Issue #8099: Add binary:join/2 to stdlib

lib/stdlib/src/binary.erl

@@ -38,7 +38,8 @@ The module is provided according to Erlang Enhancement Proposal (EEP) 31.
 %%
 %% Implemented in this module:
 -export([replace/3, replace/4,
-         encode_hex/1, encode_hex/2, decode_hex/1]).
+         encode_hex/1, encode_hex/2, decode_hex/1,
+         join/2]).
 
 -export_type([cp/0]).
 
@@ -934,6 +935,31 @@ unhex(X) ->
              no, 10, 11, 12, 13, 14, 15, no, no, no, no, no, no, no, no, no  %96
             }).
 
+-doc """
+Joins a list of binaries together by a specified `Separator`.
+
+Equivalent to `iolist_to_binary(lists:join(Separator, Binaries))` with `binary:join/2` being the faster of the two.
+
+_Example:_
+
+```erlang
+1> binary:join([<<"a">>, <<"b">>, <<"c">>], <<", ">>).
+<<"a, b, c">>
+```
+""".
+-doc(#{since => <<"OTP 27.0">>}).
+-spec join([binary()], binary()) -> binary().
+join([], _Separator) -> <<>>;
+join([H], _Separator) -> H;
+join([H | T], Separator) ->
+    Acc = <<>>,
+    join(T, Separator, <<Acc/binary, H/binary>>).
+
+-spec join([binary()], binary(), binary()) -> binary().
+join([], _Separator, Acc) -> Acc;
+join([H | T], Separator, Acc) ->
+    join(T, Separator, <<Acc/binary, Separator/binary, H/binary>>).
+
 badarg_with_cause(Args, Cause) ->
     erlang:error(badarg, Args, [{error_info, #{module => erl_stdlib_errors,
                                                cause => Cause}}]).

lib/stdlib/test/binary_module_SUITE.erl

@@ -23,7 +23,7 @@
 	 interesting/1,scope_return/1,random_ref_comp/1,random_ref_sr_comp/1,
 	 random_ref_fla_comp/1,parts/1, bin_to_list/1, list_to_bin/1,
 	 copy/1, referenced/1,guard/1,encode_decode/1,badargs/1,longest_common_trap/1,
-         check_no_invalid_read_bug/1,error_info/1, hex_encoding/1]).
+         check_no_invalid_read_bug/1,error_info/1, hex_encoding/1, join/1]).
 
 -export([random_number/1, make_unaligned/1]).
 
@@ -38,7 +38,7 @@ all() ->
      random_ref_comp, parts, bin_to_list, list_to_bin, copy,
      referenced, guard, encode_decode, badargs,
      longest_common_trap, check_no_invalid_read_bug,
-     error_info, hex_encoding].
+     error_info, hex_encoding, join].
 
 
 -define(MASK_ERROR(EXPR),mask_error((catch (EXPR)))).
@@ -260,6 +260,13 @@ badargs(Config) when is_list(Config) ->
     badarg = ?MASK_ERROR(binary:encode_hex([])),
     badarg = ?MASK_ERROR(binary:encode_hex(#{})),
     badarg = ?MASK_ERROR(binary:encode_hex(foo)),
+
+    badarg = ?MASK_ERROR(binary:join([<<"a">>], ", ")),
+    badarg = ?MASK_ERROR(binary:join([""], <<",">>)),
+    badarg = ?MASK_ERROR(binary:join([123], <<",">>)),
+    badarg = ?MASK_ERROR(binary:join(123, <<",">>)),
+    badarg = ?MASK_ERROR(binary:join(#{}, <<",">>)),
+    badarg = ?MASK_ERROR(binary:join(foo, <<",">>)),
     ok.
 
 %% Whitebox test to force special trap conditions in
@@ -1581,6 +1588,12 @@ do_hex_roundtrip(Bytes) ->
             ok
     end.
 
+%% Test join/2.
+join(Config) when is_list(Config) ->
+    <<"a, b, c">> = binary:join([<<"a">>, <<"b">>, <<"c">>], <<", ">>),
+    <<"a">> = binary:join([<<"a">>], <<", ">>),
+    <<>> = binary:join([], <<", ">>).
+
 %%%
 %%% Utilities.
 %%%