Narrative and State-Based Descriptions

Narrative and state-based descriptions

Software developers need to solve the most complex problems of the modern world using a brain that evolved mostly in the paleolithic age and earlier. How can we tap into the cognitive strengths of a hunter-gatherer to better produce and maintain software?

As we know, software development is a matter of communication as well as logic. We need to communicate clearly with the machine, of course, but just as importantly we need to communicate with other software developers and our future selves. Furthermore, clear thinking is largely a matter of communicating with our present selves. Have your thoughts ever been clarified through the act of writing? This happens to me all the time; it certainly happened while writing this essay.

In computing we often need to describe sets of sequences. Some common examples include: sets of sequences of symbols (i.e., formal languages); sets of sequences of operations on a set of objects; sets of sequences of interactions with external agents.

These sets are typically infinite and may include infinite sequences, so we need some finite way to describe the sets.

There are, I think, two modes of thought that we use when creating finite descriptions of large sets of sequences. I'll call these the narrative mode —I'm inventing the name here, I haven't seen a name put to the concept before—  and the state-based mode.

Narrative descriptions tap into the deep human habit of telling stories. We generally tell stories in order: this happened and then that. If we are giving someone directions, we might say "Keep walking north on Main Street until you get to the post-office, then turn left" or "Ring the bell, then if no one answers the door, go around to the back."

State-based descriptions describe how to react in different situations. They are a collection of rules: "If you are at the library, turn north and start walking. If you are walking north on Main Street and are south of the post-office keep walking. If you are walking north on Main Street and at the post office, turn left." The order of the rules doesn't matter because the sequencing is done via states.

With narrative descriptions the states are in the background. You can still see them in the negative space, e.g., the gap between two symbols in a regular expression or the line-break between two commands in a program. They are there, but unnamed and often unmarked.

Just as there are two modes of thought, there are two kinds of description formalism, which I'll also call narrative and state-based. Narrative formalisms have a bias toward the narrative mode of thought and state-based formalisms have a bias toward the state-based mode of thought.

State-based formalisms include finite state automata, Turing machines, flow charts, Harel's and UML's state charts, machine code, and programming languages like Fortran II. The last two might be considered hybrids, as not all states need to be named. If we add explicit stack operations to finite state automata, we get pushdown automata. If we add implicit stack operations to finite-state automata, we get syntax diagrams (sometimes called railroad diagrams); recursive transition networks are essentially the same thing.

Narrative formalisms include regular expressions, structured programming languages without recursion (say a language with just sequential composition, if-then-else commands, and while loops), process algebras, dynamic logic, and use cases.

Descriptions in narrative formalisms are trees in which the leaves are primitive narratives and the nonleaf nodes are operators on narratives. Descriptions in state-based formalisms are of sets of rules. Narratives are explicitly structured. State-based descriptions are not.

(Another name for narrative formalisms might be algebraic, emphasizing the analogy of sequential composition with multiplication, choice with addition, etc. Narrative formalisms tend to be semirings. However, in this essay, I'd like to emphasize the relationship of what I've called narrative formalisms to stories. There are lots of things that are algebraic but not are narrative; number theory is an example.)

While naming states doesn't seem to be particularly useful in the narrative mode, naming stories is. A narrator may jump from what he did in Paris to what he did in Berlin, but mention that they are leaving the story of the train ride for another day, or perhaps that is a story they already told before and they just refer to it now. In either case, giving the substory a name helps keep everything straight and promotes modularity.

Allowing named stories to refer to each other or themselves recursively is another step. Adding recursion to regular expressions gives extended-context-free grammars (a.k.a. extended BNF or EBNF). Recursive stories (unlike iterative loops) are not something that seem to be supported by our hundreds-of-thousands-of-years-old habit of story telling.

This may explain why recursion is often harder to learn for students than the other elements of the narrative mode. In a sense, though, recursion is just an extension to naming, so I'm inclined to say we are still in the narrative genera of description formalisms. And we know that, while the looping in extended CFGs reduces the need for recursion, there are still many problems that are elegantly solved by recursion. By the same token, we can add recursive procedures to a structured programming language and still be narrative.

