Add gmconfig:pure_update_config/3 (#4)

The function `pure_update_config/3` updates the config and simply returns the new config, without any side-effects.

Co-authored-by: Ulf Wiger <ulf@wiger.net>
Reviewed-on: #4

src/gmconfig.erl

@@ -38,6 +38,7 @@
 -export([update_config/1,
          update_config/2,
          update_config/3,
+         pure_update_config/3,
          silent_update_config/1,
          delete_config_value/1,
          suggest_config/2,
@@ -878,6 +879,12 @@ update_config(Map, ConfigMap, Mode) ->
     pt_set_config(ConfigMap1),
     ok.
 
+pure_update_config(Map, ConfigMap, Schema) ->
+    NewCfg = gmconfig_schema_utils:merge(Map, ConfigMap, Schema),
+    check_validation([validate_(NewCfg, Schema)],
+                     [NewCfg], pure_update_config, silent),
+    NewCfg.
+
 export_config() ->
     Config = pt_get_config(),
     JSON = json:format(Config, #{indent => 2}),