1. ERLYMOCK TUTORIAL
By chaoslawful

2. What’s ErlyMock?
 A mocking framework for Erlang
 Mock any method in any modules
 Various features:
 Argument verification
 Call order verification
 Divergence:
 Sheyll’s: http://erlymock-site.sourceforge.net
 Nialscorva’s:
https://github.com/nialscorva/erlymock

3. Looking down from 30,000ft
high
 Sheyll’s version:
M = em:new(),
em:strict(M, module, function, [arg, em:any()], {return, ok}),
em:stub(M, module, function2, [1, 2, 3], {function, fun (_) -> throw(myerror)
end}),
% replaying phase
em:replay(M),
% test
?assertMatches(ok, module:function(arg, anything)),
?assertThrow(myerror, module:function2(1, 2, 3)),
em:verify(M).

4. Looking down from 30,000ft
high
 Nialscorva version:
erlymock:start(),
erlymock:strict(module, function, [arg, '_'], [{return, ok}]),
erlymock:stub(module, function2, [1, 2, 3], [{throw, myerror}]),
% initialize
erlymock:replay(),
% test
?assertMatches(ok, module:function(arg, anything)),
?assertThrow(myerror, module:function2(1, 2, 3)),
erlymock:verify().

5. ErlyMock(sheyll’s) Funcs
 em:new/0 – Create new mock expectation instance (gen_fsm proc)
 em:strict/4,5 – Strict call expectation
 Strict calls must occur once for each, and in the order of declarations.
 em:strict/4 mocked func always return atom ‘ok’
 em:strict/5 can specify return value with:
 {return, V} – Mocked func return value V
 {function, F} – Mocked func call function F, and use its return value as return value
 em:stub/4,5 – Stub call expectation
 Stub calls can occur any times, and in any order.
 Mocked func return value can be specified as above.
 em:replay/1 – Finish expectation programming phase, start replaying phase
 Must be called before actual test code and after expectation declarataions.
 em:verify/1 – Verify expectations, and destroy mock instance
 Must be called after test code.
 em:any/0 – Return wildcard argument verifier
 em:nothing/2 – Demand no funcs in the specified module can be called

6. ErlyMock(nialscorva’s) Funcs
 erlymock:start/0 – Start mock service (gen_server proc)
 erlymock:strict/3,4 – Strict call expectation
 Strict calls must occur once for each, and in the order of declarations.
 erlymock:strict/3 mocked funcs always return atom ‘ok’
 erlymock:strict/4 can specify return value with (put in a list):
 {return, V} – Mocked func return value V
 {throw, V} – Mocked func call erlang:throw/1 with reason V
 {exit, V} – Mocked func call erlang:exit/1 with reason V
 {error, V} – Mocked func call erlang:error/1 with reason V
 {function, F} – Mocked func call function F, and use its return value as return value
 erlymock:replay/0 – Finish expectation programming, start replaying phase
 Must be called before actual test code and after expectation declarataions.
 erlymock:verify/0,1 – Verify expectations, and destroy mock instance
 Must be called after test code.
 erlymock:verify/1 can specify timeout for verification process.

7. ErlyMock(nialscorva’s) Funcs,
cont.
 erlymock:stub/3,4 – Stub call expectation
 Stub calls can occur any times, and in any order.
 Mocked func return value can be specified as above.
 Can retrict minimum and maximum invocation times of
mocked func with options:
 {min_invocations, Count} – Specify min invocation times,
default to 0.
 {max_invocations, Count} – Specify max invocation times,
default to infinity.
 erlymock:o_o/3,4 – Out of order call expectation
 Out of order calls are simply stub calls with
min_invocation=max_invocation=1, i.e. call exactly once in
any order.
 Mocked func return value can be specified as above.

8. Mock example – Missile
Launcher
 Two layer:
 launch_console– Interact with missile operator
 launcher – Called by console, control missile
hardware
 Need test launch_console:launch/0, it must comply to
the following restrictions:
 Must call launcher:confirm/0 first
 If launcher:confirm/0 return false, nothing to do
 If launcher:confirm/0 return true, call launcher:launch/2
with fixed coordinate
 Time must be retrieved through launcher:time/0

9. Mock example - Missile
Launcher
 Mocking test cases:
mock_test1() ->
M = em:new(),
em:stub(M, launcher, time, [], {function, fun () -> Old = case get(mock_time) of undefined -> 0; V -> V end, put(mock_time, Old+1), Old
end}),
em:stub(M, launcher, launch, [em:any(), em:any()], {function, fun ()->throw(should_not_happen) end}),
em:strict(M, launcher, confirm, [], {return, false}),
em:replay(M),
launch_console:launch(),
em:verify(M).
mock_test2() ->
Lat = 33.8, Lon = 45.0,
M = em:new(),
em:stub(M, launcher, time, [], {function, fun () -> Old = case get(mock_time) of undefined -> 0; V -> V end, put(mock_time, Old+1), Old
end}),
em:strict(M, launcher, confirm, [], {return, true}),
em:strict(M, launcher, launch, [Lat, Lon]),
em:replay(M),
launch_console:launch(),
em:verify(M).

10. Commons between impls
 Use beam code hot swapping mechanism to
implement module mocking.
 See compile:forms/1, code:purge/1, code:delete/1 and
erlang:load_module/2
 Note:
 Mocking must happened for entire module, restricted by
this impl method. Mocking partial funcs in a module is
netiher possible nor desirable.
 Can’t mock the same module in parallel running test cases.
 Mocked funcs are generated on the fly in memory,
according to declared expectations.
 See erl_syntax module

11. Sheyll’s compare to
Nialscorva’s
 Pros
 Can recover coverage data after mocking finished
 Shortter module name ;-)
 Support multiple expectation instance (can parallel run test cases if using
different mock module)
 Implemented in gen_fsm, cleaner than Nialscorva’s
 With ‘nothing’ expectation
 With ‘any’ arg verifier, express wildcard matching in cleaner way
 Cons
 Can’t restrict stub call min/max invocation times
 No out of order call expectation (due to above reason)
 No express way to specify throw/exit/error return value
 No built-in TCP server mocking support (can be done with self-connected socket
pair)
 No short-cut wildcard arg verifier ‘_’ (but also without its ambiguity)
 Use maven instead of rebar as building tools, not very erly…

12. Personal thoughts
 Neither impl are good enough…
 For now, recommend Sheyll’s impl prior to
Nialscorva’s
 We can contribute our efforts to make Sheyll’s
impl better, it’s not so hard.
 Sheyll’simpl: 641 lines
 Nialscorva’s impl: 843 lines

13. That’s all, folks!
Q&A