regular (WIP)

Type-safe formalisms for representing Regular Languages (Deterministic Finite Automata, Nondeterministic Finite Automata, Regular Expressions, etc.) in Haskell.

Example

Here is a small example of what FizzBuzz looks like with DFA:

-- A DFA which accepts numbers (as a string of digits) only when
-- they are evenly divisible by 5.
by5 ∷ DFA Bool Fin₁₀
by5 = DFA δ q₀ f
  where
    -- Theorem: A number is divisible by 5 iff its last digit is 0 or 5.
    δ ∷ (Bool, Fin₁₀) → Bool
    δ (_, 0) = True
    δ (_, 5) = True
    δ _      = False
    q₀ ∷ Bool
    q₀ = False
    f  ∷ Set Bool
    f = singleton True

-- A DFA which accepts numbers (as a string of digits) only when
-- they are evenly divisible by 3.
-- The transition function effectively keeps a running total modulo three by
-- totaling the numeric value of its current state and the numeric value of the
-- incoming digit, performing the modulo, and then converting that value back to a state.
-- There is a bit of overhead complexity added by the fact that an extra state, `Left ()`,
-- is introduced only to avoid accepting the empty string.
by3 ∷ DFA (Either () Fin₃) Fin₁₀
by3 = DFA δ (Left ()) (singleton (Right 0))
  where
    -- Theorem: A number is divisible by 3 iff the sum of its digits is divisible by 3.
    δ ∷ (Either () Fin₃, Fin₁₀) → Either () Fin₃
    δ = Right . toEnum . (`mod` 3) . \(q, digit) → fromEnum (fromRight 0 q) + fromEnum digit

-- FizzBuzz using DFA
main ∷ IO ()
main = mapM_ (putStrLn . fizzbuzz . toDigits) [1 .. 100]
  where
    fizz ∷ [Fin₁₀] → Bool
    fizz = accepts  by3
    buzz ∷ [Fin₁₀] → Bool
    buzz = accepts                         by5
    fbzz ∷ [Fin₁₀] → Bool
    fbzz = accepts (by3 `DFA.intersection` by5)
    fizzbuzz ∷ [Fin₁₀] → String
    fizzbuzz n | fbzz n    = "FizzBuzz"
               | fizz n    = "Fizz"
               | buzz n    = "Buzz"
               | otherwise = n >>= show

Type Safety

Consider, for example, the GNFA q s type defined in src/GNFA.hs which is the type used to represent Generalized Nondeterministic Finite Automaton in this library.

data GNFA q s where
  -- δ : (Q \ {qᶠ}) × (Q \ {qᵢ}) → Regular Expression
  GNFA ∷ { delta ∷ (Either Init q, Either Final q) → RegExp s } → GNFA q s

Striving for correctness by construction, the type alone enforces many required invariants:

• A GNFA must have only one transition between any two states
• A GNFA must have no arcs coming into its start state
• A GNFA must have no arcs leaving its final state
• A GNFA must have only one start state and one accept state, and these cannot be the same state

That means, any time you have some GNFA (and we assume pure functional programming), those invariants hold, without any extra work or runtime checks.

Try

This project currently uses stack to build. Because the project currently depends on some older packages (algebra and containers-unicode-symbols), you may have to set the allow-newer: true option in ~/.stack/config.yaml before proceeding. Then you can try out some of the code in the REPL:

git clone https://github.com/subttle/regular
cd regular
stack build
stack ghci

λ> Examples.by5'
(0∣1∣2∣3∣4∣5∣6∣7∣8∣9)★·(0∣5)
λ> by5' `matches` [1,2,3,4,5]
True
λ>

WIP status

For now it should be clear that I value correctness and simplicity over speed. Some of the code here is experimental and some is not yet structured properly, so expect major refactoring and restructuring. Once I have everything correct I can start to worry about speed. For now this code is slow.

I'm patiently (and gratefully!) waiting on a few things from some of the best projects out there right now:

• Labelled graphs in alga
• Linear types in Haskell
• Better dependent type support in Haskell

I haven't proven all class instances to be lawful yet.