Is there any kind of guidline about when to use polymorphic variants?

[silene]

There is one more downside: the runtime cost. A value Pair (x,y) occupies 3 words in memory, while a value `Pair (x,y) occupies 6 words. Indeed, a polymorphic variant uses a full-word tag instead of merging it with the GC metadata, and it carries a single payload, so there is a pointer indirection to the actual block containing the pair.