On Jan 20, 2008 12:56 AM, Thomas Hartman
The code below compiles as given, however if I uncomment the tSM function I get the overlapping instance error mentioned in the subject line.
Can someone give me advice on how to do what I want to do?
basically I want to add, for example, (USD,1) and (USD,2) and (Euro,3) and get result
fromList [(USD,3), (Euro,3)]
the datatypes below are more verbose but the above is the basic idea
thanks!
thomas.
*****
module TransactionRows {- ( mkTransactionRows,TransactionRows ) -} where import Data.Monoid import Data.List import qualified Data.Map as M
data Currency a = Currency a deriving ( Show, Eq, Ord ) data Money c a = Money ( M.Map c a ) deriving Show instance (Num a, Ord c)=> Monoid ( Sum (Money c a) ) where mempty = Sum ( Money M.empty ) Sum ( Money m1 ) `mappend` Sum ( Money m2 ) = Sum (Money m1 `plusmoney` Money m2)
This is the overlap. Data.Monoid already defines an instance for Monoid (Sum
a), so it's going to overlap with your Monoid (Sum (Money c a)).
Unless you're planning to define more than one Monoid for Money, I suggest
skipping Sum entirely.
--
Dave Menendez