I am considering what the possibilities are with declarative programming. I have a firm understanding of how to use declarative programming in practice, but, short of having examples, I don’t know if it’s possible to 100% use declarative programming all the way down to the machine code (theoretically).
Standard QA sites like this says stuff like:
it means describing the problem to be solved, but not telling the programming language how to solve it
A typical example is a SQL query which uses a where clause to define the query rather than implementing the query it step-by-step using something like an iterative loop.
An example I am interested in is an even higher level abstraction than a SQL query: defining an HTTP server. You can define the server and all of it’s GET handlers (if we just limit it to that for now) in a completely declarative way, as seen by many declarative frameworks. The declarative frameworks that do similar stuff to this are the ones like Terraform or SaltStack that define machine configurations/states. You are basically defining the state the machine should look like, and the system takes your “definition” and makes it happen.
For the HTTP server you can do like in React:
<Route path='/' component={Home}> <Route path='/about' component={About}> <Route path='/terms' component={Terms}> ... </Route> This then automagically gets wired up into a listening server for these changes, and renders the components (this API may not be exact, just pseudocoding it ftm).
In fact, there is a lot that can be done declaratively. HTML and CSS together form an animatable graphics display and is completely declarative. You can define servers, machines, parsers (PEG parser generators), graphics… I don’t know what else.
That’s mainly what my question is, what can be done declaratively. More specifically, what cannot be done in a declarative fashion. The stereotypical example I am thinking of is simply defining a step-by-step sequence of actions of some sort, but in a declarative, non-iterative way. Is it possible?
Here is my attempt…. Say we are describing some automation steps for interactively going through and clicking around a website. Here is the iterative way:
Visit "/" Click on "Button 1" Wait "2 seconds" Click on "Button 2" For each "Input" in "Fields" Type into "Input" Click on "Button 3" Wait "2 seconds" ... This is similar to assembly instructions like mov. But can this be done in a declarative way (that is also readable and easy to understand)? Does functional programming offer any insight? Maybe one could show how to do this in a functional programming paradigm, or even a logic (prolog-ish) way, if that would help illuminate how it’s done. My attempt at declarative is like this:
Current State should be "/" Button is not clicked Button has been clicked 2 seconds have passed ... Basically, just stating the “states” before and after each step sort of thing. That makes it harder to read, and it feels like you are trying to hack around having to build some iterative code anyways. It’s like it has to be iterative. Is that so? Is there any way to make this non-iterative and so it still makes sense?
To me, currently, it seems that you can use declarative programming for like 80-90% of things, but then you inevitably need “parsing” code which will take those declarative statements and interpret them, converting them into some iterative sequence. But what I’m left wondering is, can that 10-20% that seems to have “iteration” as a requirement be instead written declaratively? Can you take that 10-20% remaining iterative code and make that declarative too? If so, how would that look or what is the technique?
