Unit, Option, Pattern matching

Optional values are a pervasive programming pattern in OCaml. Since Michelson and LIGO are both inspired by OCaml, optional types are available in LIGO as well. Similarly, OCaml features a unit type, and LIGO features it as well. Both the option type and the unit type are instances of a more general kind of types: variant types.

The unit Type

The unit type in Michelson or LIGO is a predefined type that contains only one value that carries no information. It is used when no relevant information is required or produced. Here is how it used.

In JsLIGO, the unique value of the unit type is []. The global variable unit contains [] so that name can be used for clarity, but the value is the same.

let u1 : unit = [];

let u2 : unit = unit;

let eq = (u1 == u2); // true

Discriminated union type

The simplest form of pattern matching in JsLIGO is with the help of a discriminated union type, which should be familiar for developers coming from TypeScript.

type foo =

{ kind: "increment", amount: int}

| { kind: "decrement", amount: int}

| { kind: "reset"};

Here, the kind field is unique among the objects. If not, an error will be generated. Also, if multiple fields are present which can be used as unique field, only the first unique field will be used.

Creating an object from a discriminated union type requires all the fields to be fully written. So for increment that would be:

let obj = { kind: "increment", amount: 3};

or

let obj2 = { kind: "reset" };

Pattern matching over a discriminated union type works like this:

function foo (item: foo) {

let state = 0;

switch(item.kind) {

case "increment":

state += item.amount;

break

case "decrement":

state -= item.amount;

break

case "reset":

state = 0;

break

}

}

Note that all cases of the discriminated union must be handled, if not an error will be generated.

These "strict" rules on discriminated union types help prevent bugs where cases are not handled correctly.

Variant types

A variant type is a user-defined or a built-in type (in case of options) that defines a type by cases, so a value of a variant type is either this, or that or... The simplest variant type is equivalent to the enumerated types found in Java, C++, JavaScript etc.

Here is how we define a coin as being either head or tail (and nothing else):

type coin = ["Head"] | ["Tail"];

let head: coin = Head();

let tail: coin = Tail();

The names Head and Tail in the definition of the type coin are called data constructors, or variants. In this particular, they carry no information beyond their names, so they are called constant constructors.

In general, it is interesting for variants to carry some information, and thus go beyond enumerated types. In the following, we show how to define different kinds of users of a system.

type id = nat;

type user =

["Admin", id]

| ["Manager", id]

| ["Guest"];

const u : user = Admin(1000n);

const g : user = Guest();

In JsLIGO, a constant constructor is equivalent to the same constructor taking an argument of type unit, so, for example, Guest () is the same value as Guest (unit).

There are cases where several sum types match a given constructor.

In the example below, types t1 to t6 are all possible types for x.

In this case, the compiler will choose one of these types as the type of the expression, and throw a warning stating that other types are possible.

You can add a type annotation to remove this ambiguity.

NOTE : The compiler will choose in priority the latest matching sum type in the current scope, if no type is defined in this scope, it will look in the latest module, if not in the second latest etc. Below, it will choose t1, and if t1 didn't match it would have chosen t2, otherwise t3, etc.

type t2 = ["A", int] | ["B", int];

namespace MyModule {

type t5 = ["A", int] | ["C", bool];

type t4 = ["A", int] | ["D", int];

namespace MySubModule {

type t6 = ["A", int] | ["E", mav];

}

}

namespace MySecondModule {

type t3 = ["A", int] | ["F", int];

}

type t1 = ["A", int] | ["G", mav];

// The compiler will search above for sum types with an 'A' constructor

const x = A(42);

Optional values

The option type is a predefined variant type that is used to express whether there is a value of some type or none. This is especially useful when calling a partial function, that is, a function that is not defined for some inputs. In that case, the value of the option type would be None, otherwise Some (v), where v is some meaningful value of any type. An example in arithmetic is the division operation:

function div (a: nat, b: nat): option<nat> {

if (b == 0n) return None() else return Some(a/b)

};

You can extract the value of a Some (v) with the function Option.unopt (Some (v)). In case the value is None, this will fail with an error.

The proper way to deal with optional values is by means of pattern matching.

Pattern matching

Pattern matching is similar to the switch construct in JavaScript, and can be used to route the program's control flow based on the value of a variant, record, tuple, or list.

A component of a pattern can be discarded by using a wildcard _ instead of a variable name.

LIGO will warn about unused variables bound in patterns in the same way that function arguments are warned about. Variable names beginning with _ can be used as a binder to prevent warnings.

Match on variants

Here is a function that transforms a colour variant type to an int.

type color =

| ["RGB", [int, int, int]]

| ["Gray", int]

| ["Default"];

const int_of_color = (c : color) : int =>

match(c) {

when(RGB(rgb)): 16 + rgb[2] + rgb[1] * 6 + rgb[0] * 36;

when(Gray(i)): 232 + i;

when(Default()): 0 };

The right-hand sides of each when-clause is an expression. Sometimes we might need statements to be processed before a value is given to the clause. In that case, the do expression comes handy. It enables the opening of a block of statements like a function body, that is, a block ended with a return statement whose argument has the value of the block, like so:

function match_with_block () {

let x = 1;

return

match(Some(1)) {

when(None()): failwith(1);

when(Some(org)): do {

let y = x + 1;

return y

}

};

};

Matching records or tuples

Fields of records and components of tuples can be destructured. Record pattern variables can be renamed.

type my_record = { a : int ; b : nat ; c : string }

type my_tuple = [int, nat, string]

let on_record = (v : my_record) : int =>

match (v) {

when ({ a ; b : b_renamed ; c : _c }): a + int(b_renamed)

}

let on_tuple = (v : my_tuple) : int =>

match (v) {

when ([x, y, _s]): x + int(y)

}

Matching lists

let weird_length = (v : list<int>) : int =>

match(v) {

when([]): -1;

when([hd, ...tl]): 1 + int(List.length(tl))

};

Deep patterns

Pattern matching can also be used for nested patterns.

type complex_t = { a : option<list<int>> ; b : list<int> }

const complex = (x: complex_t, y: complex_t) =>

match ([x,y]) {

when ([{a:None; b:_bl}, {a:_ar; b:_br}]): -1

when ([{a:_a; b:_b}, {a: Some ([]); b: [hd,...tl]}]): hd

when ([{a:_a; b:_b}, {a: Some ([hd,...tl]); b:[]}]): hd

when ([{a: Some (a); b:_b}, _l]) : int (List.length (a))

}