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---
language: reason
filename: reason.re
contributors:
- ["Seth Corker", "https://sethcorker.com"]
---
Reason is a syntax over OCaml that is easier to get started for programmers who are familiar with C-style syntax like JavaScript. BuckleScript is part of the toolchain which compiles Reason to JavaScript so you can write statically typed code for anywhere that JavaScript runs.
```javascript
/* Comments start with slash-star, and end with star-slash */
/*----------------------------------------------
* Variable and function declaration
*----------------------------------------------
* Variables and functions use the let keyword and end with a semi-colon
* `let` bindings are immutable
*/
let x = 5;
/* - Notice we didn't add a type, Reason will infer x is an int */
/* A function like this, take two arguments and add them together */
let add = (a, b) => a + b;
/* - This doesn't need a type annotation either! */
/*----------------------------------------------
* Type annotation
*----------------------------------------------
* Types don't need to be explicitly annotated in most cases but when you need
* to, you can add the type after the name
*/
/* A type can be explicitly written like so */
let x: int = 5;
/* The add function from before could be explicitly annotated too */
let add2 = (a: int, b: int): int => a + b;
/* A type can be aliased using the type keyword */
type companyId = int;
let myId: companyId = 101;
/* Mutation is not encouraged in Reason but it's there if you need it
If you need to mutate a let binding, the value must be wrapped in a `ref()`*/
let myMutableNumber = ref(120);
/* To access the value (and not the ref container), use `^` */
let copyOfMyMutableNumber = myMutableNumber^;
/* To assign a new value, use the `:=` operator */
myMutableNumber := 240;
/*----------------------------------------------
* Basic types and operators
*----------------------------------------------
*/
/* > String */
/* Use double quotes for strings */
let greeting = "Hello world!";
/* A string can span multiple lines */
let aLongerGreeting = "Look at me,
I'm a multi-line string
";
/* A quoted string can be used for string interpolation and special chars
Use the `js` annotation for unicode */
let world = {js|🌍|js};
/* The `j` annotation is used for string interpolation */
let helloWorld = {j|hello, $world|j};
/* Concatenate strings with ++ */
let name = "John " ++ "Wayne";
let emailSubject = "Hi " ++ name ++ ", you're a valued customer";
/* > Char */
/* Use a single character for the char type */
let lastLetter = 'z';
/* - Char doesn't support Unicode or UTF-8 */
/* > Boolean */
/* A boolean can be either true or false */
let isLearning = true;
true && false; /* - : bool = false; Logical and */
true || true; /* - : bool = true; Logical or */
!true; /* - : bool = false; Logical not */
/* Greater than `>`, or greater than or equal to `>=` */
'a' > 'b'; /* - bool : false */
/* Less than `<`, or less than or equal to `<=` */
1 < 5; /* - : bool = true */
/* Structural equal */
"hello" == "hello"; /* - : bool = true */
/* Referential equal */
"hello" === "hello"; /* - : bool = false */
/* - This is false because they are two different "hello" string literals */
/* Structural unequal */
lastLetter != 'a'; /* -: bool = true */
/* Referential unequal */
lastLetter !== lastLetter; /* - : bool = false */
/* > Integer */
/* Perform math operations on integers */
1 + 1; /* - : int = 2 */
25 - 11; /* - : int = 11 */
5 * 2 * 3; /* - : int = 30 */
8 / 2; /* - : int = 4 */
/* > Float */
/* Operators on floats have a dot after them */
1.1 +. 1.5; /* - : float = 2.6 */
18.0 -. 24.5; /* - : float = -6.5 */
2.5 *. 2.0; /* - : float = 5. */
16.0 /. 4.0; /* - : float = 4. */
/* > Tuple
* Tuples have the following attributes
- immutable
- ordered
- fix-sized at creation time
- heterogeneous (can contain different types of values)
A tuple is 2 or more values */
let teamMember = ("John", 25);
/* Type annotation matches the values */
let position2d: (float, float) = (9.0, 12.0);
/* Pattern matching is a great tool to retrieve just the values you care about
If we only want the y value, let's use `_` to ignore the value */
let (_, y) = position2d;
y +. 1.0; /* - : float = 13. */
/* > Record */
/* A record has to have an explicit type */
type trainJourney = {
destination: string,
capacity: int,
averageSpeed: float,
};
/* Once the type is declared, Reason can infer it whenever it comes up */
let firstTrip = {destination: "London", capacity: 45, averageSpeed: 120.0};
/* Access a property using dot notation */
let maxPassengers = firstTrip.capacity;
/* If you define the record type in a different file, you have to reference the
filename, if trainJourney was in a file called Trips.re */
let secondTrip: Trips.firstTrip = {
destination: "Paris",
capacity: 50,
averageSpeed: 150.0,
};
/* Records are immutable by default */
/* But the contents of a record can be copied using the spread operator */
let newTrip = {...secondTrip, averageSpeed: 120.0};
/* A record property can be mutated explicitly with the `mutable` keyword */
type breakfastCereal = {
name: string,
mutable amount: int,
};
let tastyMuesli = {name: "Tasty Muesli TM", amount: 500};
tastyMuesli.amount = 200;
/* - tastyMuesli now has an amount of 200 */
/* Punning is used to avoid redundant typing */
let name = "Just As Good Muesli";
let justAsGoodMuesli = {name, amount: 500};
/* - justAsGoodMuesli.name is now "Just As Good Muesli", it's equivalent
to { name: name, amount: 500 } */
/* > Variant
Mutually exclusive states can be expressed with variants */
type authType =
| GitHub
| Facebook
| Google
| Password;
/* - The constructors must be capitalized like so */
/* - Like records, variants should be named if declared in a different file */
let userPreferredAuth = GitHub;
/* Variants work great with a switch statement */
let loginMessage =
switch (userPreferredAuth) {
| GitHub => "Login with GitHub credentials."
