mata-ll

▸ Getting Started

The compiler installs as the mll command. Install it from crates.io:

cargo install mata-ll

Write some Haskell — fibs.mll:

fibs :: [Int]
fibs = 1 : 1 : zipWith (+) fibs (tail fibs)

main :: IO ()
main = mapM_ print (take 10 fibs)

Run it directly:

$ mll -r fibs.mll
1
1
2
3
5
8
13
21
34
55

Or compile it and require the resulting single .lua file from any Lua 5.4 or LuaJIT host — see the examples below.

▸ Calling from Lua into mata-ll

local fib = require "fib"

local fibs = fib.fibonacci(8)
for i, n in ipairs(fibs) do
    print(i, n)
end

fib :: [Int]
fib = 1:1:zipWith (+) fib (tail fib)

export fibonacci :: Int -> [Int]
fibonacci = flip take fib

▸ Calling Lua library functions from mata-ll

rr :: LuaIO "math.random" Number
rr2 :: Int -> Int -> LuaIO "math.random" Int

main :: IO ()
main = do
    randNum <- rr
    putStrLn $ "A number between 0.0 and 1.0: " <> show randNum
    randNum2 <- rr2 23 42
    putStrLn $ "An integer between 23 and 42: " <> show randNum2

▸ Passing Lua callbacks to mata-ll

local wf = require "writefibs"
local writer = function(fibString)
    print("From mata-ll:", fibString)
end
wf.writeFibs(writer, 12)

export writeFibs :: (String -> LuaIO s ())
                 -> Int -> LuaIO s ()
writeFibs writer = loop 1 1
  where
    loop _ _ 0 = return ()
    loop cur next count = do
      writer (show cur)
      loop next (cur+next) (count-1)

▸ The type system

The type system is the point of the project. It provides:

• Hindley-Milner inference — signatures are checked when given, inferred when not.
• Typeclasses — Functor, Applicative, Monad and friends, with user-defined instances.
• Algebraic data types, including GADTs — model your domain in types rather than in table conventions.
• Exhaustiveness checking — a pattern match that misses a constructor is a compile error, not a runtime nil.

▸ What the compiler catches

In mata-ll, String is opaque — not [Char]. Treat it as a list and the compiler explains the deviation from GHC:

greeting :: String
greeting = "Hello, " ++ "world"

main :: IO ()
main = putStrLn greeting

Type error: Cannot unify '[a]' with 'String'
  at 2:10, in definition of 'greeting'
  note: in mata-ll String is not a list of
  characters — it is an opaque type that does
  not unify with [a]. A String cannot be passed
  where a list is expected, and list functions
  (++, map, length, …) do not accept it.

Forget a constructor and the pattern checker names the one you missed:

data Shape = Circle Number
           | Square Number
           | Rect Number Number

area :: Shape -> Number
area (Circle r) = 3.14159 * r * r
area (Square s) = s * s

Type error: Non-exhaustive patterns in 'area':
  missing patterns for Rect
  at 4:6, in definition of 'area'

▸ What the output looks like

Compiling a two-line hello.mll yields one self-contained hello.lua: a small on-demand runtime followed by the compiled definitions and an entry point. The part after the runtime:

main :: IO ()
main = putStrLn "Hello" >> putStrLn "World"

-- Generated by the mata-ll compiler (https://matall.org/)
local __MLLC_VERSION = "0.1.6"
local __MLLC_COMMIT = "da96b665cbc2a8db773c56f6ef387af7167446b8"
local __mll_fn = {}
__mll_fn[1] = function()
    print("Hello")
    return (print("World"))
end
local __mll_arg1 = ...
if __mll_arg1 == nil or (arg ~= nil and __mll_arg1 == arg[1]) then __mll_run(__mll_fn[1]()) end

▸ Project Goals

Make available a useful subset of modern Haskell to Lua. Not intended as a replacement for Haskell, but as a way to write Haskell code where you would otherwise write Lua code. Primary focus: writing embedded code in a safer way than Lua allows, without breaking boundaries to Lua.

• Add the expressiveness, fun and safety of Haskell to Lua
• Target the Lua 5.4+ VM; compile to Lua source for safe loading via mlua
• Non-strict evaluation with cheapness analysis (thunk only what's worth thunking)
• No separate runtime needed — use zero-cost abstractions
• If zero-cost abstractions don't fully work, use library functions
• Incorporate new type system research where possible and useful
• Once a stable version is reached, stay backwards compatible
• Easy interface to plain Lua via a well-defined FFI
• Portable and small; no 3rd party Rust libraries

▸ Evaluation Strategy

▸ Why Rust?

▸ Language Properties