[mercury-users] "some" and existential types

[Michael Day]

Hi,

when declaring a predicate that outputs an instance of a particular
typeclass as follows:

:- some [T] pred get_object(T) => object(T).

why is the "some [T]" necessary? I would have thought that the typeclass
constraint would be sufficient in this case; what role does the "some"
fill?

Thank you,

Michael

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