Design patterns for composability with traits (i.e. typeclasses)?

[shelby3]

@keean

but it can support mutability, a value in a Cons can be mutable

I already implied that mutability of the values is allowed in both your HList-like design pattern and in my proposal:

@shelby3

The first point that jumps out at me is that a Cons (for constructing a List) is inherently an immutable data structure w.r.t. to extension of the element data type.

@shelby3

Afaics, the immutability only applies when adding a data type to type parameter of a preexisting instance of a data type. All other operations can be mutable.