-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 12/21/2008 12:03 PM, Adam Harrison (Clojure) wrote: > > Hi folks, > > First let me say 'Thankyou very much' for Clojure - it has enabled me to > finally take the plunge into learning a Lisp without feeling like I'm > abandoning a ten year investment in the Java platform and its libraries. > I bought 'Practical Common Lisp' about eighteen months ago and read it > half heartedly, never making it to a REPL; however I now have the luxury > of working on a project of my own and decided the time was right to > revisit the promised land of Lisp. I have been aware of Clojure for > about six months, and, given my experience of Java, it seemed like the > obvious place to start. I've spent the past couple of weeks devouring > lots of general Lisp related documentation, Rich's Clojure webcasts and > Stuart's 'Programming Clojure' book. I am pleased to report that my Lisp > epiphany occurred yesterday when I found I could understand this macro > pasted to lisp.org by Rich: > > (defmacro defnk [sym args & body] > (let [[ps keys] (split-with (complement keyword?) args) > ks (apply array-map keys) > gkeys (gensym "gkeys__") > letk (fn [[k v]] > (let [kname (symbol (name k))] > `(~kname (~gkeys ~k ~v))))] > `(defn ~sym [...@ps & k#] > (let [~gkeys (apply hash-map k#) > ~@(mapcat letk ks)] > ~...@body)))) > > I am now spoiled forever, and although my powers are weak, I realise I > have become one of those smug Lisp types condemned to look down on all > other programming languages for the rest of their lives. Thankyou's and > cliched tales of enlightenment dispensed with, I can now move on to the > plea for aid :) > > My current project involves semantic web technologies, at this point > specifically SPARQL queries over RDF triple stores (for those unfamiliar > with SPARQL it will suffice to say that it's a straightforward query > language modelled after SQL, but operating over graphs rather than > tables). Like their SQL counterpart, the Java APIs for performing these > queries are verbose and cumbersome, and I am hoping that they can be > hidden behind some cunning Lisp macros to provide an expressive and > tightly integrated semantic query capability (similar to Microsoft's > LINQ). Unfortunately my ambitions far exceed my skills at this point, > and I am hoping to garner some gentle mentoring to steer me in the right > direction from the outset. > > My first inclination is to start with the simplest thing which will > work, which is to create a function which takes a SPARQL query string as > an argument and returns a list of results: > > (sparql " > PREFIX foaf: <http://xmlns.com/foaf/0.1> > SELECT ?name > WHERE > { > ?person foaf:mbox \"mailto:adam-cloj...@antispin.org\"; . > ?person foaf:name ?name > }") > > However this style is poor for several reasons: it looks ugly, quotation > marks have to be escaped manually, and interpolation of variables into > the query string is a chore which further decreases readability. What I > really want is a nice DSL: > > (let [mbox "mailto:adam-cloj...@atispin.org";] > (sparql > (with-prefixes [foaf "http://xmlns.com/foaf/0.1";] > (select ?name > (where > (?person foaf:mbox mbox) > (?person foaf:name ?name))))) > > Clearly this is going to require a macro, because for a start I don't > want the symbols representing SPARQL capture variables (the ones > starting with '?') to be evaluated - I want to take the name of the > symbol, '?' and all, and embed it into the query string which this DSL > will ultimately generate before calling into the Java API. On the other > hand, I do want some things evaluated - I want to embed the value > ('mailto:adam-clog...@antispin.org') bound to the symbol 'mbox' in the > query string, not the name of the symbol. > > From my position of total ignorance, I can see two broad approaches to > tackling this. The first is to implement (sparql ...) as a macro which > is responsible for interpreting the entire subtree of forms below it, > building a query string by selectively evaluating some forms whilst > using others as navigational markers which give context. It would honour > the grammar which defines SPARQL queries, and either signal an error or > be guaranteed to generate syntactically correct queries. The macro would > also have insight into the format of the data which would be returned > (gleaned from the 'select' part) and so could return something useful > like a list of maps where the keys are the names of the capture > variables that appear in the select clause. I have no idea how to do > this, but it feels like the 'right' way. > > The other approach, which is IMO a bit hacky, but within my reach, is to > define 'with-prefixes', 'select' and 'where' as individual macros whose > first arguments are expanded into the relevant