| Facebook => "Login with your Facebook account."
| Google => "Login with your Google account"
| Password => "Login with email and password."
};
/* > Option
An option can be None or Some('a) where 'a is the type */
let userId = Some(23);
/* A switch handles the two cases */
let alertMessage =
switch (userId) {
| Some(id) => "Welcome, your ID is" ++ string_of_int(id)
| None => "You don't have an account!"
};
/* - Missing a case, `None` or `Some`, would cause an error */
/* > List
* Lists have the following attributes
- immutable
- ordered
- fast at prepending items
- fast at splitting
* Lists in Reason are linked lists
*/
/* A list is declared with square brackets */
let userIds = [1, 4, 8];
/* The type can be explicitly set with list('a) where 'a is the type */
type idList = list(int);
type attendanceList = list(string);
/* Lists are immutable */
/* But the contents of a list can be copied using the spread operator */
let newUserIds = [101, 102, ...userIds];
/* > Array
* Arrays have the following attributes
- mutable
- fast at random access & updates */
/* An array is declared with `[|` and ends with `|]` */
let languages = [|"Reason", "JavaScript", "OCaml"|];
/*----------------------------------------------
* Function
*----------------------------------------------
*/
/* Reason functions use the arrow syntax, the expression is returned */
let signUpToNewsletter = email => "Thanks for signing up " ++ email;
/* Call a function like this */
signUpToNewsletter("hello@reason.org");
/* For longer functions, use a block */
let getEmailPrefs = email => {
let message = "Update settings for " ++ email;
let prefs = ["Weekly News", "Daily Notifications"];
(message, prefs);
};
/* - the final tuple is implicitly returned */
/* > Labeled Arguments */
/* Arguments can be labeled with the ~ symbol */
let moveTo = (~x, ~y) => {/* Move to x,y */};
moveTo(~x=7.0, ~y=3.5);
/* Labeled arguments can also have a name used within the function */
let getMessage = (~message as msg) => "==" ++ msg ++ "==";
getMessage(~message="You have a message!");
/* - The caller specifies ~message but internally the function can make use */
/* The following function also has explicit types declared */
let showDialog = (~message: string): unit => {
() /* Show the dialog */;
};
/* - The return type is `unit`, this is a special type that is equivalent to
specifying that this function doesn't return a value
the `unit` type can also be represented as `()` */
/* > Currying
Functions can be curried and are partially called, allowing for easy reuse */
let div = (denom, numr) => numr / denom;
let divBySix = div(6);
let divByTwo = div(2);
div(3, 24); /* - : int = 8 */
divBySix(128); /* - : int = 21 */
divByTwo(10); /* - : int = 5 */
/* > Optional Labeled Arguments */
/* Use `=?` syntax for optional labeled arguments */
let greetPerson = (~name, ~greeting=?, ()) => {
switch (greeting) {
| Some(greet) => greet ++ " " ++ name
| None => "Hi " ++ name
};
};
/* - The third argument, `unit` or `()` is required because if we omitted it,
the function would be curried so greetPerson(~name="Kate") would create
a partial function, to fix this we add `unit` when we declare and call it */
/* Call greetPerson without the optional labeled argument */
greetPerson(~name="Kate", ());
/* Call greetPerson with all arguments */
greetPerson(~name="Marco", ~greeting="How are you today,");
/* > Pipe */
/* Functions can be called with the pipeline operator */
/* Use `->` to pass in the first argument (pipe-first) */
3->div(24); /* - : int = 8 */
/* - This is equivalent to div(3, 24); */
36->divBySix; /* - : int = 6 */
/* - This is equivalent to divBySix(36); */
/* Use `|>` to pass in the last argument (pipe-last) */
24 |> div(3); /* - : int = 8 */
/* - This is equivalent to div(3, 24); */
36 |> divBySix; /* - : int = 6 */
/* - This is equivalent to divBySix(36); */
/* Pipes make it easier to chain code together */
let addOne = a => a + 1;
let divByTwo = a => a / 2;
let multByThree = a => a * 3;
let pipedValue = 3->addOne->divByTwo->multByThree; /* - : int = 6 */
/*----------------------------------------------
* Control Flow & Pattern Matching
*----------------------------------------------
*/
/* > If-else */
/* In Reason, `If` is an expression when evaluate will return the result */
/* greeting will be "Good morning!" */
let greeting = if (true) {"Good morning!"} else {"Hello!"};
/* Without an else branch the expression will return `unit` or `()` */
if (false) {
showDialog(~message="Are you sure you want to leave?");
};
/* - Because the result will be of type `unit`, both return types should be of
the same type if you want to assign the result. */
/* > Destructuring */
/* Extract properties from data structures easily */
let aTuple = ("Teacher", 101);
