[erlang-questions] Debug support on for guards?

[Fred Hebert]

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>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
>
>
>