1# Frontend
 2
 3The frontend interface allows to define the circuit to be folded. The available frontends are arkworks are Circom.
 4
 5We just need to fulfill the [`FCircuit` trait](https://github.com/privacy-scaling-explorations/sonobe/blob/main/sonobe/src/frontend/mod.rs):
 6
 7```rust
 8/// FCircuit defines the trait of the circuit of the F function, which is the one being folded (ie.
 9/// inside the agmented F' function).
10/// The parameter z_i denotes the current state, and z_{i+1} denotes the next state after applying
11/// the step.
12pub trait FCircuit<F: PrimeField>: Clone + Debug {
13    type Params: Debug;
14
15    /// returns a new FCircuit instance
16    fn new(params: Self::Params) -> Self;
17
18    /// returns the number of elements in the state of the FCircuit, which corresponds to the
19    /// FCircuit inputs.
20    fn state_len(&self) -> usize;
21
22    /// computes the next state values in place, assigning z_{i+1} into z_i, and computing the new
23    /// z_{i+1}
24    fn step_native(
25        // this method uses self, so that each FCircuit implementation (and different frontends)
26        // can hold a state if needed to store data to compute the next state.
27        &self,
28        i: usize,
29        z_i: Vec<F>,
30    ) -> Result<Vec<F>, Error>;
31
32    /// generates the constraints for the step of F for the given z_i
33    fn generate_step_constraints(
34        // this method uses self, so that each FCircuit implementation (and different frontends)
35        // can hold a state if needed to store data to generate the constraints.
36        &self,
37        cs: ConstraintSystemRef<F>,
38        i: usize,
39        z_i: Vec<FpVar<F>>,
40    ) -> Result<Vec<FpVar<F>>, SynthesisError>;
41}
42```
43