Entities

namespace demo;

var #hp = 0;

mcfun check() {
    #hp = @s.$Health;
    if (@s.$Health < 10) {
        @s.$Glowing = true;
    }
}

mcfun heal() {
    @s.$Health = #hp + 4;
}

A selector followed by .# or .$ names a value of an entity. @s.#mana is the entity’s score in the objective of #mana, and @s.$Health is the entity’s NBT at the path Health. Without a selector, $item is the pack’s own storage and #mana is the global score.

You can read an entity value anywhere a value is expected, and @s.$Health * 10 is one command: data get entity @s Health 10. That scaled read keeps the fraction until the end, so 1.5 * 10 is 15, and a value too large for an integer is clamped.

A write uses data modify: @s.$Glowing = true;. Minecraft refuses data modify entity on a player. A copy from other NBT keeps its type: $lastPos = @s.$Pos;. A score or math is stored as an int, and the game converts it to the type of the field, so @s.$Health = #hp; gives Health the value 7.0f.

Reads and writes need a selector for one entity: @s, @p, @r, @n or limit=1.

Costs and caveats

• The path isn’t checked.
• The compiler can’t check for a write to a player, which fails in the game.