| language | contributors | filename | |||
|---|---|---|---|---|---|
Elm |
|
learnelm.elm |
Elm is a functional reactive programming language that compiles to (client-side) JavaScript. Elm is statically typed, meaning that the compiler catches most errors immediately and provides a clear and understandable error message. Elm is great for designing user interfaces and games for the web.
-- Single line comments start with two dashes.
{- Multiline comments can be enclosed in a block like this.
{- They can be nested. -}
-}
{-- The Basics --}
-- Arithmetic
1 + 1 -- 2
8 - 1 -- 7
10 * 2 -- 20
-- Every number literal without a decimal point can be either an Int or a Float.
33 / 2 -- 16.5 with floating point division
33 // 2 -- 16 with integer division
-- Exponents
5 ^ 2 -- 25
-- Booleans
not True -- False
not False -- True
1 == 1 -- True
1 /= 1 -- False
1 < 10 -- True
-- Strings and characters
"This is a string because it uses double quotes."
'a' -- characters in single quotes
-- Strings can be appended.
"Hello " ++ "world!" -- "Hello world!"
{-- Lists, Tuples, and Records --}
-- Every element in a list must have the same type.
["the", "quick", "brown", "fox"]
[1, 2, 3, 4, 5]
-- The second example can also be written with two dots.
[1..5]
-- Append lists just like strings.
[1..5] ++ [6..10] == [1..10] -- True
-- To add one item, use "cons".
0 :: [1..5] -- [0, 1, 2, 3, 4, 5]
-- The head and tail of a list are returned as a Maybe. Instead of checking
-- every value to see if it's null, you deal with missing values explicitly.
List.head [1..5] -- Just 1
List.tail [1..5] -- Just [2, 3, 4, 5]
List.head [] -- Nothing
-- List.functionName means the function lives in the List module.
-- Every element in a tuple can be a different type, but a tuple has a
-- fixed length.
("elm", 42)
-- Access the elements of a pair with the first and second functions.
-- (This is a shortcut; we'll come to the "real way" in a bit.)
fst ("elm", 42) -- "elm"
snd ("elm", 42) -- 42
-- The empty tuple, or "unit", is sometimes used as a placeholder.
-- It is the only value of its type, also called "Unit".
()
-- Records are like tuples but the fields have names. The order of fields
-- doesn't matter. Notice that record values use equals signs, not colons.
{ x = 3, y = 7 }
-- Access a field with a dot and the field name.
{ x = 3, y = 7 }.x -- 3
-- Or with an accessor function, which is a dot and the field name on its own.
.y { x = 3, y = 7 } -- 7
-- Update the fields of a record. (It must have the fields already.)
{ person |
name = "George" }
-- Update multiple fields at once, using the current values.
{ particle |
position = particle.position + particle.velocity,
velocity = particle.velocity + particle.acceleration }
{-- Control Flow --}
-- If statements always have an else, and the branches must be the same type.
if powerLevel > 9000 then
"WHOA!"
else
"meh"
-- If statements can be chained.
if n < 0 then
"n is negative"
else if n > 0 then
"n is positive"
else
"n is zero"
-- Use case statements to pattern match on different possibilities.
case aList of
[] -> "matches the empty list"
[x]-> "matches a list of exactly one item, " ++ toString x
x::xs -> "matches a list of at least one item whose head is " ++ toString x
-- Pattern matches go in order. If we put [x] last, it would never match because
-- x::xs also matches (xs would be the empty list). Matches do not "fall through".
-- The compiler will alert you to missing or extra cases.
-- Pattern match on a Maybe.
case List.head aList of
Just x -> "The head is " ++ toString x
Nothing -> "The list was empty."
{-- Functions --}
-- Elm's syntax for functions is very minimal, relying mostly on whitespace
-- rather than parentheses and curly brackets. There is no "return" keyword.
-- Define a function with its name, arguments, an equals sign, and the body.
multiply a b =
a * b
-- Apply (call) a function by passing it arguments (no commas necessary).
multiply 7 6 -- 42
-- Partially apply a function by passing only some of its arguments.
-- Then give that function a new name.
double =
multiply 2
-- Constants are similar, except there are no arguments.
answer =
42
-- Pass functions as arguments to other functions.
List.map double [1..4] -- [2, 4, 6, 8]
-- Or write an anonymous function.
List.map (\a -> a * 2) [1..4] -- [2, 4, 6, 8]
-- You can pattern match in function definitions when there's only one case.
-- This function takes one tuple rather than two arguments.
-- This is the way you'll usually unpack/extract values from tuples.
area (width, height) =
width * height
area (6, 7) -- 42
-- Use curly brackets to pattern match record field names.
-- Use let to define intermediate values.
volume {width, height, depth} =
let
area = width * height
in
area * depth
volume { width = 3, height = 2, depth = 7 } -- 42
-- Functions can be recursive.
fib n =
if n < 2 then
1
else
fib (n - 1) + fib (n - 2)
List.map fib [0..8] -- [1, 1, 2, 3, 5, 8, 13, 21, 34]
-- Another recursive function (use List.length in real code).
listLength aList =
case aList of
[] -> 0
x::xs -> 1 + listLength xs
-- Function calls happen before any infix operator. Parens indicate precedence.
cos (degrees 30) ^ 2 + sin (degrees 30) ^ 2 -- 1
-- First degrees is applied to 30, then the result is passed to the trig
-- functions, which is then squared, and the addition happens last.
