fix bug in dict comparison

lib/compare/structures.js

@@ -77,8 +77,10 @@ export const compareDicts = compareKeys => compareValues => x => y => {
       // because of the underlying red-black tree, iteration happens in ordered fashion
       const iterate = iterX => iterY =>
         match(readDict(iterX))
-          (entryX =>
+          (_ => 0) // end of iteration (dicts are equal)
+          (entryX => // entry...
             match(readDict(iterY))
+              (_ => {throw new Error("this can never happen");})
               (entryY => compareKeys
                             (getLeft(getLeft(entryX)))
                             (getLeft(getLeft(entryY)))
@@ -87,9 +89,9 @@ export const compareDicts = compareKeys => compareValues => x => y => {
                             (getRight(getLeft(entryY)))
                        || iterate
                             (getRight(entryX))
-                            (getRight(entryY)))
-              (_ => 0))
-          (_ => 0); // we made it, dicts are equal
+                            (getRight(entryY))
+              )
+          ); 
       return iterate(firstDict(x))(firstDict(y));
     })();
 };

lib/structures/dict.js

@@ -1,3 +1,4 @@
+import { unit } from "../primitives/unit.js";
 import { RBTreeWrapper } from "../util/rbtree_wrapper.js";
 import { indent } from "../util/util.js";
 import { newProduct } from "./product.js";
@@ -32,13 +33,13 @@ export const last  = dict => dict.tree.end;
 export const read = iter => {
   if (iter !== undefined && iter.valid) {
     return newRight(
-      newProduct(
-        newProduct(iter.key)(iter.value)
+      newProduct
+        (newProduct(iter.key)(iter.value))
         (iter.clone().next())
-      )
     );
   }
   else {
+    // end of iteration
     return newLeft(unit);
   }
 };

tests/dict.js

@@ -0,0 +1,29 @@
+import assert from "node:assert";
+
+import { compareInts, compareStrings } from "../lib/compare/primitives.js";
+import { compareDicts } from "../lib/compare/structures.js";
+import { emptyDict, set } from "../lib/structures/dict.js";
+
+const cmpFn = compareDicts(compareStrings)(compareInts);
+
+const e = emptyDict(compareStrings);
+const f = set(e)("x")(1n);
+const g = set(f)("y")(2n);
+
+assert.equal(
+  cmpFn(e)(e),
+  0,
+  "empty dict should be equal to itself"
+);
+
+assert.equal(
+  cmpFn(g)(g),
+  0,
+  "dict should always equal itself"
+);
+
+assert.equal(
+  cmpFn(f)(g),
+  -1,
+  "f is smaller"
+);