Clojure core match error with variant

Question

I'm trying to solve this SICP exercise using clojure with core/match and core/typed.

I'm kind of following Jeanine Adkisson's "Variants are Not Unions" talk.

This is what I got so far:

(ns sicp.chapter2_ex29
  (:require [clojure.core.typed :as t]
            [clojure.core.match :refer [match]]))

; Type definitions
(t/defalias Mobile '{:left Branch, :right Branch})

(t/defalias Weight Number)

(t/defalias Structure
  (t/U '[(t/Value :weight) Weight]
       '[(t/Value :mobile) Mobile]))

(t/defalias Branch '{:length Number, :structure Structure})

; Constructors
(t/ann make-mobile [Branch Branch -> Mobile])
(defn make-mobile [left right]
  {:left left :right right})

(t/ann make-branch [Number Structure -> Branch])
(defn make-branch [length structure]
  {:length length :structure structure})

; Getters
(t/ann left-branch [Mobile -> Branch])
(defn left-branch [mobile]
  (:left mobile))

(t/ann right-branch [Mobile -> Branch])
(defn right-branch [mobile]
  (:right mobile))

(t/ann branch-length [Branch -> Number])
(defn branch-length [branch]
  (:length branch))

(t/ann branch-structure [Branch -> Structure])
(defn branch-structure [branch]
  (:structure branch))

; Total weight
(t/ann total-weight [Mobile -> Number])
(defn total-weight [mobile]
  (t/letfn> [structure-weight :- [Structure -> Number]
             (structure-weight [structure]
                               (do
                                 (println (str "structure -> " structure))
                                 (println (str "structure0 -> " (get structure 0)))
                                 (println (str "structure1 -> " (get structure 1)))
                                 (match structure
                                        [:weight weight] weight
                                        [:mobile mobile] (total-weight mobile))))
             branch-weight :- [Branch -> Number]
             (branch-weight [branch]
                            (structure-weight (branch-structure branch)))]
            (let
              [left (branch-weight (left-branch mobile))
               right (branch-weight (right-branch mobile))]
              (do
                (println (str "left ->" left))
                (println (str "right ->" right))
                (+ left right)))))

(t/ann mobile1 Mobile)
(def mobile1 (make-mobile
              (make-branch 3 [:weight 4])
              (make-branch 5 [:weight 2])))

(total-weight mobile1) ; <- works as expected = 6

(t/ann mobile2 Mobile)
(def mobile2 (make-mobile
              (make-branch 3 [:weight 4])
              (make-branch 5 [:mobile (make-mobile
                                       (make-branch 2 [:weight 3])
                                       (make-branch 4 [:weight 2]))])))

(total-weight mobile2) ; <- throws java.lang.IllegalArgumentException: No matching clause: [:mobile {:left {:length 2, :structure [:weight 3]}, :right {:length 4, :structure [:weight 2]}}]

The Structure type is a variant: either it is a weight, which is simply a number (it has the form [:weight weight] where weight is a Number) or it is a mobile (it has the form [:mobile mobile] where mobile is something of type Mobile).

It type-checks and the total-weight function works for a simple input: a Mobile made of two Weights.

But as you can see it fails for a Mobile with a Branch that is a Mobile.

For some reason it isn't matching the [:mobile mobile] case. Any idea on what am I doing wrong?

Answer 1

The problem appears to be that the identifier mobile in

(match structure
       [:weight weight] weight
       [:mobile mobile] (total-weight mobile))))

... is already bound as an argument in the enclosing function definition:

(defn total-weight [mobile]
  ... )

Changing the match form to

(match structure
       [:weight w] w
       [:mobile m] (total-weight m))))

... removes the error.

The rule appears to be:

If a name in a match pattern is bound, it is not rebound locally. It is interpreted as its prevailing value.

I have to say I only found the error by accident. I would prefer that the local binding took precedence, but it doesn't.

---

Notes

The syntax of your match expression does not follow the documentation, which would require ...

(match [structure]
         [[:weight w]] w
         [[:mobile m]] (total-weight m))

... but your abbreviated version works just as well.

---

The Clojure idiom is to use standard data structures - especially maps - where possible, eschewing access functions and constructors. In this way, we can write something like

; Total weight

(declare structure-weight)

(t/ann total-weight [Mobile -> Number])
(defn total-weight [mobile]
  (match [mobile]
         [{:left bl, :right br}]
           (apply + (map
                      (comp structure-weight :structure)
                      [bl br]))))

(t/ann structure-weight [Structure -> Number])
(defn structure-weight [structure]
  (match [structure]
         [[:weight w]] w
         [[:mobile m]] (total-weight m)))