Prepend to an Array

+ adds your entries to the END of an array. When order is meaningful - menu tabs, priority lists, anything rendered top-to-bottom - you sometimes need your entries at the FRONT instead. The - suffix prepends: it inserts before the existing elements, preserving the order you wrote them in. The real case: you want hexcode's Arcane category to be the FIRST tab in the Creative Library, not the last.

Goal

• Put a new Hexcode category at the front of the Creative Library's top-level Children.
• Keep Tools, Weapons, Armors and the rest exactly as they are, just after yours.

The Asset

Server/Item/Category/CreativeLibrary/Items.json

The Children array is the tab order, top to bottom; you want to land before the first entry.

{
  "Icon": "Icons/ItemCategories/Items.png",
  "Order": 2,
  "Children": [
    {
      "Id": "Tools",  // currently first; you want to sit before it
      "Name": "server.ui.itemcategory.tools",
      "Icon": "Icons/ItemCategories/Items-Tools.png"
    },
    {
      "Id": "Weapons",
      "Name": "server.ui.itemcategory.weapons",
      "Icon": "Icons/ItemCategories/Items-Weapons.png"
    },
    {
      "Id": "Armors",
      "Name": "server.ui.itemcategory.armors",
      "Icon": "Icons/ItemCategories/Items-Armor.png"
    }
  ]
}

The patch

Path: Server/Item/Category/CreativeLibrary/Items.patch

{
  "Children-": [
    {
      "Id": "Hexcode",
      "Name": "hexcode.itemcategory.hexcode.name",
      "Icon": "Icons/ItemCategories/Hexcode.png"
    }
  ]
}

The key is Children-, not Children or Children+. The - prepends to the existing array. A plain Children would REPLACE all three stock tabs; Children+ would put Hexcode last.

After

Hexcode is now the first element; the three stock categories follow in their original order.

{
  "Children": [
    {
      "Id": "Hexcode",
      "Name": "hexcode.itemcategory.hexcode.name",
      "Icon": "Icons/ItemCategories/Hexcode.png"
    },
    {
      "Id": "Tools",
      "Name": "server.ui.itemcategory.tools",
      "Icon": "Icons/ItemCategories/Items-Tools.png"
    },
    {
      "Id": "Weapons",
      "Name": "server.ui.itemcategory.weapons",
      "Icon": "Icons/ItemCategories/Items-Weapons.png"
    },
    {
      "Id": "Armors",
      "Name": "server.ui.itemcategory.armors",
      "Icon": "Icons/ItemCategories/Items-Armor.png"
    }
  ]
}

Multiple elements keep their order

Prepend preserves the order of the elements you write. "Children-": [ A, B ] against a base of [ X ] yields [ A, B, X ], not [ B, A, X ]. Front insertion does not reverse them.

Prepending with $Match

- is fully symmetric with + under $Match. On a hit it deep-merges into the matched base element in place (it does not move it); on a miss it prepends the new element. So you can upsert-to-front: merge if the element already exists, otherwise add it at the front.

{
  "Children-": [
    { "$Match": "Id", "Id": "Tools", "Order": 5 },
    { "Id": "Hexcode", "Name": "hexcode.itemcategory.hexcode.name", "Icon": "Icons/ItemCategories/Hexcode.png" }
  ]
}

Tools already exists, so it is merged in place (gains Order: 5, stays where it is). Hexcode matches nothing, so it is prepended. $Match is stripped from the output.

Notes

• - creates the array if the key is absent, exactly like +. Against no base, "Children-": [A] just yields [A].
• If the value is not an array, - does nothing (same guard as +).
• - means prepend in Patchly. It is NOT a remove operator. To remove array elements you restate the array with only the survivors (see Remove a Field or Recipe).
• Prepend and append are independent: Children- and Children+ in the same patch both apply, yielding [ prepended..., base..., appended... ]. See the array rules in the syntax reference.