Allow number indexing of struct fields.

Better error messages on failed struct access.

src/pebblisp.c

@@ -307,13 +307,22 @@ Object structAccess(Object* params, unused int length, unused struct Environment
     Object field = params[1];
 
     struct StructDef* structDef = getStructAt(structo.structObject->definition);
-    for (int i = 0; i < structDef->fieldCount; i++) {
-        if (strcmp(field.string, structDef->names[i]) == 0) {
-            return cloneObject(structo.structObject->fields[i]);
+    if (isStringy(field)) {
+        for (int i = 0; i < structDef->fieldCount; i++) {
+            if (strcmp(field.string, structDef->names[i]) == 0) {
+                return cloneObject(structo.structObject->fields[i]);
+            }
         }
+        throw(NULL_PARSE, "Could not find struct field named `%s`", field.string);
+    }
+    if (isNumber(field)) {
+        if (structDef->fieldCount < field.number || field.number < 0) {
+            throw(NULL_PARSE, "Could not find struct field at index %ld. Struct `%s` has %d fields.", field.number, structDef->name, structDef->fieldCount);
+        }
+        return cloneObject(structo.structObject->fields[field.number]);
     }
 
-    throw(NULL_PARSE, "Could not find field named `%s`", field.string);
+    throw(BAD_PARAMS, "Struct fields should be indexed with a string or a number, but received a %s", getTypeName(&field));
 }
 
 Result parse(struct Slice* slices)

src/pebblisp.h

@@ -122,7 +122,7 @@ fn(def, "def",
 );
 
 tfn(structAccess, "poss",
-    ({ expect(isStruct), expect(isStringy), anyType }),
+    ({ expect(isStruct), anyType, anyType }),
     "Get the value of a struct's field",
     "(struct Post (title body))\n    "
     "(def p (Post \"This is a title\" \"This is the body\"))\n      "