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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 option:

// 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:"-"`