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Predicate instantiation_error/1
Availability::- use_module(library(error)).(can be autoloaded)
Sourceinstantiation_error(+FormalSubTerm)
An argument is under-instantiated. I.e. it is not acceptable as it is, but if some variables are bound to appropriate values it would be acceptable.
FormalSubTerm is the term that needs (further) instantiation. Unfortunately, the ISO error does not allow for passing this term along with the error, but we pass it to this predicate for documentation purposes and to allow for future enhancement.
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LogicalCaptain said (2020-06-04T08:15:42):0 upvotes 0 0 downvotes
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The ISO exception term carries no information (it is just an atom) and thus is relatively useless (why would one throw an exception carrying no information?)

As such, the toplevel prints nothing revealing:

?- instantiation_error("Argument 2 and 3 are uninstantiated").
ERROR: Arguments are not sufficiently instantiated
ERROR: In:
ERROR:   [11] throw(error(instantiation_error,_8540))
ERROR:    [9] toplevel_call(user:user: ...) at /usr/local/logic/swipl/lib/swipl/boot/toplevel.pl:1115

One thus cannot make the difference between "error due to term being fully instantiated" and (for example) "error due to term being partially instantiated but nonground", although that information might be of interest to someone. SAD!