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Function	Function syntax in ReScript	/docs/manual/v11.0.0/function

Function

Cheat sheet for the full function syntax at the end.

ReScript functions are declared with an arrow and return an expression, just like JS functions. They compile to clean JS functions too.

letgreet= (name) =>"Hello "++name

functiongreet(name){return"Hello "+name;}

This declares a function and assigns to it the name greet, which you can call like so:

greet("world!") // "Hello world!"

greet("world!");

Multi-arguments functions have arguments separated by comma:

letadd= (x, y, z) =>x+y+zadd(1, 2, 3) // 6

functionadd(x,y,z){return(x+y|0)+z|0;}

For longer functions, you'd surround the body with a block:

letgreetMore= (name) => { letpart1="Hello"part1++" "++name }

functiongreetMore(name){return"Hello "+name;}

If your function has no argument, just write let greetMore = () => {...}.

Labeled Arguments

Multi-arguments functions, especially those whose arguments are of the same type, can be confusing to call.

letaddCoordinates= (x, y) => { // use x and y here } // ...addCoordinates(5, 6) // which is x, which is y?

functionaddCoordinates(x,y){// use x and y here}addCoordinates(5,6);

You can attach labels to an argument by prefixing the name with the ~ symbol:

letaddCoordinates= (~x, ~y) => { // use x and y here } // ...addCoordinates(~x=5, ~y=6)

functionaddCoordinates(x,y){// use x and y here}addCoordinates(5,6);

You can provide the arguments in any order:

addCoordinates(~y=6, ~x=5)

addCoordinates(5,6);

The ~x part in the declaration means the function accepts an argument labeled x and can refer to it in the function body by the same name. You can also refer to the arguments inside the function body by a different name for conciseness:

letdrawCircle= (~radiusasr, ~colorasc) => { setColor(c) startAt(r, r) // ... } drawCircle(~radius=10, ~color="red")

functiondrawCircle(r,c){setColor(c);returnstartAt(r,r);}drawCircle(10,"red");

As a matter of fact, (~radius) is just a shorthand for (~radius as radius).

Here's the syntax for typing the arguments:

letdrawCircle= (~radiusasr: int, ~colorasc: string) => { // code here }

functiondrawCircle(r,c){// code here}

Optional Labeled Arguments

Labeled function arguments can be made optional during declaration. You can then omit them when calling the function.

// radius can be omittedletdrawCircle= (~color, ~radius=?) => { setColor(color) switchradius { | None=>startAt(1, 1) | Some(r_) =>startAt(r_, r_) } }

varCaml_option=require("./stdlib/caml_option.js");functiondrawCircle(color,radius){setColor(color);if(radius===undefined){returnstartAt(1,1);}varr_=Caml_option.valFromOption(radius);returnstartAt(r_,r_);}

When given in this syntax, radius is wrapped in the standard library's option type, defaulting to None. If provided, it'll be wrapped with a Some. So radius's type value is None | Some(int) here.

Signatures and Type Annotations

Functions with optional labeled arguments can be confusing when it comes to signature and type annotations. Indeed, the type of an optional labeled argument looks different depending on whether you're calling the function, or working inside the function body. Outside the function, a raw value is either passed in (int, for example), or left off entirely. Inside the function, the parameter is always there, but its value is an option (option<int>). This means that the type signature is different, depending on whether you're writing out the function type, or the parameter type annotation. The first being a raw value, and the second being an option.

If we get back to our previous example and both add a signature and type annotations to its argument, we get this:

letdrawCircle: (~color: color, ~radius: int=?) =>unit= (~color: color, ~radius: option<int>=?) => { setColor(color) switchradius { | None=>startAt(1, 1) | Some(r_) =>startAt(r_, r_) } }

functiondrawCircle(color,radius){setColor(color);if(radius!==undefined){returnstartAt(radius,radius);}else{returnstartAt(1,1);}}

The first line is the function's signature, we would define it like that in an interface file (see Signatures). The function's signature describes the types that the outside world interacts with, hence the type int for radius because it indeed expects an int when called.

