Message Fields

Message fields define the parameters that callers pass when invoking a message variant.

Field Syntax

Fields are defined inside curly braces, similar to struct fields:

    #[selector = 0xa9059cbb]
    Transfer { to: u256, amount: u256 } -> bool,

Each field has:

• A name (to, amount)
• A type (u256)

Multiple fields are separated by commas.

Supported Types

Message fields support all Fe types:

Primitive Types

msg Example {
    #[selector = 0x00000001]
    WithPrimitives {
        flag: bool,
        count: u256,
        signed_value: i128,
    },
}

Compound Types

msg Example {
    #[selector = 0x00000002]
    WithTuple { coords: (u256, u256) } -> bool,

}

No Fields

Variants can have no fields:

msg Query {
    #[selector = 0x18160ddd]
    TotalSupply -> u256,

    #[selector = 0x06fdde03]
    Name -> String,
}

Field Order Matters

Field order is significant for ABI encoding. The order in which fields are defined determines how calldata is decoded:

// These are different!
msg Example1 {
    #[selector = 0xa9059cbb]
    Transfer { to: u256, amount: u256 },
}

msg Example2 {
    #[selector = 0xa9059cbb]
    Transfer { amount: u256, to: u256 },
}

When implementing standard interfaces like ERC20, ensure field order matches the specification.

Accessing Fields in Handlers

In recv blocks, destructure fields to access their values:

    recv TokenMsg {
        Transfer { to, amount } -> bool {
            // 'to' and 'amount' are available here
            true
        }
    }

You can also rename fields during destructuring:

    recv TokenMsg {
        Transfer { to: recipient, amount: value } -> bool {
            // Use 'recipient' and 'value' instead
            true
        }
    }

Ignoring Fields

Use _ to ignore fields you don’t need:

    recv TokenMsg {
        Transfer { to, amount: _ } -> bool {
            // Only use 'to', ignore amount
            true
        }
    }

Use .. to ignore remaining fields:

    recv TokenMsg {
        TransferFrom { from, .. } -> bool {
            // Only use 'from', ignore to and amount
            true
        }
    }

Summary

Pattern	Meaning
{ field: Type }	Named field with type
{ a, b, c }	Multiple fields
(no braces)	No parameters
{ field }	Destructure keeping name
{ field: name }	Destructure with rename
{ field: _ }	Ignore specific field
{ field, .. }	Ignore remaining fields