encoding: missing support for toml

[cueckoo]

Originally opened by @xinau in cuelang/cue#68

(Go) Tools like InfluxDB, GitLab CI (Runner), Traefik use TOML as their configuration language. Atm. CUE has no encoding support for TOML configuration files to be imported or exported. Since Go doesn't have native support for TOML, a third-party implementation needs to be imported as it's the case for YAML. Another thing to be aware of is that the native support for Date-Time, Date, Time values in TOML. Maybe this could be something CUE might want to support in the future as well. Also a mapping of array of tables to a appropriate CUE representation might be needed.

[cueckoo]

Original reply by @mpvl in cuelang/cue#68 (comment)

I'm fine with TOML, being as common as it is, to be one of the few more configuration formats that's natively supported. At the moment, YAML, JSON, Go, Proto, and OpenAPI are higher on the list to complete support for, though.

Note that generating TOML output should already not be too hard using the text/template builtin package.

CUE has some limited support for time and date parsing through the time builtin package. It is high on the list to complete this support as part of the OpenAPI support effort. With this, a time field is indicated as:

import "time"

meetingTime: time.Time

where meetingTime is a string with the respective time format.

I don't understand why arrays of tables are not just lists of values. It seems to be a special TOML notation that needs no special treatment in CUE. I do plan to add support for associative lists at some point, which would allow one to define different items of a list at different locations, though, sticking to CUE's associativity and commutativity properties, without any guarantee of order other than adhering to some topological sort.

[jmgilman]

Is this feature request on the radar for getting some attention soon? Using text/template works in simple cases but can get out of hand pretty quickly.

[myitcv]

This is not currently a priority because there are aspects like package management that are higher priority. But we would very much welcome community contributions in this space.

[yujunz]

MVP in cue.

import (
    "strings"
    "encoding/json"
)

cue: {
	tables: {
		global_tags: {
			cluster:      "foo"
			environment?: string
		}
		agent: {
			interval: "interval"
		}
	}

	table_arrays: {
		"inputs.prometheus": [
			{
				metric_version: 2
			}, {
				metric_version: 2
			},
		]
	}
}

toml: strings.Join([ for table, kv in cue.tables {
	strings.Join(["[\(table)]"]+[ for k, v in kv {"\(k) = \(json.Marshal(v))"}], "\n")
}]+[ for table, array in cue.table_arrays {
	strings.Join([ for kv in array {
		strings.Join(["[[\(table)]]"]+[ for k, v in kv {"\(k) = \(json.Marshal(v))"}], "\n")
	}], "\n\n")
}], "\n\n")

renders

[global_tags]
cluster = "foo"

[agent]
interval = "interval"

[[inputs.prometheus]]
metric_version = 2

[[inputs.prometheus]]
metric_version = 2

https://cuelang.org/play/?id=YEIedqgEzNT#cue@export@cue

Nesting tables are hard to implement and likely requires extension in cue.

[mvdan]

A TOML decoder and encoder are now in master and wired up to cmd/cue, so they should work for most basic use cases. Please let us know if you find any bugs either in master or in the upcoming alpha, or if anything doesn't behave like you would expect it to :)

Given that initial support is now merged, we have closed this issue, but we have opened a number of others for follow-up tasks:

• encoding/toml: errors do not include location information #3339
• encoding/toml: support encoding and decoding comments #3342
• encoding/toml: encoding should keep the original order and shape of CUE #3343
• pkg/encoding/toml: add package to expose TOML encoder as a CUE package #3344