# Debug and test circuits

## Common errors​

The most common error you may get while trying to create a proof is:

constraint is not satisfied: [(.. * ..) != (.. * ..) + (.. * ..) + (.. * ..)]

The error means the solver couldn't satisfy at least one of the constraints with the provided witness.

note

In some cases, you may encounter a couldn't solve computational constraint error, which means the solver couldn't perform an operation needed to verify a constraint. For example, a division by 0.

tip

You can run the program with -tags=debug to display a more verbose stack trace.

Test Engine

For a faster development workflow, it is possible (and recommended) to execute a circuit without running a zk-SNARK prover.

err := test.IsSolved(circuit, witness, field)

Under the hood it executes the instructions in the circuit, in plain Go, without generating a constraint system and calling a solver.

The easiest way to debug a circuit is to use api.Println(), which behaves like fmt.Println, except it outputs the values when they are solved. For example:

api.Println("A.X", pubKey.A.X)
note

With solving errors and api.Println, gnark outputs a stack trace which contain the exact line number to refer to in the circuit definition.

## Test​

You can implement tests as Go unit tests, in a _test.go file. For example:

// assert object wrapping testing.Tassert := test.NewAssert(t)// declare the circuitvar cubicCircuit Circuitassert.ProverFailed(&cubicCircuit, &Circuit{    PreImage:   42,    Hash:       42,})assert.ProverSucceeded(&cubicCircuit, &Circuit{    PreImage:   35,    Hash:       "16130099170765464552823636852555369511329944820189892919423002775646948828469",}, test.WithCurves(ecc.BN254))

See the test package documentation for more details.

In particular, the default behavior of the assert helper is to test the circuit across all supported curves and backends, ensure correct serialization, and cross-test the constraint system solver against a big.Int test execution engine.