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Re: [Chicken-users] Handling Errors

From: Andy Bennett
Subject: Re: [Chicken-users] Handling Errors
Date: Sun, 14 Dec 2014 13:43:45 +0000
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>   (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.

 + 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

 + 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.



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