Monday, October 18, 2010

Extending the 'Template Method' design pattern

There is this notion that you should not put semantics into names, or maybe that what your program do should not change in alpha-conversion. But of course everyone is breaking that rule if she is using the Template Method design pattern (if we agree that the library they use is somehow outside of the program and is not alpha-converted together with it). There the names of methods have a lot of semantics.

In Advanced Search in web DBIx::Class based applications I proposed an extension to DBIx::Class that would treat all methods with names search_for_* as predicate builders. This way programmers could easily extend the query language used by DBIx::Class with new predicates by adding methods to the ResultSet class. Now in WebNano all *_action controller methods also have a special meaning - these are the methods that can be called from outside via automatic dispatching from an url. Taken literally sub my_method_action is not that different from sub my_method : Action which is the way to specify that in Catalyst. The difference is only ' : A' versus '_a' (and I could use *_Action if I wanted to golf it even more) - but yeah - I am cramming the semantics into the name while Catalyst reserves it for the code attributes. Catalyst way is cleaner - but my goal was a 'minimal' framework. The 'Extended Template Method' design works in a few lines while MooseX-MethodAttributes is 13 packages with who knows how many lines of very subtle code.

What surprises me is that I have never seen a description of this 'Extended Template Method' design pattern. It looks rather obvious, maybe it is called by some other name in the literature.

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