add(max_function_arity): limit for non exported functions

doc_rules/elvis_style/max_function_arity.md

@@ -12,11 +12,13 @@ but it applies to regular functions only (not anonymous ones).
 
 - `max_arity :: non_neg_integer()`.
   - default: `8`.
+- `non_exported_max_arity :: non_neg_integer() | same`.
+  - default: `8`.
 
 ## Example
 
 ```erlang
 {elvis_style, max_function_arity}
 %% or
-{elvis_style, max_function_arity, #{max_arity => 10}}
+{elvis_style, max_function_arity, #{max_arity => 10, non_exported_max_arity => same}}
 ```

src/elvis_style.erl

@@ -197,7 +197,7 @@ default(max_module_length) ->
 default(max_anonymous_function_arity) ->
     #{max_arity => 5};
 default(max_function_arity) ->
-    #{max_arity => 8};
+    #{max_arity => 8, non_exported_max_arity => 10};
 default(max_function_length) ->
     #{max_length => 30,
       count_comments => false,
@@ -884,18 +884,31 @@ max_anonymous_function_arity(Config, Target, RuleConfig) ->
                     end,
                     Funs).
 
--type max_function_arity_config() :: #{max_arity => non_neg_integer()}.
+-type max_function_arity_config() ::
+    #{max_arity => non_neg_integer(), non_exported_max_arity => pos_integer()}.
 
 -spec max_function_arity(elvis_config:config(),
                          elvis_file:file(),
                          max_function_arity_config()) ->
                             [elvis_result:item()].
 max_function_arity(Config, Target, RuleConfig) ->
-    MaxArity = option(max_arity, RuleConfig, max_function_arity),
+    ExportedMaxArity = option(max_arity, RuleConfig, max_function_arity),
+    NonExportedMaxArity =
+        specific_or_default(option(non_exported_max_arity, RuleConfig, max_function_arity),
+                            ExportedMaxArity),
     Root = get_root(Config, Target, RuleConfig),
     IsFunction = fun(Node) -> ktn_code:type(Node) == function end,
     Functions = elvis_code:find(IsFunction, Root),
-    lists:filtermap(fun(Function) ->
+    lists:filtermap(fun(#{attrs := #{arity := Arity, name := Name}} = Function) ->
+                       IsExported =
+                           lists:member({Name, Arity}, elvis_code:exported_functions(Root)),
+                       MaxArity =
+                           case IsExported of
+                               true ->
+                                   ExportedMaxArity;
+                               false ->
+                                   NonExportedMaxArity
+                           end,
                        case ktn_code:attr(arity, Function) of
                            Arity when Arity =< MaxArity ->
                                false;

test/examples/fail_max_non_exported_function_arity.erl

@@ -0,0 +1,18 @@
+-module(fail_max_non_exported_function_arity).
+
+-export([f/0, f/1]).
+
+f() ->
+    f(1).
+
+f(1) ->
+    f(1, 2).
+
+f(1, 2) ->
+    f(1, 2, 3).
+
+f(1, 2, 3) ->
+    f(1, 2, 3, 4).
+
+f(1, 2, 3, 4) ->
+    {1, 2, 3, 4, 5}.

test/examples/pass_max_non_exported_function_arity.erl

@@ -0,0 +1,30 @@
+-module(pass_max_non_exported_function_arity).
+
+-export([f/0, f/1, f/2]).
+
+f() ->
+    f(1).
+
+f(1) ->
+    f(1, 2).
+
+f(1, 2) ->
+    f(1, 2, 3).
+
+f(1, 2, 3) ->
+    f(1, 2, 3, 4).
+
+f(1, 2, 3, 4) ->
+    f(1, 2, 3, 4, 5).
+
+f(1, 2, 3, 4, 5) ->
+    f(1, 2, 3, 4, 5, six).
+
+f(1, 2, 3, 4, 5, Six) ->
+    f(1, 2, 3, 4, 5, Six, seven).
+
+f(1, 2, 3, 4, 5, Six, seven) ->
+    f(1, 2, 3, 4, 5, Six, seven, "eight").
+
+f(1, 2, 3, 4, 5, Six, seven, "eight") ->
+    {1, 2, 3, 4, 5, Six, seven, "eight"}.

test/style_SUITE.erl

@@ -1033,6 +1033,28 @@ verify_max_function_arity(Config) ->
                               #{max_arity => -1},
                               PathFail),
 
+    PathNonExportedPass = "pass_max_non_exported_function_arity." ++ Ext,
+    [] =
+        elvis_core_apply_rule(Config,
+                              elvis_style,
+                              max_function_arity,
+                              #{max_arity => 3, non_exported_max_arity => 9},
+                              PathNonExportedPass),
+
+    PathNonExportedFail = "fail_max_non_exported_function_arity." ++ Ext,
+    [_, _] =
+        elvis_core_apply_rule(Config,
+                              elvis_style,
+                              max_function_arity,
+                              #{max_arity => 1, non_exported_max_arity => 2},
+                              PathNonExportedFail),
+
+    [_, _, _, _, _] =
+        elvis_core_apply_rule(Config,
+                              elvis_style,
+                              max_function_arity,
+                              #{max_arity => 3, non_exported_max_arity => same},
+                              PathNonExportedPass),
     ok.
 
 -spec verify_max_anonymous_function_arity(config()) -> any().