Parent comment is referencing the "let inline" construction that in addition to inline code allows to constrain generic types structurally over member definitio

Parent comment is referencing the "let inline" construction that in addition to inline code allows to constrain generic types structurally over member definition[1] instead of by name as usual. This gives us poor's man type classes which is useful nonetheless. I guess the conclusion is that the definition of duck typing is fuzzy and misleading. [1] https://msdn.microsoft.com/en-us/library/dd548046.aspx https://msdn.microsoft.com/en-us/library/dd548046.aspx