commit https://github.com/vim/vim/commit/1735bc988c546cc962c5f94792815b4d7cb79710 Author: Bram Moolenaar <Bram@vim.org> Date: Mon Mar 14 23:05:14 2016 +0100 patch 7.4.1559 Problem: Passing cookie to a callback is clumsy. Solution: Change function() to take arguments and return a partial.

--- a/src/json.c
+++ b/src/json.c
@@ -212,6 +212,7 @@ json_encode_item(garray_T *gap, typval_T
 	    break;
 
 	case VAR_FUNC:
+	case VAR_PARTIAL:
 	case VAR_JOB:
 	case VAR_CHANNEL:
 	    /* no JSON equivalent TODO: better error */