Untitled

type
    HasFoo = concept v
      v.foo is string

    WithFoo = object
      foo: string

    WithoutFoo = object
      bar: string

proc conceptual[T: HasFoo](val: T) =
  echo $(val.foo)

proc general[T](val: T) =
  echo $(val.foo)

var
  foo = WithFoo(foo:"foo")
  bar = WithoutFoo(bar:"bar")

conceptual(foo)
conceptual(bar)
general(foo)
general(bar)