introduce seed user option

src/proper.erl

@@ -239,6 +239,8 @@
 %%% <dd>This is equivalent to the {@link numtests/1} property wrapper. Any
 %%%   {@link numtests/1} wrappers in the actual property will overwrite this
 %%%   setting.</dd>
+%%% <dt>`{seed, {<Non_negative_integer>,<Non_negative_integer>,<Non_negative_integer>}}'</dt>
+%%% <dd>Pass a seed to the RNG so that random results can be reproduced.</dd>
 %%% <dt>`{start_size, <Size>}'</dt>
 %%% <dd>Specifies the initial value of the `size' parameter (default is 1), see
 %%%   the documentation of the {@link proper_types} module for details.</dd>
@@ -491,6 +493,7 @@
 		  | {'search_steps',pos_integer()}
 		  | {'search_strategy',proper_target:strategy()}
 		  | pos_integer()
+		  | {'seed',proper_gen:seed()}
 		  | {'start_size',proper_gen:size()}
 		  | {'max_size',proper_gen:size()}
 		  | {'max_shrinks',non_neg_integer()}
@@ -926,6 +929,7 @@ parse_opt(UserOpt, Opts) ->
 	{search_steps, N}    -> Opts#opts{search_steps = N};
 	{search_strategy, S} -> Opts#opts{search_strategy = S};
 	N when is_integer(N) -> Opts#opts{numtests = N};
+	{seed,Seed}          -> Opts#opts{seed = Seed};
 	{start_size,Size}    -> Opts#opts{start_size = Size};
 	{max_size,Size}      -> Opts#opts{max_size = Size};
 	{max_shrinks,N}      -> Opts#opts{max_shrinks = N};

test/proper_tests.erl

@@ -1094,6 +1094,39 @@ options_test_() ->
 		 ?FORALL(_,?SIZED(Size,integer(Size,Size)),false),
 		 [{start_size,12}])].
 
+seeded_test_() ->
+   Seed = os:timestamp(),
+   BaseOpts = [noshrink, {start_size,65536}],
+   Seeded = fun (Prop) ->
+                R = proper:counterexample(Prop, [{seed,Seed}|BaseOpts]),
+                proper:clean_garbage(),
+                R
+            end,
+   NoSeed = fun (Prop) ->
+                R = proper:counterexample(Prop, BaseOpts),
+                proper:clean_garbage(),
+                R
+            end,
+   ReSeeded = fun (Prop) ->
+                  OtherSeed = os:timestamp(),
+                  R = proper:counterexample(Prop, [{seed,OtherSeed}|BaseOpts]),
+                  proper:clean_garbage(),
+                  R
+              end,
+   [[?_assert(state_is_clean()), ?_assertEqual(Seeded(Prop),Check),
+     ?_assert(state_is_clean()), ?_assertNotEqual(NoSeed(Prop),Check),
+     ?_assert(state_is_clean()), ?_assertNotEqual(ReSeeded(Prop),Check),
+     ?_assert(state_is_clean())]
+    %% For each of these properties...
+    || Prop <- [?FORALL(_, integer(), false),
+                ?FORALL(_, integer(), ?TRAPEXIT(false)),
+                ?FORALL_TARGETED(I, integer(), begin ?MAXIMIZE(I),false end)],
+       %% Ensure that using a large enough size:
+       %% * provided a seed, another run gives the same counterexample;
+       %% * when not provided a seed: run gives out differing results to the seeded one;
+       %% * and similarly when given a different seed.
+       Check <- [Seeded(Prop)]].
+
 setup_prop() ->
     ?SETUP(fun () ->
 		   put(setup_token, true),