A new implementation of the LambdaMOO server

Latest on Hackage:

This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow to host generated Haddocks.

BSD3 licensed and maintained by Rob Leslie


This is experimental software. While it is now mostly functional, it is not yet fully complete.

Until such time as the EtaMOO database format is well tested and considered stable, please make and keep LambdaMOO-format backup copies of your EtaMOO databases.


EtaMOO is a new implementation of the LambdaMOO server written in Haskell.

LambdaMOO is a network-accessible, multi-user, programmable, interactive system well-suited to the construction of text-based adventure games, conferencing systems, and other collaborative software.

EtaMOO differs from LambdaMOO in a few significant ways:

  • EtaMOO is multi-threaded. MOO tasks run concurrently, producing network output and changes to the database in isolated transactions that are committed only when not in conflict with any other transaction. (In cases of conflict, transactions are automatically retried.) Separate threads are also used for network connection management, so for example name lookups do not block the entire server.

  • EtaMOO uses LMDB as a persistent backing store for the MOO database. Changes are committed on an ongoing basis for instantaneous crash recovery; checkpoints merely perform a quick synchronization, and are otherwise unnecessary. EtaMOO provides mechanisms for importing and exporting LambdaMOO-format databases to and from the EtaMOO-native format.

  • EtaMOO is Unicode-aware, and will eventually include support for Unicode MOO strings via compile-time build option.

  • EtaMOO supports 64-bit MOO integers via compile-time build option.

  • EtaMOO natively supports string-key association lists with efficient lookup and update operations; the list index syntax has been extended to allow alist[key] and alist[key] = value for string keys whenever alist is a well-formed association list.

  • EtaMOO supports several additional hashing algorithms besides MD5, including SHA-1, SHA-2, SHA-3, and RIPEMD-160, via optional argument to string_hash(), binary_hash(), and value_hash(). Hash digests may also optionally be returned as binary strings.

  • EtaMOO internally handles binary strings in an efficient manner, and only translates to and from the special MOO binary string syntax upon demand. For example, passing a binary string read from the network directly to decode_binary() does not suffer a round trip through the binary string representation.

  • EtaMOO supports fractional second delays in suspend() and fork.

  • EtaMOO supports IPv6.

The implementation of EtaMOO otherwise closely follows the specifications of the LambdaMOO Programmer's Manual, and should be compatible with most LambdaMOO databases as of about version 1.8.3 of the LambdaMOO server code.


EtaMOO is built with Cabal, the Haskell package manager. In the simplest case, running:

cabal install EtaMOO

should automatically download, build, and install the etamoo executable after doing the same for all of its Haskell dependencies.

Cabal itself is part of the Haskell Platform which is available for many distributions and platforms.

There are a few options you can give to cabal install to customize your build:

| Option | Feature | | --------------------- | --------------------------------------------- | | -j | Build in parallel using multiple processors | | -f llvm | Use GHC's LLVM backend to compile the code | | -f 64bit | Enable 64-bit MOO integers |

EtaMOO has non-Haskell dependencies on three external libraries: liblmdb for database persistence, libpcre (with UTF-8 support enabled) for regular expression matching, and, possibly, libcrypt (often part of the standard libraries) for the MOO crypt() built-in function. You should ensure you have these available before installing EtaMOO (e.g. on Debian-derived systems, sudo apt-get install liblmdb-dev libpcre3-dev).


etamoo is nearly a drop-in replacement for the LambdaMOO moo executable; the main difference is that etamoo takes a single database path, rather than both input and output paths. You can run etamoo --help for a command-line synopsis.

EtaMOO uses a native binary database format that allows quick loading and checkpointing, and instantaneous crash recovery. You can create a native database from a LambdaMOO-format database by using etamoo --import. You can also go the other way and convert an EtaMOO database back to a LambdaMOO-format database with etamoo --export.

If you don't already have a database, you can find LambdaMOO-format cores for various MOOs online -- for example there is the venerable LambdaCore, or you can request a character on Waterpoint and then perform a live JHCore extraction. (Note that Waterpoint's core extraction process requires running an actual LambdaMOO server executable on the precore database to obtain the final core database; EtaMOO cannot yet do this itself.)

By default, EtaMOO will make use of all available CPUs for maximum parallelism. If you'd rather limit the number of processors EtaMOO uses, you can use the command-line option +RTS -Nn -RTS where n is the number of processors to use.

If you want to enable statistics from the memory_usage() built-in function, you will need to add +RTS -T -RTS to the command line options.


The following LambdaMOO features are currently unsupported:

  • The .program intrinsic command
  • The verb_cache_stats() and log_cache_stats() built-in functions
  • Importing, exporting, or checkpointing of queued tasks in the database file
  • Task time limits (ticks are counted, but seconds are not)
  • The NP_SINGLE and NP_LOCAL network protocols (i.e. stdin/stdout, UNIX-domain sockets, and/or named pipes; only TCP/IP is supported)
  • Customizing OUT_OF_BAND_PREFIX and OUT_OF_BAND_QUOTE_PREFIX (these are currently fixed as #$# and #$", respectively)
  • The IGNORE_PROP_PROTECTED compilation option
  • $server_options.name_lookup_timeout

See also the file for other differences between EtaMOO and LambdaMOO.


Documentation is available for the various types, data structures, and functions used internally by EtaMOO.

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