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 some useful value if it ``succeeds''; such a function can be used not only as a test but also for the useful value provided in case of success. An example is member<http://clhs.lisp.se/Body/f_mem_m.htm> . On Thu, Apr 17, 2014 at 10:20 AM, Colin Yates <colin.ya...@gmail.com> wrote: > 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 (= > "someString" (is-nsf-code? "someString"))) > > I should reduce it to 1p really as I am only contributing in an abstract > design sense, not in the specifics of the coding standards you mentioned. > > *1 If this doesn't hold (i.e. caller depends upon receiving a > java.lang.Boolean or associated primitive) then I think the caller has > bigger problems. > > > On Thursday, April 17, 2014 5:33:42 PM UTC+1, Sean Corfield wrote: >> >> 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 that if you have a function that returns a boolean >> (and that is intended for use as a predicate), it should be named to end in >> '?'. Fair enough. >> >> My question is about the reverse implication: >> >> * Should a function whose name ends in '?' return a (strict) boolean >> value? >> >> Looking at the docstrings of a random selection of functions found by >> (apropos "?"), they all seem to return specifically true or false. I did >> not do an exhaustive check. >> >> Is the intent that foo? implies a result of true or false - or could foo? >> return any truthy / falsey value (and therefore any Clojure value). >> >> Concrete example that spurred this discussion from some code at work: >> >> (defn is-nsf-code? >> "Given an error code, return truthy if it is NSF." >> [code] >> (#{"BE1" "BE2"} code)) >> >> Clearly the result here could be nil or a string but it's definitely >> meant to be used as a predicate. Similarly: >> >> (defn nsf? >> "Given the result of an SBW sale, return true if it failed with NSF." >> [result] >> (and (= "failure" (:result result)) >> (some is-nsf-code? (:errors result)))) >> >> Again, the result could be false or nil or a string but is meant to be >> used as a predicate. >> >> As an aside, for core.typed, we annotate the first as [String -> Boolean] >> with ^:no-check so it type checks as a true/false predicate and then we >> annotate the second as [SBWResult -> (Nilable Boolean)] and that's all >> fine... but is it "good style"? >> >> [1] http://dev.clojure.org/display/community/Library+Coding+Standards >> [2] https://github.com/bbatsov/clojure-style-guide#naming >> >> Sean Corfield -- (904) 302-SEAN >> An Architect's View -- http://corfield.org/ >> >> "Perfection is the enemy of the good." >> -- Gustave Flaubert, French realist novelist (1821-1880) >> >> >> >> -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
