[erlang-questions] The display/1Gotcha

[Olivier BOUDEVILLE]

Hi Kenneth, Hi Lukas,

Thanks for your answers.  At first I was not able to reproduce the issue 
on a minimal example, but, as you both hinted, original code was correct 
and should not have led to a local function being shadowed by a 
"built-in": apparently the culprit is the smart_exception parse transform.

At least this is what is suggested by:

"""
-module(minimal_test).

-export([ run/0, display/1 ]).

run() ->
     display( "Smart exceptions do not seem to work properly." ).

display( T ) ->
     io:format( "My function was called, really?!?~n" ),
     throw( {called_with,T} ).
"""

Then a vanilla compilation works as expected, whereas the one with 
smart_exception behaves wrongly:

"""
> erl
Erlang R14B01 (erts-5.8.2) [source] [64-bit] [smp:8:8] [rq:8] 
[async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.8.2  (abort with ^G)
1> c(minimal_test).
{ok,minimal_test}
2> minimal_test:run().
My function was called, really?!?
** exception throw: {called_with,"Smart exceptions do not seem to work 
properly."}
     in function  minimal_test:display/1
3> c(minimal_test,[{parse_transform, smart_exceptions}]).
{ok,minimal_test}
4> minimal_test:run().
"Smart exceptions do not seem to work properly."
true
"""

It looks like that, unless in my code I use ?MODULE:display/1, 
erlang:display/1 will be called instead if smart exceptions are used.

(by the way, I am using the devel version of smart_exception that I 
downloaded a month ago, since there were an issue with the stable version 
as well, as described in 
http://erlang.2086793.n4.nabble.com/Problems-With-Smart-Exceptions-td3095862.html)

So, unless I am mistaken, I suppose smart exceptions should better be 
disabled? 
This is a bit of a pity as it is a quite nice feature, useful to avoid 
additional intellectual puzzles.

Best regards,

Olivier Boudeville.
---------------------------
Olivier Boudeville

EDF R&D : 1, avenue du Général de Gaulle, 92140 Clamart, France
Département SINETICS, groupe ASICS (I2A), bureau B-226
Office : +33 1 47 65 59 58 / Mobile : +33 6 16 83 37 22 / Fax : +33 1 47 
65 27 13



kenneth.lundin@REDACTED 
25/01/2011 17:09

A
olivier.boudeville@REDACTED
cc

Objet
Re: [erlang-questions] The display/1Gotcha






Hi,

I don't understand what you mean please provide a complete example.

If we for example have this module

-module(dtest).

-export([f/0]).

f() -> display("String").

display(String) ->
  {local_display_called_with,String}.

Then in both R13B04 and in R14A (when we introduced a change in semantics
so that autoimported BIFs no longer override local functions) the local 
function
named display will be called when you call dtest:f().

Also note that erlang:display/1 is not an autoimported BIF. You must 
always
call it giving the modulename erlang.

/Kenneth Erlang/OTP Ericsson

On Tue, Jan 25, 2011 at 4:32 PM, Olivier BOUDEVILLE
<olivier.boudeville@REDACTED> wrote:
> Hi,
>
> Wouldn't it be nice if erlang:display/1 did not silently shadow any
> user-defined display/1? I have been bitten more than once by this.
>
> Currently, defining in a module display/1 and calling it from another
> function directly, without mentioning a module (ex: 'f(Term) ->
> display(Term).') will call erlang:display/1instead of calling the
> lesser-astonishing ?MODULE:display/1.
>
> As erlang:display/1is not so very well-known and display is a rather
> common verb, maybe this would deserve forcing its call with an explicit
> module prefix (i.e. 'erlang:display(Term)'; if it is possible not to
> auto-import it) or at least issue a warning like "warning: local 
function
> display/1 will be shadowed by erlang:display/1"?
>
> Best regards,
>
> Olivier Boudeville.
> ---------------------------
> Olivier Boudeville
>
> EDF R&D : 1, avenue du Général de Gaulle, 92140 Clamart, France
> Département SINETICS, groupe ASICS (I2A), bureau B-226
> Office : +33 1 47 65 59 58 / Mobile : +33 6 16 83 37 22 / Fax : +33 1 47
> 65 27 13
>
>
>
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