In the second line, we annotate the arguments to help us remember the types of the arguments when we use them inside the function's body, here indeed radius will be an option<int> inside the function.

So if you happen to struggle when writing the signature of a function with optional labeled arguments, try to remember this!

Explicitly Passed Optional

Sometimes, you might want to forward a value to a function without knowing whether the value is None or Some(a). Naively, you'd do:

letresult=switchpayloadRadius { | None=>drawCircle(~color) | Some(r) =>drawCircle(~color, ~radius=r) }

varr=payloadRadius;varresult=r!==undefined ? drawCircle(color,Caml_option.valFromOption(r)) : drawCircle(color);

This quickly gets tedious. We provide a shortcut:

letresult=drawCircle(~color, ~radius=?payloadRadius)

varresult=drawCircle(1,undefined);

This means "I understand radius is optional, and that when I pass it a value it needs to be an int, but I don't know whether the value I'm passing is None or Some(val), so I'll pass you the whole option wrapper".

Optional with Default Value

Optional labeled arguments can also be provided a default value. In this case, they aren't wrapped in an option type.

letdrawCircle= (~radius=1, ~color) => { setColor(color) startAt(radius, radius) }

functiondrawCircle(radiusOpt,color){varradius=radiusOpt!==undefined ? radiusOpt : 1;setColor(color);returnstartAt(radius,radius);}

Recursive Functions

ReScript chooses the sane default of preventing a function to be called recursively within itself. To make a function recursive, add the rec keyword after the let:

letrecneverTerminate= () =>neverTerminate()

functionneverTerminate(_param){while(true){_param=undefined;continue;};}

A simple recursive function may look like this:

// Recursively check every item on the list until one equals the `item`// argument. If a match is found, return `true`, otherwise return `false`letreclistHas= (list, item) =>switchlist { | list{} =>false | list{a, ...rest} =>a===item||listHas(rest, item) }

functionlistHas(_list,item){while(true){varlist=_list;if(!list){returnfalse;}if(list.hd===item){returntrue;}_list=list.tl;continue;};}

Recursively calling a function is bad for performance and the call stack. However, ReScript intelligently compiles tail recursion into a fast JavaScript loop. Try checking the JS output of the above code!

Mutually Recursive Functions

Mutually recursive functions start like a single recursive function using the rec keyword, and then are chained together with and:

letreccallSecond= () =>callFirst() andcallFirst= () =>callSecond()

functioncallSecond(_param){while(true){_param=undefined;continue;};}functioncallFirst(_param){while(true){_param=undefined;continue;};}

Partial Application

Since 11.0

To partially apply a function, use the explicit ... syntax.

letadd= (a, b) =>a+bletaddFive=add(5, ...)

functionadd(a,b){returna+b|0;}functionaddFive(extra){return5+extra|0;}

Async/Await

Just as in JS, an async function can be declared by adding async before the definition, and await can be used in the body of such functions. The output looks like idiomatic JS:

letgetUserName=async (userId) =>userIdletgreetUser=async (userId) => { letname=awaitgetUserName(userId) "Hello "++name++"!" }

asyncfunctiongreetUser(userId){varname=awaitgetUserName(userId);return"Hello "+name+"!";}

The return type of getUser is inferred to be promise<string>. Similarly, await getUserName(userId) returns a string when the function returns promise<string>. Using await outside of an async function (including in a non-async callback to an async function) is an error.

Ergonomic error handling

Error handling is done by simply using try/catch, or a switch with an exception case, just as in functions that are not async. Both JS exceptions and exceptions defined in ReScript can be caught. The compiler takes care of packaging JS exceptions into the builtin JsError exception:

exceptionSomeReScriptExceptionletsomethingThatMightThrow=async () =>raise(SomeReScriptException) letsomeAsyncFn=async () => { switchawaitsomethingThatMightThrow() { | data=>Some(data) | exceptionJsError(_) =>None | exceptionSomeReScriptException=>None } }

varSomeReScriptException=/* @__PURE__ */Caml_exceptions.create("Example.SomeReScriptException");asyncfunctionsomeAsyncFn(param){vardata;try{data=awaitsomethingThatMightThrow(undefined);}catch(raw_exn){varexn=Caml_js_exceptions.internalToOCamlException(raw_exn);if(exn.RE_EXN_ID==="JsError"){return;}if(exn.RE_EXN_ID===SomeReScriptException){return;}throwexn;}returndata;}

