# Witness generation

In snarky, currently, the same code is run through again to generate the witness.

That is, the RunState contains a few changes:

• public_input: Vec<F>: now contains concrete values (instead of being empty).
• has_witness: is set to WitnessGeneration.

Additionaly, if we want to verify that the arguments are actually correct (and that the program implemented does not fail) we can also set eval_constraints to true (defaults to false) to verify that the program has a correct state at all point in time.

If we do not do this, the user will only detect failure during proof generation (specifically when the composition polynomial is divided by the vanishing polynomial).

Note

This is implemented by simply checking that each generic gate encountered is correct, in relation to the witness values observed in that row. In other words (extrapolated to the double generic gate). Note that other custom gates are not checked, as they are wrapped by gadgets which fill in witness values instead of the user. Thus there is no room for user error (i.e. the user entering a wrong private input).

Due to the has_witness variable set to WitnessGeneration, functions will behave differently and compute actual values instead of generating constraints.