Constraint-Driven Contract Interface DSL

Design and implement a self-contained constraint DSL that serves as a machine-consumable “interface layer” for Cardano smart contracts. The goal is to enable wallets, explorers, and other off-chain programs to construct transactions that are guaranteed to satisfy a script’s validation rules, without requiring bespoke per-dApp tx-building logic.

In Phase 1, the DSL will allow a contract author (or a third-party verifier) to publish a compact, deterministic specification of:

Datum and redeemer schemas (structured views over Data)

Intent/redeemer endpoints (e.g., Withdraw, Cancel, Update, etc.)

For each redeemer: the full set of constraints required for a successful spend/mint, including constraints over:

input/output UTxOs and their datum relationships (state transitions)

value preservation and token flow constraints

required signers / authorization predicates

validity interval / time bounds

reference inputs, script purpose, and required script context properties

The DSL will compile to an efficient binary format suitable for on-chain/off-chain distribution. Off-chain, we provide a reference toolchain that compiles the binary specification into a Z3 constraint system, and uses Z3 to produce witness assignments (concrete datums, redeemers, and transaction-shape parameters) that satisfy the constraints. Consumer applications can then use these witnesses to programmatically construct complete transactions that pass the target script’s validation logic.

Phase 1 is explicitly off-chain enforced: constraints are not checked by the ledger, but are intended to be sufficient for transaction construction and interoperability. The design will preserve forward compatibility for potential future extensions (e.g., on-chain commitment to interface hashes or native ledger support for typed/constraint-aware interfaces), while delivering immediate value by enabling generalized tx-building from published constraint modules.

TimelockV1 — Minimal Constraint DSL Example (Binary → Z3 → Witness → Tx)

This is a minimal, end-to-end example of a Phase 1 constraint module that describes everything an off-chain program needs to construct a transaction that will satisfy a script’s validation logic for a given redeemer.

---

DSL Module

module TimelockV1

// ----- Datum / Redeemer schemas over Data -----
datum TimelockDatum =
  Constr 0 {
    beneficiary    : Address,
    unlock_after   : Time   // POSIX millis
  }

purpose SpendingScript => redeemer Unlock = Constr 0 {}

// ----- Success conditions for redeemer Spend -----
endpoint Unlock {
  requires signer == datum.beneficiary
  requires tx.valid_from >= datum.unlock_after

  // Minimal value constraint: all value leaves the script (no continuing outputs)
  requires script.continuing_outputs.count == 0
}

---

Z3 Constraints (Solver View)

The compiler turns the endpoint into solver variables + constraints:

; Variables the builder must choose / the tx must satisfy
(declare-const signer Address)
(declare-const valid_from Int)
(declare-const continuing_outputs_count Int)

; Datum fields (these come from decoding the actual datum)
(declare-const beneficiary Address)
(declare-const unlock_after Int)

; Constraints
(assert (= signer beneficiary))
(assert (>= valid_from unlock_after))
(assert (= continuing_outputs_count 0))

(check-sat)
(get-model)

---

Z3 Witness Example

{
  "endpoint": "Unlock",
  "purpose": "SpendingScript",
  "tx": {
    "required_signers": ["addr1...beneficiary..."],
    "validity_start": 1746572854000,
    "validity_end": 1746573154000
  },
  "redeemer": { "constructor": 0, "fields": [] }
}