Friday, October 24, 2008

Interface-defined variables (or A quick idea on hybrid or diluted type systems 2)

Having expounded a vague hope in A quick idea on hybrid or diluted type systems, I came to realise that what I was really doing was defining a series of interfaces.

We can already define objects via interfaces:
ISomeInterface MyProperty = new ClassImplementingISomeInterface();

What if we limited ourselves to only defining any method or property with interfaces? Thus I would have a class MyClass, with properties IStringable NameString, IEnumerable<inumber> NumList, etc. That way the class would be maximally flexible, as we would be making the properties make only the minimum requirements of the inputs. Of course, if I pass an object that implements an interface derived from that required, it can still be cast into the minimum required interface.

Excellent, a dynamic and gregarious acceptance of many different static types - some of the innate strengths of dynamic languages in a statically-typed one.

Web Services

Let us step back from a moment and consider how this would help in the world of web services. A web service needs to be forgiving in what it will accept, and strict in its output, if it is to be used by the maximum set of other web services. If we assume that the input and output are various flavours of xml, then the static type is equivalent to the exact DTD of an xml doc, and the set of interfaces is, well, unknown.

In looking at REST interfaces, I came across a discussion of how to handle the inevitable change of interface versions when providing a RESTful API. Kalsey decides, in the end, to provide a version tag in the xml output file, containing pointers to the current, previous and latest API versions, with date-based URLs to provide access to specific versions of the API interface.

If, instead, we consider a request on a REST URI as a request for certain data conforming to a certain interface, then we can skip versioning. We define, instead, a series of smaller interfaces, and allow the user to request any subset of them. Thus my request xml should supply input data in an xml form that can be cast into any of the simpler interfaces, given an xml-generated object.

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