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Built-in Functions

StoffelVM provides a set of built-in functions that are available to all StoffelLang programs. These functions handle common operations like I/O, array manipulation, and MPC-specific functionality.

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Console Output

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print

Outputs text to the console during program execution. Signature: 
print(message: string) -> nil
Example: 
def greet() -> nil:
    print("Hello, World!")
    print("Computation complete")
Notes:
  • Output is displayed during VM execution
  • Useful for debugging and progress tracking
  • Does not reveal secret values (they remain masked)

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Array Operations

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array_push

Adds an element to the end of an array. Signature: 
array_push(array: [T], element: T) -> nil
Example: 
var numbers: [int64] = []
array_push(numbers, 10)
array_push(numbers, 20)
array_push(numbers, 30)
# numbers is now [10, 20, 30]
Notes:
  • Uses 0-based indexing (updated in recent versions)
  • Modifies the array in place
  • Works with both clear and secret element types

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array_len

Returns the length of an array. Signature: 
array_len(array: [T]) -> int64
Example: 
var items: [int64] = [1, 2, 3, 4, 5]
var count = array_len(items)  # count = 5

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ClientStore Operations

The ClientStore is a built-in singleton that provides access to client inputs in MPC computations. It acts as the bridge between external client data and the secure computation.

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ClientStore.get_number_clients

Returns the total number of clients participating in the MPC computation. Signature: 
ClientStore.get_number_clients() -> int64
Returns: A clear (non-secret) int64 value representing the client count. Example: 
main main() -> nil:
    var num_clients: int64 = ClientStore.get_number_clients()
    print("Clients connected")
Notes:
  • Returns a clear (public) value, not a secret
  • Can be assigned to regular variables
  • Useful for dynamic iteration over client inputs

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ClientStore.take_share

Retrieves a secret share from a specific client at a given index. Signature: 
ClientStore.take_share(client_id: int64, share_index: int64) -> secret int64
Parameters:
  • client_id: The ID of the client (0-indexed)
  • share_index: The index of the share from that client (0-indexed)
Returns: A secret int64 value. Example: 
main main() -> nil:
    # Get first share from client 0
    secret var input1 = ClientStore.take_share(0, 0)

    # Get first share from client 1
    secret var input2 = ClientStore.take_share(1, 0)

    # Get second share from client 0
    secret var input3 = ClientStore.take_share(0, 1)
Critical Constraint: Results from ClientStore.take_share() must be assigned to secret variables. The compiler enforces this requirement. Valid usage patterns: 
# 1. Using 'secret var' keyword
secret var share1 = ClientStore.take_share(0, 0)

# 2. Using explicit secret type annotation
var share2: secret int64 = ClientStore.take_share(1, 0)

# 3. Reassigning to existing secret variable
secret var mutable_share: int64 = ClientStore.take_share(0, 0)
mutable_share = ClientStore.take_share(1, 0)  # Valid
Invalid usage (compiler error): 
# ERROR: Cannot assign take_share result to non-secret variable
var share: int64 = ClientStore.take_share(0, 0)

# ERROR: Implicit type doesn't make it secret
var share = ClientStore.take_share(0, 0)

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MPC-Specific Builtins

These functions are available when running in MPC mode and interact with the underlying protocol.

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Share Operations

When values are declared as secret, the VM automatically handles:
  • Secret sharing: Splitting values into shares across parties
  • Share arithmetic: Computing on shares using MPC protocols
  • Reconstruction: Combining shares to produce final outputs
The programmer doesn’t directly call share operations—they happen automatically when working with secret types.

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Protocol Integration

StoffelVM integrates with the HoneyBadger MPC protocol through internal hooks:
OperationInternal FunctionDescription
Additionmpc_addAdds two secret values
Subtractionmpc_subSubtracts two secret values
Multiplicationmpc_mulMultiplies using Beaver triples
Comparisonmpc_cmpCompares two secret values
These are invoked automatically by the VM when operating on secret types.

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Type Conversion Builtins

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int_to_string (Planned)

Convert an integer to its string representation. 
var num = 42
var text = int_to_string(num)  # "42"

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string_to_int (Planned)

Parse a string as an integer. 
var text = "123"
var num = string_to_int(text)  # 123

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Memory and Lifecycle

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Automatic Memory Management

StoffelVM manages memory automatically:
  • Values are allocated on the stack or heap as needed
  • Arrays grow dynamically with array_push
  • Memory is reclaimed when values go out of scope

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Register Architecture

The VM uses a dual-register architecture:
  • Clear registers: For public/non-secret values
  • Secret registers: For MPC-protected values
Built-in functions automatically select the appropriate register set based on operand types.

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FFI Integration

StoffelVM supports calling external functions through Foreign Function Interface (FFI): 
// Rust side: Register a custom builtin
vm.register_builtin("custom_hash", |args| {
    // Implementation
});

# StoffelLang side: Call the custom builtin
var result = custom_hash(data)
See the Architecture - FFI section for details on implementing custom builtins.

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Summary Table

FunctionInput TypesReturn TypeDescription
printstringnilConsole output
array_push[T], TnilAdd element to array
array_len[T]int64Get array length
ClientStore.get_number_clients-int64 (clear)Get client count
ClientStore.take_shareint64, int64secret int64Get client share