With a lot of help from Ambrose, I managed to make Clojure’s core.logic library work with my custom Java data structures. In this posting, I’ll explain the code. I assume that you are already familiar with Clojure in general, and you know *core.logic* and relational programming at least from a user’s point of view.

Ok, so let’s start. My custom data structures are TGraphs that we develop here at our institute. You don’t need to know more than that a graph consists of vertices, and vertices can be connected by edges. Furthermore, both vertices and edges may have attributes. When you use such a graph, the graph itself, every vertex, and every edge is one Java object in your memory that implements the interface *Graph*, *Vertex*, and *Edge*, respectively.

I wrote a nice functional Clojure API for working with TGraphs (*funtg* on clojars; don’t use it for serious purposes, I’m constantly changing things without thinking about compatibility at all). So probably, if you are reading this, you are in the same situation that I was in: **You have a cool data structure, you have a cool API for it, and you are totally curious what you could do with core.logic on your data structure. So how do I get core.logic to work with my stuff???**

The answer is: you have to write relations that use your existing API to access your data structure. The topic of this posting is mainly how to doing that in a way that core.logic wants. So let’s start with the namespace declaration for the code:

```
```(ns extend.example
(:refer-clojure :exclude [==])
(:use [clojure.core.logic])
;; The following two are my functional API
(:require [de.uni-koblenz.ist.funtg.core :as core])
(:require [de.uni-koblenz.ist.funtg.funql :as funql]))

Nothing special here, except that you can see that I require my functional TGraph API using prefixes. So when you see *core/foo* or *funql/bar* in the following, you know that I’m calling my functional API there. As next, I added some helper functions for testing if a logic variable is fresh or ground. Ignore the comment about being walk-ed for now.

```
```(defn fresh?
"Returns true, if `x' is fresh.
`x' must have been `walk'ed before!"
[x]
(lvar? x))
(defn ground?
"Returns true, if `x' is ground.
`x' must have been `walk'ed before!"
[x]
(not (lvar? x)))

Then, I’ll defined a constant *+graph+* that holds some example TGraph (some route map like graph). I decided to keep the graph as a var of the namespace instead of making it a parameter of relations, because my API has no way to enumerate all graphs that happen to be in memory. If the graph was a parameter of relations, I couldn’t be fully relational, e.g., giving only fresh logic variables to my relations would have to error.

```
```(def +graph+ (core/load-graph "/home/horn/Repos/uni/funtg/test/greqltestgraph.tg"))

Now we’ll come to the actual first relation. *vertexo* is a relation where *v* is a vertex in the graph *+graph+.*

```
```(defn vertexo
"A relation where `v' is a vertex."
[v]
(fn [a] ;; (1)
(let [gv (walk a v)]
(if (fresh? gv)
(to-stream ;; (2)
(->> (map #(unify a v %)
(funql/vseq +graph+))
(remove not)))
(if (.containsVertex +graph+ gv)
a
(fail a))))))

Basically, the stuff marked with (*) is what’s important. (1) A relation must return a function which gets a so-called substitution *a*. You can think of it as something like an environment map which knows what logical variables are ground, and if so, what value they have. *(walk a v)* gets you the value of the variable (or value) *v*. If *v* is ground, then you get a value back. If *v* is fresh, you get a logical variable back. So now the functions *fresh?* and *ground?* above make sense, right?

The other important part (2) is that the function returned by a relation has to return a substitution again: *A relation returns a function that gets a substitution and returns a substitution*. *to-stream* turns a seq into a choice, which essentially say which possible values are allowed for the relation’s parameters. We declare such a possible binding using *unify*. *funql/vseq* returns the lazy seq of the graph’s vertices, each of which may be unified with the parameter *v* in the substitution *a*. If *v* was ground, then it could only be unified with the vertex it is already bound to. Because of that, it’s a good idea to filter out *false* values in the sequence before giving it to *to-stream*.

If I had put (2) directly into (1), it would work exactly the same. The additional code is for performance purposes. If *v* is already ground, there’s no reason to try to unify it with every vertex in the graph just to check if it is contained. Instead, a simple check (via the Java API) to see if that vertex is in the graph is enough. If it’s contained, then the substitution *a* is correct, so I return it “unchanged” (quotes, because of course we don’t mutate in Clojure). Else, the vertex is not in *+graph+*, so the binding is invalid and we fail.

