enhance $reduce to support 4 arg function

Author: andrew-coleman

src/functions.js

@@ -1594,7 +1594,8 @@ const functions = (() => {
 
         var result;
 
-        if (getFunctionArity(func) !== 2) {
+        var arity = getFunctionArity(func);
+        if (arity < 2) {
             throw {
                 stack: (new Error()).stack,
                 code: "D3050",
@@ -1612,7 +1613,14 @@ const functions = (() => {
         }
 
         while (index < sequence.length) {
-            result = yield* func.apply(this, [result, sequence[index]]);
+            var args = [result, sequence[index]];
+            if (arity >= 3) {
+                args.push(index);
+            }
+            if (arity >= 4) {
+                args.push(sequence);
+            }
+            result = yield* func.apply(this, args);
             index++;
         }
 

src/jsonata.js

@@ -1787,7 +1787,7 @@ var jsonata = (function() {
         "D3020": "Third argument of split function must evaluate to a positive number",
         "D3030": "Unable to cast value to a number: {{value}}",
         "D3040": "Third argument of match function must evaluate to a positive number",
-        "D3050": "First argument of reduce function must be a function with two arguments",
+        "D3050": "The second argument of reduce function must be a function with at least two arguments",
         "D3060": "The sqrt function cannot be applied to a negative number: {{value}}",
         "D3061": "The power function has resulted in a value that cannot be represented as a JSON number: base={{value}}, exponent={{exp}}",
         "D3070": "The single argument form of the sort function can only be applied to an array of strings or an array of numbers.  Use the second argument to specify a comparison function",

test/test-suite/groups/hof-reduce/case009.json

@@ -0,0 +1,6 @@
+{
+    "expr-file": "case009.jsonata",
+    "data": null,
+    "bindings": {},
+    "result": 4
+}

test/test-suite/groups/hof-reduce/case009.jsonata

@@ -0,0 +1,12 @@
+(
+ $months := [
+        "Jan", "Feb", "Mar", "Apr", "May", "Jun",
+        "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
+ ];
+ $indexof := λ($array, $value) {
+  $reduce($array, λ($acc, $v, $i) {
+   $v=$value ? $i : $acc
+  })
+ };
+ $indexof($months, 'May')
+)

test/test-suite/groups/hof-reduce/case010.json

@@ -0,0 +1,6 @@
+{
+    "expr-file": "case010.jsonata",
+    "data": null,
+    "bindings": {},
+    "result": 6
+}

test/test-suite/groups/hof-reduce/case010.jsonata

@@ -0,0 +1,9 @@
+(
+  $mean := $reduce(?, λ($acc, $v, $i, $arr) {(
+    $total := $acc + $v;
+    $length := $count($arr);
+    $i = $length - 1 ? $total / $length : $total
+  )});
+
+  $mean([7,3,8])
+)