Okay, I'm using Meck and I'm lost. My first language (that I've been writing for about 7 months) is Ruby, so I can't seem to wrap my brain around Meck mocking yet. I do get Ruby mocking though. Hoping someone can help me. Also, I've only been writing Erlang for a week.
Updated Code (but mocking still isn't working)...
I have a Erlang console_io prompter module that looks like this:
-module(prompter).
-export([prompt/1, guess/0]).
prompt(Message) ->
console_io:gets(Message).
gets() ->
{_, [Input]} = io:fread("Enter: ", "~s"),
Input.
guess() ->
Guess_Input = gets(),
Guess_List = convert_guess_to_list(Guess_Input).
convert_guess_to_list(Guess_Input) ->
re:split(Guess_Input, "", [{return, list}, trim]).
My test now looks like this:
-module(prompter_test).
-include_lib("eunit/include/eunit.hrl").
guess_1_test() ->
meck:new(prompter),
meck:expect(prompter, gets, fun() -> "aaaa" end),
?assertEqual(prompter:guess(), ["a","a","a","a"]),
?assert(meck:validate(prompter)),
meck:unload(prompter).
The error I'm getting is this:
Eshell V5.9.3.1 (abort with ^G)
1> prompter_test: guess_1_test (module 'prompter_test')...*failed*
in function prompter:guess/0
called as guess()
in call from prompter_test:guess_1_test/0 (test/prompter_test.erl, line 10)
in call from prompter_test:guess_1_test/0
**error:undef
I want to mock (stub?) the gets function in my test so that gets will return "aaaa" and then when I assert on get_guess() it should equal ["a", "a", "a", "a"].
How do I do this?
There are two problems:
The prompter module has two exported functions, but you only mock one of them (gets) with meck:expect. By default, Meck creates a new module that only contains the functions that you explicitly mock. You can change that by using the passthrough option:
meck:new(prompter, [passthrough]),
When you mock the gets function, all module-prefixed calls (i.e. prompter:gets()) are intercepted, but Meck has no way (yet?) of intercepting internal calls (e.g. the gets() call in the guess function), so you would still get the unmocked version of the function. There is no completely satisfactory way to avoid this. You could change the call in guess to prompter:gets(), or you could move gets into a separate module and mock that.
The first line says to create a new mocked module, my_library_module:
meck:new(my_library_module),
Next, we mock the function fib in my_library_module to return 21 when 8 is passed in:
meck:expect(my_library_module, fib, fun(8) -> 21 end),
We have some eunit assertions to test our mocked function. The code_under_test:run call is what you want to replace with the the function using your mocked module, and the 21 is the result you are expecting from the function call:
?assertEqual(21, code_under_test:run(fib, 8)), % Uses my_library_module
?assert(meck:validate(my_library_module)),
Then we unload the mocked module:
meck:unload(my_library_module).
If you wanted to write the same test for your module, you could write:
my_test() ->
meck:new(console_io),
meck:expect(console_io, gets, fun() -> "aaaa" end),
?assertEqual(["a", "a", "a", "a"], console_io:get_guess()), % Uses console_io
?assert(meck:validate(console_io)),
meck:unload(console_io).
Related
I'm trying to meck out a call to application:get_env, but I'm testing a function that calls it with two different sets of arguments.
I setup two separate meck:expect calls like this, but when the function I'm testing tries to call application:get_env(my_app, my_param_one) it fails and throws error:function_clause with undefined.
meck:expect(application, get_env, fun(my_app, my_param_one) -> {ok, "my_value_one"} end),
meck:expect(application, get_env, fun(my_app, my_param_two) -> {ok, "my_value_two"} end),
How can I meck out the same function, with differents sets of parameter values?
The second meck is overwriting the first.
Whenever you define a function in Erlang by pattern-matching on different sets of values, you separate each set of patterns with a semicolon, like this:
-module(sample).
-export([create_greeting/2]).
create_greeting("Grant", _Greeting) -> io:format("Good day sir!");
create_greeting(Name, Greeting) -> io:format("~s, ~s!", [Greeting, Name]).
