TSa wrote:
Note that A = (A|B) & (A|C) is the intersection type of A|B and A|C.
Note further that A|B is a subtype of A and B written A|B <: A and
A|B <: B and so on. Usually the A|B|C is called Bottom or some such.
I think it is the Whatever type of Perl6. It is the glb (greatest lower
bound) of A, B and C. In a larger lattice this type gets larger as well.
Any is the trivial supertype of all types.

Damn it! I always puzzle glb and lub (least upper bound). So it should
read lub there. This is because the intension set gets larger even
though a subtype is formed. Note that the extension set "the instances"
becomes smaller! In a limiting case the Whatever type has got no
instances at all :)


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