A Taste of Haskell(hookrace.net)

    data AST = Prim Op |
               Immediate Int 

               Variable String

    class Semantics where
        eval (Prim op) :: Prim -> a
        eval (Immediate i) :: Immediate -> a
        eval (Var name) :: Variable -> a
        

    instance Semantics Interpreter where
        eval (Prim op) = case op of
            Plus l r = eval l + eval r
            Minus l r = eval l - eval r
            ...
        eval (Immediate n) = n
            ...
        -- variables need an environment, but i think you get the idea, just look up the string in the env.

    instance Semantics Compiler where
        eval (Prim op) = case op of
            (Plus l r) = eval l ++ "MOV r1 r2 \n" 
                      ++ eval r "ADD r1 r2 -> r1\n"
            ...
        eval (Immediate a) = "STORE " ++ a ++ " r1\n"
            ...
        -- again, variables require an env to be passed around, it's just another arg to eval
        -- this time around though, instead of a simple lookup
        -- you need the memory address to load into r1
        -- you have to encode what lookup would do. 
        -- [2] talks about this a bit.