# Circuit generation

In circuit generation mode, the has_witness field of RunState is set to the default CircuitGeneration, and the program of the user is ran to completion.

During the execution, the different snarky functions called on RunState will create internal variables as well as constraints.

## Creation of variables

Variables can be created via the compute() function, which takes two arguments:

• A TypeCreation toggle, which is either set to Checked or Unsafe. We will describe this below.
• A closure representing the actual computation behind the variable. This computation will only take place when real values are computed, and can be non-deterministic (e.g. random, or external values provided by the user). Note that the closure takes one argument: a WitnessGeneration, a structure that allows you to read the runtime values of any variables that were previously created in your program.

The compute() function also needs a type hint to understand what type of snarky type it is creating.

It then performs the following steps:

• creates enough CVar to hold the value to be created
• retrieves the auxiliary data needed to create the snarky type (TODO: explain auxiliary data) and create the snarky variable out of the CVars and the auxiliary data
• if the TypeCreation is set to Checked, call the check() function on the snarky type (which will constrain the value created), if it is set to Unsafe do nothing (in which case we’re trusting that the value cannot be malformed, this is mostly used internally and it is highly-likely that users directly making use of Unsafe are writing bugs)

Note

At this point we only created variables to hold future values, and made sure that they are constrained. The actual values will fill the room created by the CVar only during the witness generation.

## Constraints

All other functions exposed by the API are basically here to operate on variables and create constraints in doing so.