[erlang-questions] feedback please

[Roelof Wobben]

Op 24-9-2015 om 08:04 schreef Roelof Wobben:
> Op 24-9-2015 om 07:56 schreef zxq9:
>> On Thursday 24 September 2015 07:41:40 Roelof Wobben wrote:
>>> Thanks,
>>>
>>> You are right. I did take a quick look and almost the whole chapter is
>>> try .. catch.  So match or chrash is better ?
>> It is not so much of a matter of which is "better" -- but that *most* 
>> of the time you want to follow the mantra of "let it crash" and this 
>> is a very natural fit for Erlang. For example, if I start a process 
>> that is supposed to read some configuration settings in its init/1 
>> section before it gets going, and the config data is all screwed up 
>> (wrong format, maybe its not even the right data -- like binary image 
>> data instead of a settings file or something) then I don't want to 
>> try..catch and then write extra code to force the process into some 
>> semi-workable state and carry on. I want to just crash right away.
>>
>> In development this might just be something that explodes in my 
>> terminal. BOOM! And then I know I need to look at something right away:
>> "Why the devil am I sending image data where a file name is supposed 
>> to go?"
>>
>> In production it may be an indicator that some remote resource you 
>> normally query has changed its API:
>> "Oh boy, our [very important to your business] service changed their 
>> protocol unannounced... again...".
>>
>> In either case the last thing you want to do is have the processes 
>> silently carry on and crash *other* processes (or other services!) 
>> *way later* for reasons that are now at least one degree removed from 
>> the actual problem.
>>
>> But occasionally you will want a try..catch. Sometimes you just want 
>> to be able to stay alive long enough to log a very specific/detailed 
>> error in a natural way; sometimes you are dealing with an unknown 
>> platform and interacting with a user at the same time, and you're not 
>> sure what sort of data you might receive -- but if the data is bad 
>> you need to stay alive long enough to tell the user about it in 
>> detail. In cases like these you *can* accomplish this without 
>> try..catch, but its really annoying. In most cases, though, 
>> try..catch is much more annoying (and downright dangerous compared to 
>> crashing immediately).
>>
>> In my experience try..catch is something you'll know when you need, 
>> and that need is infrequent. Almost any time you use try..catch you 
>> are not writing for the "success case" (or "happy path") -- the rare 
>> times that try..catch fits into the "happy path" not having it would 
>> be an annoying handicap.
>>
>> So its not that "crashing is better than try..catch" so much as 
>> "standard Erlang practice is to crash on faults, and try..catch is a 
>> very useful tool in some specific cases", so you do need to know 
>> about it.
>>
>> -Craig
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>
> Thanks for the explanation. it's very clear what point you try to make.
>
> Roelof
>
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On question pops into my mind.

You have this code for input the amount :

ask_amount() ->
     Input = io:get_line("How much? "),
     case scrub_amount(Input) of
         {ok, Number} ->
             Number;
         {error, bad_input} ->
             ok = io:format("Hrm... '~tp' does not seem to be a number~n"
                            "Now, one more time...~n",
                            [Input]),
             ask_amount()
     end.

If I want to check if the input is postive do I need to put a if then 
into  the {ok, number} piece.

Roelof