subcomponent of the query > string and whose final argument is a string to be appended to the end. > You then compose these together into the right order to get the compete > query string: > > (with-prefix [foaf "http://xmlns.com/foaf/0.1";] "...the select > statement") would evaluate to "PREFIX foaf: <http://xmlns.com/foaf/0.1> > ...the select statement" > > (select ?name "... the where clause") would evaluate to "SELECT ?name > WHERE ...the where clause" > > and so on. In this case 'sparql' would remain a function which takes a > string argument, and you build that query string recursively by nesting > calls to with-prefix, select & where in the correct order. Whist this is > much easier to implement, it has some serious drawbacks - it is up to > the user to compose these things in the correct order (since everything > gets flattened to a string at each level of recursion, we can't check > for errors), and it's no longer possible for the 'sparql' function to > glean the structure of its return value without reparsing the query > string it is passed. > > These problems aside, I decided to take a stab at implementing the > 'where' macro from the second approach because I have to start somewhere > :) Here is what I have so far: > > (defmacro where [& triples] > `(let [encode# (fn [x#] (cond (and (symbol? x#) (= (first (name x#)) > \?)) (name x#) > (integer? x#) (str "\"" x# "\"^^xsd:integer") > (float? x#) (str "\"" x# "\"^^xsd:decimal") > (string? x#) (str "\"" x# "\"")))] > (apply str > (interpose " .\n" > (for [triple# '~triples] > (apply str > (interpose " " > (map encode# triple#)))))))) > > As you can see, so far it correctly encodes SPARQL capture variables, > and literal strings, integers and floats: > > user=> (print (where (?a ?b 1) (?a ?b 2.0) (?a ?b "string"))) > ?a ?b "1"^^xsd:integer . > ?a ?b "2.0"^^xsd:decimal . > ?a ?b "string" > > I tried adding '(list? x#) (eval x#)' to the encode cond to make it cope > with expressions like this: > > (where (?a ?b (+ 1 2))) > > Unfortunately that results in an unencoded literal '3' in the query > string instead of the '"3"^^xsd:integer' I was looking for. I tried > calling encode recursively (despite the obvious infinite recursion issue > if the eval returns a list) '(list? x#) (encode# eval x#)' but I got an > error at runtime: > > user=> (where ((+ 1 2))) > java.lang.Exception: Unable to resolve symbol: encode__2605 in this context > clojure.lang.Compiler$CompilerException: NO_SOURCE_FILE:153: Unable to > resolve symbol: encode__2605 in this context > > Clealy this is due to encode# not being bound until after the function > definition completes, but I have no idea how to fix it yet (is there a > way to refer to a function during its definition?). I am also struggling > to get access to variables bound outside the macro: > > (let [v "a value"] > (where (?a ?b v)) > > I tried adding '(symbol? x#) (eval x#)' to the encode cond but that gets > me a complaint about 'v' being unresolvable in this context. > > Another problem I face is that there is no enforcement that triples are > passed - the macro just maps all the values through encode and > interposes them with spaces, so no error is raised if you have too many > or too few values to create a valid where clause. I have no idea what is > the proper way of dealing with things like this in a functional language. > > So as you can see, if you have made it heroically to the end of this > email, I am keen but facing a steep learning curve :) I am aware that > most of my troubles are well trodden issues that no doubt have thirty > year old Lisp idiomatic solutions, and for bothering you with them on a > Clojure specific mailing list I apologise. On the other hand, if you > feel like sharing some wisdom with me, either directly or through > pointers to resources I would be most grateful. I would also be most > appreciative to receive comments on the design of the DSL itself, and > suggestions for the most Lispish/Clojurish implementation thereof. In > the meantime, it's back to the REPL for me :) > > Best Regards, > > Adam Harrison >
Hello, I have also been doing work with Jena+Clojure+Sparql. I'm not sure if you're aware of it or not, but you might want to look into binding result sets. [1] I would love to compare notes with you some time. I am duck1123 on freenode, and my jabber address is the same as my email. [1]: http://www.ldodds.com/blog/archives/000251.html -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAklOtpEACgkQWorbjR01Cx53YACfZVGEON9YceoVllNgBxVmHXDt +9kAoNIRpxSfYdFu2e1F0QLmEjmHz+aF =4xnT -----END PGP SIGNATURE----- --~--~---------~--~----~------------~-------~--~----~ 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 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 -~----------~----~----~----~------~----~------~--~---
