Circuit structure

A gnark circuit must implement the frontend/Circuit interface:

type Circuit interface {
    // Define declares the circuit's Constraints
    Define(api frontend.API) error
}

The circuit must declare its public and secret inputs as frontend.Variable:

type MyCircuit struct {
    C myComponent
    Y frontend.Variable `gnark:",public"`
}

type myComponent struct {
    X frontend.Variable
}

func (circuit *MyCircuit) Define(api frontend.API) error {
    // ... see Circuit API section
}

At compile time, frontend.Compile(...) recursively parses the struct fields that contains frontend.Variable to build the frontend.constraintSystem.

By default, a frontend.Variable has the gnark:",secret" visibility.

Struct tags

Similar to standard Go packages (like encoding/json), struct fields can have tags which adds important metadata to input declarations.

Other tag options:

// omits Y, frontend.Compile will not instantiate a new variable in the ConstraintSystem
// this can be useful when a Variable is referenced in multiple places but we only wish to instantiate it once
Y frontend.Variable `gnark:"-"`

// embeds a Variable or struct
// can be helpful for test purposes, where one may want to test part of a circuit and redefine
// the Define method on another struct while keeping the same inputs
type circuitSignature struct {
    Circuit `gnark:",embed"`
}