{-# OPTIONS -fglasgow-exts #-}
data HNil = HNil
data HCons a s b = Delta s b => HCons a s b
data One = One
data Two = Two
data Three = Three
data Four = Four
data A = A
data B = B
data C = C
class Delta st sy
-- you can use HaLeX to generate minimal DFA's from
-- regular expressions.
-- http://www.di.uminho.pt/~jas/Research/HaLeX/HaLeX.html
-- the minimal deterministic finite automaton for "AB?C"
instance Delta HNil One -- One is the start state
instance Delta One (HCons A Two n)
instance Delta Two (HCons B Three n)
instance Delta Two (HCons C Four n)
instance Delta Three (HCons C Four n)
instance Delta Four HNil -- Four is an end state
-- there are two valid full sequences
x = HCons HNil One $ HCons A Two $ HCons B Three $ HCons C Four HNil
y = HCons HNil One $ HCons A Two $ HCons C Four HNil
-- if you do not terminate and/or start with HNil, you can have
-- partitial matches
z = HCons B Three $ HCons C Four HNil
-- invalid sequences do not type-check
-- b = HCons HNil One $ HCons B Three $ HCons C Four HNil
-- b' = HCons A Two $ HCons B Three HNil
--
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