The library coding standards[1] say:
* Use '?' suffix for predicates.
- N.B. - predicates return booleans
and the community Clojure style guide[2] says:
* The names of predicate methods (methods that return a boolean value) should
end in a question mark. (i.e.even?).
Both of these imply
My 2p - I interpret the contract as being boolean. Truthy values are
'polymorphically' equivalent*1 so sure. The concern would be people
relying on the implementation and treating the values as none-truthy (i.e.
in your example relying on the fact it is a string being returned, so (=
In the Common Lisp world it's common for predicates to return a useful
value instead of T, when applicable. It seems possible the same principle
could apply to clojure.
From CLtL2 http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node69.html:
Often a predicate will return nil if it ``fails'' and
While *every?* and *not-any?* return true or false, *some* returns the
first truthy value in a sequence, or nil if there are none.
Similarly, *empty?* returns true or false, while *empty* and *not-empty*return
a collection or nil.
However, it seems as if* some*, *empty*, and *not-empty *are
This is a bit tangential, but your function is-nsf-code? brings up
another style question. Is it really necessary to use the is- prefix? I
used to do this too, but I realized recently that it is a Java-ism and
seems out of place in Clojure.
Clojure's boolean functions do fine without it
My personal preference is that fns ending in ? return a strict boolean, to
be consistent with clojure.core. If I need the value I create another fn.
So I would have nsf-code? *and* get-nsf-code. Plus for more complicated
values, if you change their representation you don't have to change the
