I meant types with constraints, like this one: sum :: Num a => [a] -> a The type variable a is constrained to have implementation of interface Num (integer cons

I meant types with constraints, like this one: sum :: Num a => [a] -> a The type variable a is constrained to have implementation of interface Num (integer constant (0), addition, subtraction, etc). These constraints can be much deeper. For example, I can define a function that allow me to add two somethings if their "regions" (a property I know how to compute) is equal and the result will be produced in "region" of first argument: add :: (Region a ~ Region b, Region a ~ Region c) => a -> b -> c One use of something similar is addition of integer values represented by bit vectors in Verilog - you should not add bit vectors from different clock domains, otherwise you get unpredictable results.