Hi The one way to test this is to benchmark, everything else will just be peoples random guesses. As for my random guess, eval should be significantly faster than peval in Hugs, and probably slightly faster than peval in GHC. I don't see why you think peval is efficient - monads /= efficiency, they merely are an extra layer over standard evaluation. If you use the IO monad and some careful annotations you could hope to draw with eval, but just accept the beauty that is pure functional programming and smile :-) Thanks Neil
-----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Peter Padawitz Sent: 11 November 2008 5:49 pm To: haskell-cafe@haskell.org Cc: Hubert Wagner Subject: [Haskell-cafe] compilation question
At first a type of arithmetic expressions and its generic evaluator:
data Expr = Con Int | Var String | Sum [Expr] | Prod [Expr] | Expr :- Expr | Int :* Expr | Expr :^ Int
data ExprAlg a = ExprAlg {con :: Int -> a, var :: String -> a, sum_ :: [a] -> a, prod :: [a] -> a, sub :: a -> a -> a, scal :: Int -> a -> a, expo :: a -> Int -> a}
eval :: ExprAlg a -> Expr -> a eval alg (Con i) = con alg i eval alg (Var x) = var alg x eval alg (Sum es) = sum_ alg (map (eval alg) es) eval alg (Prod es) = prod alg (map (eval alg) es) eval alg (e :- e') = sub alg (eval alg e) (eval alg e') eval alg (n :* e) = scal alg n (eval alg e) eval alg (e :^ n) = expo alg (eval alg e) n
Secondly, a procedural version of eval (in fact based on continuations):
data Id a = Id {out :: a} deriving Show
instance Monad Id where (>>=) m = ($ out m); return = Id
peval :: ExprAlg a -> Expr -> Id a peval alg (Con i) = return (con alg i) peval alg (Var x) = return (var alg x) peval alg (Sum es) = do as <- mapM (peval alg) es; return (sum_ alg as) peval alg (Prod es) = do as <- mapM (peval alg) es; return (prod alg as) peval alg (e :- e') = do a <- peval alg e; b <- peval alg e'; return (sub alg a b) peval alg (n :* e) = do a <- peval alg e; return (scal alg n a) peval alg (e :^ n) = do a <- peval alg e; return (expo alg a n)
My question: Is peval less time- or space-consuming than eval? Or would ghc, hugs et al. optimize eval towards peval by themselves?
Peter _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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