Re: [Chicken-users] Handling Errors

[Bahman Movaqar]

On 12/14/2014 05:13 PM, Andy Bennett wrote:
> Hi,
>
>>   (define (foo lis) (cdr lis))
>>
>> Obviously LIS should neither be an empty list nor a list with only one
>> element.
>> What is the recommended way to deal with such constraints? Should I
>> enforce them by writing conditionals at the beginning of the
>> function[1]? Or is it conventional to just let the function fail and
>> catch the error somewhere else (probably in a top-level function)?
>>
>> [1] This quickly leads to dirty and not-easy-to-read code.
>>
>> What say you seasoned schemers on this?
> That depends on the scope of the accessor. I often use these kinds of
> things for quick and dirty data structures for passing seeds for fold
> around. In that case I use the even more concise '(define foo cdr)'.
>
> In the case where other modules are using the API then I might go to
> more trouble but then you have to think about whether, in the error cases:
>
>  + To return successfully with a sentinel value.
>    This is valid in some cases, depending on the data semantics. It can
>    be implemented using the conditionals that you mention.

This one is really tricky. Not only pollutes the API code but also it
also imposes some untidiness on the client side too which has to check
if a "sentinel" value has been returned or not.

>
>  + To use assert at the head of the procedure to validate the data
>    structure. This doesn't stop the error but it gives a more debuggable
>    message.

By the gods! Why did I think that "assert"s only belong to "modern'ish"
languages? :-)
Assuming you're talking about this
(http://api.call-cc.org/doc/chicken/special-forms/assert), I believe
this is the best option. Keeps the code as clean as possible while
ensuring operational soundness.


>
>  + To catch and re-throw the exception.
>    This is fraught with anti-patterns, is a lot of work and you have to
>    do it really carefully. I hardly ever do this.

Never! :-)

Thanks for the detailed answer. Much appreciated.

-- 
Bahman Movaqar

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