control-flow.mdx

title	description	canonical
If-Else & Loops	If, else, ternary, for, and while	/docs/manual/v11.0.0/control-flow

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:

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letmessage=ifisMorning { "Good morning!" } else { "Hello!" }

varmessage=isMorning ? "Good morning!" : "Hello!";

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

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ifshowMenu { displayMenu() }

if(showMenu){displayMenu();}

is basically the same as:

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ifshowMenu { displayMenu() } else { () }

if(showMenu){displayMenu()}

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

letresult=ifshowMenu { 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.

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letmessage=isMorning ? "Good morning!" : "Hello!"

varmessage=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.

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foriinstartValueInclusivetoendValueInclusive { Console.log(i) }

for(vari=startValueInclusive;i<=endValueInclusive;++i){console.log(i);}

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// prints: 1 2 3, one per lineforxin1to3 { Console.log(x) }

for(varx=1;x<=3;++x){console.log(x);}

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

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foriinstartValueInclusivedowntoendValueInclusive { Console.log(i) }

for(vari=startValueInclusive;i>=endValueInclusive;--i){console.log(i);}

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// prints: 3 2 1, one per lineforxin3downto1 { Console.log(x) }

for(varx=3;x>=1;--x){console.log(x);}

While Loops

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

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whiletestCondition { // body here }

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.

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letbreak=ref(false) while!break.contents { ifMath.random() > 0.3 { break:=true } else { Console.log("Still running") } }

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