Similarly, when you setup your meck statements, define the function once, but separate each set of patterns/arguments with a semicolon, like this:
meck:expect(application, get_env, fun(my_app, my_param_one) -> {ok, "my_value_one"};
(my_app, my_param_two) -> {ok, "my_value_two"}
end),
As a newcomer to meck, I've been putting together a test that shows the various features. I cannot, however, understand why a developer might call meck:validate. Here's my example:
-module(meck_demo).
-include_lib("eunit/include/eunit.hrl").
validate_is_of_limited_use_test_() ->
{ foreach, fun setup_mock/0, fun cleanup_mock/1,
[fun validate_does_not_fail_if_a_function_is_not_called/0,
fun validate_does_not_fail_if_a_function_is_called_with_wrong_arity/0,
fun validate_does_not_fail_if_an_undefined_function_is_called/0,
fun validate_does_fail_if_a_function_was_called_with_wrong_argument_types/0,
fun validate_does_fail_if_expectation_throws_an_unexpected_exception/0 ]}.
validate_does_not_fail_if_a_function_is_not_called() ->
meck:expect(womble, name, fun() -> "Wellington" end),
?assert(meck:validate(womble)).
validate_does_not_fail_if_a_function_is_called_with_wrong_arity() ->
meck:expect(womble, name, fun() -> "Madame Cholet" end),
?assertError(undef, womble:name(unexpected_arg)),
?assert(meck:validate(womble)).
validate_does_not_fail_if_an_undefined_function_is_called() ->
?assertError(undef, womble:fly()),
?assert(meck:validate(womble)).
validate_does_fail_if_a_function_was_called_with_wrong_argument_types() ->
meck:expect(womble, jump, fun(Height) when Height < 1 ->
ok
end),
?assertError(function_clause, womble:jump(999)),
?assertNot(meck:validate(womble)).
validate_does_fail_if_expectation_throws_an_unexpected_exception() ->
meck:expect(womble, jump, fun(Height) -> 42 = Height end),
?assertError({badmatch, 999}, womble:jump(999)),
?assertNot(meck:validate(womble)).
setup_mock() ->
meck:new(womble, [non_strict]).
cleanup_mock(_SetupResult) ->
meck:unload(womble).
What am I missing?
-- Updated to reflect the cases that Adam explains can be caught
You managed to hit just about every case not covered by validate (added better documentation in 10c5063).
Validate can detect:
When a function was called with the wrong argument types (function_clause)
When an exception was thrown
When an exception was thrown and expected (via meck:exception/2), which still results in true being return from meck:validate/1
Validate cannot detect:
When you didn't call a function
When you called a function with the wrong number of arguments
If you called an undefined function
The reason it cannot detect these cases is because of how Meck is implemented. Meck replaces the module with a mock and a process that maintains the mock. Everything Meck gets goes through that mock module. Meck does not insert itself at the caller level (i.e. in your module or in your test case), so it cannot know that you failed to call a module. All of the failures in your test case above never reaches the mock module in the first place.
Is there a straightforward way to convert an Erlang fun to a string? A call to io_lib:format only prints the function reference, e.g. something like "#Fun<erl_eval.20.67289768>". For example, I'd like to be able to do this:
1> Fun = fun() -> atom_to_list('hello world') end.
2> FunStr = fun_to_str(Fun).
"fun() -> atom_to_list('hello world') end."
I'm looking for how to implement fun_to_str. In javascript, some interpreters have a .toSource() function that can be called on any object, including functions, that print their string representation. Any info is appreciated, thanks.
First, get the environment variables for the fun (which includes the abstract code):
1> {env, [{_, _, _, Abs}]} = erlang:fun_info(Fun, env).
{env,[{[],
{eval,#Fun<shell.21.83096281>},
{value,#Fun<shell.5.83096281>},
[{clause,1,[],[],
[{call,1,{atom,1,atom_to_list},[{atom,1,hello_world}]}]}]}]}
Pretty print the abstract code using erl_pp:
3> Str = erl_pp:expr({'fun', 1, {clauses, Abs}}).
[[[["fun",
[[[[["()"]," ->"],
["\n ",
[["atom_to_list",[[40,["'hello world'",41]]]]]]]]]]],
[10,["end"]]]]
4> io:format([Str|"\n"]).
fun() ->
atom_to_list('hello world')
end
ok
(You have to add {'fun', 1, {clauses, ...}} around it to make it a complete Erlang expression)
You might be able to use erlang:fun_info/2 for that, atleast i get some information from the shell when doing
1> erlang:fun_info(fun() -> test,ok end, env).
