[erlang-questions] Debug support on for guards?

[Henning Diedrich]

Thanks a lot,

also for clarifying the use of throw and error on the occasion!

Henning

Fred Hebert wrote:
> Untested:
>
> milli_epoch({Mega, Sec, Micro}) when
>   is_integer(Mega), is_integer(Sec),  
>   is_integer(Micro),
>   Mega >= 0, Sec >= 0, Micro >= 0 ->
>    Mega * ?BILLION + Sec * 1000 + trunc(Micro / 1000);
> milli_epoch({Mega, Sec, Micro}) when
>   is_integer(Mega), is_integer(Sec), is_integer(Micro) ->
>    erlang:error(negative_input).
>
> Notice that I used erlang:error/1 rather than throw/1. The reason is 
> that throw/1 is meant to  be used mainly for control flow or very 
> common exceptions you know how to fix, and should never be seen by the 
> programmer -- they ought to be restricted to the module where they 
> happen. Errors represent a piece of code you want to fail but you 
> can't expect to fix without changing the code.
>
> On Sun, May 2, 2010 at 3:52 AM, Henning Diedrich <hd2010@REDACTED 
> <mailto:hd2010@REDACTED>> wrote:
>
>     Hi folks,
>
>     is there a way to add detail to a function_clause error messages?
>     Be it guards or patterns that don't match?
>
>     On a crash, I would like to clarify what guard failed, or even
>     warn the programmer that this guard exists when it may not be
>     intuitive.
>
>     1
>
>     This is written, I believe, as it should:
>
>     milli_epoch({Mega, Sec, Micro}) when
>       is_integer(Mega), is_integer(Sec), is_integer(Micro),
>       Mega >= 0, Sec >= 0, Micro >= 0 ->
>        Mega * ?BILLION + Sec * 1000 + trunc(Micro / 1000).
>
>     2
>
>     This fails the way I would like it to, giving "negative_input" as
>     reason.
>
>     But it is written wrong for two (2) counts I believe (I am NOT
>     trying to sneak in a point about ifs):
>
>     milli_epoch({Mega, Sec, Micro}) when is_integer(Mega),
>     is_integer(Sec), is_integer(Micro) ->
>
>       if (Mega >= 0), (Sec >= 0), (Micro >= 0) -> ok;
>       true -> throw(negative_input) end,
>         Mega * ?BILLION + Sec * 1000 + trunc(Micro / 1000).
>
>     Thank you very much for taking a look,
>
>     Henning
>
>
>