Add generators for UTF-8 strings

As reported in #318, `proper_types:string()` may generate invalid Unicode
strings; the Erlang type language overapproximates the set of lists that
represent valid Unicode strings.

This `proper_unicode` module is therefore extended with functions that
generate valid UTF-8 character lists.

include/proper.hrl

@@ -1,6 +1,6 @@
 %%% -*- coding: utf-8; erlang-indent-level: 2 -*-
 %%% -------------------------------------------------------------------
-%%% Copyright 2010-2022 Manolis Papadakis <manopapad@gmail.com>,
+%%% Copyright 2010-2025 Manolis Papadakis <manopapad@gmail.com>,
 %%%                     Eirini Arvaniti <eirinibob@gmail.com>,
 %%%                 and Kostis Sagonas <kostis@cs.ntua.gr>
 %%%
@@ -19,7 +19,7 @@
 %%% You should have received a copy of the GNU General Public License
 %%% along with PropEr.  If not, see <http://www.gnu.org/licenses/>.
 
-%%% @copyright 2010-2022 Manolis Papadakis, Eirini Arvaniti, and Kostis Sagonas
+%%% @copyright 2010-2025 Manolis Papadakis, Eirini Arvaniti, and Kostis Sagonas
 %%% @version {@version}
 %%% @author Manolis Papadakis
 %%% @doc User header file: This file should be included in each file containing
@@ -70,7 +70,8 @@
 %% Unicode
 %%------------------------------------------------------------------------------
 
--import(proper_unicode, [utf8/0, utf8/1, utf8/2]).
+-import(proper_unicode, [utf8/0, utf8/1, utf8/2,
+                         utf8_string/0, utf8_string/1, utf8_string/2]).
 
 
 %%------------------------------------------------------------------------------

src/proper_unicode.erl

@@ -1,5 +1,4 @@
-%%% -*- coding: utf-8 -*-
-%%% -*- erlang-indent-level: 2 -*-
+%%% -*- coding: utf-8; erlang-indent-level: 2 -*-
 %%% -------------------------------------------------------------------
 %%% Copyright 2014 Motiejus Jakstys <desired.mta@gmail.com>
 %%%
@@ -24,10 +23,10 @@
 
 %%% @doc Unicode generators for PropEr
 %%%
-%%% This module exposes utf8 binary generator.
+%%% This module exposes utf8 binary and string generators.
 %%%
-%%% Makes it easy to create custom-encoded unicode binaries. For example,
-%%% utf16 binary generator:
+%%% Makes it easy to create custom-encoded unicode binaries and strings.
+%%% For example, utf16 binary generator:
 %%%
 %%% ```
 %%% utf16() ->
@@ -40,18 +39,20 @@
 %%% ?FORALL(S, utf16(),
 %%%         size(S) >= 2*length(unicode:characters_to_list(S, utf16))).
 %%% '''
-%%% Only utf8 generation is supported: {@link utf8/0}, {@link utf8/1}, {@link
-%%% utf8/2}. Unicode codepoints and other encodings are trivial to get with
-%%% utf8 generators and {@link unicode} module in OTP.
+
+%%% Only utf8 generation is supported: see {@link utf8/0}, {@link utf8/1},
+%%% {@link utf8/2} which generate binaries and the corresponding functions
+%%% generating strings. Unicode codepoints and other encodings are trivial
+%%% to get with utf8 generators and the {@link unicode} module in OTP.
 -module(proper_unicode).
 
--export([utf8/0, utf8/1, utf8/2]).
+-export([utf8/0, utf8/1, utf8/2, utf8_string/0, utf8_string/1, utf8_string/2]).
 
 -include("proper_common.hrl").
 
 %% @private_type
 %% @alias
--type nonnegextint()  :: non_neg_integer() | 'inf'.
+-type nonnegextint() :: non_neg_integer() | 'inf'.
 
 
 %% @doc utf8-encoded unbounded size binary.
@@ -78,6 +79,21 @@ utf8(N, MaxCodePointSize) ->
          unicode:characters_to_binary(Str)).
 
 
+%% @doc utf8-encoded unbounded size string.
+-spec utf8_string() -> proper_types:type().
+utf8_string() ->
+    utf8_string(inf, 4).
+
+%% @doc utf8-encoded bounded upper size string.
+-spec utf8_string(nonnegextint()) -> proper_types:type().
+utf8_string(N) ->
+    utf8_string(N, 4).
+
+%% @doc Bounded upper size utf8 string, `codepoint length =< MaxCodePointSize'.
+-spec utf8_string(nonnegextint(), 1..4) -> proper_types:type().
+utf8_string(N, MaxCodePointSize) ->
+    vector_upto(N, unicode_codepoint_upto(MaxCodePointSize)).
+
 %% =============================================================================
 %% Internal functions
 %% =============================================================================

test/proper_tests.erl

@@ -937,7 +937,10 @@ native_type_props_test_() ->
      ?_passes(?FORALL(B, utf8(2, 1), byte_size(B) =< 2)),
      ?_passes(?FORALL(B, utf8(4), byte_size(B) =< 16)),
      ?_passes(?FORALL(B, utf8(),
-                      length(unicode:characters_to_list(B)) =< byte_size(B)))
+                      length(unicode:characters_to_list(B)) =< byte_size(B))),
+     ?_passes(?FORALL(S, utf8_string(), unicode:characters_to_list(S) =:= S)),
+     ?_passes(?FORALL(S, utf8_string(4),
+		      byte_size(unicode:characters_to_binary(S)) =< 16))
     ].
 
 -type bin4()   :: <<_:32>>.