# Build systems á la carte

https://www.microsoft.com/en-us/research/uploads/prod/2018/03/build-systems.pdf

Andrey Mokhov, Neil Mitchell, and Simon Peyton Jones published this already-classic paper in 2018.

It breaks down build systems (including ${ Excel}$!) in several dimensions to look at exactly what they do, and think about where the gaps are that productive build systems might fit in.

I don't follow much of the haskell code, but it's still a useful read.

${ Excel}$ is a build system in disguise. Consider the following simple spreadsheet:

```
A1: 10
A2: 20
B1: A1 + A2
```

There are two input cells A1 and A2, and a single task that computes the sum of their values,

producing the result in cell B1. If either of the inputs change, $Excel$ will recompute the result.

Unlike $Make$, ${ Excel}$ does not need to know all task dependencies upfront. Indeed, some dependencies may change dynamically according to computation results.

To support dynamic dependencies, ${ Excel}$’s calc engine is significantly different from $Make$. $Excel$ arranges the cells into a linear sequence, called the calc chain. During the build, $Excel$ processes cells in the calc-chain sequence, but if computing a cell C requires the value of a cell D that has not yet been computed, $Excel$

abortscomputation of C, moves D before C in the calc chain, and resumes the build starting with D. When a build is complete, the resulting calc chain respects all the dynamic dependencies of the spreadsheet. When an input value (or formula) is changed, $Excel$ uses the final calc chain from thepreviousbuild as its starting point so that, in the common case where changing an input value does not change dependencies, there are no aborts.

They define these axes on which build systems differ:

- Scheduling
- Topological: pre-compute the task list to build
- Restarting: just start building, and if your task requires some $dep$, stop what you're doing, build $dep$, and restart when it's done
- Suspending: just start building, and if your task requires some $dep$, pause what you're doing while you build $dep$, then carry on

- Rebuilding
- A dirty bit: there is some marker on an object of whether it needs rebuilt; in make it's the file's
`mtime`

compared to its build output - Verifying traces: record a hash of what you built last time, and only build if the hash is different
- Constructive traces: record the actual built value instead of a hash

- A dirty bit: there is some marker on an object of whether it needs rebuilt; in make it's the file's

a classic paper I hadn't read probably since it came out, it was nice to remember it!

I got to it this time from a link on this article, "Build system schism: the curse of meta build systems"