Compact Under the Hood: How Midnight’s Smart Contract Language Encodes Privacy by Default

The Thesis

Compact is not a general-purpose programming language. It is a domain-specific language designed to express zero-knowledge circuits that execute on Midnight Network. Its core value lies in enforcing privacy by default, where all data is treated as confidential unless explicitly disclosed, enabling developers to build applications that balance verifiability with data protection.

The Constraint That Shapes Everything

If you've written JavaScript or TypeScript, Compact feels familiar at first. The syntax looks recognizable, but the execution model is completely different.

Compact doesn’t run like traditional smart contract languages such as Solidity. It compiles into circuits that generate proofs using Zero-Knowledge Proof systems.

That single constraint changes everything.

You’re not writing instructions for a virtual machine. You’re defining what must be proven.

The Anatomy of a Compact Program

Every Compact contract follows a predictable structure. Let’s start with the minimal example:

pragma language_version >= 0.16 && <= 0.18;

import CompactStandardLibrary;

export ledger message: Opaque<"string">;

export circuit storeMessage(customMessage: Opaque<"string">): [] {
  message = disclose(customMessage);
}

Six lines. Fully deployable.

Let’s break it down.

Pragma: Version Control for Determinism

pragma language_version >= 0.16 && <= 0.18;

This ensures your contract compiles against a known version of the language.

Why it matters:

Compact evolves quickly
Minor updates can introduce breaking changes
Deterministic compilation is critical for reproducible proofs

Imports: Accessing the Standard Library

import CompactStandardLibrary;

This gives you access to:

Data structures (Map, Set, Counter)
Cryptographic helpers
Utility circuits

Unlike traditional languages, libraries are minimal and tightly scoped to what can be efficiently proven.

Ledger: Where Public State Lives

Compact does not use a contract body like Solidity. Instead, state is declared at the top level.

export ledger message: Opaque<"string">;

This defines:

A persistent, on-chain variable
Stored on Midnight’s public ledger

Key properties:

export → readable by external applications
sealed → immutable after deployment

Example:

export sealed ledger owner: Bytes<32>;

This is set once and never changed.

Circuits: Functions That Become Proofs

In Compact, functions are called circuits:

export circuit storeMessage(customMessage: Opaque<"string">): [] {
  message = disclose(customMessage);
}

What actually happens:

The user executes this locally
A proof is generated
The network verifies the proof
The state updates if valid

No raw computation is re-executed on-chain.

This is the difference between:

Execution-based systems (Ethereum)
Proof-based systems (Midnight)

Data Types: Designed for Proof Efficiency

Compact’s type system is intentionally restrictive.

Primitive Types

Field → base unit for computation
Uint<N> → bounded integers
Boolean → true/false
Bytes<N> → fixed-length data

Opaque Types

Opaque<"string">

These are critical:

Data is not readable inside circuits
Treated as hashed values
Preserves privacy while enabling verification

Witnesses: Private Data Without Exposure

This is where things diverge sharply from traditional models.

witness get_user_input(): Field;

A witness:

Exists off-chain
Runs locally on the user’s machine
Supplies private inputs to the circuit

The network never sees the actual value.

It only verifies that:

“A valid value exists and was used correctly.”

The `disclose` Mechanism: Explicit Transparency

Compact enforces a strict rule:

Everything is private unless explicitly made public.

message = disclose(customMessage);

Without disclose, the compiler rejects the code.

Why this matters:

Prevents accidental data leaks
Forces intentional design decisions
Aligns with Midnight’s selective disclosure model

Control Flow and Privacy Constraints

Even logic must respect privacy boundaries.

if (disclose(caller == owner)) {
  // allowed
}

Without disclose, this fails.

Because:

Control flow can leak information
The compiler enforces safe disclosure

Assertions: Enforcing Validity

assert(amount > 0, "Amount must be positive");

If the condition fails:

The proof is invalid
The transaction is rejected

This guarantees correctness without exposing internal values.

The Execution Model: Proof, Not Execution

Traditional blockchains:

Execute every transaction
Replicate computation across nodes

Midnight:

One party computes
Generates a proof
Network verifies

This improves:

Efficiency (less computation per node)
Throughput (parallelizable validation)
Privacy (no raw data exposure)

The "Why": What Problem Compact Solves

Most smart contract platforms force a tradeoff:

Transparency → no privacy
Privacy → no verifiability

Compact removes that constraint.

It enables:

Private financial logic
Confidential business processes
Verifiable off-chain computation

Without exposing:

Inputs
Intermediate state
Sensitive relationships

Contextual Analysis: Where Compact Fits

Compact is not competing with Solidity or Rust directly.

It operates in a different category:

ZK-native smart contract languages

Compared to alternatives:

Solidity → execution-based
Cairo (Starknet) → circuit-focused but lower-level
Compact → higher-level abstraction with privacy enforcement

Its advantage:

Developers don’t need deep cryptography knowledge
Privacy is built into the language itself

Roadmap & Hurdles

Compact is still evolving alongside Midnight Network, and adoption depends on a few key factors:

Challenges

Proof generation latency (especially on consumer devices)
Developer onboarding (new mental model)
Tooling maturity

Opportunities

Growing demand for privacy-preserving applications
Integration with regulated environments
Expansion of ZK infrastructure

Conclusion

Compact represents a shift in how developers think about smart contracts.

You’re no longer writing code to be executed publicly.

You’re defining logic that can be proven without being revealed.

That shift matters.

Because the future of blockchain isn’t just about transparency.

It’s about selective disclosure, where privacy and verification coexist.

Compact is the language that makes that possible.