Schema example: Non-Inflatable Assets

In this section, we will look more closely at an actual example of an RGB Contract Schema written in Rust and contained in the nia.rs file from the RGB Schemata Repository. The Repository contains an example set of schema templates related to other types of contracts. This Schema, which we will be using as an example in this chapter, allows for the contract setup of Non-Inflatable Assets (NIA) that can be considered as the RGB analog to Ethereum's fungible tokens created with the ERC20 standard.

First, it's helpful to provide a general layout that is common to all Schemata.

General Layout of a Schema. Global States and Assignments of Owned States are declared and preceded by their own Type which makes possible their referencing in Contract Operations which are declared just after them. Finally, the Validation Scripts and the strict type system used inside the Schema are declared.

We can observe that a Schema can be divided into several general sections:

• A header which contains:

  • An optional Root SchemaId which indicates a limited form of inheritance from some master schema.

  • A reserved Feature field which provides room for additional future extensions of contract schema.

• A first section in which all the State Types and related variables (both pertaining to Global and Assignments) and the Valencies are declared.
• A second section where all the possible Contract Operations referencing the previously declared State Types are encoded.
• A field containing the declaration of the Strict Type System being used in the whole schema.

• A last section containing the validation scripts for all the operations.

After this layout indication, we provide below the actual Rust Code of the schema. Each code section is provided with a numbered reference to an explanation paragraph reported below.

fn nia_schema() -> SubSchema {                      //  --->    (1) 
    
    // definitions of libraries and variables

    Schema {
        ffv: zero!(),                                                                           // --+                                                      
        subset_of: None,                                                                        //   |  (2) 
        type_system: types.type_system(),                                                       // --+     
        global_types: tiny_bmap! {                                                              // --+
            GS_NOMINAL => GlobalStateSchema::once(types.get("RGBContract.DivisibleAssetSpec")), //   |  
            GS_DATA => GlobalStateSchema::once(types.get("RGBContract.ContractData")),          //   |
            GS_TIMESTAMP => GlobalStateSchema::once(types.get("RGBContract.Timestamp")),        //   |  (3)
            GS_ISSUED_SUPPLY => GlobalStateSchema::once(types.get("RGBContract.Amount")),       //   |
        },                                                                                      // --+
        owned_types: tiny_bmap! {                                               // --+
            OS_ASSET => StateSchema::Fungible(FungibleType::Unsigned64Bit),     //   |  (4)
        },                                                                      // --+                    
        valency_types: none!(),                          //  --->   (5)
        genesis: GenesisSchema {                         //  --+ -------> Contract Operations declaration start here
            metadata: Ty::<SemId>::UNIT.id(None),        //    |
            globals: tiny_bmap! {                        //    |
                GS_NOMINAL => Occurrences::Once,         //    |  
                GS_DATA => Occurrences::Once,            //    |
                GS_TIMESTAMP => Occurrences::Once,       //    |
                GS_ISSUED_SUPPLY => Occurrences::Once,   //    |   (6) 
            },                                           //    |
            assignments: tiny_bmap! {                    //    |
                OS_ASSET => Occurrences::OnceOrMore,     //    | 
            },                                           //    |
            valencies: none!(),                          //  --+                 
        },
        extensions: none!(),                             //  --->  (7) 
        transitions: tiny_bmap! {                        //  --+      
            TS_TRANSFER => TransitionSchema {            //    |
                metadata: Ty::<SemId>::UNIT.id(None),    //    |
                globals: none!(),                        //    |
                inputs: tiny_bmap! {                     //    |
                    OS_ASSET => Occurrences::OnceOrMore  //    |   (8)
                },                                       //    |
                assignments: tiny_bmap! {                //    |
                    OS_ASSET => Occurrences::OnceOrMore  //    |
                },                                       //    |
                valencies: none!(),                      //    |
            }                                            //  --+ 
        },
        script: Script::AluVM(AluScript {                                                                 // -+
            libs: confined_bmap! { alu_id => alu_lib },                                                   //  |
            entry_points: confined_bmap! {                                                                // (9)
                EntryPoint::ValidateGenesis => LibSite::with(FN_GENESIS_OFFSET, alu_id),                  //  |
                EntryPoint::ValidateTransition(TS_TRANSFER) => LibSite::with(FN_TRANSFER_OFFSET, alu_id), // -+
            },
        }),
    }
}

(1) It is possible to observe that the nia_schema() function has an output of type SubSchema which indicates the application an optional single level of inheritance from a more general template. This way, a generic Schema that has many useful features, can be partially reused according to the needs of the issuer.

(2) In this section:

• ffv statement indicates the version of the contract.

• subset_of statement reflects the optional inheritance from a master contract template described at point 1. The type_system statement connects the strict type definition to the StandardType library of RGB.

(3) In this section global_state and its variables are declared, in particular:

• The token's GS_NOMINAL set of specifications which according to the Strict Type Library contain: the token full name , the ticker, some additional details, the digit precision of the asset.
• GS_DATA containing some additional contract data such as a contract disclaimer.

• GS_TIMESTAMP referring to the issuance date.

• GS_ISSUED_SUPPLY which defines the maximum cap to the token issuance.

• The Once statement guarantees that all these declarations are associated with a single value.

(4) In owned_type section, through the OS_ASSET statement, we can find the StateType declaration of the fungible token being transferred through the owned state assignment. The quantity of token used in the transfer is declared as a Fungible Type represented by a 64-bit unsigned integer.

(5) In this line a declaration of non-existence of valencies for the contract is made: valency_types: none!()

(6) This section of the contract schema marks the beginning of Contract Operations' declaration section. Starting from the operation allowed within genesis :

• No metadata are declared.

• The instantiation, inside the Genesis state, of all the variables of the Global State variables previously defined in code section (3).

• The declaration of the first assignment of the token using the previously declared type OS_ASSET.

• No Valencies are declared for this Genesis through the valencies: none!() statement.

(7) With extensions: none!() statement the schema embeds the absence of any State Extension operation.

(8) The transitions section provides the declaration of a single TS_TRANSFER operation which:

• Contains no metadata.

• Doesn't update the global state (it was defined only in Genesis).

• Takes as inputs at leas one or more OS_ASSET types.

• Declare the assignments of the very same OS_ASSET type as those of the inputs.

• Declare absence of Valencies committed inside the operation.

(9) In this final code section we can find the declaration of the a single AluVm script which is responsible for validating:

• The issuance of the maximum number of token in genesis.

• The validation of each TS_TRANSFER operation where the number of tokens in inputs must match the number declared in the assignment.