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2011-11-11T02:18:04.083
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2011-11-11T02:18:04.083
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<p>This question has been closed for a while, but I recently solved this problem and thought people might wish to see how I ended up solving it.</p> <p>Doing it this way, the print statements are actually printed to the screen as soon as the list of expressions descended from it are finished being evaluated:</p> <pre><code>module IOInterpreter where import Data.Char ------------------------------------------------------------------------------- type Id = [Char] type Output = [Char] type Value = Int type Table = [(Id, Value)] data Stm = CompoundStm Stm Stm | AssignStm Id Exp | PrintStm ExpList deriving Show data Exp = IdExp Id | NumExp Value | OpExp Exp Op Exp | EseqExp Stm Exp deriving Show type ExpList = [Exp] data Op = Plus | Minus | Times | Div deriving Show ------------------------------------------------------------------------------- example :: Stm example = CompoundStm (AssignStm "a" (OpExp (NumExp 5) Plus (NumExp 3))) (CompoundStm (AssignStm "b" (EseqExp (PrintStm [IdExp "a", OpExp (IdExp "a") Minus (NumExp 1)]) (OpExp (NumExp 10) Times (IdExp "a")))) (PrintStm [IdExp "b"])) ------------------------------------------------------------------------------- tableUpdate :: Table -> Id -> Value -> Table tableUpdate t i v = (i,v):t tableLookup :: Table -> Id -> Value tableLookup ((x,v):t) i | x == i = v tableLookup ((x,v):t) i | x /= i = tableLookup t i ------------------------------------------------------------------------------- execute :: Stm -> IO() execute s = do interpStm s [] return () interpStm :: Stm -> Table -> IO Table interpStm (CompoundStm s1 s2) t = do t' <- interpStm s1 t interpStm s2 t' interpStm (AssignStm i e) t = do (v, t') <- interpExp e t return $ tableUpdate t' i v interpStm (PrintStm es) t = do (s, t') <- interpExpList es t putStrLn s return t' interpExp :: Exp -> Table -> IO (Value, Table) interpExp (NumExp v) t = return (v, t) interpExp (IdExp i) t = return (tableLookup t i, t) interpExp (EseqExp s e) t = do t' <- interpStm s t interpExp e t' interpExp (OpExp e1 o e2) t = do (v1, t') <- interpExp e1 t (v2, t'') <- interpExp e2 t' return (f o v1 v2, t'') where f Plus v1 v2 = v1 + v2 f Minus v1 v2 = v1 - v2 f Times v1 v2 = v1 * v2 f Div v1 v2 = v1 `div` v2 interpExpList :: ExpList -> Table -> IO (String, Table) interpExpList [] t = return ("", t) interpExpList (e:es) t = do (v, t') <- interpExp e t (s, t'') <- interpExpList es t' return $ (show v ++ " " ++ s, t'') </code></pre>
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1. POSimple interpreter written in Haskell, saves up print output until the end, instead of when it comes across a print statement
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1. USTom Busby
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