Hi Aditya,
Your modifications are very nice. I didn't know about the modify
function. Now the code looks much better.
Thanks.
On Sun, Jul 24, 2011 at 12:55 AM, aditya siram
Hi Rohit, I've refactored your program a little bit. Hope you don't mind. The main change I made was, since there can only be four registers, to create explicit datatypes for each register and turn your RegisterFile datatype into a 4-tuple. The corresponding setter and getter, store_operand and get_operand have also been changed to reflect the new datatypes. This way there can never be errors with list indices etc.
I've also used two new State monad functions of which you might be unaware: 1. modify :: modifies state with the given function. 2. mapM_ :: is like a "map" but takes a monadic action and applied it to the given ist.
The output is stil the same as your code, but I've added a little type safety.
I haven't covered every detail of the changes but if you're reading through it and get stuck, please let me know.
-deech
import Control.Monad.State
type RegisterVal = Int data RegisterID = R1 | R2 | R3 | R4 deriving Show type RegisterFile = (RegisterVal, RegisterVal, RegisterVal, RegisterVal)
get_operand :: RegisterFile -> RegisterID -> RegisterVal get_operand (r1,_,_,_) R1 = r1 get_operand (_,r2,_,_) R2 = r2 get_operand (_,_,r3,_) R3 = r3 get_operand (_,_,_,r4) R4 = r4
store_operand :: RegisterFile -> RegisterID -> RegisterVal -> RegisterFile store_operand (r1,r2,r3,r4) r v = case r of R1 -> (v,r2,r3,r4) R2 -> (r1,v,r3,r4) R3 -> (r1,r2,v,r4) R4 -> (r1,r2,r3,v)
eval_inst :: Instruction -> RegisterFile -> RegisterFile eval_inst inst rf = let store_op' = store_operand rf get_op' = get_operand rf in case inst of Add dest rid1 rid2 -> store_op' dest (get_op' rid1 + get_op' rid2) Sub dest rid1 rid2 -> store_op' dest (get_op' rid1 - get_op' rid2) Mov dest rid -> store_op' dest (get_op' rid) Movc dest v -> store_op' dest v
--destination comes first data Instruction = Add RegisterID RegisterID RegisterID | Sub RegisterID RegisterID RegisterID | Mov RegisterID RegisterID | Movc RegisterID RegisterVal deriving (Show)
initial_rf :: RegisterFile initial_rf = (0,0,0,0)
insts = [Movc R1 231, Movc R2 (-42), Add R3 R2 R1, Sub R4 R2 R1]
execute_program2 :: [Instruction] -> State RegisterFile () execute_program2 = mapM_ (modify . eval_inst)
main :: IO () main = putStrLn $ show $ execState (execute_program2 insts) initial_rf
-- Rohit Garg http://rpg-314.blogspot.com/