mini-Kanren, core.logic, clojure: Reasoned Scheme Exercise 60

Question

This is NOT homework -- the solution is already in the text. I just failed to understand the solution. Problem ``` (run* (q) (let [a (== true q) b (== false q)] b)) ``` Correct Solution ``` (false) ``` My believed solution ``` () ``` My Confusion Apparently the "a (== true q)" line is NOT executed, since only b is the goal. This confuses me. My mental model so far for logic programming has been: - consider all possible assignemnts to q output the ones that manages to pass through the entire program Thus, the "a (== true q)" forces q = true, which makes it impossible to satisfy the "b (== false q)" line. However, apparently only "thigns needed to compute the goal" are executed. What's going on? What's the right mental execution model for core.logic / mini-kanren? Thanks (BTW, I'm clearly in the wrong, since mini-karen + core.logic agre with each other -- I just want to understand what I'm doing wrong.)

Accepted Answer

`==` produces a goal. But you don't pass the `a` goal to run. So run doesn't know about it. A comparable situation is this:

```
(defn call [f] (f))

(call
  (let [a #(println "a")
        b #(println "b")]
    b))
```

The `a` function is created but not passed to `call`. So it is never executed.