Hello all, Quick one for you. I have a list of (<timestamp>,<value>) pairs, and I have defined lots of nice functions for doing various tricks with them. I'd like to define a custom data type which is an instance of 'Num' so I can conveniently do arithmetic on them. (I have written some tools for merging two time series etc...) e.g. t (x,y) = x v (x,y) = y mergeSkip _ [] = [] mergeSkip [] _ = [] mergeSkip (x:xs) (y:ys) | t x == t y = ( t x, (v x, v y) ) : (mergeSkip xs ys) | t x > t y = mergeSkip xs (y:ys) | t x < t y = mergeSkip (x:xs) ys binaryValueFunc f [] = [] binaryValueFunc f ((t,(a,b)):xs) = (t, f a b):binaryValueFunc f xs add xs = binaryValueFunc (+) xs addSkip xs ys = add $ mergeSkip xs ys So addSkip will nicely take two of my time series, merge them by throwing out any pairs which don't have a time stamp that exists in both and then add the values to return a new series. All is well. But how do I make it so that I can use the + operator to add two of them? Well of course, I tried making a custom data type: data Ts a b = Ts [(a,b)] intance Ts Num where <blaa blaa blaa> But then I have to rewrite all of my functions to pattern match on Ts instead of just the list. Worse, when I am writing recursive functions I have to construct a Ts again for the recursive call. I guess I could make my own 'list-like' datastructure data Ts a b = Empty | Cons ((a,b), Ts) But that seems rather clumsy. So I tried to make 'Ts' a type synonym for [(a,b)] which worked, but then I couldn't declare it an instance of 'Num'. So I guess the question is, is there any way of making a type synonym for a type which I can define as an instance of another class? So I can use my values of the type synonym for functions which expect the 'vanilla' type, but define 'extra' stuff my type synonym can do? Any help or advice greatly appreciated. - Philip