[mercury-users] existential types

[Michael Day]

Can anyone explain what is wrong with this code:

:- type builder
    --->    some [S, T] builder(S, node(T), doc(T)) => builder(S).

:- func new_builder(S, node(T), doc(T)) = builder <= builder(S).

new_builder(S, Node, Doc) = 'new builder'(S, Node, Doc).

: In clause for `new_builder(in, in, in) = out':
:   in function result term of clause head:
:   mode error: variable `TypeInfo_9' has instantiatedness `free',
:   expected instantiatedness was `ground'.

??

Thanks,

Michael

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