On 2016-05-23 at 12:06, Silent Leaf
Say there's a class, many types that can instantiate it. Then, i in fact need to be able to concatenate (mappend would be ideal!), make lists of values of different types, all instantiating the class.
In most cases, when Haskell beginners want to make a list that contains several types from a single type class, there's a better way to organize the code. If you post your code, I'll try to suggest a specific solution. In general, try to find a simple data type that captures the same fields & functions as an unknown type that is part of the type class. Here's an example. We have a type class for vectors in a metric space, and instances for 2D, 3D, etc.
class Metric v where length :: v -> Double (*^) :: Double -> v -> v
This class has the law: s * length v == length (s *^ v) Instead of a heterogeneous list [ V2 1 2, V3 3 4 5, V4 6 7 8 9], we make a list that just has the length of each vector, and lets us multiply those lengths by a scalar. In this case, we don't even need to write a new data type, the type is simply Double. We can write:
[ length (V2 1 2), length (V3 3 4 5), length (V4 6 7 8 9) ]
And (*) gives us what Metric does with (*^). Of course, with your class, it's probably not so obvious how to transform the class this way. It's certainly possible to make a record with functions as members, moving in the object-oriented direction. Existential types (with a class constraint inside the data constructor) are even more rarely used. cheers, bergey