Control Flow

ReScript supports if, else, ternary expression, 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:

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:

if showMenu {
  displayMenu()
}

is basically the same as:

if showMenu {
  displayMenu()
} else {
  ()
}

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

RES
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.

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.

for i in startValueInclusive to endValueInclusive {
  Js.log(i)
}

// 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.

for i in startValueInclusive downto endValueInclusive {
  Js.log(i)
}

// 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.

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.

let break = ref(false)

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