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NPL Cheat Sheet

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This is a non-exhaustive list of the most important NPL features and how to use them, as briefly as possible. Non-standard whitespace and short identifiers are used to support printability, so the code style should not be emulated.

Protocols

Declaration

protocol[party1, party2] Foo(
  var arg: Number,
  var arg2: Text
) { /*..*/ };

Instantiation

var f = Foo[p, q](42, "b");
// With named arguments:
var f2 = Foo[
  party1 = p,
  party2 = q
](arg2 = "b", arg = 42);

@api-annotated protocols can also be instantiated from the API.

Permissions

Declaring permissions

protocol[a, b] Bar() {
  permission[a] foo(
    x: Text
  ) returns Text {
    /*..*/
    return "foo";
  }
  permission[a | b] bar() { /*..*/ }
}

Invoking permissions

Permissions can be invoked from within NPL:

var b = Bar[alice, bob]();
var x = b.foo[alice]("x");
b.bar[bob]();

@api-annotated permissions can also be invoked from the API.

Require conditions

Permissions can be safeguarded by require conditions, and are great for validating input:

permission[p] spend(
  amount: Number
) {
  require(
    (amount > 0),
    "Amount must be positive"
  );
}

States

Declaring states

protocol[p, q] MyProtocol() {
  initial state unpaid;
  state billed;
  state reminded;
  final state paid;
  final state forgiven;
}

States as guards

States can be used to only allow permission when the parent protocol is in specific states:

permission[p] pay(
  amount: Number
) | unpaid, billed, reminded {
  //...
};

State transitions

permission[p] pay2(
  amount: Number
) | unpaid, billed, reminded {
  if (amount > 42) { become paid; };
}

Interacting with states

Protocol types expose a States enum, and protocol instances expose some helper methods:

protocol[p] Qux() {
  initial state a;
  final state b;
}
var q = Qux[p]();
test.assertEquals(
  listOf(
    Qux.States.a,
    Qux.States.b
  ),
  Qux.States.variants()
);
test.assertEquals(
  optionalOf(Qux.States.a),
  q.initialState()
);
test.assertEquals(
  setOf(Qux.States.b),
  q.finalStates()
);
test.assertEquals(
  optionalOf(Qux.States.a),
  q.activeState()
);

Party system

Claims

Claims are runtime concepts that are used to determine whether a party is allowed to interact with a protocol instance or permission.

Refer to the standard library reference for examples.

Observers

Observers are parties that are allowed to observe and read a protocol instance. They need to be explicitly added or removed using permissions. All protocol instances have an implicit (empty) observers field of type Map<Text, Party>.

permission[*newObs & issuer] addObs(
  name: Text
) {
  observers =
    observers.with(name, newObs);
}
permission[issuer] removeObs(
  name: Text
) {
  observers =
    observers.without(name);
}

Types

This section is non-exhaustive and only covers the most important types. For a complete list, refer to the standard library reference.

Basic types

var a: Number = 42 + 0.01 * 5;
var b: Text =
  "foo" + "bar" + a.toText();
var c: Boolean =
  true || ((5 > 3) && false);
var d: LocalDate =
  localDateOf(2021, 1, 1);
var e: DateTime = dateTimeOf(
  2021, 1, 1, 12, 0,
  valueOfZoneId(
    ZoneId.EUROPE_ZURICH
  )
);
var f: Duration = millis(4)
  .plus(seconds(2))
  .multiplyBy(50)
  .plus(minutes(59))
  .minus(hours(1));
var g: Period = days(42)
  .plus(weeks(2))
  .minus(months(1))
  .multiplyBy(2);

Optionals

Optional<T> is a typesafe wrapper for values that may be either Some<T> or None, where T is the type of the wrapped value.

var s: Optional<Number> =
  optionalOf(42);
var n = optionalOf<Number>();
t.assertEquals(42, s.getOrElse(1));
t.assertEquals(1, n.getOrElse(1));
t.assertEquals(42, s.getOrFail());
t.assertFails(
  function() -> { return n.getOrFail(); }
);
t.assertTrue(s.isPresent());
t.assertFalse(n.isPresent());

Collections

var l: List<Number> = listOf(1, 1);
var s: Set<Number> = setOf(1, 2);
var m: Map<Text, Number> = mapOf(
  Pair("foo", 1),
  Pair("bar", 2)
);

See the relevant docs or use the IDE to explore the supported receiver methods.

User-defined types

Structs

Structs store arbitrary data in an organized way. They are immutable and belong to particular protocol instances.

// Declaration
struct St {
  a: Number,
  b: Text
}
// Instantiation
var st: St = St(42, "foo");
var st2: St = St(b = "s", a = 42);
// Copying/overwriting
var st3: St = st.copy(b = "t");

Enums

// Declaration
enum Color { Red, Blue }
// Instantiation
var blue: Color = Color.Blue;
// Matching
var c: Text = match(blue) {
  Color.Red -> "red"
  Color.Blue -> "blue"
};

Unions

// Declaration
union U { Number, Text }
// Instantiation
var u1: U = U(42);
var u2: U = U("foo");

Identifiers

Identifiers define opaque, typed identifiers.

// Declaration
identifier Id;
// Instantiation
var id = Id();
// Usage
var m = mapOf(Pair(id, "foo"));

Symbols

The Symbol type is used to represent Number values with a unit.

// Declaration
symbol usd;
// Instantiation
var x: usd = usd(4);

Control flow

if (a == b) {/*..*/};
if (a != b) {/*..*/} else {/*..*/};
for (item in collection) {/*..*/};
match (c) {
  Color.Blue -> {/*..*/}
  Color.Red -> {/*..*/}
};
match(c) {
  Color.Red -> {/*..*/}
  else -> {/*..*/}
};

Functions and lambdas

Functions can be defined on the top-level or within protocols. They support type inference and the body can be an expression.

function e(a: Text) returns Text -> {
  return a + "!";
}
function p2(a: Number) -> a + 2;

Anonymous functions use the same syntax but omit the name, and can be used as values:

var l = function(a: Number) -> a * 3;
t.assertEquals(
  listOf(1, 2, 3).map(l),
  listOf(3, 6, 9)
);

Logging

Log the text representation of any value:

debug(42);
info("foo");
error(Foo[p, q](42, "b"));

Testing

@test
function testStuff(t: Test) -> {
  t.assertEquals(1, 1);
}

Read the relevant docs or use the IDE autocompletion to learn more about other supported assertions