On 18 March 2010 05:48, Brandon S. Allbery KF8NH
They can be mixed only with significant effort. And you can't really prevent it once your users know the magic of existential quantification; *but* those lists won't typecheck when passed to your routines expecting a Polygon, because you will be expecting a (Point a => Polygon [a]) but they will be passing a (Polygon [forall a. Point a => a]) or something similar.
If you can live without constructors, an ADT view is another possibility... Jeremy Gibbons gives a clear explanation of using an ADT to implement complex numbers with polar and cartesian representations. Both representations share the same type, so can be stored together in lists - figuratively speaking the the existential type has been moved "further down" inside the type. See section 2: http://www.comlab.ox.ac.uk/jeremy.gibbons/publications/adt.pdf Best wishes Stephen