Use @munificent's single-argument reduce

builtin/core.wren

@@ -29,7 +29,18 @@ class Sequence {
     return acc
   }
 
-  reduce(f) { this[1..-1].reduce(this[0], f) }
+  reduce(f) {
+    var iter = iterate(null)
+    if (!iter) Fiber.abort("Can't reduce an empty sequence.")
+
+    // Seed with the first element.
+    var result = iteratorValue(iter)
+    while (iter = iterate(iter)) {
+      result = f.call(result, iteratorValue(iter))
+    }
+
+    return result
+  }
 
 }
 

src/wren_core.c

@@ -72,7 +72,18 @@ static const char* libSource =
 "    return acc\n"
 "  }\n"
 "\n"
-"  reduce(f) { this[1..-1].reduce(this[0], f) }\n"
+"  reduce(f) {\n"
+"    var iter = iterate(null)\n"
+"    if (!iter) Fiber.abort(\"Can't reduce an empty sequence.\")\n"
+"\n"
+"    // Seed with the first element.\n"
+"    var result = iteratorValue(iter)\n"
+"    while (iter = iterate(iter)) {\n"
+"      result = f.call(result, iteratorValue(iter))\n"
+"    }\n"
+"\n"
+"    return result\n"
+"  }\n"
 "\n"
 "}\n"
 "\n"

test/list/reduce_single_item.wren

@@ -1 +1,2 @@
-[1].reduce {|a, b| a } // expect runtime error: Range start out of bounds.
+IO.print([1].reduce {|a, b| 42 }) // expect: 1
+IO.print([].reduce(1) {|a, b| 42 }) // expect: 1