feat: add example for printing constraints

examples/print_constraints/circuit.go

@@ -0,0 +1,23 @@
+// Copyright 2020-2025 Consensys Software Inc.
+// Licensed under the Apache License, Version 2.0. See the LICENSE file for details.
+
+package print_constraints
+
+import (
+	"github.com/consensys/gnark/frontend"
+)
+
+// CubicCircuit defines a simple cubic equation circuit
+// x**3 + x + 5 == y
+type CubicCircuit struct {
+	X frontend.Variable `gnark:"x"`
+	Y frontend.Variable `gnark:",public"`
+}
+
+// Define declares the circuit constraints
+// x**3 + x + 5 == y
+func (circuit *CubicCircuit) Define(api frontend.API) error {
+	x3 := api.Mul(circuit.X, circuit.X, circuit.X)
+	api.AssertIsEqual(circuit.Y, api.Add(x3, circuit.X, 5))
+	return nil
+}

examples/print_constraints/doc.go

@@ -0,0 +1,10 @@
+// Package print_constraints demonstrates how to print constraints from a compiled constraint system.
+//
+// This example shows how to:
+//  1. Compile a circuit using frontend.Compile
+//  2. Type assert the compiled constraint system to access constraint-specific methods
+//  3. Retrieve and print constraints in a human-readable format
+//
+// This is useful for debugging circuits and understanding the constraint structure,
+// similar to the visual view available on https://play.gnark.io/
+package print_constraints

examples/print_constraints/print_constraints_test.go

@@ -0,0 +1,103 @@
+// Copyright 2020-2025 Consensys Software Inc.
+// Licensed under the Apache License, Version 2.0. See the LICENSE file for details.
+
+package print_constraints
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/consensys/gnark-crypto/ecc"
+	cs_bn254 "github.com/consensys/gnark/constraint/bn254"
+	"github.com/consensys/gnark/frontend"
+	"github.com/consensys/gnark/frontend/cs/r1cs"
+	"github.com/consensys/gnark/frontend/cs/scs"
+)
+
+// Example demonstrates how to print constraints from an R1CS constraint system.
+//
+// This example shows how to:
+//  1. Compile a circuit using frontend.Compile with r1cs.NewBuilder
+//  2. Type assert the compiled constraint system to cs_bn254.R1CS
+//  3. Retrieve constraints using GetR1Cs()
+//  4. Print each constraint using String() method with the constraint system as Resolver
+func Example() {
+	// Compile the circuit
+	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &CubicCircuit{})
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to compile circuit: %v\n", err)
+		return
+	}
+
+	// Type assert to get access to R1CS-specific methods
+	r1csSystem, ok := ccs.(*cs_bn254.R1CS)
+	if !ok {
+		fmt.Fprintf(os.Stderr, "constraint system is not an R1CS\n")
+		return
+	}
+
+	// Get all constraints
+	constraints := r1csSystem.GetR1Cs()
+
+	// Print constraint system statistics
+	fmt.Printf("Constraint System Type: R1CS\n")
+	fmt.Printf("Number of constraints: %d\n", len(constraints))
+	fmt.Printf("Number of public variables: %d\n", r1csSystem.GetNbPublicVariables())
+	fmt.Printf("Number of secret variables: %d\n", r1csSystem.GetNbSecretVariables())
+	fmt.Printf("Number of internal variables: %d\n", r1csSystem.GetNbInternalVariables())
+	fmt.Println()
+
+	// Print each constraint
+	fmt.Println("Constraints:")
+	fmt.Println("-----------")
+	for i, r1c := range constraints {
+		// The String() method requires a Resolver (the constraint system implements this)
+		// This formats the constraint as "L ⋅ R == O"
+		fmt.Printf("Constraint %d: %s\n", i, r1c.String(r1csSystem))
+	}
+
+}
+
+// ExampleSparseR1CS demonstrates how to print constraints from a SparseR1CS constraint system.
+//
+// This example shows how to:
+//  1. Compile a circuit using frontend.Compile with scs.NewBuilder (PLONK/SparseR1CS)
+//  2. Type assert the compiled constraint system to cs_bn254.SparseR1CS
+//  3. Retrieve constraints using GetSparseR1Cs()
+//  4. Print each constraint using String() method with the constraint system as Resolver
+func ExampleSparseR1CS() {
+	// Compile the circuit using SparseR1CS (PLONK) builder
+	ccs, err := frontend.Compile(ecc.BN254.ScalarField(), scs.NewBuilder, &CubicCircuit{})
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to compile circuit: %v\n", err)
+		return
+	}
+
+	// Type assert to get access to SparseR1CS-specific methods
+	scsSystem, ok := ccs.(*cs_bn254.SparseR1CS)
+	if !ok {
+		fmt.Fprintf(os.Stderr, "constraint system is not a SparseR1CS\n")
+		return
+	}
+
+	// Get all constraints
+	constraints := scsSystem.GetSparseR1Cs()
+
+	// Print constraint system statistics
+	fmt.Printf("Constraint System Type: SparseR1CS (PLONK)\n")
+	fmt.Printf("Number of constraints: %d\n", len(constraints))
+	fmt.Printf("Number of public variables: %d\n", scsSystem.GetNbPublicVariables())
+	fmt.Printf("Number of secret variables: %d\n", scsSystem.GetNbSecretVariables())
+	fmt.Printf("Number of internal variables: %d\n", scsSystem.GetNbInternalVariables())
+	fmt.Println()
+
+	// Print each constraint
+	fmt.Println("Constraints:")
+	fmt.Println("-----------")
+	for i, sparseR1c := range constraints {
+		// The String() method requires a Resolver (the constraint system implements this)
+		// This formats the constraint as "qL⋅xa + qR⋅xb + qO⋅xc + qM⋅(xaxb) + qC == 0"
+		fmt.Printf("Constraint %d: %s\n", i, sparseR1c.String(scsSystem))
+	}
+
+}