This mod is an API that provides useful extensions to existing vanilla classes. They are grouped based on the class that is primarily dealt with or returned by the function.

Block Extensions

Block.makeItem() - creates a new BlockItem based on the block. Optionally takes an Item.Settings.
Block.matches(Identifier) - checks whether the block is contained in the tag with the given id
Block.matches(String) - converts the String to an Identifier then calls matches(Identifier)

Identifier Extensions

String.id - converts the String to an Identifier
Pair.id - converts the Pair to an Identifier
Identifier.component1() and .component2() - allows for destructuring of Identifiers
String.validId() - checks if the String is a valid Identifier
Pair.validId() checks if the Pair is a valid Identifier

Item Extensions

Item.stack() - creates a new ItemStack with the Item. Optionally takes an Int for the count.
Item.matches(Tag or Identifier or String) - checks if the Item is in the Tag. Same idea as Block::matches

NBT Tag Extensions

compooundTag(vararg Pair) - creates a new CompoundTag with the given elements
CompoundTag.set(String, Tag) - operator for CompoundTag::put
Map.toCompoundTag() - creates a new CompoundTag from the map's elements
Any?.toTag() - Creates a new Tag from the object. Note: if the object cannot be converted into a tag, this will crash the game.
interface NBTSerializable - Allows you to create the NBT representation for your classes. Not needed for Entity and BlockEntity subclasses.

Registry Extensions

Registry.get(String) - converts to an Identifier and gets the value in the Registry
Registry.set(Identifier or String, T) - Registers the value in the Registry using Registry.register
Identifier.item() and .block() and .entityType() and .blockEntityType() and .registry() - gets the value at the Identifier in the appropriate Registry

Tag Extensions

Identifier.tag() - gets a Tag at the Identifier. Only valid for Item, Block, EntityType, and Fluid. Any other type will crash the game.
String.tag() - converts to an Identifier and gets the Tag at it
Tag.isEqual(Tag) - Tag.equals is not properly overridden. Use this to compare equality with tags.

World Extensions

World.runOnClient(()-Unit) - Runs the passed function only on the client
World.runOnServer(()-Unit) - Runs the passed function only on the server

Kotlin Extensions [Fabric]