How to extend Shake with additional command-line arguments?

I'm trying to extend the Shake command line argument, with two goals in mind:

To take extra flags, e.g. --foo.
To treat non-flag arguments specially depending on the current working directory, so go does one thing in the root, and another in the foobar directory.

How can I do that?

Solution

There are four relevant functions to extend the Shake command line:

shake is the main entry point to Shake, which takes a ShakeOptions structure describing all the options. If you want an entirely custom command line interface, use any command-line parser to populate these fields.
shakeArgs runs Shake filling in the ShakeOptions using the command line arguments, so is useful to get a standard command line with Shake.
shakeArgsWith takes a list of extra options as its second argument, and logic for how to deal with any non-flag arguments as its third. You can inject new flags, and modify how a non-flag like go gets resolved based on any other information you have to hand, including current directory.
shakeOptDescrs gives data types describing flags corresponding to fields in ShakeOptions.

As to their relationship, shake is the actual build system. shakeArgsWith uses the information from shakeOptDescrs and any custom flags to build and run a command line parser. shakeArgs calls shakeArgsWith using no additional flags.

In the specific case of adding a flag or customising the behaviour of a non-flag, shakeArgsWith is sufficient. If you need more control, you can build your own parser, perhaps incorporating the existing flags found in shakeOptDescrs or not.