Liquorice

Liquorice is a system for constructing maps for the computer game Doom by writing algorithms to describe the geometry. A Liquorice program is a series of instructions to move a virtual "turtle" or "pen", which define the walls, floors, rooms and monsters as it goes.

More specifically, Liquorice is an embedded domain-specific language (eDSL) within the pure functional programming language Haskell.

Quick example

-- simple example, triangle (for orientation); unique texture per line
import Liquorice.Monad
import Liquorice.Render

main = buildWad "example1.wad" $ runWadL $ do
    mid "ZZWOLF1"
    draw 128 0
    mid "ZZWOLF2"
    draw 0 128
    turnaround
    mid "ZZWOLF3"
    draw 128 128
    rightsector 0 128 160
    turnaround
    step 64 32
    thing

More detailed example

See examples/birds.hs, for an example program that generates a complete playable map. The map targets Doom 1 / The Ultimate Doom (map slot E2M8), and requires a doom engine with raised engine limits, such as Crispy Doom.

The generated PWAD, with nodes, ready to play: birds.zip

birds.hs is a re-implementation/transformation of "Bird Cage" for WadC.

Getting Started

For Doom novices

Doom's engine source code was open-sourced in 1996, but you need a copy of the game's data files to use them. You can buy The Ultimate Doom and Doom II at Gog, amongst other places. Failing that, you could try FreeDoom, a free-content game for Doom engines.

Examples of powerful open source engines include Crispy Doom, Eternity Engine and GZDoom.

For Haskell novices

…

Evaluation

The commands that a typical Liquorice program will use are predominantly monadic, and so make use of Haskell's "do-notation" for ordering. Internally, the majority of these monadic functions are wrappers around pure equivalents which transform an input Context type into an output, e.g.:

xoff :: Int -> Context -> Context
xoff x c = c { paletteXoff = x }

place :: Int -> Int -> (Context -> Context) -> Context -> Context
place x y fn c = c & step x y
                   & fn
                   & step (-1 * x) (-1 * y)

The pure functions can be combined using the infix operator & (from Data.Function), or the more usual composition operator (which reads back-to-front). However, the monadic versions are probably more user- friendly, and so the separate pure implementations might go away or stop being explicitly exported at some point. We also probably need to use the monadic versions if we want to introduce randomness or debug IO in the middle of a program.

A series of monadic Liquorice statements are converted into a final Context via runWadL. A Context is written to an output PWAD file via

buildWad :: FilePath -> Context -> IO ()

Internally, buildWad first converts a Context into an intermediate data structure WadMap, which closely resembles the binary structure of a PWAD. (WadMap itself is a specialisation of Wad, imposing the presence of map specific lumps)

Pros/Cons

Pros:

• Unlike WadC, you have full access to the Context at any point in your program, which you can inspect or transform as you wish.
• You also have the full power of the Haskell programming language.
• In particular simply infix numeric operators are hugely useful.

Cons:

• There's no GUI.
• The Pure-versus-Monadic functions are a bit awkward and I might get rid of the pure versions altogether in the future
• There are some slow parts (line splitting in particular)
• If you don't know Haskell, this will be very hard to use and most error messages will make little sense.
• You need a Haskell development environment installed to use Liquorice.

Code overview

The following three source files are the ones that you will want to import as modules to your Liquorice program:

• Liquorice.hs — The main Context definition and most basic pure operations
• Liquorice/Monad.hs — monadic wrappers around the above, + some more
• Liquorice/Render.hs — exports buildWad for exporting a PWAD

These are back-end implementation details:

• Liquorice/Pure.hs — most of the "standard library", in pure functions
• Liquorice/Line.hs — Line type, line splitting and tests
• Liquorice/Wad.hs — Wad and WadMap definitions and binary serialisation
• TestMain.hs — HTF test harness

Author

Liquorice was created by Jonathan Dowland and is distributed under the terms of the GNU Public License, version 3 (see COPYING).

The design of Liquorice is heavily influenced by Wad Language (WadC), which in turn owes a debt to LOGO.

See also

• Jonathan Dowland's homepage
• WadC
• The Doom Wiki