6 ms·

I'm great at excel and terrible at programming. Can you break down what's meant here?

klodolph 3y ago
In Excel, every cell has some number of inputs (which are other cells), and a rule for calculating the cell’s value from its input. That is basically how software build systems work, except usually they use files instead of spreadsheet cells, and they run commands (like “compile this source code”, or “combine these seven files into one program”) instead of Excel formulas.
ufo 3y ago
In a build system, when you update a source code file, it should recompile the binaries that depend on it. Similarly, in excel when you update a cell in the spreadsheet, it should recompute the cells that refer to it. The paper is about thinking about build systems in a more modular way. Are dependencies specified statically or dynamically? What algorithm is used to decide what should get rebuilt after an update? What order do we rebuild things? What happens if we pick different combinations of algorithm+order?
- dayjaby 3y ago
  Is that actually a thing? What algorithms do you mean? I thought everything that depends on a change gets rebuilt.
  - ufo 3y ago
    The devil is in the details. For example, Make decides to rebuild based on timestamp, while Bazel decides to recompute based of file hashes. Thus, if you touch a file but don't change its contents, Make might rebuild things that Bazel wouldn't. Another example in the paper is the INDIRECT function in Excel. This makes it so that Excel cannot compute the dependency graph before the build starts. Excel has ways to deal with this, but they might cause some cells to be recomputed even when they didn't actually need to be. The paper has a bunch of interesting examples like this, it's worth the read :) The problem of dynamic dependencies is particularly tricky. For example, suppose that you want to teach your makefile that your compilation depends on the source C file, but also on all the headers that are #included by that C file. And you want to actually look inside the C file to get that list of headers, not just copy paste the list into the makefile. There's more than one way to handle this.

In Excel, every cell has some number of inputs (which are other cells), and a rule for calculating the cell’s value from its input. That is basically how software build systems work, except usually they use files instead of spreadsheet cells, and they run commands (like “compile this source code”, or “combine these seven files into one program”) instead of Excel formulas.

ufo 3y ago

In a build system, when you update a source code file, it should recompile the binaries that depend on it. Similarly, in excel when you update a cell in the spreadsheet, it should recompute the cells that refer to it. The paper is about thinking about build systems in a more modular way. Are dependencies specified statically or dynamically? What algorithm is used to decide what should get rebuilt after an update? What order do we rebuild things? What happens if we pick different combinations of algorithm+order?

dayjaby 3y ago
Is that actually a thing? What algorithms do you mean? I thought everything that depends on a change gets rebuilt.
- ufo 3y ago
  The devil is in the details. For example, Make decides to rebuild based on timestamp, while Bazel decides to recompute based of file hashes. Thus, if you touch a file but don't change its contents, Make might rebuild things that Bazel wouldn't. Another example in the paper is the INDIRECT function in Excel. This makes it so that Excel cannot compute the dependency graph before the build starts. Excel has ways to deal with this, but they might cause some cells to be recomputed even when they didn't actually need to be. The paper has a bunch of interesting examples like this, it's worth the read :) The problem of dynamic dependencies is particularly tricky. For example, suppose that you want to teach your makefile that your compilation depends on the source C file, but also on all the headers that are #included by that C file. And you want to actually look inside the C file to get that list of headers, not just copy paste the list into the makefile. There's more than one way to handle this.