JSON | CUE

Intro

CUE is a superset of JSON: any valid JSON file is a valid CUE file. There is not much more integration you can get than that. The main motivation to make it a superset was to promote familiarity.

JSON’s influence on CUE

There are many design decisions in CUE that are a result of CUE being a strict super set of JSON. For instance, interpolations in CUE use the "\(str)" notation, rather than the somewhat more common "${str}". The latter is, however, a valid JSON string and cannot be used interchangeably with normal strings, as one cannot tell from the string itself if it is to be used as a template or a verbatim string. CUE’s notation is not valid JSON, meaning it is safe for CUE to give it special treatment outside of the JSON spec. This way 100% compatibility is guaranteed.

Generate JSON

The cue export command outputs CUE in a data format. By default this is JSON:

$ cat >> coordinates.cue << EOF
x: 43.45
y: -34.12
EOF

$ cue export coordinates.cue
{
    "x": 43.45,
    "y": -34.12
}

JSON in CUE

It may also be needed to deal with JSON inside a CUE file. For instance, a Kubernetes ConfigMap may specify an embedded JSON file. CUE provides a builtin JSON package to parse, generate, or validate JSON from within CUE.

Create

The builtin encoding/json.Marshal generates JSON from within CUE.

import "encoding/json"

configMap: data: "point.json":
	json.Marshal({
		x: 4.5
		y: 2.34
	})

{
    "configMap": {
        "data": {
            "point.json": "{\"x\":4.5,\"y\":2.34}"
        }
    }
}

Parse

The reverse is also possible.

import "encoding/json"

data:  #"{"x":4.5,"y":2.34}"#
point: json.Unmarshal(data)

{
    "data": "{\"x\":4.5,\"y\":2.34}",
    "point": {
        "x": 4.5,
        "y": 2.34
    }
}

Validate

import "encoding/json"

#Dimensions: {
	width:  number
	depth:  number
	height: number
}

// Validate JSON configurations embedded strings.
configs: [string]: json.Validate(#Dimensions)

configs: bed:      #"{ "width": 2, "height": 0.1, "depth": 2 }"#
configs: table:    #"{ "width": "34", "height": 23, "depth": 0.2 }"#
configs: painting: #"{ "width": 34, "height": 12, "depht": 0.2 }"#

configs.painting: invalid value "{ \"width\": 34, \"height\": 12, \"depht\": 0.2 }" (does not satisfy encoding/json.Validate({width:number,depth:number,height:number})): error in call to encoding/json.Validate: field not allowed:
    ./in.cue:10:20
    ./in.cue:3:14
    ./in.cue:14:20
    json.Validate:1:1
    json.Validate:1:30
configs.table: invalid value "{ \"width\": \"34\", \"height\": 23, \"depth\": 0.2 }" (does not satisfy encoding/json.Validate({width:number,depth:number,height:number})): error in call to encoding/json.Validate: conflicting values number and "34" (mismatched types number and string):
    ./in.cue:10:20
    ./in.cue:4:10
    ./in.cue:13:20
    json.Validate:1:1
    json.Validate:1:12