Modify build.Instance before building it?

[cueckoo]

Originally opened by @shykes in cuelang/cue#440

TLDR: I need to combine the import support of load.Instances and the flexibility of cue.Compile. Can I alter build.Instance.Files to achieve that?

In theory one can produce their own cuelang.org/go/cue/build.Instance without relying on cuelang.org/go/cue/load.Instances. But in practice that is difficult, because proper handling of external imports all happens in undocumented private functions inside load. So I use load.Instances... But it is flexible enough for my needs.

Specifically I would like to dynamically change the “root key” for some of the files in the build, so that they are merged at a key different than what their source file says. I need to honor the imports in those files, so I can’t use cue.Compile.

I found the following fields in the cue/build.Instancd struct:

• Files
• BuildFiles
• IncludeFiles

Of those, the comments tell me that only BuildFiles will exist eventually. But for now all three are used. From reading the docs and the source, it’s not entirely clear how.

I am hoping to modify these fields to transform the correct fioes in the correct way. My best bet seems to be modifying the contents of Files using the ast package. Does that seem 1) possible and 2) a good idea?

Thanks!

[cueckoo]

Original reply by @mpvl in cuelang/cue#440 (comment)

You can modify build.Instance files, like adding additional Files, at will. The compiler will only look at Files.

It is also fine to modify the contents of existing Files.

You do need to ensure the Imports maps is updated accordingly to ensured imported packages are loaded as well if the added files introduce any imports, I believe. I may remove this requirement in the future, but for now it can't hurt to add it.

More info:
The building goes in two stages: first it collects the set of files, then it converts those to syntax trees in Files.

By the time load is done, all is converted to Files, so it safe to ignore any of the other file fields.

[cueckoo]

Original reply by @shykes in cuelang/cue#440 (comment)

Thank you. Does this mean that some of the fields of build.Instance are specific to load, and will be ignored by cue.Build? If so, that means I could also ignore those fields when producing my own custom build.Instance, correct?

[cueckoo]

Original reply by @mpvl in cuelang/cue#440 (comment)

Indeed. Most of the fields are just informative and are ignored by cue.Build.

I believe the only thing that is being used is Files and Imports. The latter could really also be eliminated as a requirement.