{-# 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 -- View this message in context: http://www.nabble.com/Code-snippet%3A-constraint-a-heterogenous-list-by-a-de... Sent from the Haskell - Haskell mailing list archive at Nabble.com.