構造化代入 · Reason

Destructuring is a concise way to extract data from a structure. Destructuring can be used anywhere a variable name might appear.

Tuples

let pair = (1, "Hello");

/* Destructuring */
let (one, hello) = pair;

/* Prints: Hello */
print_endline(hello);

Records

Destructuring is very commonly used with records.

type person = {
  name: string,
  age: int,
};

let alice = {
  name: "Alice",
  age: 42,
};

/* Destructuring */
let {name, age} = alice;

/* Prints: Alice */
print_endline(name);

Note: When destructuring a record the type needs to be in scope or the record needs to be explicitly annotated.

Renaming Record Fields

While destructuring record fields, it is possible to bind variable names that are different than the record field names:

let {name: legalName, age} = alice;

/* Prints: Alice */
print_endline(legalName);

Function Arguments

/* Without destructuring */
let hello = (person) => {
  print_endline("Hello " ++ person.name);
};

/* With destructuring */
let hello = ({name}) => {
  print_endline("Hello " ++ name);
};

/* With destructuring and aliasing */
let hello = ({name: legalName}) => {
  print_endline("Hello " ++ legalName);
};