Avoiding false Dialyzer warnings for NIFs

[Kostis Sagonas]

Björn Gustavsson wrote:
> I have implemented a new erlang:nif_error/1 BIF that can
> be used in the stub for NIFs to avoid false Dialyzer
> warnings:
> 
> http://github.com/bjorng/otp/commit/826c9574776ae931917e55d441ec0cfcb0acaf84
> 
> In a second commit, I have modified the crypto
> module to use erlang:nif_error/1.
> 
> I have run the crypto test suite, but I have not yet
> verified that the false Dialyzer warnings disappear.
> 
> Feedback welcome.

Bjorn,

I've verified that your patch is OK and does what you want it to as far 
as dialyzer is concerned.

However, a slightly better addition to the type/4 function of 
erl_bif_types is the following:

=========================================================================
type(erlang, nif_error, 1, _) ->
   t_unit();	% this BIF and the next one are stubs for NIFs and never return
type(erlang, nif_error, 2, Xs) ->
   strict(arg_types(erlang, nif_error, 2), Xs, fun (_) -> t_unit() end);
=========================================================================

I suggest you use it.  Make sure you also add t_unit/0 at the end of the 
list of imported functions from erl_types (i.e., immediately after 
t_tuple_subtypes/1).


FYI, the unit() type is an internal type somewhere between any() and 
none().  It's main purpose is to be used for functions that do not 
return e.g. because they go into a loop.  However, unlike none(), this 
return type can be overwritten by a spec that says that the function 
returns some other type.

Also, the strict(...) call in the second BIF will make dialyzer catch 
accidental wrong uses of this BIF (those where the second argument to 
erlang:nif_error/2 is something other than a list).  Obviously, the BIF 
with only one argument does not need this check since the type of its 
argument is any().


For your fix to be complete, I suggest you also add -specs for all 
crypto functions that are implemented as NIFs.  With this patch, these 
can be added directly to the crypto.erl file and dialyzer will handle 
them properly.

Kostis