default, the keys are (probably) strings, see above about how atoms are mapped. Values are mapped into the appropriate Lua type according to the above table.

The table will have a metatable set to indicate it was a map - etf.map_mt.

Ports

The various PORT types (PORT_EXT, NEW_PORT_EXT, V4_PORT_EXT) will be decoded into a table with the following fields:

• node - a string.
• id - a number or integer.
• creation - a number or integer.

The table will have a metatable set to etf.port_mt.

PIDs

The PID types (PID_EXT, NEW_PID_EXT) will be decoded into a table with the following fields:

• node - a string.
• id - a number or integer.
• serial - a number or integer.
• creation - a number or integer.

The table will have a metatable set to etf.pid_mt.

FUN_EXT

FUN_EXT will be decoded into a table with the following fields:

• numfree - a number or integer.
• pid - the previously-mentioned PID type.
• module - a string.
• index - a number or integer.
• uniq - a number or integer.
• free_vars - an array like table of terms.

The table will have a metatable set to etf.fun_mt.

NEW_FUN_EXT

NEW_FUN_EXT will be decoded into a table with the following fields:

• size - a number or integer.
• arity - a number.
• uniq - a string.
• index - a number or integer.
• numfree - a number or integer.
• module - a string.
• oldindex - a number or integer.
• olduniq - a number or integer.
• pid - the previously-mentioned PID type.
• free_vars - an array like table of terms.

The table will have a metatable set to etf.new_fun_mt.

EXPORT_EXT

EXPORT_EXT will be decoded into a table with the following fields:

• module - a string.
• function - a string.
• arity - a number or integer.

The table will have a metatable set to etf.export_mt.

References

The REFERENCE types (REFERENCE_EXT, NEW_REFERENCE_EXT, NEWER_REFERENCE_EXT) will be decoded into a table with the following fields:

• node - a string.
• creation - a number or integer.
• id - an array-like table of numbers or integers.

The table will have a metatable set to etf.reference_mt.

Encoding

local encoder = etf.encoder(options) -- create a encoder
local encoded = encoder:encode({ a = 1, b = 2 })
-- encoded will be a MAP_EXT with BINARY_EXT keys and SMALL_INT_EXT values

options is an optional table with the following keys, all optional:

• version - specify the Erlang Term Format version you wish to encode. As far as I can tell, 131 is the only version in existence.
• compress - set to true to enable compression at the default level, or 0 through 9 to specify a compression level.
• value_map - customize how values are encoded, this can be a table or a function that accepts the value to be encoded, and a boolean indicating if the value is a table key.

Lua Types

Here's how various Lua types are mapped to Erlang Term Format by default:

Supported	Lua Type	Erlang Type
[x]	nil	a nil SMALL_ATOM_UTF8_EXT
[x]	number	NEW_FLOAT_EXT, SMALL_INTEGER_EXT, INTEGER_EXT, SMALL_BIG_EXT, LARGE_BIG_EXT (as appropriate)
[x]	boolean	SMALL_ATOM_UTF8_EXT
[x]	string	BINARY_EXT
[x]	table	NIL_EXT, LIST_EXT, or MAP_EXT
[x]	userdata	(see details below)

A note on tables

A table is determined to either be map-like or list-like. If a table has integer keys starting at 1, with no gaps, it's considered to be list-like and will be encoded as a LIST_EXT.

If a table has no keys at all, it will be treated as a list-type with zero items and encoded as a NIL_EXT (Erlang's version of an empty list).

Otherwise, the table is considered map-like, and will be encoded as a MAP_EXT. All table keys will be encoded as strings (specifically BINARY_EXT). This is meant to be compatible with erlpack.

Userdata types

etf allows creating various userdata to force a specific encoding:

Userdata	Erlang Type
etf.integer	SMALL_INTEGER_EXT, INTEGER_EXT, SMALL_BIG_EXT, LARGE_BIG_EXT as appropriate
etf.float	NEW_FLOAT_EXT
etf.string	STRING_EXT
etf.binary	BINARY_EXT
etf.atom	SMALL_ATOM_UTF8_EXT or ATOM_UTF8_EXT
etf.tuple	TUPLE_EXT
etf.list	LIST_EXT
etf.map	MAP_EXT
etf.port	NEW_PORT_EXT or V4_PORT_EXT
etf.pid	NEW_PID_EXT
etf.export	EXPORT_EXT
etf.reference	NEWER_REFERENCE_EXT

The etf.integer type will encoded to the smallest-possible integer. So, a integer in the range of an 8-bit unsigned integer will be encoded as a SMALL_INTEGER_EXT value, a integer in the range of a 32-bit signed integer will be encoded as an INTEGER_EXT value, and so on.

Using these userdata with a custom value_map function allows precise control over mapping. For example, if you want to use Atom types for all table keys, you could do:

local function value_map(val, is_key)
  if is_key then
    return etf.atom(val)
  end
  return val
end

local encoder = etf.encoder({value_map = value_map })
local binary = encoder:encode({ a = 1, b = 2 })
-- will return a MAP_EXT with atom keys and integer values

Full listing of fields in the etf module:

Decoding Functions

• decoder - function that returns a decoder userdata.
• decode - convenience function to decode without creating a decoder.

Encoding Functions

• encoder - function that returns an encoder userdata.
• encode - convenience function to encode without creating an encoder.

Userdata-creating Functions

String-like types

• atom - function that returns an atom userdata (requires a string).
• binary - function that returns a binary userdata (requires a string).
• string - a function that returns a string userdata (requires a string).

Integer types

• integer - function that returns a integer userdata (accepts a number, string, or none).

Float types

• float - function that returns a float userdata (accepts a number, string, or none).

Table-like types

• list - function that returns a list userdata (optionally accepts a table).
• map - function that returns a map userdata (optionally accepts a table).
• tuple - a function that returns a tuple userdata (optionally accepts a table).

Other tpyes

• export - function that returns an export userdata (requires a table matching EXPORT_EXT above).
• pid - a function that returns a pid userdata (requires a table matching PID_EXT above).
• port - a function that returns a port userdata (requires a table matching PORT_EXT above).
• reference - a function that returns a reference userdata (requires a table matching REFERENCE_EXT above).

Pre-created Userdatas

• maxinteger - a integer value representing the maximum integer that can be represented by Lua natively.
• mininteger - a integer value representing the minimum integer that can be represented by Lua natively.
• null - an atom that represents a nil atom.

Metatables

• atom_mt - the atom userdata's metatable.
• integer_mt - the integer userdata's metatable.
• float_mt - the float userdata's metatable.
• binary_mt - the binary userdata's metatable.
• decoder_131_mt - the decoder userdata's metatable.
• encoder_131_mt - the encoder userdata's metatable.
• export_mt - the export userdata's metatable.
• fun_mt - the fun userdata's metatable.
• list_mt - the list userdata's metatable.
• map_mt - the map userdata's metatable.
• new_fun_mt - the new_fun userdata's metatable.
• pid_mt - the pid userdata's metatable.
• port_mt - the port userdata's metatable.
• reference_mt - the reference userdata's metatable.
• string_mt - a string userdata's metatable.
• tuple_mt - the tuple userdata's metatable.

Version Info

• _VERSION - the module version as a string.
• _VERSION_MAJOR - the module's major version as a number.
• _VERSION_MINOR - the module's minor version as a number.
• _VERSION_PATCH - the module's patch version as a number.

Misc

• numsize - the size of a Lua number, in bytes.