The ignore() Function

Occasionally you may want to ignore the return value of a function. ReScript provides an ignore() function that discards the value of its argument and returns ():

mySideEffect()->Promise.catch(handleError)->ignoresetTimeout(myFunc, 1000)->ignore

$$Promise.$$catch(mySideEffect(),function(prim){returnhandleError(prim);});setTimeout(function(prim){myFunc();},1000);

Tips & Tricks

Cheat sheet for the function syntaxes:

Declaration

// anonymous function (x, y) =>1// bind to a nameletadd= (x, y) =>1// labeledletadd= (~firstasx, ~secondasy) =>x+y// with punning sugarletadd= (~first, ~second) =>first+second// labeled with default valueletadd= (~firstasx=1, ~secondasy=2) =>x+y// with punningletadd= (~first=1, ~second=2) =>first+second// optionalletadd= (~firstasx=?, ~secondasy=?) =>switchx {...} // with punningletadd= (~first=?, ~second=?) =>switchfirst {...}

With Type Annotation

// anonymous function (x: int, y: int): int=>1// bind to a nameletadd= (x: int, y: int): int=>1// labeledletadd= (~firstasx: int, ~secondasy: int) : int=>x+y// with punning sugarletadd= (~first: int, ~second: int) : int=>first+second// labeled with default valueletadd= (~firstasx: int=1, ~secondasy: int=2) : int=>x+y// with punning sugarletadd= (~first: int=1, ~second: int=2) : int=>first+second// optionalletadd= (~firstasx: option<int>=?, ~secondasy: option<int>=?) : int=>switchx {...} // with punning sugar// note that the caller would pass an `int`, not `option<int>`// Inside the function, `first` and `second` are `option<int>`.letadd= (~first: option<int>=?, ~second: option<int>=?) : int=>switchfirst {...}

Application

add(x, y) // labeledadd(~first=1, ~second=2) // with punning sugaradd(~first, ~second) // application with default value. Same as normal applicationadd(~first=1, ~second=2) // explicit optional applicationadd(~first=?Some(1), ~second=?Some(2)) // with punningadd(~first?, ~second?)

With Type Annotation

// labeledadd(~first=1: int, ~second=2: int) // with punning sugaradd(~first: int, ~second: int) // application with default value. Same as normal applicationadd(~first=1: int, ~second=2: int) // explicit optional applicationadd(~first=?Some(1): option<int>, ~second=?Some(2): option<int>) // no punning sugar when you want to type annotate

Standalone Type Signature

// first arg type, second arg type, return typetypeadd= (int, int) =>int// labeledtypeadd= (~first: int, ~second: int) =>int// labeledtypeadd= (~first: int=?, ~second: int=?, unit) =>int

In Interface Files

To annotate a function from the implementation file (.res) in your interface file (.resi):

letadd: (int, int) =>int

The type annotation part is the same as the previous section on With Type Annotation.

Don't confuse let add: myType with type add = myType. When used in .resi interface files, the former exports the binding add while annotating it as type myType. The latter exports the type add, whose value is the type myType.

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function.mdx

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Function

Labeled Arguments

Optional Labeled Arguments

Signatures and Type Annotations

Explicitly Passed Optional

Optional with Default Value

Recursive Functions

Mutually Recursive Functions

Partial Application

Async/Await

Ergonomic error handling

The ignore() Function

Tips & Tricks

Declaration

With Type Annotation

Application

With Type Annotation

Standalone Type Signature

In Interface Files

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Function

Labeled Arguments

Optional Labeled Arguments

Signatures and Type Annotations

Explicitly Passed Optional

Optional with Default Value

Recursive Functions

Mutually Recursive Functions

Partial Application

Async/Await

Ergonomic error handling

The ignore() Function

Tips & Tricks

Declaration

With Type Annotation

Application

With Type Annotation

Standalone Type Signature

In Interface Files