Value types

A field is its type. The type tag decides which other keys are legal — constraints live where the type is, so minLength can only appear on text and min only on a number.

The same nine types are used on both sides of the config, and nest inside each other.

The nine

Type	Holds	Own keys
text	A string	minLength, maxLength, pattern
integer	A whole number	min, max, step
number	A number, whole or fractional	min, max, step
boolean	true / false	—
date	A date	—
enum	One of a fixed set	choices
object	A record with named members	fields, table
array	A list	items
map	A dictionary	keys, values

Scalars

{ "name": "title",   "type": "text", "minLength": 1, "maxLength": 200 },
{ "name": "email",   "type": "text", "pattern": "^[^@\\s]+@[^@\\s]+$" },
{ "name": "rating",  "type": "integer", "min": 1, "max": 5, "step": 1 },
{ "name": "score",   "type": "number", "min": 0, "max": 1, "step": 0.01 },
{ "name": "flagged", "type": "boolean" },
{ "name": "created", "type": "date" },

pattern is a JavaScript regular expression. It is compiled when the config loads, so a malformed one is a config error rather than a surprise at render time.

step is not a constraint

step sizes the increment on a number box or slider. It is never used for validation — three clicks of a 0.1 slider legitimately produce 0.30000000000000004, and rejecting that would be absurd. If you need a real constraint, use min and max.

enum

{
  "name": "sentiment",
  "type": "enum",
  "choices": [
    { "name": "positive", "display": "Positive", "shortcut": "p" },
    { "name": "neutral", "display": "Neutral" },
    { "name": "negative", "display": "Negative", "selectedStyle": { "tone": "danger" } },
  ],
}

A choice’s name is the literal string written to the file; display.title is only what the labeler sees. Choice names are trimmed and cannot be empty — an empty choice would read back as no value at all, making it both unselectable and impossible to round-trip.

See Shortcuts for shortcut, and Display for selectedStyle.

array

// A list of strings — renders as chips.
{ "name": "tags", "type": "array", "items": { "type": "text" } },

// A multi-select — an array of enum is the only way to declare one.
{
  "name": "issues",
  "type": "array",
  "items": {
    "type": "enum",
    "choices": [{ "name": "toxic" }, { "name": "off-topic" }, { "name": "unhelpful" }],
  },
},

An array of enum is a genuinely important idiom: it is how you get checkboxes where more than one answer is allowed. Selected values are written in declaration order, not click order, so the exported cell is stable however the labeler arrived at it.

object

{
  "name": "checks",
  "type": "array",
  "items": {
    "type": "object",
    "fields": [
      { "name": "name",   "type": "text",    "display": "Check" },
      { "name": "passed", "type": "boolean", "display": "Passed" },
    ],
  },
},

An array of object renders as a table, one row per element. object.table controls how its members group into displayed columns — see TableView.

Names inside an object are JSON keys from your source payload, not column headers, so they are deliberately unconstrained: colons, slashes and unicode are all fine.

map

{ "name": "tokenCounts", "type": "map", "values": { "type": "integer" } },

{
  "name": "annotators",
  "type": "map",
  "values": {
    "type": "object",
    "fields": [
      { "name": "role",       "type": "text" },
      { "name": "confidence", "type": "number" },
    ],
  },
},

keys defaults to text and is rarely worth stating. A map of object renders as a table keyed by its entries.

object and map are display-only

Neither can be filled by a person. There is no widget that could sensibly capture an arbitrary nested structure, so an output field of either type must declare a fill that does not involve asking:

{ "name": "meta", "type": "object", "fields": [ /* … */ ] }
// ✗ "object" fields cannot be filled by a user; give the field a `fill`.

{ "name": "meta", "type": "object", "fields": [ /* … */ ], "fill": { "kind": "copy" } }
// ✓

How raw values become typed ones

CSV cells are strings. Each is coerced according to its declared type when the file loads.

Type	Accepts
text	Anything; non-strings are stringified.
number	Anything Number() parses, after trimming.
integer	The same, but rejects a fractional result.
boolean	true false 1 0 yes no y n, case-insensitive.
date	Anything new Date() parses. ISO-8601 is what MLabel writes.
enum	Must be exactly one of the choice names.
composite	A JSON string at the top level, then recursively coerced.

An empty cell becomes null, for every type. Empty is not zero, not false, and not an empty list — it is the absence of a value, which is what makes required-ness meaningful.

A cell that will not coerce does not refuse the file. The value shows as empty with a note, and the labeler carries on — they cannot fix the source data, so it is information rather than a task. See Troubleshooting.

Full reference

InputField · OutputField · ValueType · NestedField