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Agent skill
nw-pbt-erlang-elixir
Erlang/Elixir property-based testing with PropEr, PropCheck, and StreamData frameworks
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npx add-skill https://github.com/nWave-ai/nWave/tree/main/plugins/nw/skills/nw-pbt-erlang-elixir
SKILL.md
PBT Erlang/Elixir -- PropEr, PropCheck, StreamData
Framework Selection
| Framework | Language | Stateful | Parallel | Choose When |
|---|---|---|---|---|
| PropEr | Erlang | Yes | Yes (linearizability) | Erlang projects needing full stateful/parallel testing |
| PropCheck | Elixir | Yes (via PropEr) | Yes (via PropEr) | Elixir projects needing stateful/parallel testing |
| StreamData | Elixir | No | No | Elixir projects needing only stateless PBT |
PropCheck wraps PropEr with Elixir syntax. StreamData is pure Elixir but lacks stateful testing.
Quick Start
%% PropEr (Erlang)
-include_lib("proper/include/proper.hrl").
prop_sort_preserves_length() ->
?FORALL(List, list(integer()),
length(lists:sort(List)) =:= length(List)).
%% Run: proper:quickcheck(my_module:prop_sort_preserves_length()).
Syntax Differences
| Concept | PropEr (Erlang) | PropCheck (Elixir) | StreamData (Elixir) |
|---|---|---|---|
| Import | -include_lib("proper/include/proper.hrl"). |
use PropCheck |
use ExUnitProperties |
| Property | ?FORALL(X, gen(), body) |
forall x <- gen() do body end |
check all x <- gen() do assert body end |
| Integer | integer() |
integer() |
integer() |
| List | list(integer()) |
list(integer()) |
list_of(integer()) |
| Run | proper:quickcheck(prop()) |
mix test |
mix test |
Generator Cheat Sheet (PropEr)
Primitives
integer() % any integer
integer(0, 100) % bounded
float()
binary() % binary data
boolean()
atom()
Collections
list(integer()) % list of integers
non_empty(list(integer())) % non-empty list
vector(5, integer()) % fixed-length list
{integer(), binary()} % tuple (direct syntax)
Combinators
oneof([integer(), binary()]) % union
elements([a, b, c]) % pick from list
frequency([{80, integer()}, {20, atom()}]) % weighted
%% ?LET (map/transform)
even() ->
?LET(N, integer(), N * 2).
%% ?SUCHTHAT (filter -- use sparingly)
non_empty_list() ->
?SUCHTHAT(L, list(integer()), L =/= []).
%% Nested ?LET (dependent generation)
list_and_element() ->
?LET(List, non_empty(list(integer())),
?LET(Elem, elements(List),
{List, Elem})).
Recursive
tree(Type) ->
?SIZED(Size, tree(Size, Type)).
tree(0, Type) -> {leaf, Type};
tree(Size, Type) ->
frequency([
{1, {leaf, Type}},
{5, ?LAZY({node, Type, tree(Size div 2, Type), tree(Size div 2, Type)})}
]).
Shrinking
year() ->
?SHRINK(integer(0, 9999), [integer(1970, 2000)]).
date() ->
?LETSHRINK([Y, M, D],
[integer(1, 9999), integer(1, 12), integer(1, 31)],
{Y, M, D}).
Stateful Testing (proper_statem)
-behaviour(proper_statem).
initial_state() -> #{items => #{}}.
command(#{items := Items}) ->
oneof([
{call, ?MODULE, put, [key(), value()]},
{call, ?MODULE, get, [elements(maps:keys(Items))]}
|| maps:size(Items) > 0
]).
precondition(#{items := Items}, {call, _, get, [Key]}) ->
maps:is_key(Key, Items);
precondition(_, _) -> true.
postcondition(#{items := Items}, {call, _, get, [Key]}, Result) ->
Result =:= maps:get(Key, Items);
postcondition(_, _, _) -> true.
next_state(State = #{items := Items}, _Var, {call, _, put, [Key, Val]}) ->
State#{items := Items#{Key => Val}};
next_state(State, _, _) -> State.
prop_store() ->
?FORALL(Cmds, commands(?MODULE),
begin
{History, State, Result} = run_commands(?MODULE, Cmds),
cleanup(),
Result =:= ok
end).
StreamData: No stateful testing. PropCheck: Same proper_statem callbacks with Elixir syntax.
Parallel Testing (Linearizability)
prop_store_parallel() ->
?FORALL(Cmds, parallel_commands(?MODULE),
begin
{Sequential, Parallel, Result} = run_parallel_commands(?MODULE, Cmds),
cleanup(),
Result =:= ok
end).
Swap commands for parallel_commands, run_commands for run_parallel_commands. Framework generates sequential prefix + parallel branches and checks all linearizations.
Test Runner Integration
%% PropEr: {deps, [{proper, "1.4.0"}]}. Run: rebar3 proper
%% PropCheck: {:propcheck, "~> 1.4", only: :test}. Run: mix test
%% StreamData: {:stream_data, "~> 1.0", only: :test}. Run: mix test
Unique Features
- proper_fsm: Finite state machine testing module (distinct from generic state machines)
- PULSE integration: Controlled scheduling for parallel test race detection
- Symbolic references: First-class support with two-phase (abstract + concrete) execution
- collect/aggregate: Built-in distribution analysis for generator quality verification
- Linearizability checking: One of the most mature implementations in any PBT framework
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