December 3, 2018

Clojure 1.10's datafy and nav

One of the more mysterious new features in Clojure 1.10 seems to be the pairing of datafy and nav (and their underlying protocols, Datafiable and Navigable). Interest in these new functions has been piqued after Stuart Halloway showed off REBL at Clojure/conj (video). Stu presented this functionality as "generalized laziness": datafy produces a "data representation" of things and nav lets you (lazily) navigate around that data.

The REBL "is a graphical, interactive tool for browsing Clojure data". And in Clojure we're used to the concept of "it's just data" so a graphical browser might sound useful but not exactly earthshaking. But REBL is just an example of what can be built with the new functionality in 1.10 and, indeed, Stu's claim of "generalized laziness" is well made but a little hard to grok until you actually build something with the new protocols and functions.

Since I've done exactly that -- with experimental support for lazy navigation of related records in java.jdbc -- and spent some time on Slack today explaining how it all works, I figured it would be worth writing down in a more permanent place, as a blog post.

The Datafiable protocol (new in clojure.core.protocols) is defined for nil and Object to just return those values as-is, and then extended in clojure.datafy to cover:

  • Throwable -- producing a simple hash map by calling Throwable->map
  • clojure.lang.IRef -- producing a vector containing the dereferenced value, with the original metadata from the reference attached to that vector
  • clojure.lang.Namespace -- producing a hash map that is the data representation of the namespace (:name, :publics, :imports, and :interns) with the metadata of the namespace attached
  • java.lang.Class -- producing the clojure.reflect/reflect representation of the class, with an additional member :name, and the :members of the class grouped by name and sorted

The Navigable protocol (new in clojure.core.protocols) is defined just for Object and, given some sort of object or collection, some sort of "key", and a value, it will just return the value itself.

Based on that, it may not be clear how to implement the protocols or use the functions. The key thing that may not be obvious here is that datafy is intended to convert an arbitrary "thing" of any type into a pure Clojure data representation and, from that starting point, you can then use nav to "navigate" to a value derived from the original "thing", which you would then process with datafy to get a pure Clojure data representation again. Those data representations may contain pieces that satisfy the Navigable protocol so that navigation (via nav) provides more than just a simple data-level lookup.

Using the code as an example, you would call query (from that new namespace) and get back a result set. That looks like a sequence of hash maps (rows) but adds metadata to the rows that provides an implementation of Datafiable (protocol extension via metadata is also new in Clojure 1.10). The row is the "arbitrary thing" that we are starting with. One or more of the columns in that row may be a foreign key into another table. When you turn the row into a pure Clojure data representation -- by calling datafy on it -- it still looks like a hash map but now it has metadata that provides an implementation of Navigable. That supports calling (nav row column value) and, if the column is considered to be a foreign key to another table, it will fetch the relevant row(s) from that table and return that as the next "arbitrary thing", otherwise it will just return the column's value as passed in. The cycle of converting that to data (via datafy) and navigating through it (by navigating the Clojure data and then calling nav on that) can be continued indefinitely, until you bottom out to simple values.

You can sum this up as:

  • Starting with a "thing"...
  • convert it to data (with datafy)...
  • ...and walk it with simple Clojure data access...
  • ...and, at each stage, you can navigate to the corresponding "new thing" by calling nav...
  • ...which may return just that value or may do something more complex...
  • ...and from that "new thing" you convert it to data (with datafy) and continue the process.

Relating this back to REBL, it works by taking some arbitrary value produced in the REPL and converting it to data (via datafy) so that it can be displayed in the UI. With any part of that data highlighted you can "drill down", at which point REBL calls nav to perform the (potentially lazy, complex) navigation and then converts that to data (via datafy) and displays that as the next "level" of data. Given an associative data representation, it does (get coll k) first to get v, and then it calls (nav coll k v) to allow the underlying navigation to return an updated value.

You go from "thing" to "data representation" of "thing" (via datafy), and then you can do the simple associative lookup in the data representation, not the original thing, and then you use nav to get back to the relevant equivalent part of the "thing", and then you datafy that again to get pure data. For built-in Clojure data types that are already pure data, datafy does nothing and nav just returns the selected value. For hash maps, navigation is just simple key lookup. For vectors, navigation is also simple key (index) lookup. This new machinery only starts to shine in more complex situations.

Tags: clojure jdbc open source