feat: add --wrap flag to apply a jsonnet expression to the top-level value

[vivainio]

Closes #195.

Motivation

My primary use case is emitting YAML output directly from jsonnet, without piping through an external converter. The --wrap flag makes this possible with --wrap std.manifestYamlDoc, while also being general enough to support any output format via stdlib or custom functions.

Summary

Adds a --wrap <expr> flag that applies a jsonnet expression to the top-level evaluated value before output, with string output mode implied.

This is the "simple wrapping" variant discussed in #195 — it constructs a snippet (expr)(value) and evaluates it with string output. This is equivalent to the existing jsonnet -S -e 'std.manifestINI(import "file.jsonnet")' pattern but more convenient.

Known limitation: stack traces

As discussed in #195, when an error occurs during manifestation through a stdlib function like std.manifestYamlDoc, the stack trace includes stdlib internals, which is noisier than errors during normal evaluation. A future enhancement could add a "deep evaluation" variant that fully evaluates the value before passing it to the manifester, producing cleaner error traces for the custom manifester use case.

Examples

# YAML output
jsonnet --wrap std.manifestYamlDoc file.jsonnet

# YAML without quoted keys
jsonnet --wrap 'function(x) std.manifestYamlDoc(x, quote_keys=false)' file.jsonnet

# Pretty JSON via stdlib
jsonnet --wrap std.manifestJson file.jsonnet

# Extract a single field for use in shell scripts
jsonnet --wrap 'function(x) x.version' file.jsonnet

Notes

• --wrap is incompatible with --multi, --yaml-stream, and --string
• For file input, builds (expr)(import @'file') as a snippet
• For -e / stdin, inlines the input as (expr)(input)
• The wrap expression must evaluate to a function of one argument

Test plan

• jsonnet --wrap std.manifestYamlDoc file.jsonnet produces YAML
• jsonnet --wrap 'function(x) std.manifestYamlDoc(x, quote_keys=false)' file.jsonnet produces unquoted-key YAML
• Works with stdin (-) and -e code input
• --wrap + --multi / --yaml-stream / --string each produce a clear error

[He-Pin]

is there a pr in the cpp implementation too?

[vivainio]

No, I wasn't aware this is needed

[vivainio]

I am closing this for lack of personal need (using jrsonnet for this)