/* We can extract the values of a tuple */
let (name, classNum) = aTuple;
/* The properties of a record can be extracted too */
type person = {
firstName: string,
age: int,
};
let bjorn = {firstName: "Bjorn", age: 28};
/* The variable names have to match with the record property names */
let {firstName, age} = bjorn;
/* But we can rename them like so */
let {firstName: bName, age: bAge} = bjorn;
let {firstName: cName, age: _} = bjorn;
/* > Switch
Pattern matching with switches is an important tool in Reason
It can be used in combination with destructuring for an expressive and
concise tool */
/* Lets take a simple list */
let firstNames = ["James", "Jean", "Geoff"];
/* We can pattern match on the names for each case we want to handle */
switch (firstNames) {
| [] => "No names"
| [first] => "Only " ++ first
| [first, second] => "A couple of names " ++ first ++ "," ++ second
| [first, second, third] =>
"Three names, " ++ first ++ ", " ++ second ++ ", " ++ third
| _ => "Lots of names"
};
/* - The `_` is a catch all at the end, it signifies that we don't care what
the value is so it will match every other case */
/* > When clause */
let isJohn = a => a == "John";
let maybeName = Some("John");
/* When can add more complex logic to a simple switch */
let aGreeting =
switch (maybeName) {
| Some(name) when isJohn(name) => "Hi John! How's it going?"
| Some(name) => "Hi " ++ name ++ ", welcome."
| None => "No one to greet."
};
/* > Exception */
/* Define a custom exception */
exception Under_Age;
/* Raise an exception within a function */
let driveToTown = (driver: person) =>
if (driver.age >= 15) {
"We're in town";
} else {
raise(Under_Age);
};
let evan = {firstName: "Evan", age: 14};
/* Pattern match on the exception Under_Age */
switch (driveToTown(evan)) {
| status => print_endline(status)
| exception Under_Age =>
print_endline(evan.firstName ++ " is too young to drive!")
};
/* Alternatively, a try block can be used */
/* - With Reason exceptions can be avoided with optionals and are seldom used */
let messageToEvan =
try (driveToTown(evan)) {
| Under_Age => evan.firstName ++ " is too young to drive!"
};
/*----------------------------------------------
* Object
*----------------------------------------------
* Objects are similar to Record types but aren't as rigid
* An object resembles a class
*/
/* An object may be typed like a record but contains a dot */
type surfaceComputer = {
.
color: string,
capacity: int,
};
/* - A single dot signifies a closed object, an object that uses this type
must have the exact shape */
let surfaceBook: surfaceComputer = {pub color = "blue"; pub capacity = 512};
/* But an object doesn't require a type */
let house = {
/* A private property */
val temp = ref(18.0);
/* Public properties */
pub temperature = temp;
/* A private method only accessible from within house */
pri setThermostat = v => temp := v;
/* A public method that calls the private setThermostat method */
pub arriveHome = () => this#setThermostat(22.0)
};
house#temperature; /* - : float = 18. */
house#arriveHome();
house#temperature; /* - : float = 22. */
/*----------------------------------------------
* Module
*----------------------------------------------
* Modules are used to organize your code and provide namespacing.
* Each file is a module by default
*/
/* Create a module */
module Staff = {
type role =
| Delivery
| Sales
| Other;
type member = {
name: string,
role,
};
let getRoleDirectionMessage = staff =>
switch (staff.role) {
| Delivery => "Deliver it like you mean it!"
| Sales => "Sell it like only you can!"
| Other => "You're an important part of the team!"
};
};
/* A module can be accessed with dot notation */
let newEmployee: Staff.member = {name: "Laura", role: Staff.Delivery};
/* Using the module name can be tiresome so the module's contents can be opened
into the current scope with `open` */
open Staff;
let otherNewEmployee: member = {name: "Fred", role: Other};
/* A module can be extended using the `include` keyword, include copies
the contents of the module into the scope of the new module */
module SpecializedStaff = {
include Staff;
/* `member` is included so there's no need to reference it explicitly */
let ceo: member = {name: "Reggie", role: Other};
let getMeetingTime = staff =>
switch (staff) {
| Other => 11_15 /* - : int = 1115; Underscores are for formatting only */
| _ => 9_30
};
};
```
## Further Reading
- [Official Reason Docs](https://reasonml.github.io/docs/en/what-and-why)
- [Official BuckleScript Docs](https://bucklescript.github.io/docs/en/what-why)
- [Try Reason](https://reasonml.github.io/en/try)
- [Get Started with Reason by Nik Graf](https://egghead.io/courses/get-started-with-reason)
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