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The Quint specification language

Quint is a modern specification language that is a particularly good fit for distributed systems, such as blockchain protocols, distributed databases, and p2p protocols. Quint combines the robust theoretical basis of the Temporal Logic of Actions (TLA) with state-of-the-art type checking and development tooling.

Example code in Quint 🤶 🎁 🎅

Here is a small, partial, holiday special specification of the Secret Santa game:

module secret_santa {
  const participants: Set[str]

  /// get(recipient_for_santa, S) is the recipient for secret santa S
  var recipient_for_santa: str -> str

  /// the bowl of participants, containing a paper piece for each participant name
  var bowl: Set[str]

  val santas = recipient_for_santa.keys()
  val recipients = participants.map(p => get(recipient_for_santa, p))

  /// The initial state
  action init = all {
    recipient_for_santa' = Map(), // No santas or recipients
    bowl' = participants,         // Every participant's name in the bowl
  }

  action draw_recipient(santa: str): bool = {
    nondet recipient = oneOf(bowl)
    all {
      recipient_for_santa' = put(recipient_for_santa, santa, recipient),
      bowl' = bowl.exclude(Set(recipient)),
    }
  }

  action step = all {
    bowl.size() > 0,
    nondet next_santa = oneOf(participants.exclude(santas))
    draw_recipient(next_santa)
  }

  val everyone_gets_a_santa = (bowl.size() == 0).implies(participants == recipients)

  val no_person_is_self_santa = santas.forall(person =>
    get(recipient_for_santa, person) != person
  )

  val invariant = everyone_gets_a_santa and no_person_is_self_santa
}

module quint_team_secret_santa {
  import secret_santa(participants = Set("Gabriela", "Igor", "Jure", "Shon", "Thomas")).*
}

We can use this specification to check whether certain properties needed for a good game hold:

Checking if everyone gets a santa

Quint (with the help of Apalache) can check to ensure that after the bowl is empty, every participant has a santa! No kids crying when the gifts are exchanged 🎁.

echo '{ "checker": { "no-deadlocks": true } }' > config.json
quint verify quint_team_secret_santa.qnt --invariant=everyone_gets_a_santa --apalache-config=config.json
[ok] No violation found (2119ms).
You may increase --max-steps.

Checking if no one gets themself

This specification has no safeguards against people being their own santa! Quint (with the help of Apalache) can easily find a minimal example where this happens. Sorry kids, I hope you don't mind buying your own present 😢!

quint verify quint_team_secret_santa.qnt --invariant=no_person_is_self_santa
An example execution:

[State 0]
{
  quint_team_secret_santa::secret_santa::bowl: Set("Gabriela", "Igor", "Jure", "Shon", "Thomas"),
  quint_team_secret_santa::secret_santa::recipient_for_santa: Map()
}

[State 1]
{
  quint_team_secret_santa::secret_santa::bowl: Set("Igor", "Jure", "Shon", "Thomas"),
  quint_team_secret_santa::secret_santa::recipient_for_santa: Map("Gabriela" -> "Gabriela")
}

[violation] Found an issue (2047ms).
error: found a counterexample

Features

A simple and familiar syntax

to support engineers reading and writing specifications

An expressive type system

to ensure the domain model is coherent

A novel effect system

to ensure state updates are coherent

IDE support via LSP

giving real time feedback when writing specifications

A REPL

enabling interactive exploration of specifications

A simulator

enabling tests, trace generation, and exploration of your system

A symbolic model checker

to verify your specifications via Apalache

Motivation

Quint is inspired by TLA+ (the language) but provides an alternative surface syntax for specifying systems in TLA (the logic). The most important feature of our syntax is that it is minimal and regular, making Quint an easy target for advanced developer tooling and static analysis (see our design principles and previews of the tooling).

The syntax also aims to be familiar to engineers:

• At the lexical level, it borrows many principles from C-like languages.
• At the syntax level, it follows many principles found in functional languages.
• At the semantic level, Quint extends the standard programming paradigm with non-determinism and temporal formulas, which allow concise specification of protocol environments such as networks, faults, and time.

Thanks to its foundation in TLA and its alignment with TLA+, Quint comes with formal semantics built-in.

An example that highlights differences between Quint and TLA⁺

Quint:

type Status = Working | Prepared | Committed | Aborted

const ResourceManagers: Set[str]
var statuses: str -> Status

action init = {
  statuses' = ResourceManagers.mapBy(_ => Working)
}

val canCommit: bool = ResourceManagers.forall(rm => statuses.get(rm).in(Set(Prepared, Committed)))
val notCommitted: bool = ResourceManagers.forall(rm => statuses.get(rm) != Committed)

action prepare(rm) = all {
  statuses.get(rm) == Working,
  statuses' = statuses.set(rm, Prepared)
}

TLA⁺:

CONSTANT ResourceManagers
VARIABLE statuses

TCTypeOK == statuses \in [ResourceManagers -> {"working", "prepared", "committed", "aborted"}]

TCInit == statuses = [rm \in ResourceManagers |-> "working"]

canCommit == \A rm \in ResourceManagers : statuses[rm] \in {"prepared", "committed"}

notCommitted == \A rm \in ResourceManagers : statuses[rm] # "committed"

Prepare(rm) == /\ statuses[rm] = "working"
               /\ statuses' = [statuses EXCEPT ![rm] = "prepared"]

To learn more about Quint's motivation and design philosophy, watch this 15 minute presentation, delivered at Gateway to Cosmos in 2023.

Installation

1. Install the latest published version from npm:

   npm i @informalsystems/quint -g

2. Install IDE support for your editor:

   • VSCode
   • Emacs
   • Vim

3. Optionally, you may also install the VSCode plugin for visualizing traces.

Community

• Join the chat in the Telegram group or in the Zulip stream
• Join the Quint discussions on GitHub
• Contribute your spell to the collection of Quint spells
• Contribute to the development of Quint
• Join or co-design meetings: We hold fortnightly meetings with users and those interested in contributing to the design and development of Quint. Contact us if you would like an invitation.

Documentation

View the Quint documentation.

We aspire to having great, comprehensive documentation. At present, we have a good start, but still far to go. Please try what we have available and share with us any needs we have not yet been able to meet.

On "Quint"

Quint is short for 'quintessence', from alchemy, which refers to the fifth element. A lot of alchemy is about transmutation and energy, and Quint makes it possible to transmute specifications into executable assets and empower ideas to become referenced artifacts.

Acknowledgments

Quint has been designed and developed by the Apalache team: Gabriela Moreira, Igor Konnov, Jure Kukovec, Shon Feder, and Thomas Pani. ❤️

Thanks for notable contributions goes to Romain Ruetschi, Philip Offtermatt, Ivan Gavran, and, Ranadeep Biswas.

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Quint is developed at Informal Systems.

Supported by the Vienna Business Agency.