Elm (programming language)


Elm is a domain-specific programming language for declaratively creating web browser-based graphical user interfaces. Elm is purely functional, and is developed with emphasis on usability, performance, and robustness. It advertises "no runtime exceptions in practice", made possible by the Elm compiler's static type checking.

History

Elm was initially designed by Evan Czaplicki as his thesis in 2012. The first release of Elm came with many examples and an online editor that made it easy to try out in a web browser. Evan Czaplicki joined Prezi in 2013 to work on Elm, and in 2016 moved to NoRedInk as an Open Source Engineer, also starting the Elm Software Foundation.
The initial implementation of the Elm compiler targets HTML, CSS, and JavaScript. The set of core tools has continued to expand, now including a REPL, package manager, time-traveling debugger, and installers for macOS and Windows. Elm also has an ecosystem of community created libraries and an advanced online editor that allows saved work and inclusion of community libraries.

Features

Elm has a small set of language constructs, including traditional if-expressions, let-expressions for local state, and case-expressions for pattern matching. As a functional language, it supports anonymous functions, functions as arguments, and partial application by default. Its semantics include immutable values, stateless functions, and static typing with type inference. Elm programs render HTML through a virtual DOM, and may interoperate with other code by using "JavaScript as a service".

Immutability

All values in Elm are immutable, meaning that a value cannot be modified after it is created. Elm uses persistent data structures to implement its Array, Dict, and Set libraries.

Static types

Elm is statically typed. Type annotations are optional but strongly encouraged. Annotations exist on the line above the definition. Elm uses a single colon to mean "has type".
Types include primitives like integers and strings, and basic data structures such as lists, tuples, and records. Functions have types written with arrows, for example round : Float -> Int. Custom types allow the programmer to create custom types to represent data in a way that matches the problem domain.
Types can refer to other types, for example a List Int. Types are always capitalized; lowercase names are type variables. For example, a List a is a list of values of unknown type. It is the type of the empty list and of the argument to List.length, which is agnostic to the list's elements. There are a few special types that programmers create to interact with the Elm runtime. For example, Html Msg represents a DOM tree whose event handlers all produce messages of type Msg.
Rather than allow any value to be implicitly nullable, Elm's standard library defines a Maybe a type. Code that produces or handles an optional value does so explicitly using this type, and all other code is guaranteed a value of the claimed type is actually present.

Module system

Elm has a module system that allows users to break their code into smaller parts called modules. Modules can hide implementation details such as helper functions, and group related code together. Modules serve as a namespace for imported code, such as Bitwise.and. Third party libraries consist of one or more modules, and are available from the . All libraries are versioned according to semver, which is enforced by the compiler and other tools. That is, removing a function or changing its type can only be done in a major release.

Interoperability with HTML, CSS, and JavaScript

Elm uses an abstraction called ports to communicate with JavaScript. It allows values to flow in and out of Elm programs, making it possible to communicate between Elm and JavaScript.
Elm has a library called elm/html that a programmer can use to write HTML and CSS within Elm. It uses a virtual DOM approach to make updates efficient.

Backend

Elm does not officially support server-side development. The core development team does not consider it as their primary goal and prefers to focus development on the enhancement of front-end development experience. Nevertheless, there are several independent projects, which attempt to explore possibilities to use Elm for the back-end. The projects are mainly stuck on Elm version 0.18.0 since newer ones do not support "native" code and some other utilized features. There are two attempts to use Elm with BEAM. One of the projects executes Elm directly on the environment while another one compiles it to Elixir. Also, there was an attempt to create a back-end framework for Elm powered by Node.js infrastructure. None of the projects are production-ready.

Limitations

Elm does not support higher-kinded polymorphism,, which related languages Haskell and PureScript offer in the form of type classes.
This means that, for example, Elm does not have a generic map function which works across multiple data structures such as List and Set. In Elm, such functions are typically invoked qualified by their module name, for example calling List.map and Set.map. In Haskell or PureScript, there would be only one function map. This is a known feature request that is on Czaplicki's rough roadmap since at least 2015.
Another outcome is a large amount of boilerplate code in medium to large size projects as illustrated by the author of "Elm in Action" in their single page application example with almost identical fragments being repeated in update, view, subscriptions, route parsing and building functions.

Example code


-- This is a single line comment
-- Here we define a value named greeting. The type is inferred as a String.
greeting =
"Hello World!"
-- It is best to add type annotations to top-level declarations.
hello : String
hello =
"Hi there."
-- Functions are declared the same way, with arguments following the function name.
add x y =
x + y
-- Again, it is best to add type annotations.
hypotenuse : Float -> Float -> Float
hypotenuse a b =
sqrt
-- Functions are also curried; here we've curried the multiplication
-- infix operator with a `2`
multiplyBy2 : number -> number
multiplyBy2 =
2
-- If-expressions are used to branch on values
absoluteValue : number -> number
absoluteValue number =
if number < 0 then negate number else number
-- Records are used to hold values with named fields
book :
book =

-- Record access is done with `.`
title : String
title =
book.title
-- Record access `.` can also be used as a function
author : String
author =
.author book
-- We can create entirely new types with the `type` keyword.
-- The following value represents a binary tree.
type Tree a
= Empty
| Node a
-- It is possible to inspect these types with case-expressions.
depth : Tree a -> Int
depth tree =
case tree of
Empty ->
0
Node value left right ->
1 + max