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    <p>What you see on the screen is the top of the iceberg. Components are different size, difficult or simple programs, with public interfaces. In dataflow programming, these interfaces are producers and consumers (outputs and inputs), so the component can visualised as a black box with pins on its input and output sides. When you connect pins (ports), you lead one program's output to another program's input. The components are pre-compiled for you, they are ready to run, you have just set their consumers (inputs) and producers (outputs) by connecting them. That's why they are black boxes: they're programs, which you can't change (except if you got the source code).</p> <p>The components are designed to be connected to others. Some cases, components can run stand-alone, but usually they have to be connected to do the complete work. Basicly, there are three kind of components: - source: generates output (which requires further processing or displaying), - process: receives input, processes it, then passes it to further processing or displaying, - sink: receives input, displays or saves it, and doesn't pass it to anyone.</p> <p>A typical complete dataflow construction contains a source-process-process-sink chain, where the number of process-type components can be even zero (the data generated by source is displayed by a sink component). You can think for that three components as they were one program before, but they've had broken, and you can now re-assemble them.</p> <p>One of the most well known dataflow system is Unix shell. The CLI commands are the components. They're precompiled, you just define a chain by putting "|" between them. Also, most "source" commands can be used stand-alone, like <em>ls</em>, and most "sink" components can receive input from file defined as argument, e.g. <em>more</em>.</p>
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    1. POHow do visual programming languages work?
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    1. COGreat explanation. So, in a system designed like Yahoo Pipes (in PHP, let's say), these sources, processes and sinks could be just visual representations of actual PHP classes which are called and run on the fly? Would that be a good way to go? Or would - after a user edits this dataflow let's say - compiling and storing and then fetching and running these classes be a better way to go?
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    2. COSry, I'm not familiar with Yahoo Pipes. I've written a dataflow system in C++, where the dataflow program (component defs & connection list) is first compiled to C++, then to .o, then it's linked together with pre-compiled component and dispatcher .o files. Or see Unix shell, where components are executable files, and the connections are file descriptors (stdin, stdout). Dataflow is just an approach, it does not define any technique. Spreadsheets are also DF systems, where components are formulas in cells, connections are references - in this case the original question is even not applicable.
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