An area I've started exploring is expressive power. Which paradigm allows us to talk about a domain in a richer way?
Many years ago, usability gurus Don Gentner and Jakob Nielsen wrote a provocatively-titled article called The Anti-Mac Interface. They hastened to explain that they were fans of the Mac, and that their paper was an attempt to explore what the human user interface would be like if each of the Macintosh's design principles was systematically and deliberately violated.
I will quote a couple of paragraphs from that paper here, especially where they talk about the limitations of the friendly point-and-click paradigm.
Note three important classes of objects that the "see-and-point" paradigm is unable to cater for:
The see-and-point principle states that users interact with the computer by pointing at the objects they can see on the screen. It's as if we have thrown away a million years of evolution, lost our facility with expressive language, and been reduced to pointing at objects in the immediate environment. Mouse buttons and modifier keys give us a vocabulary equivalent to a few different grunts. We have lost all the power of language, and can no longer talk about objects that are not immediately visible (all files more than one week old), objects that don't exist yet (future messages from my boss), or unknown objects (any guides to restaurants in Boston).
1. resources that are not immediately visible
2. resources that don't exist yet
3. resources that are unknown
I've deliberately used the term "resources" there. Open-ended question: does REST's insistence on identifiable resources expose it to an analogous shortcoming of the kind Gentner and Nielsen have identified with GUI interfaces?
Gentner and Nielsen go on to say
Again, I can't help wondering, does SOAP's free-format style give it greater expressiveness than REST with its deliberately constrained interface? Certainly the composition of services into processes backed up by rules engines provides the ability to formulate complex conditional expressions.
If we want to order food in a country where we don't know the language at all, we're forced to go into the kitchen and use a see-and-point interface. With a little understanding of the language, we can point at menus to select our dinner from the dining room. But language allows us to discuss exactly what we would like to eat with the waiter or chef. Similarly, computer interfaces must evolve to let us utilize more of the power of language. Adding language to the interface allows us to use a rich vocabulary and gives us basic linguistic structures such as conditionals. Language lets us refer to objects that are not immediately visible. For example, we could say something like "Notify me if there is a new message from Emily." Note we are not advocating an interface is based solely on language. Neither does the interface have to understand full natural language. Real expressive power comes from the combination of language, examples, and pointing.
To continue with Gentner and Nielsen's delightful analogy, is homo restus then a caveman who has to point to something visible in his immediate environment and make one of four different grunts to indicate what he means? And is his opposite number a glib-tongued SOAP salesman unconstrained by language?