Non Inflatable Fungible Asset Schema

In this section we will look more closely to an actual example of an RGB Contract Schema written in Rust and contained in the nia.rs file from the RGB Schemata Repository containing other Schema template as well. This Schema allows for the contract setup of non-inflationary fungible assets (NIA) that can be considered as the RGB analogue to Ethereum's fungible tokens created with ERC20 standard.

First it's helpful to provide a general layout which is common to all schema.

General Layout of a Schema. Global States and Assignments of Owned States are declared each one preceded by their own Type which make 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 in several general parts:

• An header which contain:

  • A Root SchemaId which indicates an optional 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 {                         //  --+
            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 feature, 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 reflect the optional inheritance from a master contract template described at point 1. The type_system statement connect the strict type definition to the StandardType library of RGB.

3. In this section global_state and its variable are declared, in particular:

   • The token's GS_NOMINAL set of specification which tanks to the strict type library contain inside:

     • 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 this declaration pertain to an updatable State rather then an accumulating State.

4. In owned_type section, through the OS_ASSET statement, we can find the State type declaration of the fungible token being transferred through owned state assignment. The quantity of token used in 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 the declaration of Contract Operations:

   • Starting from the genesis :

     • No metadata are declared.

       • the declaration, inside the Genesis state, of all the variable 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 provide 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 to validate:

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

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