Unexpected results in otp 26

[WHYZN]

Describe the bug
Unexpected results

To Reproduce

-module(abc).

%% API
-export([a/1, b/1]).

-record(abc, {data}).

-record(kv, {k, v}).

a(Data) ->
	F =
		fun(Map) ->
			{maps:get(a, Map, 0),
				#kv{
					k = maps:get(a, Map, 0),
					v = maps:get(b, Map, 0)
				}}
		end,
	
	#abc{data = maps:from_list(lists:map(F, Data))}.

b(Data) ->
	F =
		fun(Map) ->
			#kv{
				k = maps:get(a, Map, 0),
				v = maps:get(b, Map, 0)
			}
		end,
	#abc{data = maps:from_list(lists:map(F, Data))}.

Expected behavior

(game_ccc_1@127.0.0.1)24> Data = [#{a => 1, b => 2}, #{a => 3, b => 4}].
[#{a => 1,b => 2},#{a => 3,b => 4}]
(game_ccc_1@127.0.0.1)25> abc:a(Data).
{abc,#{1 => {kv,1,2},3 => {kv,3,4}}}
(game_ccc_1@127.0.0.1)26> abc:b([]).
#{}
(game_ccc_1@127.0.0.1)27> abc:b(Data).
* exception error: bad argument
in function  maps:from_list/1
called as maps:from_list([{kv,1,2},{kv,3,4}])
in call from abc:b/1 (apps/mmo_server/src/role/abc.erl, line 43)

why ach:b([]). return #{} not return #abc{}
Alternatively, an error can be reported instead of receiving unexpected results

Affected versions
Erlang/OTP 26 [erts-14.2.5.4]

Additional context
Add any other context about the problem here. If you wish to attach Erlang code you can either write it directly in the post using code tags, create a gist, or attach it as a zip file to this post.

[garazdawi]

Hello! This is not a bug in Erlang, but a bug in your code. The only input to maps:from_list/1 is a list of two tuples, but you are passing the a list of the #kv{} record. To make it work you need to update b to look like this:

b(Data) ->
	F =
		fun(Map) ->
			{
				maps:get(a, Map, 0),
				maps:get(b, Map, 0)
			}
		end,
	#abc{data = maps:from_list(lists:map(F, Data))}.

If you need more help understanding the Erlang and its APIs, please visit the https://www.erlangforums.com and people there will be happy to answer your questions.

[WHYZN]

i know,but when data is empty list,why can't return #abc{} or report error,
It's difficult to find where the error lies when the code is very long
@garazdawi

[garazdawi]

Aha, now I see what you mean! Yes, that is indeed a bug. The compiler seems to assume that the call to lists:map/2 will fail becase the fun will never return a correct value, but when called with the empty list it never calls the fun and thus does not fail.

[jhogberg]

Thanks for reporting this, I've fixed it in #9481 :)