Build a contract schema

This guide will show you how to build a smart contract schema, how to export it to a file, and/or embed the schema into the smart contract module, all using cargo-concordium.

Preparation

First, ensure you have cargo-concordium installed and if not the guide Install tools for development will help you.

We also need the Rust source code of the smart contract you wish to build a schema for.

Setup the contract for a schema

In order to build a contract schema, we first have to prepare our smart contract for building the schema.

You can choose which parts of the smart contract to include in the schema. For each init function, you can choose to include a schema for the parameter. And for each receive function, you can choose to include a schema for the parameter, the return value, or both.

Every type we want to include in the schema must implement the SchemaType trait. This is already done for all base types and some other types (see list of types implementing the SchemaType). For most other cases, it can also be derived automatically, using #[derive(SchemaType)]:

#[derive(SchemaType)]
struct SomeType {
    ...
}

Implementing the SchemaType trait manually only requires specifying one function, which is a getter for a schema::Type, which essentially describes how this type is represented as bytes and how to represent it.

Including schemas for init

To generate and include the schema for parameters for init functions, set the optional parameter attribute for the #[init(..)]-macro:

#[derive(SchemaType)]
enum InitParameter { ... }

#[init(contract = "my_contract", parameter = "InitParameter")]
fn contract_init<...> (...){ ... }

Including schemas for receive

To generate and include the schema for parameters or return values for receive functions, set the optional parameter or return_value attribute for the #[receive(..)]-macro:

#[derive(SchemaType)]
enum ReceiveParameter { ... }

#[derive(SchemaType)]
enum ReceiveReturnValue { ... }

#[receive(contract = "my_contract", name = "just_param", parameter = "String")]
fn contract_receive_just_param<...> (...) -> ReceiveResult<String> { ... }

#[receive(contract = "my_contract", name = "just_return", return_value = "Vec<u64>")]
fn contract_receive_just_return<...> (...) -> ReceiveResult<Vec<u64>> { ... }

#[receive(
    contract = "my_contract",
    name = "param_and_return",
    parameter = "ReceiveParameter",
    return_value = "ReceiveReturnValue"
)]
fn contract_receive_param_and_return<...> (...) -> ReceiveResult<ReceiveReturnValue> { ... }

Building the schema

Now, we are ready to build the actual schema using cargo-concordium, and we have the options to embed the schema and/or write the schema to a file.

See also

For more on which to choose see here.

Embedding the schema

In order to embed the schema into the smart contract module, we add --schema-embed to the build command

$cargo concordium build --schema-embed

If successful the output of the command will tell you the total size of the schema in bytes.

Outputting a schema file

To output the schema into a file, we can use the --schema-out=FILE where FILE is a path of the file to create:

$cargo concordium build --schema-out="/some/path/schema.bin"