Definition of relation-slot? looks very strange.

[robert-strandh]

The definition of relation-slot? looks very strange.
A minor issue is that the Scheme convention for
predicates is used. But the major strangeness is
that the function is presumably a predicate, but then
it returns the value of a call to slot-type->cardinality,
which presumably returns a cardinality which does
not look like a Boolean value. So this definition is
not understandable by itself and forces the reader to
understand the details of a different function. A
cardinality is usually a number, but this library also
defines a relation-cardinality, which is also not a
Boolean value.

[scymtym]

Would writing the body as

(and (not (null (c2mop:slot-definition-initargs slot)))
     (not (null (slot-type->cardinality (c2mop:slot-definition-type slot)))))

make the function easier to understand? The function is used as a predicate, is named like a predicate and would, when changed like that, also behave like a predicate.

[robert-strandh]

Not much.

It looks to me like there ought to be a name for the case where the
the type of the slot is NIL, like SLOT-HAS-CARDINALITY-P (I am just
guessing, I don't know the right name), so that slot-type->cardinality
would signal an error if this new thing returns NIL.

In other words, I think it is confusing to overload a function with a
name that suggests that it returns a cardinality so that it also
returns NIL when there is no cardinality available. You could of
course rename it to
slot-type->cardinality-or-nil-if-there-is-no-cardinality, but when
names must look like this to explain what they do, it indicates to me
that there is something wrong with the abstraction.

I know that there are plenty of standard functions that do this, and
this practice was no doubt kept for reasons of backwards
compatibility. We all learn how these functions work after many years
of practice, but I don't think it is a good idea to continue this
practice in new code that new people must read and understand.

I also know that a library such a Clostrum contains many cases of NIL
being returned as a default value, like for FDEFINITION. However, the
Clostrum functions mimic standard functions which is part of the
reason. And, I don't use a call to FDEFINITÎON in a context where a
Boolean value is expected.

Then, if the universal builder is not a good idea, it doesn't matter
much.