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:
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:
RESlet 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")
}
}