That’s it. Now let’s turn to edges which are accessed with a relation of 3 parameters denoting the edge itself, its start vertex, and its end vertex.

```
```(defn edgeo
"A relation where `e' is an edge from `alpha' to `omega'."
[e alpha omega]
(fn [a]
(let [ge (walk a e)
galpha (walk a alpha)
gomega (walk a omega)]
(cond
(ground? ge) (unify a [alpha omega]
[(core/alpha ge) (core/omega ge)])
(ground? galpha) (to-stream
(->> (map #(unify a [e omega] [% (core/omega %)])
(funql/iseq galpha nil :out))
(remove not)))
(ground? gomega) (to-stream
(->> (map #(unify a [e alpha] [% (core/alpha %)])
(funql/iseq gomega nil :in))
(remove not)))
:else (to-stream
(->> (for [edge (funql/eseq +graph+)]
(unify a [e alpha omega]
[edge (core/alpha edge) (core/omega edge)]))
(remove not)))))))

Again, basically the relation would work fine if we delete everything except the expression of the *:else* clause. That unifies all edges in the graph including their start and end vertices with the parameters of the relation. The three first clauses of the *cond* are for performance only. The first says, if an edge is already given, then we only check the start and end vertex, which is a constant time operation. The second and third clause say, if either the start or end vertex are already given, then we don’t need to unify all edges in the graph with the parameters but only the edges incident to the given vertex (*eseq* vs. *iseq*), which is again much faster.

The last relation I’ll show is concerned with attributes.

```
```(defn valueo
"A relation where `ae' has value `val' for its `at' attribute."
[ae at val]
(fn [a]
(let [gae (walk a ae)
gat (walk a at)
gval (walk a val)]
(cond
(and (ground? gae)
(ground? gat)) (or (unify a [ae at val]
[gae gat (core/value gae gat)]))
(ground? gae) (to-stream
(->> (for [attr (seq (.getAttributeList
(core/attributed-element-class gae)))
:let [an (keyword (.getName attr))]]
(unify a [ae at val]
[gae an (core/value gae an)]))
(remove not)))
:else (to-stream
(->> (for [elem (concat (funql/vseq +graph+)
(funql/eseq +graph+))
attr (seq (.getAttributeList
(core/attributed-element-class elem)))
:let [an (keyword (.getName attr))]]
(unify a [ae at val]
[elem an (core/value elem an)]))
(remove not)))))))

Again, only the *:else* part is neccessary. I unify every vertex and every edge together with every attribute that is defined for them with the given parameters. Clearly, that’s quite some effort. The first clause of the *cond* simply looks up the attribute value if the element and the attribute name are already given. The second clause deals with the case when at least the graph element is given, in which case only all its attribute/value pairs have to be unified.

So here are some example applications.

What are the 3 first vertices in the graph?

```
```(run 3 [q]
(vertexo q))
;=> (#<v1: localities.Village>
#<v2: localities.Village>
#<v3: localities.Town>)

Is this a vertex of the graph?

```
```(let [v1 (core/vertex +graph+ 1)]
(run* [q]
(vertexo v1)))
;=> (_.0) ; succeeded, so yes, it's a vertex of +graph+

What are the first 3 edges of the graph?

```
```(run 3 [q]
(fresh [a o]
(edgeo q a o)))
;=> (#<e1: connections.Footpath>
#<e2: connections.Footpath>
#<e3: connections.Footpath>)

What edges end at the vertex *v1*?

```
```(let [v1 (core/vertex +graph+ 1)]
(run* [q]
(fresh [o]
(edgeo q o v1))))
;=> (#<e-22: localities.ContainsLocality>)

Which attribute of what element has the value 251?

```
```(run* [q]
(fresh [e a]
(valueo e a 251)
(== q [e a])))
;=> ([#<v1: localities.Village> :inhabitants])

So that village has 251 inhabitants. Pretty small. What’s the name of that village?

```
```(run* [q]
(fresh [e a]
(valueo e a 251)
(valueo e :name q)))
;=> ("Kammerforst")

Ah, it’s Kammerforst. Oh, wikipedia says its population has increased to 253. ;-)

Ok, that’s it. I hope this helps you to make your custom data structures work with *core.logic*. Have fun!

Thank you for an excellent set of practical examples using core.logic with a set of custom data structures and API’s. This definitely helps bridge the gap between using core.logic in “toy” scenarios vs. practical applications.

I’m really enjoying the design and layout of your site. It’s a very easy on the eyes which makes it much more pleasant for me to come here and visit more often.

Did you hire out a developer to create your theme? Superb work!

Nope, that’s a standard wordpress theme called “Greyzed”.

Could you fix the indentation in vertexo? I had a hard time understanding what was going on because the line “(if (.containsVertex +graph+ gv)” looks like it is inside to-stream.

Thanks for the heads-up, Adrian. I’ve fixed it now.