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Subject: Re: [boost] Flow-based programming library for Boost?
From: Devchandra L Meetei (dlmeetei_at_[hidden])
Date: 2012-12-15 04:04:10

I will have some time to contribute, Please let me know If you want some
coding help.


On Sat, Dec 15, 2012 at 6:52 AM, Topher Cooper <topher_at_[hidden]> wrote:

> I've only had time to read over the documentation quickly, but I found the
> discussion interesting, and I thought I would throw out some thoughts, with
> the caveat that I might have missed something.
> 1) /Simplicity is a simplistic goal/. Generally good design means that
> doing simple things is simple and doing more complex things is more complex
> proportionately (and no more than proportionately) to the degree of
> additional complexity. This may mean that there are many "components"
> (methods, parameters, libraries, classes, or whatever) and options but that
> at any given time most can be ignored. Part of the design process is to be
> clear what can be ignored when, and part of the implementation is
> documentation (whatever form that that takes) that makes it easy to focus
> on what one needs for a task and to be barely aware that there is more
> available (this, by the way, is my one complaint about the use of "literate
> programming" mechanisms -- there is a tendency to over-rely on them with a
> resulting mass of undifferentiated information).
> One of the tools for accomplishing this are careful selection early on of
> an explicit set of use cases. Careful use of defaults, especially of
> defaults that interact intelligently with explicits (this can lead to a
> system that from a usage viewpoint that "just does what is expected" but
> which can be a bear to implement and formally describe, with lots of
> "unlesses" and "with this combination of factors this, and with this
> combination that"), as well as things like policies and pre-specified
> frameworks that specify lots of things all at once.
> 2) /Single vs multiple inputs?/ There seems to be a natural way to handle
> a need for multiple inputs -- one interposes a component with an expandable
> set of individual inputs and a single output.
> That's a good, well integrated /mechanism/ but a weak /interface/. The
> reason is that the combining component is conceptually very closely tied to
> the input. I would suggest that a library of common ways of handling
> multiple inputs ("tuplers", "ands", "ors", "averagers", etc. as well as the
> current "only one allowed"), and that these be "declared" (or, given the
> run-time reconfigurability "redeclared") along with the input itself. Any
> wire addressed to the input actually gets attached to the combiner, and
> redeclaration would automatically pass the existing inputs to the new
> combiner.
> Of course, in line with my previous comment, there should be a default, as
> well as a way to declare the default over some sense of scope.
> 3) /Typed inputs, outputs -- and thus wires and signals/? Flexibility and
> performance may be primary in your use-cases, but that doesn't mean
> reliability has zero value. I really should not be able to attach an RGB
> wire to an input meant to process an aerial heading just because they both
> use a 3-tuple of numbers for representation. Its conceivable that one
> could generate tons of nonsense data without detection this way. Typing
> won't protect you, of course, from the result of attaching an RGB wire to
> the wrong RGB input, but decades of experience with strong typing has shown
> that it can radically cut down on the frequency of errors. (Note that this
> is not an argument against C++ template style duck typing generally: its
> compile time nature makes for a different circumstances, especially in
> situations where there is less of a tendency to use general purpose types
> with more specific operations -- e.g., R(), G() and B() rather than first,
> second, third or get<1>(), get<2>() and get<3>() -- than I suspect is the
> case with many uses of this package).
> Note that this is a run time type -- whether or not it is implemented via
> run-time type labels or as a reflection (no pun intended) of the C++ type
> system is not the issue. Performance wouldn't be an issue since the
> checking would occur when a wire is attached and wouldn't have any overhead
> during "ticks". If it is purely run-time, however, there wouldn't be any
> obvious way to enforce that the type placed on an output has any relation
> to the type label. On the other hand, dynamically modifying the output of
> a component would not be possible. Maybe a mix would be the right thing,
> though I'm not sure exactly how that would be done.
> Of course, I would think that a "not type specified" type, providing the
> present behavior, would be appropriate, and is a reasonable default.
> However, specifying a different default, within some scope, would be
> valuable as well -- if you are simulating logic circuits then either
> 2-state or 3-state logic signals would be a good default, even if parts of
> the system, dealing with edge-triggered circuitry or A/D/A, or analog
> sensors, need other types.
> 4) /Compile-time vs run-time configuration? /(a.k.a., declaration vs
> execution?) There seems to be the potential for large gains in fixed
> compile-time configuration and in some cases, in reliability. On the other
> hand, its clear that the primary intended use cases require more
> flexibility. It would seem that, once again, a hybrid system would useful.
> What occurs to me is that one should be able to create components by
> combining "primitive" components (classes derived from DspComponent) at
> compile time using the same concepts (wires, inputs, outputs) as the
> run-time system. Prototypes could be turned into fixed sets of components
> when fully understood and debugged, and also compiled units could be
> replaced by the same set of primitive components wired together dynamically
> (in fact, it might be possible for the class representing the hard-wired
> version to automatically include a static method that could be called to
> create a soft wired version of itself, an instance method instead of or in
> addition to the static one would allow compiled components to be
> hot-swapped out for a run-time configurable version of itself).
> That's a lot of work, of course, and I'm certainly not in a position to do
> it.
> 5) /Object-Oriented vs Generic Interface?/ -- I'm not going to take sides
> here, but it seems unlikely that the small overhead of run-time-bound calls
> would make much of a difference except in the limited case of a large
> network of simple components (e.g., a large, low-level, logic gate system)
> with high sequentiality and very little I/O or logging. In any other
> circumstances, I would say that the time for the indirection would be
> completely swamped by the component internals, by other kinds of system
> overhead and by I/O. That doesn't mean that generic programming isn't
> preferable but only that the performance overhead of virtual calls isn't an
> argument for it unless one can show that the non-monitored, large
> logic-gate type system requiring high-performance is an important design
> case.
> Just some thoughts, hope someone finds them useful or at least interesting.
> Topher
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