Opinion on how typeclasses would look in Elm

interface.elm

interface Numerical a = {
  (+) : a -> a -> a,
  (-) : a -> a -> a,
  (*) : a -> a -> a,
  negate : a -> a,
  abs : a -> a
}

implementation Numerical Float = {
  (+) = Native.Float.add,
  (-) = Native.Float.sub,
  (*) = Native.Float.mul,
  negate = Native.Float.negate,
  abs = Native.Float.abs
}

square : a -> a where a : Numerical

a : Numerical
square : a -> a 
square numerical = 
  numerical * numerical


-- If a type only requires one interface, it can be inlined in the type annotation
square : Numerical -> Numerical

-- You can have types that implement multiple interfaces 
a : Numerical, Comparable
compareSquares : a -> a -> Bool

compareSquares : a -> a -> Bool where a : Numerical, Comparable

a : Numerical, StringConvertible
b : Sequential, StringConvertible
concatNumberAndListAsString : a -> b -> String

concatNumberAndListAsString : a -> b -> String where a : Numerical, StringConvertible, b : Sequential, StringConvertible