Question

I’m currently working on my first major project in clojure and have run into a question regarding coding style and the most “clojure-esque” way of doing something. Basically I have a function I’m writing which takes in a data structure and a template that the function will try to massage the data structure into. The template structure will look something like this:

{
:key1 (:string (:opt :param))
:key2 (:int (:opt :param))
:key3 (:obj (:tpl :template-structure))
:key4 (:list (:tpl :template-structure))
}

Each key is an atom that will be searched for in the given data structure, and it’s value will be attempted to be matched to the type given in the template structure. So it would look for :key1 and check that it’s a string, for instance. The return value would be a map that has :key1 pointing to the value from the given data structure (the function could potentially change the value depending on the options given).

In the case of :obj it takes in another template structure, and recursively calls itself on that value and the template structure, and places the result from that in the return. However, if there’s an error I want that error returned directly.

Similarly for lists I want it to basically do a map of the function again, except in the case of an error which I want returned directly.

My question is what is the best way to handle these errors? Some simple exception handling would be the easiest way, but I feel that it’s not the most functional way. I could try and babysit the errors all the way up the chain with tons of if statements, but that also doesn’t seem very sporting. Is there something simple I’m missing or is this just an ugly problem?

Answer 1

You might be interested in schematic, which does pretty similar stuff. You can see how it’s used in the tests, and the implementation.

Basically I defined an error function, which returns nil for correctly-formatted data, or a string describing the error. Doing it with exceptions instead would make the plumbing easier, but would make it harder to get the detailed error messages like "[person: [first_name: expected string, got integer]]".