How to view compiled constraint system?

[sshravan]

Hi! Is there a way to view the compiled constraint system in gnark?

I am looking for something similar to the visual view available on https://play.gnark.io/, as mentioned in this PR discussion.

[sshravan]

Looks like one can use GetR1Cs() or GetSparseR1Cs() as follows:

	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), scs.NewBuilder, circuit)
	scs := ccs.(*cs.R1CS)
	constraints := scs.GetR1Cs()
	// scs := ccs.(*cs.SparseR1CS)
	// constraints := scs.GetSparseR1Cs()
	for _, r1c := range constraints {
		fmt.Println(r1c.String(scs))
	}

Happy to learn if there are better ways to do this!

[ivokub]

In general there is no better method, unfortunately. Another option to avoid type asserting particular types is to see if the circuit implements the interfaces a la:

import (
	"testing"

	"github.com/consensys/gnark-crypto/ecc"
	"github.com/consensys/gnark/constraint"
	"github.com/consensys/gnark/frontend"
	"github.com/consensys/gnark/frontend/cs/r1cs"
	"github.com/consensys/gnark/frontend/cs/scs"
	"github.com/consensys/gnark/test"
)

type Circuit struct {
	A, B frontend.Variable `gnark:",secret"`
}

func (c *Circuit) Define(api frontend.API) error {
	res := api.Add(c.A, c.B)
	for i := 0; i < 8; i++ {
		res = api.Mul(res, res)
	}
	api.AssertIsEqual(res, 10)
	return nil
}

func TestPrint(t *testing.T) {
	assert := test.NewAssert(t)
	circuit := &Circuit{}
	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, circuit)
	assert.NoError(err)
	tscs, ok := ccs.(interface{ GetR1Cs() []constraint.R1C })
	assert.True(ok)
	constraints := tscs.GetR1Cs()
	for _, r1c := range constraints {
		t.Log(r1c.String(ccs))
	}
}

func TestPrintPlonk(t *testing.T) {
	assert := test.NewAssert(t)
	circuit := &Circuit{}
	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), scs.NewBuilder, circuit)
	assert.NoError(err)
	tscs, ok := ccs.(interface{ GetSparseR1Cs() []constraint.SparseR1C })
	assert.True(ok)
	constraints := tscs.GetSparseR1Cs()
	for _, r1c := range constraints {
		t.Log(r1c.String(ccs))
	}
}

I think we could add a shorthand (a la ccs.PrintConstraint(i int)), and imo we historically even had it, but removed for some reason.

The least we could do is to add a good example in the examples package.