[Chicken-users] Handling Errors
Consider the following simple function: (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? -- Bahman Movaqar http://BahmanM.com - https://twitter.com/bahman__m https://github.com/bahmanm - https://gist.github.com/bahmanm PGP Key ID: 0x6AB5BD68 (keyserver2.pgp.com) signature.asc Description: OpenPGP digital signature ___ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
Re: [Chicken-users] Handling Errors
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 signature.asc Description: OpenPGP digital signature ___ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
Re: [Chicken-users] Handling Errors
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 asserts 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 http://BahmanM.com - https://twitter.com/bahman__m https://github.com/bahmanm - https://gist.github.com/bahmanm PGP Key ID: 0x6AB5BD68 (keyserver2.pgp.com) signature.asc Description: OpenPGP digital signature ___ Chicken-users mailing list Chicken-users@nongnu.org https://lists.nongnu.org/mailman/listinfo/chicken-users
