?LET macro throws 'proper_transformer' exception when including a markdown file with `-moduledoc`.

[sthames42]

This is my first experience with proper and Property Based Testing so this may be about my inexperience.

The Code

-module(my_module).
-moduledoc({file, "../doc/my_module.md"}).

...

%*------------------------------------------------*
%* `proper.hrl` must be included before functions *
%* because it contains `-import` attributes.      *
%*------------------------------------------------*
-ifdef(TEST).
-include_lib("proper/include/proper.hrl").
-include_lib("eunit/include/eunit.hrl").
-endif.

...

%*******************************************************************************
% Tests
%*******************************************************************************
-ifdef(TEST).

int_range_test() -> 
  ?assert(proper:quickcheck((fun() ->
  numtests(1000, 
    ?FORALL({From, To, Value}, ?LET(S, oneof([integer(), undefined]), {S, S, S}),
      begin
        Result   = ?TEST_VALIDATOR({int_range, {From, To}}, Value),
        Expected = 
          if 
            not is_integer(From)           -> {error, ?ERR_BAD_RANGE};
            not is_integer(To)             -> {error, ?ERR_BAD_RANGE};
            not is_integer(Value)          -> {error, ?ERR_NOT_IN_RANGE};
            Value < From orelse Value > To -> {error, ?ERR_NOT_IN_RANGE};
            true -> ok
          end,
        ?WHENFAIL(io:format(user, "int_range_test() failed~nFrom: ~p, To: ~p, Value: ~p~nExpected: ~p~nResult:   ~p~n~n", 
                            [From, To, Value, Expected, Result]), Result =:= Expected)
      end))
  end)(), [{on_output, fun(_,_) -> ok end}])).

-endif.

The Exception

> rebar3 eunit -m mymodule
===> Verifying dependencies...
===> Analyzing applications...
===> Compiling proper
===> Analyzing applications...
===> Compiling myapp
===> Compiling src/my_module.erl failed
src/my_module.erl:none: error in parse transform 'proper_transformer':
exception error: no case clause matching {epp_reply,<0.512.0>,{eof,{291,8}}}
  in function  proper_transformer:rewrite_type/2 (.../_build/default/lib/proper/src/proper_transformer.erl, line 364)
  in call from proper_transformer:rewrite_expr/2 (.../_build/default/lib/proper/src/proper_transformer.erl, line 277)
  in call from proper_transformer:'-rewrite_expr/2-lc$^4/1-1-'/2 (.../_build/default/lib/proper/src/proper_transformer.erl, line 288)
  in call from proper_transformer:'-rewrite_expr/2-lc$^4/1-1-'/2 (.../_build/default/lib/proper/src/proper_transformer.erl, line 288)
  in call from proper_transformer:rewrite_expr/2 (.../_build/default/lib/proper/src/proper_transformer.erl, line 288)
  in call from proper_transformer:'-rewrite_clause/2-lc$^1/1-1-'/2 (.../_build/default/lib/proper/src/proper_transformer.erl, line 234)
  in call from proper_transformer:rewrite_clause/2 (.../_build/default/lib/proper/src/proper_transformer.erl, line 234)
  in call from proper_transformer:'-rewrite_expr/2-lc$^17/1-10-'/2 (.../_build/default/lib/proper/src/proper_transformer.erl, line 335)

The Analysis

1. Removing this line at the top of the file fixes the problem and tests are successful:

   -moduledoc({file, "../doc/my_module.md"}).

   It doesn't matter what is in the Markdown file. Even including an empty file results in the error.

2. Replacing the -moduledoc() line with:

   -moduledoc """
     This is a great module!
   """.

   Also eliminates the error.

3. The int_range_test() uses the ?LET macro to set the testing values:

   int_range_test() -> 
     ?assert(proper:quickcheck((fun() ->
     numtests(1000, 
       ?FORALL({From, To, Value}, ?LET(S, oneof([integer(), undefined]), {S, S, S}), ...

   Replacing the ?LET macro eliminates the exception:

   int_range_test() -> 
     Random = fun() -> oneof([integer(), undefined]) end,
     ?assert(proper:quickcheck((fun() ->
     numtests(1000, 
       ?FORALL({From, To, Value}, {Random(), Random(), Random()}, ...

   FWIW, I misunderstood how ?LET works and now realize {S, S, S} was giving all the same values for S. So this needed to be fixed, anyway.

Conclusion

It looks like there is a problem with compilation of the ?LET macro when an external file is included using -moduledoc().

I hope this helps with the diagnosis.