[erlang-questions] string:substr/2 gives confusing error message

Robert Virding <>
Fri Dec 5 14:50:42 CET 2008


I think in cases like this you will find that the person who wrote the
module (me most likely) was just trying to be kind and return something
"sensible" if the arguments are of the right type and not too crazy. Like
asking for more characters from a list than exists.

I have now thought this over and come to the conclusion that you probably
shouldn't be kind but generate an error in these situations. Being kind will
often come back and bite you later in an unpleasant way.

I personally don't see any problem with a function_clause error as these
means that there was a bad argument. It could be helpful if the error came
from the function which is called, but as long as it comes from a function
which is cleary related to the call, as in this case, I don't worry about
it.

How to return errors is a difficult problem. If you catch them at a higher
level how do you then return a helpful error? And helpful fo whom?

Robert

2008/12/5 Valentin Micic <>

> I think that depends how you want to look at it...
> One may argue that
>        string:substr([], 2 )
> does not make much sense as a statement, but then, I'd say that:
>        string:substr("1", 2 )
> does not make to much sense either, and yet it returns [].
> IMO, both functions should behave the same way, and to be pragmatic, return
> an empty list (not a badarg exception).
>
> V.
>
>
> -----Original Message-----
> From: 
> [mailto:] On Behalf Of Vlad Dumitrescu
> Sent: 05 December 2008 12:12 PM
> To: erlang-questions
> Subject: [erlang-questions] string:substr/2 gives confusing error message
>
> Hi!
>
> Calling string:substr/2 with erroneous arguments gives a
> non-informative error message
>
> ()1> string:substr([], 2).
> ** exception error: no function clause matching string:substr2([],2)
>
> Shouldn't it be badarg instead, or something similar?
>
> Also, the strings module documentation doesn't mention what are the
> cases when the function will fail. (A quick glance shows that there
> are other functions in the same situation).
>
> best regards,
> Vlad
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