Composite Types

Composite types allow composing simpler types into more complex types, i.e., they allow the composition of multiple values into one. Composite types have a name and consist of zero or more named fields, and zero or more functions that operate on the data. Each field may have a different type.

Composite types can only be declared within a contract and nowhere else.

There are two kinds of composite types. The kinds differ in their usage and the behaviour when a value is used as the initial value for a constant or variable, when the value is assigned to a variable, when the value is passed as an argument to a function, and when the value is returned from a function:

• Structures are copied, they are value types.

  Structures are useful when copies with independent state are desired.

• Resources are moved, they are linear types and must be used exactly once.

  Resources are useful when it is desired to model ownership (a value exists exactly in one location and it should not be lost).

  Certain constructs in a blockchain represent assets of real, tangible value, as much as a house or car or bank account. We have to worry about literal loss and theft, perhaps even on the scale of millions of dollars.

  Structures are not an ideal way to represent this ownership because they are copied. This would mean that there could be a risk of having multiple copies of certain assets floating around, which breaks the scarcity requirements needed for these assets to have real value.

  A structure is much more useful for representing information that can be grouped together in a logical way, but doesn't have value or a need to be able to be owned or transferred.

  A structure could for example be used to contain the information associated with a division of a company, but a resource would be used to represent the assets that have been allocated to that organization for spending.

Nesting of resources is only allowed within other resource types, or in data structures like arrays and dictionaries, but not in structures, as that would allow resources to be copied.