This is the eighth in a series of articles of me writing up my notes from from RailsConf Europe 2006. They are all first drafts, probably technically inaccurate and slanderously misquoting people. Let me know and I’ll fix them. You can follow this series of posts by looking at articles in the RailsConfEurope category.
Jim was talking about safe programming practices in Ruby, in particular for library writers where their code will be reused by others. Probably best to just summarise his tips:
Use namespaces for libraries. In fact, use namespaces for everything you write. For example, how many libraries define a class called
Node? Lots.Don’t use generic project names or namespaces since they are prone to collision. For example, a namespace of
HTMLis probably too generic. (In fact, there’s already a library which uses that namespace, and has a classHTML::Node…)Avoid modifying existing classes if you can. There are two possible ways of modifying an existing class:
Adding. If you have to change an existing class, add new behaviour instead of modifying existing behaviour. You should always ‘namespace’ (which in the case of methods means ‘add your own prefix’) methods that you add so that there’s no conflict with other library writers. Check if an added method already exists and, if so, pop up a warning. Better to warn the user than go blindly onwards! Apparently there’s a technique called ‘selector namespaces’, which might go into 2.0 which will mitigate these problems.
Replacing. Hooks (for example
method_missing) should always chain to the next hook instead of assuming that hook’s behaviour and reimplementing it. After all, you might not be the first person to replace that hook… Don’t break people’s expectations of behaviour — always preserve existing behaviour and meet contracts.
He went on to discuss contracts a little. A contract for a method is a set of preconditions and postconditions. New behaviour for a method must have the same or weaker preconditions than the original, and must have the same or stronger postconditions.
I can completely understand adding namespaces to libraries, but I’m perplexed by his suggestion to namespace “everything you write”. Does he really want you to namespace all models, controllers, helpers, etc. in a Rails project? That seems unconventional and would no doubt be difficult to do. The way I see it, why bother namespacing something you won’t be using outside of your application. It’s just easier that way.