Hi Cafe, I am trying to embed x86 assembly in Haskell. I'd like the EDSL to not allow invalid movements into registers - for example, should not allow moving into RIP. I was not able to get it to work. I ended up using DataTypeContexts - which is considered misfeature anyway. I was wondering if I could get some suggestions. {-# LANGUAGE DatatypeContexts #-} data SREG = RIP data DREG = RBX data SNDREG = RAX data (Source s, Destination d) => Instruction s d = MOV s d class Source a class Destination a instance Source SREG instance Source SNDREG instance Destination DREG instance Destination SNDREG move :: (Source s, Destination d) => s -> d -> Instruction s d move s d = MOV s d hello = [move RAX RAX, move RAX RAX] hello = [move RAX RAX, move RAX RBX] -- this is still not allowed. Regards, Kashyap