> Interfaces act as constraints on what a module
> exports. These constraints permit eliding some symbols.
> 
> They also permit abstracting types, or replacing types
> with sub/supertypes (depending on variance) or specialisations.
> <SNIP> relevant examples </SNIP>
> ... for no apparent reason (there is,
> of course, a reason).

Thank you Skaller for this neat explanation and clear conclusion.
My answer was much less informative, I admit.


--Luc