By restricting extended-context-free grammars to only use sequential composition, we get context-free grammars (a.k.a. Backus-Naur Form, a.k.a. BNF). It would seem that adding a restriction to a formalism can't change it from narrative to state-based.  After all any description we can make in the restricted formalism we could have made in the unrestricted formalism, so if the description was narrative before it must still be narrative in restricted formalism. However, in fact we've started down a slippery slope: If we restrict further that all productions have the form A --> xB, we arrive at finite state automata.

So the lesson here is that narrative and state-based formalisms aren't clearly separated. Rather, as I said before, there are narrative and state-based modes of thought and different formalisms promote one mode over another. Because EBNF is so flexible, we can use it to write descriptions in either a narrative or a  state-based mode, but, because it makes narratives easy, it tends to promote a narrative mode of thought. How does it make narrative easy? Because, by using a more narrative style, I don't have to introduce as many nonterminals.

For me EBNF is a nice compromise. The same compromise is made in many modern programming languages. We have structured constructs (sequencing, choice, repetition) to construct our narratives, but also subroutines to give our narratives names. In theory we could use the recursive subroutine mechanism to write completely unstructured code, but in practice this rarely happens.

UIs, asynchronous programming, and inversion of control

So what motivated these thoughts? A few years about I asked myself: How do we deal with a sequence of interactions with the user? Back in the days of command-line programs, we used a narrative form. We used to write code like this

     ask the user for input

exit when there is no more input

     process the input

     output the result

end loop
More recently, use cases have become a popular way of describing sets of sequence of interactions during system specification. Use cases are very much narratives.

However, with graphical user interfaces has come event-driven input. Here we are expected to set up a set of event handlers and then the main program exits. From that point on the code executes a series of reactions to the various possible input events. The reaction to an event may depend on what has happened in the past and so we have a state dependent response. This is particularly true with actions like mouse movements or mouse clicks whose interpretation depends on past inputs.
onStart :  showQuestion(); showAnswerField() ; state := 1

onInput: if state=1 then hideQuestion(); hideAnswerField() ;
                 showOutput( process(getInput()) ) ; showNextButton() ; state := 2 end if

onDone: if state=1 then hideQuestion(); hideAnswerField() ; state := 3 end if

onNext: if state=2 then hideOutput() ; hideNextButton() ;
                 showQuestion() ; showAnswerField() ; state := 1 end if
There is a name for this sort of programming: Inversion of Control.

Writing programs using inversion of control essentially means throwing out the narrative formalism provided by our programming language: We no longer use whiles to create loops, nor ifs decide choices, at least not when the choice is dictated by the user's choice of the next input event. The result is an approach to programming that is unstructured and inviting of ad hoc solutions. It is difficult to write and difficult to read. As a result it is difficult to verify. We can fight the ad hoc solutions by using principled approaches to inversion of control such as the state pattern. This improves the situation somewhat, but it underscores that we are now living in the land of state-based thinking.

User interfaces are just one area where we are forced by our technology away from narrative and towards state-based modes of thought. Others include interacting with network communications. In general we call this asynchronous programming.

Computer scientists developed narrative formalisms to cope with asynchronous programming in the 70s and 80s, starting with threads and monitors and later with rendezvous and process algebras. Process algebras are very close to --and inspired by-- context-free grammars, especially in how they deal with choices imposed by external agents; however ideas from process algebras have not yet made it into mainstream language design nor into libraries for mainstream languages.

When I started thinking about a way to design and implement user interfaces in a more narrative way, I thought about using context-free grammars and perhaps generating UI code using something similar to a parser generator. However I then realized that I could follow an approach similar to monadic parsing, i.e., writing a library rather than a tool. But backtracking is more constrained in an interactive system than when parsing; we can't backtrack over output events that the user has already seen. Thus some changes from parsing monads are required; taking a monadic leads to implements a process algebra as a library or an embedded domain specific language; using such an embedded DSL, we can hide the inversion of control under an abstraction layer. Above that layer we write applications in a narrative style. I call my library "take back control". I've written more about it in another blog post.

Elsewhere I've described the difference between inversion of control and upright control as a difference between pushing and pulling. With inversion of control, a sequence of events is being pushed at out program and it must react to each one as it arrives. Pulling events means the code is written to pull events into the program, events are dealt with only when the program is ready to deal with them. This contrast between push and pull does not really explain why I find pull code preferable to code that simply reacts to a pushy world. This post is meant to fill that gap: The pull approach fits a narrative mode of thinking, which I think is easier for us humans, with our paleolithic brains, to cope with.

No comments:

Post a Comment