{-- Types and Type Annotations --}
-- The compiler will infer the type of every value in your program.
-- Types are always uppercase. Read x : T as "x has type T".
-- Some common types, which you might see in Elm's REPL.
5 : Int
6.7 : Float
"hello" : String
True : Bool
-- Functions have types too. Read -> as "goes to". Think of the rightmost type
-- as the type of the return value, and the others as arguments.
not : Bool -> Bool
round : Float -> Int
-- When you define a value, it's good practice to write its type above it.
-- The annotation is a form of documentation, which is verified by the compiler.
double : Int -> Int
double x = x * 2
-- Function arguments are passed in parentheses.
-- Lowercase types are type variables: they can be any type, as long as each
-- call is consistent.
List.map : (a -> b) -> List a -> List b
-- "List dot map has type a-goes-to-b, goes to list of a, goes to list of b."
-- There are three special lowercase types: number, comparable, and appendable.
-- Numbers allow you to use arithmetic on Ints and Floats.
-- Comparable allows you to order numbers and strings, like a < b.
-- Appendable things can be combined with a ++ b.
{-- Type Aliases and Union Types --}
-- When you write a record or tuple, its type already exists.
-- (Notice that record types use colon and record values use equals.)
origin : { x : Float, y : Float, z : Float }
origin =
{ x = 0, y = 0, z = 0 }
-- You can give existing types a nice name with a type alias.
type alias Point3D =
{ x : Float, y : Float, z : Float }
-- If you alias a record, you can use the name as a constructor function.
otherOrigin : Point3D
otherOrigin =
Point3D 0 0 0
-- But it's still the same type, so you can equate them.
origin == otherOrigin -- True
-- By contrast, defining a union type creates a type that didn't exist before.
-- A union type is so called because it can be one of many possibilities.
-- Each of the possibilities is represented as a "tag".
type Direction =
North | South | East | West
-- Tags can carry other values of known type. This can work recursively.
type IntTree =
Leaf | Node Int IntTree IntTree
-- "Leaf" and "Node" are the tags. Everything following a tag is a type.
-- Tags can be used as values or functions.
root : IntTree
root =
Node 7 Leaf Leaf
-- Union types (and type aliases) can use type variables.
type Tree a =
Leaf | Node a (Tree a) (Tree a)
-- "The type tree-of-a is a leaf, or a node of a, tree-of-a, and tree-of-a."
-- Pattern match union tags. The uppercase tags will be matched exactly. The
-- lowercase variables will match anything. Underscore also matches anything,
-- but signifies that you aren't using it.
leftmostElement : Tree a -> Maybe a
leftmostElement tree =
case tree of
Leaf -> Nothing
Node x Leaf _ -> Just x
Node _ subtree _ -> leftmostElement subtree
-- That's pretty much it for the language itself. Now let's see how to organize
-- and run your code.
{-- Modules and Imports --}
-- The core libraries are organized into modules, as are any third-party
-- libraries you may use. For large projects, you can define your own modules.
-- Put this at the top of the file. If omitted, you're in Main.
module Name where
-- By default, everything is exported. You can specify exports explicity.
module Name (MyType, myValue) where
-- One common pattern is to export a union type but not its tags. This is known
-- as an "opaque type", and is frequently used in libraries.
-- Import code from other modules to use it in this one.
-- Places Dict in scope, so you can call Dict.insert.
import Dict
-- Imports the Dict module and the Dict type, so your annotations don't have to
-- say Dict.Dict. You can still use Dict.insert.
import Dict exposing (Dict)
-- Rename an import.
import Graphics.Collage as C
{-- Ports --}
-- A port indicates that you will be communicating with the outside world.
-- Ports are only allowed in the Main module.
-- An incoming port is just a type signature.
port clientID : Int
-- An outgoing port has a definition.
port clientOrders : List String
port clientOrders = ["Books", "Groceries", "Furniture"]
-- We won't go into the details, but you set up callbacks in JavaScript to send
-- on incoming ports and receive on outgoing ports.
{-- Command Line Tools --}
-- Compile a file.
$ elm make MyFile.elm
-- The first time you do this, Elm will install the core libraries and create
-- elm-package.json, where information about your project is kept.
-- The reactor is a server that compiles and runs your files.
-- Click the wrench next to file names to enter the time-travelling debugger!
$ elm reactor
-- Experiment with simple expressions in a Read-Eval-Print Loop.
$ elm repl
-- Packages are identified by GitHub username and repo name.
-- Install a new package, and record it in elm-package.json.
$ elm package install evancz/elm-html
-- See what changed between versions of a package.
$ elm package diff evancz/elm-html 3.0.0 4.0.2
-- Elm's package manager enforces semantic versioning, so minor version bumps
-- will never break your build!The Elm language is surprisingly small. You can now look through almost any Elm source code and have a rough idea of what is going on. However, the possibilities for error-resistant and easy-to-refactor code are endless!
Here are some useful resources.
-
The Elm website. Includes:
- Links to the installers
- Documentation guides, including the syntax reference
- Lots of helpful examples
-
Documentation for Elm's core libraries. Take note of:
-
The Elm Architecture. An essay by Elm's creator with examples on how to organize code into components.
-
The Elm mailing list. Everyone is friendly and helpful.
-
Scope in Elm and How to Read a Type Annotation. More community guides on the basics of Elm, written for JavaScript developers.
Go out and write some Elm!