diff options
| -rw-r--r-- | elm.html.markdown | 107 |
1 files changed, 64 insertions, 43 deletions
diff --git a/elm.html.markdown b/elm.html.markdown index 8c191509..944ab770 100644 --- a/elm.html.markdown +++ b/elm.html.markdown @@ -38,11 +38,10 @@ not False -- True 1 < 10 -- True -- Strings and characters -"This is a string." -'a' -- character -'You cant use single quotes for strings.' -- error! +"This is a string because it uses double quotes." +'a' -- characters in single quotes --- Strings can be appended +-- Strings can be appended. "Hello " ++ "world!" -- "Hello world!" {-- Lists, Tuples, and Records --} @@ -53,10 +52,10 @@ not False -- True -- The second example can also be written with two dots. [1..5] --- Append lists just like strings +-- Append lists just like strings. [1..5] ++ [6..10] == [1..10] -- True --- To add one item, use "cons" +-- 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 @@ -64,6 +63,7 @@ not False -- True 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. @@ -74,23 +74,28 @@ List.head [] -- Nothing fst ("elm", 42) -- "elm" snd ("elm", 42) -- 42 --- Records are like tuples but the fields have names. --- Notice that equals signs, not colons, are used. -{ x = 3, y = 7} +-- 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 +{ x = 3, y = 7 }.x -- 3 --- Or with an accessor fuction, a dot and then the field name. -.y { x = 3, y = 7} -- 7 +-- Or with an accessor fuction, 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" } -{ physics | - position = physics.position + physics.velocity, - velocity = physics.velocity + physics.acceleration } +-- Update multiple fields at once, using the current values. +{ particle | + position = particle.position + particle.velocity, + velocity = particle.velocity + particle.acceleration } {-- Control Flow --} @@ -111,11 +116,16 @@ else -- 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" + Nothing -> "The list was empty." {-- Functions --} @@ -126,7 +136,7 @@ case List.head aList of multiply a b = a * b --- Apply (call) a function by passing it arguments (no commas necessay). +-- 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. @@ -151,8 +161,8 @@ area (width, height) = area (6, 7) -- 42 --- Use curly brackets to pattern match record field names --- Use let to define intermediate values +-- Use curly brackets to pattern match record field names. +-- Use let to define intermediate values. volume {width, height, depth} = let area = width * height @@ -161,21 +171,22 @@ volume {width, height, depth} = volume { width = 3, height = 2, depth = 7 } -- 42 --- Functions can be recursive +-- 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] +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 application happens before any infix operation +-- 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. @@ -191,7 +202,7 @@ cos (degrees 30) ^ 2 + sin (degrees 30) ^ 2 -- 1 True : Bool -- Functions have types too. Read -> as "goes to". Think of the rightmost type --- as the type of the return value. +-- as the type of the return value, and the others as arguments. not : Bool -> Bool round : Float -> Int @@ -220,34 +231,41 @@ 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 } +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 +otherOrigin = + Point3D 0 0 0 --- But it's still the same type, you can equate them +-- 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 +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 - +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 +root = + Node 7 Leaf Leaf -- Union types (and type aliases) can use type variables. -type Tree a = Leaf | Node a (Tree a) (Tree a) +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." --- You can pattern match union tags. The uppercase tags must be matched exactly. --- The lowercase variables will match anything. Underscore also matches --- anything, but signifies that you aren't using it. +-- 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 @@ -260,21 +278,20 @@ leftmostElement tree = {-- Modules and Imports --} --- The core libraries are organized into modulues, as are any third-party --- libraries you may use. For large projects, you can define your own modulues. +-- 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. --- Limit what values and types are exported -module Name (Type, value) 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 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 @@ -318,6 +335,8 @@ $ elm repl -- 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! ``` @@ -335,12 +354,14 @@ Here are some useful resources. * Documentation for [Elm's core libraries](http://package.elm-lang.org/packages/elm-lang/core/latest/). Take note of: * [Basics](http://package.elm-lang.org/packages/elm-lang/core/latest/Basics), which is imported by default - * Data structures like [Array](http://package.elm-lang.org/packages/elm-lang/core/latest/Array), [Dict](http://package.elm-lang.org/packages/elm-lang/core/latest/Dict), and [Set](http://package.elm-lang.org/packages/elm-lang/core/latest/Set) + * [Maybe](http://package.elm-lang.org/packages/elm-lang/core/latest/Maybe) and its cousin [Result](http://package.elm-lang.org/packages/elm-lang/core/latest/Result), commonly used for missing values or error handling + * Data structures like [List](http://package.elm-lang.org/packages/elm-lang/core/latest/List), [Array](http://package.elm-lang.org/packages/elm-lang/core/latest/Array), [Dict](http://package.elm-lang.org/packages/elm-lang/core/latest/Dict), and [Set](http://package.elm-lang.org/packages/elm-lang/core/latest/Set) * JSON [encoding](http://package.elm-lang.org/packages/elm-lang/core/latest/Json-Encode) and [decoding](http://package.elm-lang.org/packages/elm-lang/core/latest/Json-Decode) -* [The Elm Architecture](https://github.com/evancz/elm-architecture-tutorial#the-elm-architecture). An essay with examples on how to organize code into components. +* [The Elm Architecture](https://github.com/evancz/elm-architecture-tutorial#the-elm-architecture). An essay by Elm's creator with examples on how to organize code into components. * The [Elm mailing list](https://groups.google.com/forum/#!forum/elm-discuss). Everyone is friendly and helpful. +* [Scope in Elm](https://github.com/elm-guides/elm-for-js/blob/master/Scope.md#scope-in-elm) and [How to Read a Type Annotation](https://github.com/elm-guides/elm-for-js/blob/master/How%20to%20Read%20a%20Type%20Annotation.md#how-to-read-a-type-annotation). More community guides on the basics of Elm, written for JavaScript developers. Go out and write some Elm! |