{env,[[],
{value,#Fun<shell.7.37281544>},
{eval,#Fun<shell.24.85590193>},
[{clause,1,[],[],[{atom,1,test},{atom,1,ok}]}]]}
2>
You want the last list with the clause atom and then pretty print it using for instance erl_pp
I want to call xyz with the name of a function to be invoked.
-module(sample).
-export([xyz/1]).
xyz(Name) -> Name().
p() -> "you called p".
g() -> "you called g".
But I get the following error:
1> c(sample.erl).
./sample.erl:6: Warning: function p/0 is unused
./sample.erl:7: Warning: function g/0 is unused
{ok,sample}
2> sample:xyz('p').
** exception error: bad function p
in function sample:xyz/1
3>
It is correct that you have to export p and g. You can then use apply/3 to call it.
erlang:apply(sample, p, []).
Only fun-values are usable with the Fun(...) syntax. You are passing in an atom-value. An atom is a 'bad function' as the error message go. You could do something similar to
xyz(p) -> fun p/0;
xyz(g) -> fun g/0.
Then go ahead and call
Fun = xyz(p),
Fun()
-module(sample).
-export([xyz/1, p/0, g/0]).
xyz(Name) -> ?MODULE:Name().
p() -> "you called p".
g() -> "you called g".
1> sample:xyz(p).
"you called p"
Pattern match is the idiom to use:
-module(sample).
-export([xyz/1]).
xyz(p) -> p();
xyz(q) -> g().
p() -> "you called p".
g() -> "you called g".
If you want to be dynamic you can use a gen_event server.
Essentially what this is is a server that holds a state which consists of key/function pair like so:
[{p, #func1},
{g, #func2},
{..., ...},
...]
You can then essentially bind events to functions. (there is, needless to say, a bit more to it than that.
The easiest way to do is to try exporting p and g along with xyz.
-export([xyz/1, p/0,g/0]).
After exporting the function p and g can be called as follows :
1> sample:xyz(fun sample:p/0).
"you called p"
2> sample:xyz(fun sample:g/0).
"you called g"
Another way to look at it is that (depending on the problem you're solving) dynamic calls to functions isn't necessarily the right approach. Given that processes and message passing are the way you organize your code in Erlang since it's a "concurrency oriented language", maybe you could just use message passing with a selective receive rather than mimicking idioms of a sequential language? Send a message for what you want and get the custom reply based on that. It's about the result of each function, not the function itself, after all. (Plus there's the flexibility and scalability of message passing, etc.)
Although processes aren't totally free compared to calling from a library module, Erlang-level processes are dirt cheap (especially if the message communication is within the same node). They're not OS-level processes. The overhead would be comparable (or better) to dynamic function calls and object instantiation in heavier scripting languages.
The following code gives me an error: "syntax error before: Some_ets"
-module(tut).
-export([incr/1]).
Some_ets = ets:new(?MODULE, [bag]).
incr(X) ->
X+1.
But I am able to declare the ETS within a function, like:
-module(tut).
-export([incr/1]).
incr(X) ->
Some_ets = ets:new(?MODULE, [bag]),
X+1.
Can't I declare a ETS outside a function?
No - unlike other languages there isn't a concept of static initialization - there's no appropriate time for an Erlang system to execute that piece of code.
Erlang does have the concept of a parameterized module however, and that may be what you're after. Have a look here http://www.lshift.net/blog/2008/05/18/late-binding-with-erlang which is a good write up of that - it would allow you to instantiate an "instance" of your tut module bound to a given ets table and save passing around that handle explicitly in your module function calls.
Or if you are into OTP you could have the handle to the ets table passed around in the state variable:
init(_) ->
Some_ets = ets:new(?MODULE, [bag]),
{ok, Some_ets}.
and then use it in your handle_call methods:
get_ets_handle() ->
gen_server:call(?MODULE, {getETSHandle}, infinity).
handle_call({getETSHandle}, _From, Some_ets) ->
{reply, Some_ets, Some_ets}.
You can't do variable assignments like that in a module. See here.