Docs / Language Manual / If-Else & Loops
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You are currently looking at the < v8.2.0 docs (Reason v3.6 syntax edition). You can find the latest manual page here.

(These docs cover all versions between v3 to v8 and are equivalent to the old BuckleScript docs before the rebrand)

If-Else & Loops

ReScript supports if, else, ternary expression (a ? b : c), for and while.

ReScript also supports our famous pattern matching, which will be covered in its own section

If-Else & Ternary

Unlike its JavaScript counterpart, ReScript's if is an expression; they evaluate to their body's content:

Reason (Old Syntax)ML (Older Syntax)JS Output
 
let message = if (isMorning) {
  "Good morning!"
} else {
  "Hello!"
};

Note: an if-else expression without the final else branch implicitly gives () (aka the unit type). So this:

Reason (Old Syntax)ML (Older Syntax)JS Output
 
if (showMenu) {
  displayMenu();
};

is basically the same as:

Reason (Old Syntax)ML (Older Syntax)JS Output
 
if (showMenu) {
  displayMenu();
} else {
  ();
};

Here's another way to look at it. This is clearly wrong:

RE
let result = if (showMenu) { 1 + 2; };

It'll give a type error, saying basically that the implicit else branch has the type unit while the if branch has type int. Intuitively, this makes sense: what would result's value be, if showMenu was false?

We also have ternary sugar, but we encourage you to prefer if-else when possible.

Reason (Old Syntax)ML (Older Syntax)JS Output
 
let message = isMorning ? "Good morning!" : "Hello!"

if-else and ternary are much less used in ReScript than in other languages; Pattern-matching kills a whole category of code that previously required conditionals.

For Loops

For loops iterate from a starting value up to (and including) the ending value.

Reason (Old Syntax)ML (Older Syntax)JS Output
 
for (i in startValueInclusive to endValueInclusive) {
  Js.log(i);
};
Reason (Old Syntax)ML (Older Syntax)JS Output
 
// prints: 1 2 3
for (x in 1 to 3) {
  Js.log(x);
  Js.log(" ");
};

You can make the for loop count in the opposite direction by using downto.

Reason (Old Syntax)ML (Older Syntax)JS Output
 
for (i in startValueInclusive downto endValueInclusive) {
  Js.log(i);
};
Reason (Old Syntax)ML (Older Syntax)JS Output
 
// prints: 3 2 1
for (x in 3 downto 1) {
  Js.log(x);
  Js.log(" ");
};

While Loops

While loops execute its body code block while its condition is true.

Reason (Old Syntax)ML (Older Syntax)JS Output
 
while (testCondition) {
  // body here
};

Tips & Tricks

There's no loop-breaking break keyword (nor early return from functions, for that matter) in ReScript. However, we can break out of a while loop easily through using a mutable binding.

Reason (Old Syntax)ML (Older Syntax)JS Output
 
let break = ref(false);

while (!break.contents) {
  if (Js.Math.random() > 0.3) {
    break := true;
  } else {
    Js.log("Still running");
  };
};