On Sun, Mar 25, 2012 at 2:01 PM, TP
Hello,
My primary problem may be reduced to adding elements of two lists: [1,2,3] + [4,5,6] = [5,7,9]
My first idea was to declare a list of Int as an instance of Num, and define (+) in the correct way. However, it seems it is not possible to do that:
------------------- instance Num [Int] where l1 + l2 = .... -------------------
Why? It seems it is necessary to do:
------------------ newtype ListOfInt = ListOfInt { getList :: [Int] } deriving (Show, Eq)
instance Num ListOfInt where l1 + l2 = ... -------------------
Am I correct? Is it the best way to do that?
Now, what is the most usual way to implement l1+l2? I have just read about applicative functors, with which I can do:
------------------- import Control.Applicative let l1 = [1,2,3] let l2 = [4,5,6] print $ getZipList $ (+) <$> ZipList l1 <*> ZipList l2 [5,7,9] -------------------
Is it the correct way to do that? I have tried:
------------------- instance Num ListOfInt where l1 + l2 = ListOfInt $ getZipList $ (+) <$> ZipList (getList l1) <*> ZipList (getList l2) -------------------
Isn't it too much complicated?
Thanks
TP
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A simple solution is to use the zipWith[1] function: zipWith (+) [1,2,3] [4,5,6] == [5,7,9] It takes a bit of time to get acquainted with all of the incredibly convenient functions in base, but once you know them, it can greatly simplify your code. Michael [1] http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Prelude.ht...