Gabi wrote:
I've tried to use Data.Tree as a computation tree (each node is numerical function, each leaf is a terminal) It kinda works, but the code seems very verbose. How can it made more concise ? I am sure I missed a lot of shortcuts and idioms.
-- file t.hs import qualified Data.Tree as T
data Term = TInt Int| TDouble Double deriving (Show, Eq)
data Func = Plus | Minus | Mult | Div deriving (Show, Eq)
data ANode = GFunc Func | GTerm Term deriving (Show, Eq)
fNode :: Func -> T.Forest ANode-> T.Tree ANode fNode f = T.Node (GFunc f)
tNode:: Term -> T.Tree ANode tNode t = T.Node (GTerm t) []
calc :: T.Tree ANode -> Double calc (T.Node (GTerm (TInt n))[]) = fromIntegral n :: Double calc (T.Node (GFunc Plus) xs ) = foldl1 (+) (map calc xs) calc (T.Node (GFunc Minus) xs ) = foldl1 (-) (map calc xs) calc (T.Node (GFunc Mult) xs ) = foldl1 (*) (map calc xs) calc (T.Node (GFunc Div) xs ) = foldl1 (/) (map calc xs)
How about calc :: T.Tree ANode -> Double calc (T.Node (GTerm sym) []) = term sym where term (TInt n) = fromIntegral n term (TDouble d) = d calc (T.Node (GFunc sym) xs) = foldl1 (op sym) (map calc xs) where op Plus = (+) op Minus = (-) op Mult = (*) op Div = (/) By the way, Data.Tree is not used very often, people usually roll their own syntax trees because it's so easy. data Expr = V Value | App Fun [Expr] data Value = VInt Int | VDouble Double data Fun = Plus | Minus | Mult | Div eval :: Expr -> Double eval (V (VInt n)) = fromIntegral n eval (V (VDouble d)) = d eval (App sym xs ) = foldl1 (op sym) (map eval xs) where op Plus = ... Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com