Docs / Language Manual / Let Binding

You are currently looking at the v6.0 - v8.2 docs (Reason v3.6 syntax edition). You can find the latest manual page here.

(These docs are equivalent to the old BuckleScript docs before the ReScript rebrand)

Let Binding

A "let binding", in other languages, might be called a "variable declaration". let binds values to names. They can be seen and referenced by code that comes after them.

Reason (Old Syntax)ML (Older Syntax)JS Output
let greeting = "hello!";
let score = 10;
let newScore = 10 + score;

Block Scope

Bindings can be scoped through {}.

Reason (Old Syntax)ML (Older Syntax)JS Output
let message = {
  let part1 = "hello";
  let part2 = "world";
  part1 ++ " " ++ part2;
// `part1` and `part2` not accessible here!

The value of the last line of a scope is implicitly returned.

Design Decisions

ReScript's if, while and functions all use the same block scoping mecanism. The code below works not because of some special "if scope"; but simply because it's the same scope syntax and feature you just saw:

Reason (Old Syntax)ML (Older Syntax)JS Output
if (displayGreeting) {
  let message = "Enjoying the docs so far?";
// `message` not accessible here!

Bindings Are Immutable

Let bindings are "immutable", aka "cannot change". This helps our type system deduce and optimize much more than other languages (and in turn, help you more).

Binding Shadowing

The above restriction might sound unpractical at first. How would you change a value then? Usually, 2 ways:

The first is to realize that many times, what you want isn't to mutate a variable's value. For example, this JavaScript pattern:

var result = 0; result = calculate(result); result = calculateSomeMore(result); really just to comment on intermediate steps. You didn't need to mutate result at all! You could have just written this JS:

var result1 = 0; var result2 = calculate(result1); var result3 = calculateSomeMore(result2);

In ReScript, this obviously works too:

Reason (Old Syntax)ML (Older Syntax)JS Output
let result1 = 0;
let result2 = calculate(result1);
let result3 = calculateSomeMore(result2);

Additionally, reusing the same let binding name overshadows the previous bindings with the same name. So you can write this too:

Reason (Old Syntax)ML (Older Syntax)JS Output
let result = 0;
let result = calculate(result);
let result = calculateSomeMore(result);

(Though for the sake of clarity, we don't recommend this).

As a matter of fact, even this is valid code:

Reason (Old Syntax)ML (Older Syntax)JS Output
let result = "hello";
Js.log(result); // prints "hello"
let result = 1;
Js.log(result); // prints 1

The binding you refer to is whatever's the closest upward. No mutation here! If you need real mutation, e.g. passing a value around, have it modified by many pieces of code, we provide a slightly heavier mutation feature.

Private let bindings

Private let bindings are introduced in the release 7.2.

In the module system, everything is public by default, the only way to hide some values is by providing a separate signature to list public fields and their types:

module A: { let b: int } = { let a = 3 let b = 4 }

%private gives you an option to mark private fields directly

module A = { %%private(let a = 3) let b = 4 }

%private also applies to file level modules, so in some cases, users do not need to provide a separate interface file just to hide some particular values.

Note interface files are still recommended as a general best practice since they give you better separate compilation units and also they're better for documentation.

Still, %private is useful in the following scenarios:

  • Code generators. Some code generators want to hide some values but it is sometimes very hard or time consuming for code generators to synthesize the types for public fields.

  • Quick prototyping. During prototyping, we still want to hide some values, but the interface file is not stable yet, %private provide you such convenience.