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. + 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. + 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. Regards, @ndy -- andy...@ashurst.eu.org http://www.ashurst.eu.org/ 0x7EBA75FF
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